The external frame function in the control of pitch, register, and singing mode: Radiographic observations of a female singer

Journal of Voice

Vol. 13, No. 3, pp. 319-340 © 1999 Singular Publishing Group, Inc.

The External Frame Function in the Control of Pitch, Register, and Singing Mode: Radiographic Observations of a Female Singer *Aatto Sonninen, *Pertti Hurme, and tAnne-Maria Laukkanen *Department of Communication, Universityof Jyviiskylii, Jyviiskyl& Finland; ?Institute of Speech Communication and VoiceResearch, Universityof Tampere, Tampere, Finland

Summary: This study investigates pitch control, register, and singing mode related movements of the laryngo-pharyngeal structures by radiographic methods. One trained female singer served as the subject. The results show that singing voice production involves complex movements in the laryngeal structures. Pitch related increase in the thyro-arytenoid distance (vocal fold length) is nonlinear, slowing down as pitch rises. Similar observations have been made earlier. At the highest pitches, a shortening of the distance can be seen, suggesting the use of alternative pitch control mechanisms. The various observations made support the existence of three registers in this trained female singing voice. Open and covered modes of singing seemed to be distinguishable on the basis of different amounts of inner and outer forces acting on the larynx. Therefore, caution must be exercised when generalizing from the results. Key Words: Radiography-Mode of singing--Pitch control--Registers.

Singing is based on a complex synchronous and successive function of various muscles. There is most likely considerable variation in this function between different singing styles. Covered singing can be regarded as the established way of producing the voice quality valued in Western operatic singing. (Covered singing is associated with flow phonation by eg, Hertegfird, Gauffin, & Sundberg. 1) Open singing, in turn, is the speechlike singing which, in the Western operatic tradition, is regarded as unacceptable and typical of untrained singers.

According to the external frame function theory, a the activity of various extrinsic laryngeal muscles greatly affects voice production by controlling the circumstances for vocal fold vibration. The basic components in the external frame function theory are identified in Figure 1. Hyoid muscles may pull the thyroid cartilage in the anterior-superior direction. Sternothyroid muscles and the inhalation-related tracheal pull may translocate the thyroid cartilage in the anteriorinferior direction. These changes in the location of the thyroid cartilage may assist the cricothyroid (CT) muscle in the elongation of the vocal folds if the cricopharyngeal (CP) muscle stabilizes the cricoid cartilage. Vocal fold elongation, in turn, increases the stiffness of the tissue and thus raises the fundamental frequency of vibration.

Accepted for publication October 20, 1998. Address correspondence and reprint requests to Aatto Sonninen, Gummeruksenkatu 3 B, FIN-40100, Jyv~iskyl~i,Finland, e-mail: [email protected]

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In this study, changes in the distances between laryngeal structures are supposed to give information about the state of the vocal folds as well as about the function of various muscles and muscle groups. The validity of this method can be questioned. However, it should be kept in mind that a muscle is an organ that produces movements by contraction, never by extension. Extension is caused by the contraction of an antagonist muscle. Muscle contraction causes either a shortening of the distance between the insertion points of the muscle (isotonic contraction) or an increase in the inner tension of the muscle without affecting the distance between the insertion points (isometric contraction). In the latter case, measurement of the distance between the insertion points of the muscle does not reveal anything about the degree of activation in the muscle. Nevertheless, isometric contraction of one muscle may result in changes in the length of another muscle sharing the insertion point with the former, thereby revealing the extent of isometric contraction. Figure 2 describes the interdependency, "functional chains" (cf. 3) of selected distances between laryngeal and extrinsic laryngeal structures. A change in one distance is usually reflected in other distances. Distance 1-2 illustrates the distance between the larynx and the anterior parts of the hyoid and the mandible. Distance 2-3 represents the length of the vocal folds. Distance 3-4 is the distance between the Journal of Voice, Vol. 13, No.3, 1999

E FIG. 2. Interdependency of distances in a functional chain of laryngeal and extrinsic laryngeal structures. The complement indicates the difference between the minimum and maximum sum of the distances.

larynx and the cervical spine. Suppose that a shorter distance indicates a stronger contractile force between two insertion points of a muscle and a longer distance a weaker contractile force. In Figure 2 the letters R Q, and R refer to muscles affecting the distances 1-2, 2-3, and 3-4, respectively. The top constellation (A) illustrates a case with relaxed muscles resulting in maximal distances. The constellation below (B) shows a case with much contraction in all the muscles resulting in minimal distances. Therefore, the complement describing the difference between maximum (A) and minimum (B) states of contraction is large. This line of reasoning is further illustrated in the lower part of Figure 2: Even though the distance 2-3 is equal in cases C and D, contraction in Q must be greater in case D (indicating greater stiffness in the vocal folds) than in case C, because the distances 1-2 and 3-4 are shorter, implying that the contractile forces of P and R are stronger. In case D, the increased isotonic contraction of Q prevents any increase in the distance 2-3. In case E, the distance 2-3 is larger in spite of isotonic contraction of Q, because the contractile forces of P and R are maximal. The present study aims at describing how the distances between laryngeal structures change as a function of vocal pitch and as a function of singing mode.

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 321 The modes of singing studied were covered singing both in piano and forte and open singing in forte. (Soft phonation most likely cannot be produced in the open mode.) An attempt will be made to explain the observations through a functional chain existing in voice production. The study also aims at finding out how "good" and "poor" singing differ on the level of muscle activity in the laryngeal region.

METHODS Subject and registration The material for the present study was obtained from earlier radiographic registrations by the first author. Comparable radiographic studies have not been made. The results of the registrations have so far been reported only partly.2, 4 A trained female singer (dramatic soprano, age 40 years) served as the subject in the study. She is also an actress and a skilled mimic. Therefore, she was highly capable of simulating various voice qualities. The subject was investigated by radiography while phonating samples of the sustained /a/ vowel for about 5 seconds each. The samples were in semitone intervals throughout the entire physiological frequency range of phonation, going up the scale in 3 modes of phonation: in piano andforte loudness in the covered mode of singing and in forte loudness in the open mode of singing (piano covered, forte covered, and forte open). The singer started with forte covered mode (singing the whole series of notes), whereafter she sang the series in forte open and last in piano covered mode. (In piano singing there were only a limited number of measurement points.) Each task was performed once. The maximal frequency range of phonation was C#3-G#6 (139-1661 Hz) for forte covered, F#3-A#4 (185-466 Hz) for forte open and G#3-D#6 (208-1246 Hz) for piano covered singing. The subject's primo passaggio was between D#4-F4 and secondo passaggio was between D#5-F5. Radiograms were taken in approximately the middle of the production of each vowel. Audio recordings were also made. The subject stood in her normal singing position, with the forehead touching a head rest to stabilize the head position. The movements of the mandible were restricted by instructing the subject to phonate with equal subjective loudness in each singing mode and at every pitch.

The subject generally regarded the tasks as easy to fulfill. However, she felt open singing was very unpleasant and strenuous. The singing teacher of the subject was present in the recording sessions to perceptually control the quality of the samples. In a confirmational but not definitive exercise, 5 singing teachers listened to the samples of covered and open singing (at least 5 seconds each). The productions that the singer intended to be covered were unanimously accepted as representative of that mode of singing. The same applied to open singing. There is a clear singer's formant in the forte covered samples, except for the lowest (below F#3) and highest (above C6) pitches. 2 Other systematic spectral differences between the singing modes (eg, the spectrum was less steep in open than in covered singing) have also been found. 5 Sound pressure level measured with a Brtiel & Kja~r level meter at the distance of 75 cm from the subject's lips varied between 68-81 dB inpiano covered, 67-104 dB inforte covered and 80-104 dB infbrte open (cf. Fig. 8C).

