Broadband measurements on ultrasonic tank lining materials

Broadband measurements on ultrasonic tank lining materials R.C. CHIVERS,

A.D. SMITH

and P.R. FILMORE

The need for good absorbing materials in ultrasonic test tanks is described, although few systematic measurements appear to have been reported to date. Measurements of transmission and reflection on neoprene, paraffin wax, rubber car mat and plastic door mat are reported in the frequency range 1 - 5 MHz and for angles of incidence up to 45”. Neoprene and paraffin wax appear to have excellent characteristics. The paper includes a discussion of experimental problems, a brief comparison with similar techniques in audio acoustics, and identifies two important areas for further work.

Introduction The most important component of an ultrasonic system is the transducer, whether the system is to be used for diagnostic applications or for active effect. The accurate determination of the details of the field actually radiated by a transducer is an extremely complex problem’ and at present both the measurement procedures and the indices used to characterize the field are essentially those described by Hill in 1970.’ A measurement of total power is made (for continuous waves) with a radiation force technique or a calorimeter,3 from which, with a knowledge of the effective area of the transducer,4 the intensity may be estimated. Alternatively the ultrasonic field distribution may be assessed with a small probe. For many reasons the miniature hydrophone is popular,’ although the optimum design has not yet been established.6 If the transmitting transducer is continuously excited (as it will be if single frequency measurements are required and a gating system is unavailable; if high ultrasonic levels are used; or if the equipment, such as a therapy apparatus, has no facilities for interrupting the excitation), particular attention should be paid to the acoustic properties of the enclosure in which the measuring device is housed. At the low megahertz frequencies of interest in materials inspection and medical applications, this may be a tank of water of reasonable size, or it may be a relatively small chamber surrounding the crucial element, such as the radiation force balance target.’ Similar considerations apply to equipment in which continuous wave experiments under closely controlled conditions are to be performed.’ The removal of unwanted reverberations, and thus the simulation of free field conditions, has been considered of importance, as may be seen by the variety of methods that have been employed. 2Y9-13These techniques appear to have been successful - in some cases 99% of the incident energy is estimated to have been absorbed. However very few details of the relevant measurement procedures have been presented and the conclusions of different investigators The authors are at the Department of Physics, University of Surrey, Guildford, Surrey GU2 5XH. Paper received 13 March 1980. Revised 7 November 1980.

ULTRASONICS.

MAY

1981

0041-624X/81

are not always consistent. This has led to a lack at present of any general agreement on the most suitable materials to use as ultrasonic absorbers at megahertz frequencies, despite some independent work conducted in the high kilohertz frequency range. l4 -I6 The range of materials employed to date includes neoprene, car matting and shoe brushes. The present report describes a broad-band technique for assessing tank lining materials and gives the results of measurements of some easily available materials in the frequency range 1 - 5 MHz. It is hoped that this may lay the basis for a better informed use of the materials. Although the work has been confined to measuring reflection and transmission coefficients of samples of the materials, it has been based, broadly speaking, on the techniques used in audio (room) acoustics. A brief comparison of the two areas is given in the following section. Comparison with room acoustics The physical basis of the comparison can be seen in Table 1, in which the magnitudes of different parameters are estimated and compared. It can be seen that there is an approximate scaling factor of 10: 1 in the dimensions in the two cases, with the exception of the wavelengths, for which the scaling is significantly greater. Two standard procedures exist for the measurement of the absorption coefficient of materials in audio-acoustics - the standing-wave tube method,17’18 and the determination of changes in reverberation time resulting from the introduction of a given amount of a particular material into an enclosure. The absorption coefficient in audio acoustics is defined as the ratio of the sound energy absorbed by the material (or passing through it) to the total incident energy (that is, it is equal to 1 - r, where r is energy reflection coefficient). (This differs from the usual use of the absorption coefficient in ultrasonics, which expresses the reduction in amplitude - or intensity - as a plane wave travels through a homogeneous medium, and is thus a fundamental acoustical property of the medium.) Thus the results obtained from the measurement of reverberation times are likely to vary considerably with positioning of the sample, and other geometrical constraints.”

/030125-09/$02.00

0 1981

I PC Business Press

125

Table --

1.

