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The influence of absorbent linings on the transmission loss of double-leaf partitions
J. Sound VS. (1967)
(5) (I), 22-28
THE INFLUENCE
OF ABSORBENT
ON THE TRANSMISSION DOUBLE-LEAF
LININGS
LOSS OF
PARTITIONS
R. D. FORD, I’. LORD ANDI’. c. WILLIAMS Acoustics Group, Department of Pure and Applied Physics, Royal College of Advanced Technology, Salford, England (Recebed
22
March 1966)
Under certain circumstances the insertion of a porous blanket into the cavity of a double-leaf partition can produce a pronounced increase in transmission loss, particularly when the leaves have a low critical frequency and a broad coincidence plateau. In the measurements reported, improvements of between 7 and IO dB have been obtained, depending on the size of the cavity. The authors show that a similar improvement in transmission loss is obtainable, even with quite small quantities of absorbent, if placed either round the reveals or in other limited regions in the cavity.
I. INTRODUCTION
Much has been written previously about the use of absorbent linings in double-leaf partitions for attenuating the high frequency air resonances, which are due to standing waves between the leaves (I, 2). The beneficial effects reported as having been gained by this method only appear to be in the region of 3 dB, however, for partitions with a superficial weight of IO lb/ft* or more. There is sufficient evidence in the literature to indicate that much higher insulation gains may be obtained, particularly at low frequencies under certain specified conditions, and that careful positioning of the absorbent may lead to a reduction in the total volume of material that is generally considered to be desirable. 2. BRIEF
SURVEY
OF PREVIOUS
WORK
Figure I (3) shows the theoretical transmission loss for an isolated double-leaf partition with an air space of 3 in. and each leaf has a superficial mass of 2 lb/ft*, for sound waves at normal incidence and for frequencies up to one hundred times the mass air resonance frequency. It shows the reduction in transmission loss which occurs at high frequencies due to standing waves in the cavity normal to the leaves and which occurs whenever the separation I between the leaves is an integral multiple of half the wavelength X of sound in air: i.e., 1=
nil/z,
n =
1,2,3 . . .
(I)
When, as is the case in practice, the waves impinge on the leaves over a wide range of angles then a range of both mass air resonances and standing wave resonances occur, giving rise to a transmission loss curve which represents the average of all these resonance dips. The introduction, however, of a porous blanket into the cavity brings about a considerable improvement in the transmission loss at high frequencies. As an example of
ofincidence
22
ABSORBENT LININGS IN PARTITIONS
a3
this, Figure 3 (4) shows the improvement observed when 5.3 lb/ft3 vermiculite is used to fill the panel shown in Figure 2. Meyer (5) and Beranek and Work (6) h ave indicated that there is a possibility of increasing the transmission loss of the empty cavity structure by the use of sound absorbing I
’
’
’
’
’
’
I
I 20 c lOO-D :
,‘_
SO.-g a ‘6 60“r g 40-
I I I 125 250 500 1000 2000 Frequency
Figure I. Theoretical normal incidence.
I 4000
(c/s)
transmission loss for an isolated double-leaf partition for sound waves at
material placed around the periphery between the two leaves. Reference (7) only deals with measurements obtained on samples which are 18 in. x 18 in. under conditions of normal incidence over a range of frequencies IOO to 10,000 c/s. Here it is intended that the 237
Panel
/
Panel
238
Figure 2. Panel 237: Staggered z x 4 in. wood studs, each set 16 in. in OC and with + in. offset. On each side + in. plain gypsum lath and 3 in. of gypsum vermiculite plaster. Panel 238 same with 6.3 lb/f@ vermiculite filling. ET 23
I _o .P= 15 z 7 s IO s .c t
5
x 6 5
0
250
500
1000 Frequency
2000
4000
8000
(c/s)
Figure 3. Difference between the transmission losses measured for NBS panels Nos 237 and 238.
absorbent should reduce the amplitude of the standing waves parallel to the leaves. However, because of the rather limited sample size which gives a standing wave mode, the lowest frequency of which is 370 c/s, and the limitation of normally incident sound waves, it is difficult to extrapolate these results to cover the case of a full-scale partition under conditions of random incidence.