Measurements Figure 3A shows the anatomical structures and the ossification/calcification centers which were chosen as measurement points since they were easily recognizable: M = mandible (a fixed distance from the 2nd cervical vertebra), H = hyoid bone (the anterior-inferior corner), H' -- hyoid bone (a fixed distance (30 mm) from the body of the hyoid bone along the greater cornua), T = thyroid cartilage, A = arytenoid cartilage, and C = cricoid cartilage. The exact measurement points have been marked with the black centers in the unfilled circles. The radiograms were analyzed as follows. The radiogram was perforated with a sharp needle at two points, the anterior-inferior corners of the 5th and 6th cervical vertebrae. The film was placed on a transparency so that the line connecting these two points was vertical (thus, the position of all radiograms was standardized). To measure sagittal and vertical movements of the larynx a reference point has to be determined. The mandible is not a reliable reference, as it moves during voice production. The upper sternum (the jugular notch) would be a more reliable reference point, but it was not visible in these radiograms. There was also some movement in the cervical spine during singing. Nevertheless, the line connecting the Journal of Voice, Vol. 13, No. 3, 1999

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bodies of the 2nd to the 5th cervical vertebra was chosen as the vertical coordinate (y) in the measurements (see Figure 3A), as these vertebrae were well visible in the radiograms. The x-coordinate was drawn at an angle of 90 ° across the y-coordinate at the 5th cervical vertebra. Further steps were taken to produce the illustrations: all the measurement points were perforated through the film to the transparency, the measurement points were marked with a narrow felt pencil on the transparency and essential structures in the area were sketched on the transparency, the transparency was digitized with a scanner, and the scans were read into a drawing program where the figure was magnified. The measurement points were strengthened and the lines representing laryngeal and extrinsic laryngeal structures were smoothed. Changes in the distances between laryngeal cartilages were used to give indirect information on changes in the length of the vocal folds. The changes in distances between the larynx and the mandible as well Journal of Voice, Vol. 13, No.3, 1999

as between the larynx and the cervical spine were also examined. The technique of measuring vocal fold length and larynx position by means of lateral radiograms has been described in detail in earlier publications by the present authors. 2,4 When measuring changes in the length of the vocal folds, the anterior-inferior part of the thyroid cartilage was chosen to represent the anterior insertion of the vocal folds (point T). The posterior-superior part of the cricoid cartilage (point C) was chosen to represent the posterior insertion point of the vocal folds in all cases selected for this study. An average of the ossification centers of both arytenoid cartilages (point A) was measured. The choice of reference length is critical when comparing strain values. Because of projection errors in lateral radiograms, it is not possible to measure the thyroid-arytenoid distance during respiration. Therefore, the distances obtained in all measurements at pitch D#4 in forte covered singing were set as the reference strain. Throughout the study, the measure-

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 323 ments reported are unadjusted for projection error. They are about 10% higher than real values. Nevertheless, absolute values are not essential for the purposes of the present study: Changes in laryngeal distances are reflected fairly well by relative values. Length-changes in the vocal folds can be measured in absolute values such as millimeters. However, it is often useful to employ a normalized measure of elongation such as strain. Strain means standardized elongation: the change in length divided by the original starting length of the vocal fold body multiplied by 100. The relation between longitudinal stress and strain is nonlinear and follows the stress-strain curve of organic tissue.6, 7 In singing the relationship between vocal fold strain and vocal pitch is also typically nonlinear. 8-12 To standardize the degree of strain, an index of strain per semitone has been proposed by Sonninen and Hurme. 13 This Semitone Strain Index (SSI) is calculated by dividing the difference in the strain values of the lowest and highest pitch of a certain pitch range by the number of semitones in the range. The semitone strain thus indicates the average amount of strain per 1 semitone of pitch increase or decrease. A low value on the semitone strain index indicates much stiffness, and a high SSI value indicates slight stiffness in the vocal folds. The index has been shown to be affected by severn factors: gender, singing training, singing technique, voice class, age, and status of muscle function as well as methodological decisions. ~3 To compare singing modes, SSI was calculated in the pitch range common to them. Furthermore, SSI was compared below the first passaggio (G3-C#4) and above it (F4-A#4). (Strain values at 1=#3 were excluded, as the value in open singing clearly differs from the others, probably because it approaches the physiological limit.) In piano covered singing SSI was calculated between G#3-D#4 and D#4-G#5. The vertical size of the laryngeal ventricle (the maximum distance between upper margin of the vocal folds and lower margin of the ventricular folds, indicated by V) was also measured. To examine the forces influencing the size of the laryngeal ventricle, a new ratio is introduced: the Ventricle Quotient (VQ), equaling V in mm divided by T - A in mm. Additional qualitative observations were made of the width and vertical position of the pharynx.

To obtain a more general picture of laryngeal constellations (and muscle activity, as far as possible), tracings shown in Figure 3B were made of the pharyngeal and tracheal walls. In this way, cervical bending, shape and position of the pharynx, shape and position of the larynx, and the position of the thyroid cartilage and the hyoid bone could be examined at various pitches and singing modes. The tracings were standardized in relation to the 5th and 6th cervical vertebrae, as shown in Figure 3B. The figure also shows the epiglottis and the anterior extremity of the laryngeal ventricle. The lower end of the pharynx was often seen, especially at low pitches. Anatomically, what is seen as the lower end of the pharynx (hypopharynx) are the aryepiglottic folds or pharyngeal constrictor in the vicinity of the orifice of the oesophagus. To examine the reliability of measurements, 3 sets of distance measurements were taken twice (47 Cx measurements by one person at the interval of 30 years, and 20 Hx measurements, ie, the distance between the hyoid bone and the 6th cervical vertebra by two persons, and 16 T - A measurements by two persons). Relative error was less than 10% in almost all cases (89%) and less than 3% in the majority of the cases (57%). Significant correlation between the sets of measurements was found (r = .81-.94). Thus, the measurements can be regarded as reliable. Absolute values naturally vary in different distances (eg, the distance M - T varied between 37-70 mm and Cx between 5.0-12.5 mm). Therefore the margin of error is different for each distance. The margin of error for Hx measurement was 0.2 mm and that for T - A measurement was 0.13 mm. The remeasurement of the vertical size of the laryngeal ventricle accomplished by one person showed a significant correlation, but two sets of measurements done by two persons did not correlate. The average difference between the measurements was 2 mm. Thus, the V and VQ measures should be regarded as tentative.