A comparison -

Ultrasonics

Room acoustics

Parameter --__

--~-

Frequency Velocity

of dimensions

range

50 Hz-15

kHz

1 MHz-10

MHz

of

sound

330 m s-’

(in air)

1500

m s-’

6.6 m-Z.2

cm

1.5 mm-O.15

(in water)

Corresponding wavelength range Source diameter

20 cm (loudspeaker)

mm

2.5 cm (transducer)

Receiver diameter Typical

1.3 cm (microphone)1

enclosure

dimensions

mm (hydrophone)

3 m x 3 m x 5 m

30cmx3Ocmx

(room)

(water tank)

1m

-li_----__

8

This comparison ly IO:1

transducer holder23 until the maximum echo was received from the far wall of the tank. To check that the frequency range would be adequately covered, the transducers were excited with the same impulse, and the echo from a polished Perspex target 12.5 cm from the transducer face gated and fed to a spectrum analyser. The holder for the Perspex block permits orientation of the block in two directions. Before taking the spectral response the block was adjusted to maximize the echo from the front surface, and this setting of the holder was used with the samples of absorbers as the orientation of normal incidence. The loop spectral responses are shown in Fig. 1. They are, to some extent, a function of the excitation pulse shape, but clearly cover the range required (the significance of the noise level shown is reduced by virtue of the fact that it is the actual radiated field that is used for most of the measurements rather than the loop response).

clearly

1

shows that there is an approximate-

ratio in the dimensions

the wavelengths

for the two cases but that ratio is considerably greater.

The measurement of an ultrasonic reverberation time should be possible in principle. Use of the figures in Table 1 and the Eyring formulation2’ suggests that the reverberation time should be less than 10 ms. Potential difficulties may arise in connection with producing enough ultrasonic energy to fill the enclosure, and in cutting this off in such a way that the ringing decay of the transducer element is much shorter than the smallest reverberation time to be measured. This may be achievable, depending on the actual values of the reverberation times, with a suitable clamping circuit.21 The only other requirements are a hydrophone, a peak (level) detector with a short time constant and an appropriate means of recording the decay.

+A__&--_-

Naselevel

/a

Oo

The tube method yields only a value of the (audio) absorption coefficient for a specific configuration (normal incidence). However this is easily repeatable and is suitable when a comparison of materials is required. The main problem of its application to the low MHz frequencies for waves in liquids arises from the disparity of wavelength ratio to the other dimension ratios mentioned earlier. The prime requirement of having a tube diameter small compared to the wavelength and a probe which is sufficiently small compared to this diameter that it does not significantly affect the wave pattern, is clearly impossible to fulfil. Recourse has therefore been made to the techniques described by Bobber 22 for underwater acoustics for the measurement of insertion loss (which is related to the reflection coefficient). Experimental

spectm1

of reflectm

I

1

05

I

1.0

I

20

15 Frequency[MHz3

I

I.25

I

-1

175 FrequencyCMMI

225

275

8

arrangements

Au the measurements were conducted in a tank of distilled water, the tank dimensions being 17 cm x 50.6 cm x 28.3 cm. The water was maintained at 28°C k 05°C by circulation through a heat exchanger system. Transmission

transducers

To cover the frequency range from 1 to 5 MHz, three transmitting transducers were used, of nominal frequencies 1, 2 and 5 MHz, and nominal radii 0.75 cm, 0.75 cm and 0.5 cm. When in use, each was aligned by exciting it with a short impulse (’ 200 V in 20 ns) and orienting the

126

Frequency [MHz1 Loop frequency response of the transmitting transducers Fig. 1 using: a - nominal 1 MHz; b -nominal 2 MHz; c -nominal 5 MHz

ULTRASONICS.

MAY

1981

Hydrophone testing and wave-form selection

The use of an omnidirectional hydrophone has considerable potential advantages, 24 however, the size of the currently avaiIable ultrasonic hydrophones (see Table 1) makes the assumption an uncertain one, and the hydrophone to be used (a commercially available device of nominal 1 mm diameter element) was subject to some rudimentary tests of its directionality. This was achieved by aligning it to receive maximum signal from the pulse-excited transmitter, with its tip 17.9 cm from the transmitter face, and using a facility on the test tank (used subsequently with the absorbing samples) for rotating the hydrophone about an axis through the middle of the front face of the element. A comparison of the directionality at 1 MHz with continuous excitation and a seven-cycle pulse is shown in Fig. 2a, clearly illustrating the need for using pulses or for using absorbing material in the tank! The hydrophone clearly was unsuitable and a second one (nominally identical, from the same manufacturer) chosen which had rather better characteristics (Fig. 2b shows the response to seven cycles of 1.5 MHz). It is possible that the mechanical construction of the hydrophone may affect its apparent frequency response, and this will be influenced by the pulse length used. However, for the experiments performed here, approximate symmetry of response and maximum sensitivity approximately along the axis of the probe were adequate criteria for its use.