24
R. D. FORD, P. LORD AND P. C. WILLIAMS
London (2) reported on the effect of placing a 3 in. thick glass fibre blanket having a density of about 1.0 lb/ft2 in the airspace between panels, of 8 in. plasterboard, I in. plasterboard and 2 in. plasterboard. He observed an average improvement in transmission loss for the frequency range 128 to 4096 c/s over the untreated cavity of 9.6 dB, 3.0 dB and 3.5 dB, respectively. In general he found that the insulation improvement was greater for light than for heavy structures. London also tried the effect of placing glass fibre round the boundaries of a double wall with the intention of attenuating standing waves parallel to the wall surfaces. He found that there was no significant difference in the transmission loss between the case of an empty cavity and one with the boundaries lined. He concluded therefore that the effect of transverse modes was negligible. Kurtze (7) also has commented on the fact that waves striking a double-leaf partition at oblique incidence give rise to standing waves in the cavity which lie parallel to the leaves. He points out that the spacing between the boundaries, which is very much larger than the spacing between the leaves, gives rise to resonances that are much lower in frequency. These resonances are particularly disastrous if they occur in the same frequency range as the bending wave coincidence. However, because this type of standing wave arises in what is effectively a long narrow channel it is more easily damped out than a standing wave normal to the leaves. Kurtze (7) suggests that, in order to avoid a deep trough in the transmission loss curve due to the combined effects of low frequency resonance and coincidence, it is advisable to use leaves with different bending wave velocities. 3. THEORY
The overlapping of the bending wave coincidence frequency and the low frequency resonances associated with the standing waves between the boundaries can assume important practical significance. For structural reasons it is desirable to have a statically stiff panel as an element in a double-leaf partition. Acoustically this is undesirable as, in general, it leads to a bending wave coincidence frequency in the middle of the working range (IOO to 3 I 50 c/s). On the other hand, if the panel is made so stiff in relation to its superficial mass that the coincidence frequency is very low, i.e. around 200 to 300 c/s, then the coupling between the two leaves due to the wavelength of the standing waves matching the wavelength of the bending wave can be very much reduced by the introduction of an absorbent into the cavity and the undesirable plateau associated with coincidence greatly reduced. It appears that the use of materials of different bending wave velocities for the two leaves is not essential and, in any case, it is practically simpler to construct partitions with similar leaves. The use of dissimilar materials would be justified when it is necessary to add a second leaf to a partition that is already in existence and that has proved unsatisfactory as a sound insulator (7). The cavity in a double leaf partition should be thought of as a rectangular room with length breadth and height IX,&,I, but with I,, which represents the distance between the leaves, very small. The lower normal frequencies of such an enclosure are given by
fnzn, = :[ (g+(T)‘]“’ wherefn,,,, is the normal frequency of the nXnv mode,
nxn,, are the integers which can be separately chosen taking values anywhere between o and w, and c is the propagation velocity of sound in air. Morse (8) shows that the pressure distribution in such an enclosure has the form Pnzn, tc
cos(~)cos(yq
ABSORBENT LININGS
and the velocity distributions
IN PARTITIONS
25
in the x and y directions have the form %.ny cc sin(y)cos(u) (4) Gzn, x cOS(y)sin(y)
1
respectively. If the velocity distribution (as given by (4)) is examined for the simplest and lowest order modes, some clue is obtained as to the most advantageous positioning of the absorbent, working on the principle that it ought to be situated where the air molecules have their maximum velocities in order to provide maximum damping. 4. EXPERIMENTAL
RESULTS
The investigations have been carried out on two materials whose bending wave coincidence frequencies are vastly different. One material consisted of panels of hardboard separated by a honeycomb of stiff paper. It had a bending wave coincidence frequency of approximately 6000 c/s. The other material was heavier and very much stiffer giving a coincidence frequency of about 250 c/s. 4. I.
EXPERIMENTS
WITH
HARDBOARD/HONEYCOMB
PANELS
Three panels 2 in. thick and measuring 8 ft. x 4 ft with a superficial weight of 1.6 lb/f@ were erected in the 8 ft x 12 ft opening on a timber frame in a transmission suite, to form a single leaf partition. The transmission loss was measured using random noise in one-third octave bands as the source, as per BS 2750, 1956.
70-
125 250 500 1000 2000 Freauency (c/s)
4000
Figure 4. Transmission loss measurements on hardboard/honeycomb panels 1-6 lb/ft* superficial mass : (a) single leaf; (b) double leaf empty; (c) double leaf with mineral wool of 3-9 lb/f@ density arranged as in Figure s(c) ; (d) double leaf, cavity containing mineral wool, arranged as in Figure s(a).