RESULTS Distances and positions of laryngeal structures Hyoid bone The vertical and sagittal movements of the hyoid bone can be seen in Figure 4. In all modes of singing Journal of Voice, Vol. 13, No. 3, 1999

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Vertical hyoid position H y m m • Forte Open oForte Covered Respiration Piano Covered

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the vertical position of the hyoid bone is higher than during respiration (Figure 4A). The sagittal position is also more anterior in singing than in respiration with the exception of the lowest pitches (Figure 4B). The hyoid bone moves considerably both vertically and sagittally with increasing pitch (Figure 4C). In general, the hyoid bone moves in the anteriorsuperior direction as the pitch rises. However, there are exceptions, especially in covered singing in the Journal of Voice, Vol. 13, No.3, 1999

passaggio areas. The hyoid bone moves in the anterior-superior direction up to the primo passaggio area, the pitch range where the primary register shift takes place (in the figures the two passaggio areas are marked with gray columns). The rise culminates at D#4 for the open mode and at F4 for the covered mode. Immediately above these frequencies, the hyold bone is lowered and retracted. Anterior-superior movement resumes above the first passaggio and

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 325 continues approximately up to the second passaggio area. At and below the firstpassaggio, the position of the hyoid bone is lower and more posterior in open singing than in covered singing, whereas the opposite is true above the first passaggio. In the covered mode the anterior movement of the hyoid bone ends at F#5, while the upward movement continues until G#5. Above these pitches, the hyoid bone moves in the posterior-inferior direction. At the highest pitch (G#6), the hyoid bone makes a sudden forward movement. When singing in piano covered mode, the vertical and sagittal position of the hyoid bone stays constant at the lowest pitches. Between A4-E5, the hyoid bone moves slightly up and forward to adopt a fairly constant position at pitches above E5. Below the first passaggio, the position of the hyoid bone is higher and more anterior in piano covered singing than in the 2 other modes of singing. At the first passaggio, the opposite can be seen. Between the first and secondpassaggio, the hyoid bone stays higher and more anterior compared to forte covered singing, but lower and more posterior than in forte open singing. At and above the second passaggio, the position of the hyoid bone is lower and more posterior in piano covered than forte covered singing.

Pharynx and epilarynx Figure 5 illustrates the vertical position and shape of the lower part of the pharynx (hypopharynx) in re-

lation to pitch and mode of singing (with the line connecting the 5th and 6th cervical vertebrae as the reference, see Figure 3B). There are differences between the singing modes. The vertical movement of the pharynx naturally follows the vertical movements of the larynx. The sagittal changes of the pharynx may reflect differences either in the activity of the pharyngeal constrictors or in the positions of the epiglottis and the spine. The pharynx is wider in forte open singing and piano covered singing than in forte covered singing and respiration. Figure 5 shows that there is much pitch-related variation in the width of the lower pharynx: the width seems to increase at the passaggio regions. This is probably related to an anterior movement of the hyoid bone and the larynx (as indicated by the sagittal measurements of the cricoid cartilage (see Figure 6B). There is much variation in the position of the cervical spine during singing (compared to the line connecting the 5th and 6th vertebra); see Appendix 1 for details. During singing (in all singing modes and at all pitches) the cervical spine is more bent in the anterior direction than during respiration. An increase in pitch generally results in increased anterior bending. Additionally, Figure 5 shows sudden increases in bending at some pitches: in forte open singing between F4 and F#4, in forte covered singing between A#4 and C5, and in piano covered singing between

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Journal of Voice, Vol. 13, No. 3, 1999

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Primo Secondo passaggio Journal of Voice, Vol. 13, No.3, 1999

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 327 G#4 and D#5. (As can be seen in the figures, there are only few measurement points in piano covered singing.) At the two first mentioned pitch areas, the cricoid-spine distance is relatively small (see Figure 6B), which suggests strong activity in the cricopharyngeal muscle. In piano covered singing, in turn, the larynx moves slightly forward around the second passaggio, and thus the bending of the spine may result from the forces pulling the larynx in the anterior direction. At high pitches, the cervical spine bends about as much in covered and in piano covered singing. At the highest pitches in forte open singing, between F#4 and A#4, there is more bending than in forte covered and piano covered singing and the cricoid-spine distance is larger. At that pitch area, the position of the larynx (Cy) is also higher and slightly more anterior in the forte open mode (see Figures 6A and 6B), suggesting the existence of strong forces pulling the larynx in the anterior-superior direction. Larynx

Figure 6 illustrates pitch-related changes in vertical and sagittal laryngeal position (6A, 6B) through the movements of the cricoid cartilage, as well as changes in the mandible-hyoid (6C), mandible-thyroid (6D) and hyoid-thyroid (6E) distances. Appendix 2 shows the position of the hyoid bone and the thyroid cartilage in relation to the line connecting the 5th and 6th cervical vertebrae. The data are given for each singing mode at each semitone and for respiration. It can be seen in Appendix 2 that there are irregularities especially in the passaggio areas. Mandible-hyoid and mandible-thyroid distances mainly seem to reflect movements of the larynx, while the hyoid-thyroid distance seems to reflect somewhat independent changes in the vertical hyoid position (see Figure 6). The vertical and sagittal position of the larynx in singing shows clear deviation from that during respiration. In piano covered singing the larynx moves smoothly upward throughout the frequency range. In .forte open and forte covered singing the vertical position of the larynx (VLP, indicated by Cy in Figure 6A) rises with pitch up to or close to the first passaggio, and above that an abrupt drop (more marked in covered than open singing) takes place. After the sudden drop VLP shows a tendency for a downward movement in covered mode as pitch rises close to the sec-

ond passaggio. At B4, however, a sudden rise in VLP can be seen. After the second passaggio, VLP goes constantly up with pitch. In open singing VLP continues to rise after the disturbance in the area of the first passaggio. VLP is generally higher both in open and forte covered modes of singing than in respiration except for the lowest pitches and in forte covered singing pitches close to the second passaggio. VLP is higher in open singing than in covered singing between G#3-C4 and above the first passaggio, while the opposite is true at the first passaggio region. In piano covered singing, VLP is lower than in the other modes below and at the first passaggio. At the secondpassaggio, it is higher than in forte covered mode. The sagittal position of the larynx (see Figure 6B) is generally more anterior in covered singing than in respiration and in open singing. In open mode, the larynx is mainly more posterior than in respiration except for the highest pitches. In piano covered singing, the larynx is more anterior than in respiration only at the lowest pitches. The largest difference in the sagittal position of the cricoid between covered and open singing is at E4, the subjectively reported register transition area. Further, Figure 6B shows that in both the open mode and in the forte covered mode, the cricoid cartilage moves backward up to the first passaggio where a sudden anterior movement can be seen, followed by a posterior movement. Above the first passaggio, the cricoid cartilage moves in an anterior direction and then in forte covered singing continues up to the second passaggio, after which it starts moving backward again. At the highest pitch, a strong anterior movement can be seen. In piano covered singing the larynx moves backward up to G#4, takes a more anterior position at the second passaggio, and makes a small back-and-forth movement at pitches above that. In forte open singing, the mandible-thyroid distance (M-T, see Figure 6C) is shortened up to the primo passaggio and then the two structures stay close to each other. In forte covered singing, the distance is shortened up to the primo passaggio, albeit not as much as in open singing. From primo to secondo passaggio, the distance is enlarged, and above that shortened again. Piano covered singing resemblesforte covered singing above the secondo passaggio; below that the mandible-thyroid distance is similar to the Journal of Voice, Vol. 13, No. 3, 1999

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AATTO SONNINEN ET AL

distance during respiration. Mandible-hyoid distances (M-H, see Figure 6D) are similar to mandible-thyroid distances. The hyoid-thyroid (H-T, see Figure 6E) distances decrease as pitch increases in all singing modes. Usually the distances are shorter than in respiration. Below the primo passaggio, the distance is largest in piano covered singing and shortest in forte open singing, with forte covered singing in between. Figure 7 describes the sagittal and vertical position (Cx, Cy) of the larynx by means of coordinates based on the line connecting the 2nd and 5th cervical vertebra (see Figure 3A). For clarity, the data in Figure 7A are presented in groups (4 to 15) of four semitones each. In general, the larynx rises and moves in a posterior direction until the primo passaggio area.