A second leaf was then erected, separated from the first by the timber frame at a distance of 8 in., and the transmission loss again determined. The results for the single- and doubleleaf systems are shown in Figure 4, curves (u) and (b), respectively. Now consideration of the velocity distributions shows that the first normal modes above IOO c/s will be the 3,0 and the 0,2 both of which have the same value of 138 c/s. The
26
R. D. FORD,
P. LORD
AND
P. C. WILLIAMS
existence of these was confirmed by the use of pure tone and exploration of the cavity with a probe microphone which disclosed the position of pressure antinodes. Absorbent, in the form of mineral wool of 3.9 lb/ft3, was then placed in the cavity in the region of the 3,0 velocity antinodes, as shown in Figure 5(a). Because the strips were wide they would naturally tend to damp out particle motion for many other antinodes that arise at higher frequencies. With strips z ft wide and 8 ft long exactly half the area of the partition cavity was covered. The resulting transmission loss (Figure 4, curve (d)) shows a marked increase from about 200 c/s onwards of the order of IO dB. In this series of experiments the absorbent was removed from the cavity and the same quantity arranged around the reveals to a depth of approximately 9 in., as shown in Figure
(bl
(a) Figure
5. Arrangement
of absorbent
in cavity.
5(c). The transmission loss obtained with this arrangement is shown in curve (c) Figure 4. There is a noticeable loss in insulation at low frequencies but only a slight reduction at high frequencies. 4.2.
EXPERIMENTS
WITH
HEAVIER,
STIFFER
MATERIAL
These panels 2 in. thick and each measuring 8 ft x 4 ft with a superficial weight of 5.45 lb/ft2 were erected in the 8 ft x 12 ft opening of the transmission suite and were held in position by small 2 in. x I in. timber battens and the transmission loss measured. A second leaf was then erected at a distance of 4 in. from the first and the transmission
70
(cl
lo-“‘ll’lI”l’,,‘tl’l’ 125
250
500
1000
Frequency
2000
-
4000
k/s)
Figure 6. Transmission loss measurements on stiff panels : (a) x-x, single leaf; (b) A-A, double leaf empty; (c) O--O, cavity filled with polyurethane foam 1.4 lb/ft3 density; (d) A****A, cavity partially filled with foam as shown in arrangement Figure s(b) ; (e) O-O, cavity partially filled with foam as shown in arrangement Figure s(c).
loss again measured. The results for the single and double leaf systems are shown in Figure 6, curves (LZ)and (b) respectively. The cavity was then completely filled with polyurethane foam of 1.4 Ib/ft3 density and a transmission loss obtained as described by curve (c) in Figure 6. The most noticeable
ABSORBENT LININGS IN PARTITIONS
27
feature of these results is the marked coincidence plateau in the double-leaf partition between 250 c/s and IOOO c/s and the large improvement obtained between these frequencies on the introduction of an absorbent lining. Curve (d) shows the effects of placing only 2 ft x 4 ft strips (Figure 5, arrangement (b)) at the 3,0 velocity antinodes and indicates that the loss in insulation is only I dB overall. Curve (e) is obtained when the same quantity of absorbent is placed round the reveals. It shows that some insulation is lost at low frequencies but that there is a marginal improvement over the situation in (d).
(b)
70-
:i:L
125 250 500 1000 2000 Frequency (c/s)
4000
Figure 7. Transmission loss measurements obtained by T.N.O., Delft on stiff panels: (a) empty double-leaf partition ; (b) cavity filled with superfine glass wool 0.77 lb/f@ density.
Figure 7 shows the measurements taken by the T.N.O. Laboratories, Delft, on the same material for a double leaf partition constructed in the same way with its cavity, which in this case was 5 in., empty (Figure 7, curve (a)), and with the cavity lined with superfine glass wool of 0.77 Ib/ft3 density (curve (b)).