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However, in open singing, the larynx is higher and closer to the spine than in forte covered singing. Above that area, in open singing, the larynx continues to rise, whereas in covered singing it abruptly lowers. Above the secondo passaggio, the VLPs of piano and forte covered singing coincide. The position of the larynx in groups 8 and 9 (D#4-A#4) differs dramatically in covered and open singing. Figure 7B shows that the position of the larynx at E4 is very posterior in open singing and very anterior in covered singing. In covered singing, the singer makes a radical readjustment when singing at F4; the larynx moves into a lower and more posterior position (close to the position during respiration).

Intralaryngeal distances Distances between thyroid, arytenoid and cricoid cartilages are given in Figure 8. Figure 8A shows that the thyro-arytenoid (T-A) distance increases with pitch in all modes of singing indicating increasing length of the vocal folds. This increase in the thyro-arytenoid distance is fastest below the first passaggio and ends above the second passaggio; at the highest pitches a shortening of the distance can even be seen. However, there are also similar drops in the T-A distance at lower pitches. In both open andforte covered singing a sudden decrease in the T - A distance can be seen at the lowest pitch range (at E3-F3 for covered mode and at G3-G#3 for open mode), as well as at or slightly below the passaggio regions. T - A distance is higher in piano covered singing at all pitches below the second passaggio. No special passaggio events can be seen in piano covered singing. Forte covered and forte open modes of singing do not differ from each other in the mean T-A distance, ie, the vocal fold length is approximately the same in both modes. The T - A distance is significantly larger in piano covered singing than in forte covered singing (t = 4.9, P < .0002***, calculated from measurements in the area common to both, ie, F#3-A#4). Thus the vocal folds are longer in piano covered singing. The mandible-thyroid distance is larger in forte covered thanforte open singing (t = 10.6, P < .0001"**), and even larger in piano covered. The crico-arytenoid distance is larger in open than forte covered singing (t = -12.3, P < .0001"**). In piano covered the distance is slightly larger than in forte covered, but clearly smaller than in forte open singing. The

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C4 F4 C5 C6 Primo Secondo passaggio

G#6

cricoid-spine distances, in tum, are significantly smaller in the forte open mode of singing than in piano and forte covered singing (t = 3.0, P < .009**). Figure 8C shows the SPL as a function of fundamental frequency. The SPL measurements will be related to T - A measurements in the discussion. The mean T-A differences between covered and open singing in the area common to both were not statistically significant. An attempt was made to examine the T-A differences more closely by means of the Semitone Strain Index. Figure 9 gives the SSI values in piano and forte covered and in forte open singing. In all three modes of singing, SSI decreases with increasing pitch, even though the change is minimal in piano covered singing. The SSI values are lower in forte covered than in forte open and the decrease of the values with pitch rise is greater. Below the first passaggio, SSI is .93 in forte covered, .98 in forte open, and .76 in piano covered singing. Above the first passaggio, SSI is .22 in forte covered, .72 in forte open, and .75 in piano covered singing. Figure 8B indicates that, below the first passaggio, there is a tendency for the crico-arytenoid distance to increase with pitch in all modes of singing, although the change is not linear. In forte covered singing, the distance A-C is rather large at both passaggio areas. In the pitch range between them, the crico-arytenoid distance increases. The distance A-C is larger in forte open singing than in piano and forte covered singing at any particular pitch.

Semitone Strain Index

C 110

o Forte C ~ e n

oForte Covered ~>Piano Covered

1-

,91 ,81

100

,71

o Forte Covered • Forte Open o Piano Covered

,6". ,51 t4"

,31

7O

,2

60

C3

C4

C5

C6

G#6

FIG. 8. Distances between thyroid, arytenoid and cricoid cartilage (A, B) in different modes of singing. SPL is shown in Figure 8C.

Below first passaggio

Above first Passaggio

FIG. 9. Semitone strain index (SSI) and change in it with pitch rise in forte covered, forte open and in piano covered singing. SSI is compared for 2 pitch ranges below (G3-C#4) and above (F4-A#4) the first passaggio in forte covered and forte open modes. In piano covered the ranges G#3, D#4, and G#4 are compared.

Journal of Voice, Vol. 13, No. 3, 1999

330

AATTO SONNINEN ET AL

Laryngeal ventricle The vertical size of the laryngeal ventricle is indicated with a solid line in Figure 10. The figure also shows the cricoid-spine distance (Cy) and the hyoidthyroid distance (H-T) in the 3 singing modes. In general, the ventricle becomes smaller as the pitch rises. However, in forte open voice the size of the ventricle appears to have a complex relation to pitch. Nevertheless, it is small compared to the size of the ventricle in covered singing at similar pitches. Changes in the vertical laryngeal position explain the majority of changes in the size of the ventricle (see Cy in Figure 10); as the laryngeal position rises the ventricle becomes smaller. On the other hand, the size of the ventricle also reflects changes in vocal fold thickness as well as distances between hyoid bone and the thyroid cartilage. As the distance decreases, glottal tissue is folded and the ventricle becomes smaller. At the lowest pitches, the pitch-synchronous diminishing of the ventricle may suggest increased activity of the TA muscle, which increases thickness of the vocal folds. The fact that between F4-G#4 the ventricle is relatively large in open singing even though VLP is rising may be related to a decrease of thickness in the vocal folds as well as to the fact that the hyoid bone moves markedly forward (see the hyoid-spine distance in Figure 4B). Between E4-F4 in forte covered singing the ventricle is small although VLP is lower; the hyo-thyroid distance also increases (see H - T in Figure 10). However, at the same time the hyoid bone moves in a posterior direction which probably causes folding of the laryngeal tissue by loosening the aryepiglottic fold, thus explaining the decrease in the ventricular size. Between A#4-B4 in forte covered singing, the ventricle is relatively large, although VLP rises and the hyo-thyroid distance decreases. The explanation may be that the hyoid bone moves in an anterior direction at the same time, most likely decreasing the folding of the laryngeal tissue. At the second passaggio, the ventricle is small in covered mode although VLP is low, the hyo-thyroid distance is relatively large, and the hyoid bone moves forward. One possible explanation may be a strong anterior tilting of the thyroid cartilage, related to an anterior bending of the cervical spine (see Appendix 2); the tilting might also increase the folding of the glottal tissue. At pitches above the second passaggio, the rising vertical posiJournal of Voice, Vol. 13, No.3, 1999

A

Forte Covered .... V * Cy

.......................~ H - T

4 30

3

25 20

1

15

0

B

4 ( Primo Secondo passaggio

C#

Forte O p e n ~ V 4 60 -° 3

o Cy

10

..................H - T

50 ........................., 40:

~"

30..~......................................... I 20:

C#3

C

'k

C4 F4 C5 C6 G#64 Primo Secondo passaggio

Piano Covered

116o

~, Cy

.......... H - T 30 25 20 15

0

k..~g d

10 C4 P4

C,

l~imo Secondo passaggio FIG. 10. Changes in the vertical size of the laryngeal ventricle (V), in the vertical laryngeal position (Cy) and hyoidthyroid distance (H-T) during different modes of singing.