5. CONCLUSIONS
(i) Both partition systems with superficial weights of 3.2 lb/ft2 and 10.9 lb/ft2 show large improvements in transmission loss with the introduction of an absorbent lining into their cavities. The lighter partition has the higher insulation but it also possesses the wider cavity. (ii) If the material from which the leaves of the partition are constructed has a low bending wave coincidence frequency, then the decoupling between the leaves, which occurs when the low frequency standing waves are heavily attenuated, is very marked. In the case of the particular sample under investigation the improvement in insulation is 7 dB for a 4 in. cavity and IO dB for a 5 in. cavity and this for a partition with a superficial mass of 10.4 Ib/ft2. (iii) If only half the area of the cavity is lined with an absorbent, whether it is placed round the reveals or in other strategic positions, there is still a considerable improvement over the case of the empty cavity. In fact, the overall difference between the full and partially filled cavity is not significant. (iv) Lining the reveals does not appear to be quite so effective at low frequencies as placing the absorbent at the 3,0 velocity antinodes.
28
R. D. FORD,
P. LORD AND P. C. WILLIAMS
ACKNOWLEDGMENTS
The authors wish to acknowledge the considerable help and encouragement which they received from P. A. de Lange and G. J. Van OS of the T.N.O., Delft, and for the technical information which they provided. Thanks are also given to Messrs. Turner and Newall Ltd., and Messrs. Stramit Ltd., for the supply of samples. REFERENCES I. H. J. PURKIS 1966 Building Physics: Acoustics. Pergamon Press, p. 66. 2. A. LONDON195oJ. acoust. Sot. Am. 22, No. 2,270. Transmission of reverberant sound through double walls. 3. LEO L. BERANEK1960 N&e Reduction. New York: McGraw-Hill. 4. R. K. COOK and P. CHRZANOWSKI1957 Handbook of Noise Control. Edited by C. M. Harris. New York: McGraw-Hill. Ch. 20, Transmission of noise through walls and floors. 5. E. MEYER 1935 Elek. Nachr. Tech. 393 Die Mehrfachwand als Akustische Drosselkette. 6. LEO L. BERANEK and E. A. WORK 1949r. acoust. Sot. Am. 21,419. Sound transmission through multiple structures containing flexible blankets. 7. G. KURTZE1964 Physik und Tech&k der Liirmbekfimpfung. Karlsruhe: G. Braun, p. I 16. 8. P. M. MORSE1948 Vibration and Sound. New York: McGraw-Hill, second edition, pp. 389-390.
(5) (I), 22-28
THE INFLUENCE
OF ABSORBENT
ON THE TRANSMISSION DOUBLE-LEAF
LININGS
LOSS OF
PARTITIONS
R. D. FORD, I’. LORD ANDI’. c. WILLIAMS Acoustics Group, Department of Pure and Applied Physics, Royal College of Advanced Technology, Salford, England (Recebed
22
March 1966)
Under certain circumstances the insertion of a porous blanket into the cavity of a double-leaf partition can produce a pronounced increase in transmission loss, particularly when the leaves have a low critical frequency and a broad coincidence plateau. In the measurements reported, improvements of between 7 and IO dB have been obtained, depending on the size of the cavity. The authors show that a similar improvement in transmission loss is obtainable, even with quite small quantities of absorbent, if placed either round the reveals or in other limited regions in the cavity.
I. INTRODUCTION
Much has been written previously about the use of absorbent linings in double-leaf partitions for attenuating the high frequency air resonances, which are due to standing waves between the leaves (I, 2). The beneficial effects reported as having been gained by this method only appear to be in the region of 3 dB, however, for partitions with a superficial weight of IO lb/ft* or more. There is sufficient evidence in the literature to indicate that much higher insulation gains may be obtained, particularly at low frequencies under certain specified conditions, and that careful positioning of the absorbent may lead to a reduction in the total volume of material that is generally considered to be desirable. 2. BRIEF
SURVEY
OF PREVIOUS
WORK
Figure I (3) shows the theoretical transmission loss for an isolated double-leaf partition with an air space of 3 in. and each leaf has a superficial mass of 2 lb/ft*, for sound waves at normal incidence and for frequencies up to one hundred times the mass air resonance frequency. It shows the reduction in transmission loss which occurs at high frequencies due to standing waves in the cavity normal to the leaves and which occurs whenever the separation I between the leaves is an integral multiple of half the wavelength X of sound in air: i.e., 1=
nil/z,
n =
1,2,3 . . .