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 331 tion of the larynx may explain the decrease in the size of the ventricle. At the highest pitches, the ventricle becomes larger despite further rising of the larynx. At the same time, however, the hyoid bone moves pronouncedly forward. In general, the ventricle is smaller in forte open than in forte covered singing at any particular pitch. This may partly be due to a somewhat higher vertical position of the larynx in the forte open mode (at some pitch ranges) and partly due to the smaller hyothyroid distance (see Figure 6E). It is also possible and likely that vocal fold thickness is greater in forte open singing (see below), thus diminishing the ventricle. However, the ventricle is relatively small in piano covered singing where vocal fold thickness is supposed to be small. This may be due to the shorter hyo-thyroid distance in piano covered singing (see Figure 6E). Figure 11 displays the Ventricle Quotient across the frequency range from C3 to G#6 in forte and piano covered singing and forte open. In piano and forte covered singing, there is a strong increase in the quotient below the first and the second passaggi, while a sudden drop in the quotient can be seen at the passaggio region. In piano covered singing, the quo2 tient decreases linearly with pitch after the first passaggio area. VQ is in general highest in the forte covered and smallest in the forte open mode. The pitch-related decrease in VQ reflects an increase in T - A and a decrease in the size of the ventricle, which,

oForte Covered

• Forte Open

o Piano Covered

~ C3 C4 C5 C6 FIG. 11. Changes in Ventricle Quotient (the vertical size of the ventricle V in mm divided by distance T-A in ram) in different modes of singing.

in turn, seems to be related to the rising vertical position of the larynx.

Distances in relation to singing modes Figure 12 shows the M-T, T-A, A-C, and Cx distances in the pitch area F#3-A#4 in forte and piano covered andforte open singing. In forte open singing, especially above A3 and in the register transition area D#4-F4, the vocal folds are longer and the external distances (M-T and Cx) shorter than in forte covered singing. The Cx distances above F#4 in relation to the distances at the first passaggio region and below are larger in forte open than in covered piano and forte singing, possibly due to shorter M - T distances in forte open than in piano and forte covered singing. The complement is largest at C4 in forte open singing, giving evidence for considerable external forces pulling the vocal folds. To examine the relations of pitch and the measured laryngeal distances, Spearman Rank Correlations were computed, Figure t3 shows the statistically significant correlations resulting from the computations. The correlation between pitch and T - A is very strong both in covered (r = .97***) and open (r = .86***) singing. Pitch and several laryngeal distances show a significant correlation. The vertical and sagittal positions of the hyoid bone (Hy, Hx) show a positive correlation with pitch (ie, the distances increase as pitch rises) in both singing modes (Hy covered r = .65** and Hy Open r = .95***, Hx covered r = .51", Hx open r = .93***). However, covered and open singing also differ. Covered singing shows a negative correlation between pitch and the cricoid-spine distance (Cx r = -.70**), the mandible-thyroid distance (M-T r = -.78**), and the hyoid-thyroid distance (H-T r = -.75**), ie, these distances decrease as pitch rises. Forte open singing shows a positive correlation between pitch and the cricoid-arytenoid distance (A-C r = .60*) and with the vertical size of the laryngeal ventricle (V r = .67**), and a negative correlation between pitch and the mandible-hyoid (M-H r = - . 5 2 " ) distance. The rank correlation between the T - A distance and the other distances was also computed. The lower part of Figure 13 shows that the T-A distance has a negative correlation with the cricoid-spine distance (Cx r = -.63*) in covered and a positive correlation in open (Cx r = .52*) and with the vertical position Journal of Voice, Vol. 13, No. 3, 1999

332

AATTO SONNINEN ET AL M-T

T-A

A-C

Cx

Forte C o v e r e d

2.% -y---2® H-T/ M-H ~ M-T.\

Forte Open M-H

I-Ix , , ' N ~ s s S C Y

cx

H y .. - - - - k ~ ) ~ • I I

i I

I

H y --" ~ . T - A -

I-Ix ~ Cx

Hy

Forte ()pen T-A

A-C

Negative c o r r e l a t i o n - -

Cx

~'V

I

H-+~ I

M-T

A-C

*

sCy

I ,,"

. - ... ',T - A -- - - -Cx % %. V p < .05

** p ~ .01 *** p a .001 Positive correlation - - -- -- * p a .05 ..... p a .01 -- - - " *** p < .001 FIG. 13. Rank correlations of pitch (F0) and T-A distance with laryngeal distances. Upper graph shows the correlations between pitch and the measured distances and the lower graph between T-A and the measured distances. Positive correlation is indicated with a solid line, negative with a dashed line. Line thickness indicates the strength of the correlation.

Piano Covered M-T

T-A

A-C

Cx o f the hyoid bone in covered (Hy r = .58*) and in open singing m o d e (Hy r = .79**). In covered singing, T - A has a negative correlation with the mandible-hyoid (M-H r = -.49"), mandible-thyroid (M-T r = - . 7 4 " * ) , and hyoid-thyroid ( H - T r = - . 7 1 " * ) distances. In forte open singing, T - A has a positive correlation with the vertical and sagittal position ot the hyoid b o n e (Hy r = .79"*, H x r = .76"*), the vertical size o f the laryngeal ventricle (V r -- .65**) and the vertical laryngeal position (Cy r =.75**).

M-T 0

30

T-A

20 22 24 26 28 3 0 m m

0

40

50

A-C 0

8

9

10

Cx

4

6

8

0

60 m m

11

12mm

10 m m

FIG. 12. Distances M-T, T-A, A-C and Cx at each pitch in the area common to the 3 modes of singing (F#3-A#4). Calculated in absolute millimeter values. The gray area on the left indicates the complement, i.e. the difference between the minimum and maximum sum of the distances (relative to F#3 in covered voice).

Journalof Voice,Vol.13, No.3, 1999

DISCUSSION Above, w e have presented details on laryngopharyngeal positions and distances during singing. In the figures, the m e a s u r e m e n t s fall in overall patterns. There are also minute deviations ("ripple") f r o m one m e a s u r e m e n t point to another in the patterns. Deviations are to be expected, as w e are m e a s u r i n g h u m a n behavior. T h e ripple is m a i n l y due to the perform a n c e o f the subject, as the m e a s u r e m e n t error was very small (on average .16 m m in T - A measurements). It should be kept in m i n d that the observa-

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 333 tions have been made only for one singer and no repetitions were made. Thus, caution must be exercized when generalizing from the results. However, both the overall patterns and the changes from one measurement point to another provide insights into singing voice production. Autonomous, separate measurements as such are not valuable (and their sheer number can be bewildering) unless interpreted in a wider framework. In our opinion, laryngeal distances (and the muscles involved) should be considered as links in a chain, a Mandible-Hyoid-Thyroid-Arytenoid-Cricoid-Cervical Spine (MHTACC) functional chain. Below, our discussion centers on the vocal folds, as represented by the thyroid-arytenoid distance, a part of the sagittal functional chain. In singing, biomechanical control follows the principle of trade-off or motor equivalence between the actitivity of muscles. Pitch control Pitch is raised by increasing the stiffness of the vocal folds. Stiffness can be increased by increasing the length of the vocal folds by activating the cricothyroid muscle (CT) and extrinsic laryngeal muscles or by introducing isometric contraction of the thyroarytenoid (TA) muscle. Increased subglottic pressure also increases stiffness of the vocal fold tissue and thus increases F 0 because the amplitude of vibration of the vocal folds is increased.7 Therefore, if F 0 is to be kept constant, the activity of the CT muscle must be reduced if subglottic pressure is increased; contrastively, if subglottic pressure is decreased, the activity of the CT muscle must be increased to prevent lowering of F0.14 The role of the TA in fundamental frequency control is dependent on the amplitude of vibration of the vocal folds. If the amplitude is large enough to involve the muscular layer of the vocal fold, then contraction of TA raises F 0 since it increases the effective stiffness of the vibrating portion of the vocal fold. This holds true for phonation at lower pitches and when the intensity of phonation is sufficiently large. In contrast, when the amplitude of vibration of the vocal folds is low and the vibration concentrates mainly on the cover part, contraction of the TA lowers F 0, since it loosens the cover. Increased activity of the TA muscle thus tends to lower F 0 at high frequencies and when intensity is low. 15

In this study, the thyroarytenoid distance (corresponding to the length of the vocal folds) changed with an increase in pitch in all modes of singing. The change was seen as an increase in T-A distance (relatively fast) below the first passaggio, and leveling above the second passaggio. At the highest pitches, a shortening of the distance could be seen. Thus, the change in vocal fold length is pitch range dependent. Reflecting this, the semitone strain index decreases with F 0. This can be explained by (1) that the stiffness of the vocal fold tissue increases through longitudinal stretch, and a smaller increase in stiffness is needed to increase pitch comparably, and (2) that the effective mass participating in vibration decreases as the length of the vocal folds increases. Nonlinearities in the relation between pitch and the T-A distance most likely indicate changes in the pitch control mechanism. A decreased T-A distance in relation to a pitch increase may suggest that the pitch increase is brought about by increased activity in the thyroarytenoid muscle and thus increased stiffness of the vocal fold muscular layer. Decreasing T - A distance together with pitch rise above the second passaggio may be related to the adoption of another pitch control mechanism, shortening of the vibrating glottis by increased adduction. This may include activation of the sphincter mechanism, which usually is supposed to be related to pitch lowering at the low pitch. However, also at the highest pitches it is possible to see pitch rise related narrowing of the laryngeal tube, eg, through nasoendoscopy. Changes in subglottic pressure could also affect the nonlinearities in vocal fold length changes. Subglottic pressure was not measured in this study. Results from SPL measurements seem to suggest that subglottic pressure did not have a decisive role in F 0 control. SPL decreased both as T - A distance decreased and increased strongly at the first passaggio, while at the second passaggio region, SPL increased both when T-A distance decreased and increased (see Figure 8). Thus, in these samples, voice intensity seemed to be related neither to shortening nor to lenghening of the T-A distance. Zenker and Zenker3 claims that the cricopharyngeal muscle shortens the vocal folds. This may be true at low pitches, but at least for the subject of this study, the cricoarytenoid distance increases and the Cx distance decreases at high pitches, which is not in Journal of Voice, Vol. 13, No. 3, 1999

334

AATTO SONNINEN ET AL

accordance with Zenker's claim. No muscle other than the cricopharyngeal can pull the cricoid cartilage in the posterior direction, elongating the A-C distance and shortening the Cx distance. The A-C distance alone can also be increased by the anterior pull of the thyroarytenoid muscle. It has been suggested that the posterior cricoarytenoid muscle (PCA), the abductor, assists in raising the pitch by stretching the vocal folds in the posterior direction. 16 However, increasing A-C distance in the present study shows that the PCA muscle has not been able to participate in the elongation of the vocal folds. Interesting pitch-related movements were also seen in laryngeal positions and distances other than T - A and A-C. They may be related to pitch control. However, because these differences seemed to be related not only to pitch change, but also to the pitch range where the change was accomplished and to the singing mode, they will be discussed below in connection with register and singing mode.

Forte Covered o

oT-A

Cx A-C

C3 A

C4 F4 C5 C6 Primo Secondo

G#6

passaggio

Forte Open o

o T-A

Cx A-C

Register

From the perspective of artistic perception, singing in one register only is often desirable. Some singers feel that only poor singers have register boundaries or shifts. However, from a physiological perspective, there are two or more registers. A variety of pitch-related movements were observed in the laryngeal structures. These changes were different at different pitch ranges. Abrupt and large changes seem to center in the passaggio regions, especially in the forte covered mode. This seems to suggest that there are 3 biomechanical areas or registers in this singer. Figure 14 qualitatively summarizes values for T-A, A-C and Cx in forte covered singing (upper graph) and forte open singing (lower graph) on an arbitrary scale. In forte covered singing, in the passaggio regions, the T - A distance, (ie, the length of the vocal folds), first shortens as pitch rises from C#4 to D4 and then again as pitch rises from D#5 to E5. Above these frequencies, a remarkable lengthening in T - A distance takes place both when pitch further rises from D4 to D#4 and from F5 to F#5. Shortening of the T - A is related to increased A-C and Cx distances, while the marked lengthening of T - A is related to decreases in A-C and Cx distances. It Journal of Voice, Vol. 13, No.3, ]999

Primo B

Secondo passaggio

FIG. 14. Distances between thyroid and arytenoid cartilages and the cervical spine on the entire pitch range in .forte covered (A) and around the first passaggio region in forte open singing (B). The scale is arbitrary.

seems plausible that, in the former case, strong activity in the TA muscle overrules the capacity of the CP muscle to stabilize the cricoid cartilage, while in the latter case, the function of CP is assisted by partial relaxation in the TA muscle. Changes in the distances between the laryngeal structures at the first passaggio region were somewhat different in the open mode. A decrease followed by an increase in T-A was seen, but as T - A decreases, A-C does not change and Cx decreases, suggesting that CP activity opposes TA activity. As T - A increases, A-C and Cx both increase. This may suggest that the lengthening of T - A is not related to relaxation of TA, but instead to stronger longitudinal

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 335 stretching. These observations are in line with the perception that no register shift takes place in the open mode. In the other distances, the clearest differences between forte open and forte covered singing were in VLP and H-T. VLP continues rising after the first passaggio in forte open, whereas in forte covered there is a sudden drop. In forte open singing, H-T distance is small at the first passaggio (related to low vertical hyoid position). In whole, mainly qualitatively similar changes in the distances between the laryngeal structures at the first passaggio in forte open and in forte covered were observed. This may be related to the fact that a simulation made by a trained singer was in question. In piano covered mode, no clear changes were seen at the passaggi, most likely indicating that in singing in piano, no register transitions occur. The balancing of the stiffness of the body (TA muscle) with respect to the cover (membrane) of the vocal folds is a major factor in register control. Electromyographic observations have shown that when singing a rising pitch series, TA activity is marked within registers but slight across registers, 17 or that there is a marked decrease in TA activity at register transition. 18 This is supported in this study by the increase in T - A distance and a decrease in A-C and Cx distance at both passaggio locations for the subject studied here. This reduction of TA activity together with pitch rise is most likely what the singers refer to as "covering" or "thinning" (Finnish ohentaminen). The abrupt decrease in T - A distance observed immediately below the passaggi can be interpreted as indicating a trade-off in the function of muscles. It is likely that the activity of various muscles controlling pitch increases simultaneously as pitch rises. When the register transition is approached, the activity level is relatively high in all the muscles. A further increase in pitch may be accomplished with the TA muscle, since increased activity in CT might lead to an overshoot of F 0 when the external forces pulling the larynx are strong (VLP rises, M - T decreases, see Figures 6 and 12). However, if the larynx is strongly pulled in the anterior-superior direction, it may be easier to raise the pitch by means of TA than CT. Another interpretation is that the TA muscle is activated to prevent an abrupt register transition. Titze has proposed that the primary register transition can be related to the acoustic-mechanic effects

of the first subglottic resonance impeding vocal fold vibration. The effects of the subglottic resonance can be opposed by increasing adduction, eg, by activating TA, or they can be smoothed out by lowering the supraglottic F 1 close to F0.7 This, in turn, could be accomplished by lowering the larynx (or by rounding the lips), and this leads to a darker voice timbre. This technique may be called covering by singers. The abrupt lowering of the larynx (reduced Cy distance) observed at the first passaggio could be related to an attempt of introducing acoustic-mechanic register equalization.