(I)
When, as is the case in practice, the waves impinge on the leaves over a wide range of angles then a range of both mass air resonances and standing wave resonances occur, giving rise to a transmission loss curve which represents the average of all these resonance dips. The introduction, however, of a porous blanket into the cavity brings about a considerable improvement in the transmission loss at high frequencies. As an example of
ofincidence
22
ABSORBENT LININGS IN PARTITIONS
a3
this, Figure 3 (4) shows the improvement observed when 5.3 lb/ft3 vermiculite is used to fill the panel shown in Figure 2. Meyer (5) and Beranek and Work (6) h ave indicated that there is a possibility of increasing the transmission loss of the empty cavity structure by the use of sound absorbing I
’
’
’
’
’
’
I
I 20 c lOO-D :
,‘_
SO.-g a ‘6 60“r g 40-
I I I 125 250 500 1000 2000 Frequency
Figure I. Theoretical normal incidence.
I 4000
(c/s)
transmission loss for an isolated double-leaf partition for sound waves at
material placed around the periphery between the two leaves. Reference (7) only deals with measurements obtained on samples which are 18 in. x 18 in. under conditions of normal incidence over a range of frequencies IOO to 10,000 c/s. Here it is intended that the 237
Panel
/
Panel
238
Figure 2. Panel 237: Staggered z x 4 in. wood studs, each set 16 in. in OC and with + in. offset. On each side + in. plain gypsum lath and 3 in. of gypsum vermiculite plaster. Panel 238 same with 6.3 lb/f@ vermiculite filling. ET 23
I _o .P= 15 z 7 s IO s .c t
5
x 6 5
0
250
500
1000 Frequency
2000
4000
8000
(c/s)
Figure 3. Difference between the transmission losses measured for NBS panels Nos 237 and 238.
absorbent should reduce the amplitude of the standing waves parallel to the leaves. However, because of the rather limited sample size which gives a standing wave mode, the lowest frequency of which is 370 c/s, and the limitation of normally incident sound waves, it is difficult to extrapolate these results to cover the case of a full-scale partition under conditions of random incidence.
24
R. D. FORD, P. LORD AND P. C. WILLIAMS
London (2) reported on the effect of placing a 3 in. thick glass fibre blanket having a density of about 1.0 lb/ft2 in the airspace between panels, of 8 in. plasterboard, I in. plasterboard and 2 in. plasterboard. He observed an average improvement in transmission loss for the frequency range 128 to 4096 c/s over the untreated cavity of 9.6 dB, 3.0 dB and 3.5 dB, respectively. In general he found that the insulation improvement was greater for light than for heavy structures. London also tried the effect of placing glass fibre round the boundaries of a double wall with the intention of attenuating standing waves parallel to the wall surfaces. He found that there was no significant difference in the transmission loss between the case of an empty cavity and one with the boundaries lined. He concluded therefore that the effect of transverse modes was negligible. Kurtze (7) also has commented on the fact that waves striking a double-leaf partition at oblique incidence give rise to standing waves in the cavity which lie parallel to the leaves. He points out that the spacing between the boundaries, which is very much larger than the spacing between the leaves, gives rise to resonances that are much lower in frequency. These resonances are particularly disastrous if they occur in the same frequency range as the bending wave coincidence. However, because this type of standing wave arises in what is effectively a long narrow channel it is more easily damped out than a standing wave normal to the leaves. Kurtze (7) suggests that, in order to avoid a deep trough in the transmission loss curve due to the combined effects of low frequency resonance and coincidence, it is advisable to use leaves with different bending wave velocities. 3. THEORY