Singing modes Figure 1 described the interdependency of the distances between laryngeal and extrinsic laryngeal structures. Here we have reported measurements of such distances. We discuss the significance of the observations in relation to (1) the averages computed, (2) pitch, (3) the forces acting on the vocal folds, and (4) the Ventricle Quotient and epilaryngeal narrowing. Forte open singing differed from forte covered singing by (1) shorter cricoid-spine distance, (2) larger cricoid-arytenoid distance, and (3) shorter mandible-thyroid distance. (4) The ventricle quotient was lower in forte open than in piano and forte covered. On the other hand, (5)forte covered and open modes of singing did not differ from each other in the mean T - A distance, ie, the vocal fold length was approximately the same in both modes. These differences suggest that the forces pulling the laryngeal structures both in the anterior-superior direction and the posterior direction are stronger in forte open mode. This, in turn, suggests that the activity of the TA muscle is greater in forte open mode and thus more longitudinal stretching in the vocal fold tissue is needed to increase pitch. The Semitone Strain Index was greatest in forte open singing and decreased less with pitch rise. It is obvious that the mechanical stress on the vocal folds should be higher in the open mode. These results are in line with the theoretical constellations presented in Figure 1. Case D, where the increased isotonic contraction of Q (TA) moderately opposes an increase in the distance 2-3 (vocal fold length) and the anterior (P) and posterior (R) forces (M-T and Cx, respectively) are moderate, resembles forte covered singing. Case E, where the distance 2-3, (T-A) + (A-C), is larger Journal of Voice, Vol. 13, No. 3, 1999

336

AATTO SONNINEN ET AL

than in Case D in spite of strong isotonic contraction of Q (TA), because the contractile forces of P (anterior forces, M-T) and R (posterior forces, Cx) are maximal, illustrates forte open singing. SSI is smaller in forte covered than in forte open singing. It also decreases more with pitch in forte covered singing. Thus, the vocal folds elongate less per semitone in forte covered singing. This could be explained by suggesting a gradual relaxation of the TA, reducing the effective mass in vibration. The T - A distance was significantly larger in piano covered singing than in the other two modes of singing (see Figure 11); thus, the vocal folds are longer in piano covered. The A-C distance, in turn, is shorter in piano covered singing than in forte open singing. At lower pitch range the M-T, M - H and H-T distances are longer in piano covered singing than in forte covered singing and the cricoid-spine distance is the same in both modes. It seems obvious that the external forces stretching the vocal folds are smallest in piano covered singing. These differences are most likely due to lower activity in the thyroarytenoid muscle in piano covered singing. As the activity of the TA is relatively low and not opposing longitudinal stretch, a smaller force is needed to lengthen the vocal folds. It is known that as the subglottic pressure is lower, the activity of the CT muscle has to be increased in order to prevent F 0 from lowering. 7 Therefore, the vocal folds are longer in piano covered than forte covered and open singing. At the higher pitch range the A-C distance is larger and the Cx, M-T, M-H, and H-T distances somewhat smaller in piano covered than forte covered singing. Could this be related to the fact that as the effective mass of the vocal folds cannot be reduced in piano covered mode by deactivation of the TA muscle, and subglottic pressure cannot be increased in order to keep SPL low, the extrinsic laryngeal muscles have to be activated more in order to raise F0? In piano covered singing, SSI decreased very little with pitch rise. As the activity of TA is relatively low, the cover is stiffer and less elongation is needed to raise fundamental frequency. In forte open singing, the complement (see Figure 12) is larger than in covered singing. This indicates that, in general, there is more muscle activity in open than covered singing. The force that stretches the vocal folds increases with pitch in all the singing modes Journal of Voice, Vol. 13, No.3, 1999

studied (Figure 12). In the highest notes sung in the forte covered mode, the Cx distance is shorter than in the forte open mode. It can be hypothesized that in forte open singing, the forces stretching the vocal folds to the posterior direction have already reached the maximum at lower pitches, and that the cricopharyngeal muscle is no longer able to oppose the forces pulling the larynx in the anterior direction. Forte covered and .forte open singing were compared from the point of view of the size of the pharynx and epilarynx. The lower pharynx and the epilaryngeal region were narrower in piano and forte covered singing, most likely resulting from the more anterior position of the larynx and stronger bending of the cervical spine. Our observations are in contrast to the results of Hertegfird, Gauffin, and Sundberg, 1 according to whom the larynx is wider in covered than in open singing. However, the two studies are not directly comparable. Radiography gives sagittal pharyngeal distances, whereas fiberoptic investigation (the method used by Herteg~rd and his co-workers1), gives information about the pharyngeal opening. A wide pharynx (in relation to the laryngeal tube) has been regarded as important for the establishment of the singer's formant. 19A singer's formant is essential especially for the male singers in order to be heard over orchestral accompaniment. Female singers, in turn, can be heard over the orchestra at least at higher pitch range (over 500 Hz) since they boost the intensity of the fundamental by raising the supralaryngeal F 1 at the same frequency.19 However, the results of Bloothooft and Plomp 20 suggest that a singer's formant can be seen also in female singers, except for sopranos. The subject of this study has a clear singer's formant in the forte covered samples except for the lowest (below F#3) and the highest (above C6) pitches. 2 A wide pharynx, however, is not the absolute prerequisite for the establishment of a singer's formant. 21 According to the calculations by Titze and Story, a narrowing at the epilaryngeal region clusters the third, fourth, and fifth formant to form a singer's formant. 22 A wide pharynx has been regarded as important in the production of economic and healthy phonation, so-called flow phonation, while a narrow pharynx has been related to the production of uneconomic, hyperfunctional voice. Open singing is traditionally regarded as an uneconomical, strenuous way of voice pro-