The overlapping of the bending wave coincidence frequency and the low frequency resonances associated with the standing waves between the boundaries can assume important practical significance. For structural reasons it is desirable to have a statically stiff panel as an element in a double-leaf partition. Acoustically this is undesirable as, in general, it leads to a bending wave coincidence frequency in the middle of the working range (IOO to 3 I 50 c/s). On the other hand, if the panel is made so stiff in relation to its superficial mass that the coincidence frequency is very low, i.e. around 200 to 300 c/s, then the coupling between the two leaves due to the wavelength of the standing waves matching the wavelength of the bending wave can be very much reduced by the introduction of an absorbent into the cavity and the undesirable plateau associated with coincidence greatly reduced. It appears that the use of materials of different bending wave velocities for the two leaves is not essential and, in any case, it is practically simpler to construct partitions with similar leaves. The use of dissimilar materials would be justified when it is necessary to add a second leaf to a partition that is already in existence and that has proved unsatisfactory as a sound insulator (7). The cavity in a double leaf partition should be thought of as a rectangular room with length breadth and height IX,&,I, but with I,, which represents the distance between the leaves, very small. The lower normal frequencies of such an enclosure are given by
fnzn, = :[ (g+(T)‘]“’ wherefn,,,, is the normal frequency of the nXnv mode,
nxn,, are the integers which can be separately chosen taking values anywhere between o and w, and c is the propagation velocity of sound in air. Morse (8) shows that the pressure distribution in such an enclosure has the form Pnzn, tc
cos(~)cos(yq
ABSORBENT LININGS
and the velocity distributions
IN PARTITIONS
25
in the x and y directions have the form %.ny cc sin(y)cos(u) (4) Gzn, x cOS(y)sin(y)
1
respectively. If the velocity distribution (as given by (4)) is examined for the simplest and lowest order modes, some clue is obtained as to the most advantageous positioning of the absorbent, working on the principle that it ought to be situated where the air molecules have their maximum velocities in order to provide maximum damping. 4. EXPERIMENTAL
RESULTS
The investigations have been carried out on two materials whose bending wave coincidence frequencies are vastly different. One material consisted of panels of hardboard separated by a honeycomb of stiff paper. It had a bending wave coincidence frequency of approximately 6000 c/s. The other material was heavier and very much stiffer giving a coincidence frequency of about 250 c/s. 4. I.
EXPERIMENTS
WITH
HARDBOARD/HONEYCOMB
PANELS
Three panels 2 in. thick and measuring 8 ft. x 4 ft with a superficial weight of 1.6 lb/f@ were erected in the 8 ft x 12 ft opening on a timber frame in a transmission suite, to form a single leaf partition. The transmission loss was measured using random noise in one-third octave bands as the source, as per BS 2750, 1956.
70-
125 250 500 1000 2000 Freauency (c/s)
4000
Figure 4. Transmission loss measurements on hardboard/honeycomb panels 1-6 lb/ft* superficial mass : (a) single leaf; (b) double leaf empty; (c) double leaf with mineral wool of 3-9 lb/f@ density arranged as in Figure s(c) ; (d) double leaf, cavity containing mineral wool, arranged as in Figure s(a).
A second leaf was then erected, separated from the first by the timber frame at a distance of 8 in., and the transmission loss again determined. The results for the single- and doubleleaf systems are shown in Figure 4, curves (u) and (b), respectively. Now consideration of the velocity distributions shows that the first normal modes above IOO c/s will be the 3,0 and the 0,2 both of which have the same value of 138 c/s. The
26
R. D. FORD,
P. LORD
AND
P. C. WILLIAMS
existence of these was confirmed by the use of pure tone and exploration of the cavity with a probe microphone which disclosed the position of pressure antinodes. Absorbent, in the form of mineral wool of 3.9 lb/ft3, was then placed in the cavity in the region of the 3,0 velocity antinodes, as shown in Figure 5(a). Because the strips were wide they would naturally tend to damp out particle motion for many other antinodes that arise at higher frequencies. With strips z ft wide and 8 ft long exactly half the area of the partition cavity was covered. The resulting transmission loss (Figure 4, curve (d)) shows a marked increase from about 200 c/s onwards of the order of IO dB. In this series of experiments the absorbent was removed from the cavity and the same quantity arranged around the reveals to a depth of approximately 9 in., as shown in Figure
(bl
(a) Figure
5. Arrangement
of absorbent
in cavity.
5(c). The transmission loss obtained with this arrangement is shown in curve (c) Figure 4. There is a noticeable loss in insulation at low frequencies but only a slight reduction at high frequencies. 4.2.