EXTERNAL FRAME FUNCTION IN CONTROL OF PITCH, REGISTER, AND SINGING MODE 337 duction, and practical observations of phoniatricians and speech pathologists suggest that singers using the open mode often develop vocal nodules. Therefore, it may seem strange that in this study a wider pharynx was seen in the forte open than in forte covered mode of singing. The singer was well-trained and her forte covered singing sounded perfectly acceptable to represent classical operatic singing. On the other hand, it should be borne in mind that the forte open mode was a simulation made by a trained singer. Therefore, it is possible that all features that may be typical of really untrained singer's voice production are not included in the simulation. It should also be borne in mind that we are focusing on the lower part of the pharynx, actually just the epilaryngeal region. Titze and Story 22 have presented a hypothesis on the positive effects of epilaryngeal narrowing on voice production. Increased input impedance of the vocal tract seems to lower the phonatory threshold and increase the amplitude of the higher harmonics of the voice. A narrow epilarynx seems to function in the same way as the mouthpiece of the trumpet to shape the flow and affect the mode of vibration of the voice source. Obviously, a sufficiently high input impedance improves voice quality and at the same time diminishes the burden that voice production places on the vibrating vocal folds or the lips of the trumpet player. Epilaryngeal narrowing can thus also be seen as very positive from the point of view of vocal hygiene. Interestingly, both in forte open and in piano and Jbrte covered modes of singing, the epilaryngeal region becomes narrower below the passaggio and opens up at the passaggio region. This opening is probably related to the anterior movement of the hyoid bone and the larynx. Is this to be regarded as a sign of the struggle between opposing inner and outer forces at the passaggio? Or, could it be a maneuver that makes a register shift easier, given that sufficiently increased input impedance could improve the modal register vibration of the vocal folds and, contrastively, decreased input impedance could assist a more falsettolike vibration? CONCLUSIONS 1. Roentgenological observations suggest that the thyroarytenoid distance, corresponding to the length

of the vocal folds, increases nonlinearly with pitch. At higher pitches, the change in length per an increase in F 0 is smaller. 2. Forte open and .forte covered modes of singing (corresponding to the singing modes used by untrained and classically trained singers, respectively) do not differ from each other in the length of the vocal folds (T-A distance). 3. The length of the vocal folds is longest in piano covered singing. 4. There are marked pitch-related changes in laryngeal structures. These changes concentrate especially at the passaggio regions in the forte covered mode of singing. This finding supports the existence of at least 3 registers in the singer investigated, and possibly the female trained singing voice in general. In piano covered singing, no clear changes were seen at the passaggi (suggesting that no passaggi occur in piano singing). 5. Changes in the laryngeal structures can be seen as part of external frame function in the control of fundamental frequency. 6. Open and covered modes of singing showed different patterns in the movements of the laryngeal structures. 7. As only one singer participated in the study and no repetitions were made, generalizations have to be made with caution.

Acknowledgement: We wish to thank three anonymous reviewers for their critical and constructive comments on an earlier version of this article. We also thank Paavo Malinen, Ph.D. and Virginia Mattila, M.A. REFERENCES 1. Herteg~rd S, Gauffin J, Sundberg J. Open and covered singing as studied by means of fiberoptics, inverse filtering, and spectral analysis. J Voice. 1990;4:220-230. 2. Sonninen A. The external frame function in the control of pitch in the human voice. Ann N Y A c a d Sci. 1968;155:68-90. 3. Zenker W, Zenker A. ~oer die Regelung der Stimmlippenspannung durch von aussen eingreifende Mechanismen. Folia Phoniatr. 1960;12:1-36. 4. Sonninen A, Hurrne P, Vilkman E. Roentgenological observations on vocal fold length-changes with special reference to register transition and open/covered voice. Scand J Logo Phoniat. 1992;17:95-106. 5. Hurme, P. 1996. Acoustic studies of voice variation. Jyvdskyld Studies in Communication 7. [Jyv~iskyl/i: University of Jyv~skyl~]. Journal of Voice, Vol. 13, No. 3, 1999

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6. Sonninen A, Damst6 PH, Jol J, Fokkens J, Roelofs J. Microdynamics in vocal fold vibration. Acta Oto-Laryng. 1974; 78:129-134. 7. Titze IR. Principles of Voice Production. Englewood Cliffs, NJ: Prentice-Hall; 1994. 8. Sonninen A. Is the length of vocal cords the same at all different levels of singing? Acta Otolaryngol. 1954;ll(suppl):219-231. 9. Sonninen A. Paratasis-gram of the vocal folds and the dimensions of the voice. In: Proceedings of the Fourth International Congress of Phonetic Sciences. The Hague: Mouton; 1962:250-258; also in: Large J, ed. Contributions of Voice Research to Singing. Houston, Tex: College-Hill Press; 1980:134-145. 10. Hollien H, Moore R Measurements of the vocal folds during changes in pitch. J Speech Hear Res. 1960;3:157-165. 11. Luchsinger R, Pfister K. Die Messung der Stimmlippenverl~ingerung beim Steigern der Tonh6he. Folia Phoniatr. 1961;13:1-12. 12. Wendler J. Stimmlippenl~ingeund Tonh6he. ZLaryngol Rhinol. 1966;45:355-369. 13. Sonninen A, Hurme R Vocal fold strain and vocal pitch in singing: radiographic observations of singers and nonsingers. J Voice. 1998;12:274-286.

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14. Hirano M, Vennard W, Ohala J. Regulation of register, pitch, and intensity of voice. Folia Phoniatr. 1970;27:1-20. 15. Titze I, Luschei E, Hirano M. Role of the thyroarytenoid muscle in regulation of fundamental frequency. J Voice. 1989;3:213-224. 16. Hollien H. In search of vocal frequency control mechanims. In: Bless D, Abbs J, eds. Vocal Fold Physiology. San Diego, Calif: College-Hill Press; 1983:361-378. 17. Faaborg-Andersen K. Electromyographic Investigation of Intrinsic Laryngeal Muscles in Humans. Copenhagen, Den: 1957;121. 18. Hirano M. Phonosurgery basic and clinical investigations. Otologia (Fukuoka). 1975; 21(suppl):283. 19. Sundberg J. The Science of the Singing Voice. DeKalb, Ill: Northern Illinois University Press; 1987. 20. Bloothooft G, Plomp R. The sound level of the singer's formant in professional singers. J Acoust Soc Am. 1986;79: 2028-2033. 21. Detweiler RE An investigation of the laryngeal system as the resonance source of the singer's formant. J Voice. 1994;8:303-313. 22. Titze IR, Story BH. Acoustic interactions of the voice source with the lower vocal tract. J Acoust Soc Am. 1997; 101:2234-2243.

E X T E R N A L F R A M E F U N C T I O N I N C O N T R O L O F PITCH, REGISTER, A N D S I N G I N G M O D E

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APPENDIX 1. Superimposed Laryngeal Constellations of the Pharyngeal and Tracheal Walls

Cervical bending, shape and position of the pharynx, shape and position of the larynx are shown at various pitches and singing modes. The figure also shows the epiglottis and the anterior extremity of the laryngeal ventricle. The tracings were standardized in relation to the 5th and 6th cervical vertebrae. Forte covered low

Forte covered mid

G#3 - F3

F#3 - A#4

Forte covered high B4 - G#6

2nd

E4~

5th ~

~

\\ Piano covered G#3 - D#6

Forte open mid

~ ~ 6th

Respiration

F#3 - A#4

\

\ \

2nd %

3rd H

~J

4th

e

5th ~

'

\

/

6th

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340

AATTO SONNINEN ET AL APPENDIX 2

Position of the hyoid bone (as indicated by H and H') and the thyroid cartilage (T) line connecting the 5th and 6th cervical vertebra serves as the reference. The data are given for each semitone in forte covered, forte open, and piano covered singing, as well as in inspiration and expiration.

Piano covered H

,

H I, G#6

F5

E x p ~

C5

G#4

F4

C4

3

H' ion

H,

\,\

T C5

i G#4

F4

C5

I G#4

F4

~

C4

G#3

C4

G#3 i

Forte covered H,

G#6

F6

Journal of Voice, Vol. 13, No.3, 1999

G#5

F5

C#3