EXPERIMENTS
WITH
HEAVIER,
STIFFER
MATERIAL
These panels 2 in. thick and each measuring 8 ft x 4 ft with a superficial weight of 5.45 lb/ft2 were erected in the 8 ft x 12 ft opening of the transmission suite and were held in position by small 2 in. x I in. timber battens and the transmission loss measured. A second leaf was then erected at a distance of 4 in. from the first and the transmission
70
(cl
lo-“‘ll’lI”l’,,‘tl’l’ 125
250
500
1000
Frequency
2000
-
4000
k/s)
Figure 6. Transmission loss measurements on stiff panels : (a) x-x, single leaf; (b) A-A, double leaf empty; (c) O--O, cavity filled with polyurethane foam 1.4 lb/ft3 density; (d) A****A, cavity partially filled with foam as shown in arrangement Figure s(b) ; (e) O-O, cavity partially filled with foam as shown in arrangement Figure s(c).
loss again measured. The results for the single and double leaf systems are shown in Figure 6, curves (LZ)and (b) respectively. The cavity was then completely filled with polyurethane foam of 1.4 Ib/ft3 density and a transmission loss obtained as described by curve (c) in Figure 6. The most noticeable
ABSORBENT LININGS IN PARTITIONS
27
feature of these results is the marked coincidence plateau in the double-leaf partition between 250 c/s and IOOO c/s and the large improvement obtained between these frequencies on the introduction of an absorbent lining. Curve (d) shows the effects of placing only 2 ft x 4 ft strips (Figure 5, arrangement (b)) at the 3,0 velocity antinodes and indicates that the loss in insulation is only I dB overall. Curve (e) is obtained when the same quantity of absorbent is placed round the reveals. It shows that some insulation is lost at low frequencies but that there is a marginal improvement over the situation in (d).
(b)
70-
:i:L
125 250 500 1000 2000 Frequency (c/s)
4000
Figure 7. Transmission loss measurements obtained by T.N.O., Delft on stiff panels: (a) empty double-leaf partition ; (b) cavity filled with superfine glass wool 0.77 lb/f@ density.
Figure 7 shows the measurements taken by the T.N.O. Laboratories, Delft, on the same material for a double leaf partition constructed in the same way with its cavity, which in this case was 5 in., empty (Figure 7, curve (a)), and with the cavity lined with superfine glass wool of 0.77 Ib/ft3 density (curve (b)).
5. CONCLUSIONS
(i) Both partition systems with superficial weights of 3.2 lb/ft2 and 10.9 lb/ft2 show large improvements in transmission loss with the introduction of an absorbent lining into their cavities. The lighter partition has the higher insulation but it also possesses the wider cavity. (ii) If the material from which the leaves of the partition are constructed has a low bending wave coincidence frequency, then the decoupling between the leaves, which occurs when the low frequency standing waves are heavily attenuated, is very marked. In the case of the particular sample under investigation the improvement in insulation is 7 dB for a 4 in. cavity and IO dB for a 5 in. cavity and this for a partition with a superficial mass of 10.4 Ib/ft2. (iii) If only half the area of the cavity is lined with an absorbent, whether it is placed round the reveals or in other strategic positions, there is still a considerable improvement over the case of the empty cavity. In fact, the overall difference between the full and partially filled cavity is not significant. (iv) Lining the reveals does not appear to be quite so effective at low frequencies as placing the absorbent at the 3,0 velocity antinodes.
28
R. D. FORD,
P. LORD AND P. C. WILLIAMS
ACKNOWLEDGMENTS
The authors wish to acknowledge the considerable help and encouragement which they received from P. A. de Lange and G. J. Van OS of the T.N.O., Delft, and for the technical information which they provided. Thanks are also given to Messrs. Turner and Newall Ltd., and Messrs. Stramit Ltd., for the supply of samples. REFERENCES I. H. J. PURKIS 1966 Building Physics: Acoustics. Pergamon Press, p. 66. 2. A. LONDON195oJ. acoust. Sot. Am. 22, No. 2,270. Transmission of reverberant sound through double walls. 3. LEO L. BERANEK1960 N&e Reduction. New York: McGraw-Hill. 4. R. K. COOK and P. CHRZANOWSKI1957 Handbook of Noise Control. Edited by C. M. Harris. New York: McGraw-Hill. Ch. 20, Transmission of noise through walls and floors. 5. E. MEYER 1935 Elek. Nachr. Tech. 393 Die Mehrfachwand als Akustische Drosselkette. 6. LEO L. BERANEK and E. A. WORK 1949r. acoust. Sot. Am. 21,419. Sound transmission through multiple structures containing flexible blankets. 7. G. KURTZE1964 Physik und Tech&k der Liirmbekfimpfung. Karlsruhe: G. Braun, p. I 16. 8. P. M. MORSE1948 Vibration and Sound. New York: McGraw-Hill, second edition, pp. 389-390.