AXIS of hydrophme mc dwei3lon of madent wove -

CcntlnuYJs -- - Pulsed sme w(N

The choice of seven cycles (at 1 kHz pulse repetition frequency) was selected as a working compromise. The compromise arose from having a long enough transmitter hydrophone distance to avoid near-field effects. The maximum distance used was 25 cm. To avoid interference of the direct transmitted pulse with any reflections of containing surfaces, the pulse length must decrease as the transmitter - hydrophone separation increases. The effect is most significant at low frequencies (for a given number of cycles) and for the particular dimensions of the present work seven cycles of 1 MHz gave a 9 ps safety margin.

Am of hydqhone

Material samples

The samples investigated (Fig. 3) were selected on the basis of their availability and recurrence in the literature. All had largest dimensions of 12 cm x 18 cm to ensure that the ultrasonic beam was completely intercepted. The neoprene sample had smooth parallel faces and a thickness of 3 mm, and the paraffin wax a thickness of 1.2 cm with rather rougher faces which were shaved to be parallel. Both the car matting (of rubber) and plastic door mat had extreme thicknesses of 1.3 cm. The former consisted of a flat base of 3 mm thickness with tapered pimples, and the latter of a foam base attached to lines of plastic spikules. Support effects and geometry variation

Bobber noted22 that the sample in coefficient measurement should be mounted asymmetrically with respect to the mid-line between the transmitter and the receiver (to minimize unwanted diffraction effects), and that any support framework should be as acoustically transparent as possible. The geometrical constraints of the construction of the tank used for the measurements reported permitted only a slight asymmetry. Table 2 indicates the effect of varying the geometry on the measurement of the transmission coefficient of the rubber car mat. The errors quoted are discussed in a later section. Whereas it can be seen that the results are

ULTRASONICS.

MAY 1981

bL-.ulr,

I

I

I

I

I

I

Fig. 2 Hydrophone directivity plots: a -a comparison of continuous wave and pulsed sine wave responses for hydrophone 1 at 1 MHz (curves have different scales); b - hydrophone 2, pulsed sine waves at 1.5 MHz. All scales are linear. The transmitterhydrophone distance was identical for each hydrophone

dependent to some extent on the geometry used, no significant trends can be seen and the symmetrical geometry was used for all the experiments reported. The transparency of the sample support was tested using the transmission coefficient measurement procedure (see next section) with and without the support and with the support at different angles to the transmitted beam axis (up to 604, at 1.5 MHz. Within the reproducibility of the basic measurement system (see later section) the presence of the support was not detectable.

127

Fig. 3

The samples tested: a - neoprene;

Measurement Transmission

b -paraffin

wax; c -

rubber car mat; d - plastic door mat

procedures

coefficient

greater precision of measurement, and the transmission coefficient, T, was calculated from the following expression

measurement

The experimental arrangement is indicated schematically in Figs 4a and 5. Each sample was mounted in the support frame and positioned as shown. Spectra were recorded of the hydrophone output with the sample orientated at angles (every 10”) between 0” and 60”) and with no sample present. The transmitters were excited with seven cycles of (approximately) 1, 1.5, 2.5, 3.5 and 4.5 MHz. The spectra were displayed on an amplitude scale to achieve Table

2.

Variation

of transmission

to the sample and geometric -.---I_

coefficient

coefficient

at 1.55 MHz

t/h = 21 cm

t/h = 25 cm

t/h = 25 cm

Configuration

t/s = 7 cm

t/s=

t/s = 7 cm

0”

0.218+0.041

0.171+0.028

0.186+0.032

30”

0.172+0.029

0.144+0.022

0.125+0.018

0.107+0.014

0.111+0.015

0.050+0.004

0.040+0.003

11 cm

(amplitude

For the non-normal configurations readings were taken only at the frequencies corresponding to the spectral peaks, but for normal incidence extra readings were taken at other frequencies in the main peak. At first the plastic door mat exhibited zero transmission, for all configurations, which was attributed to air in the thin layer of foam with which it was backed. The sample was then heated in water under vacuum and the measurements reported are those obtained after this procedure. Reflection

45” 60”

0.058+0.006

t/h is the transducerlhydrophone t/s is the transducer/sample

128

separation

separation

level in absence of sample at same frequency)*

with incidence

configuration

-~ Transmission

T=

(amplitude level (in V) for a given sample configuration and frequency)*

coefficient

measurement

The reflection coefficient was calculated by a formula analogous to the one given above for the transmission coefficient, with appropriate modifications of the source receiver geometry. Different measurement procedures were adopted for normal reflection and non-normal reflection. For the latter the experimental configuration is shown in Figs 4b and 5, the tank size restricting measurements to 15”, 30” and 45”, and data analysis being performed as

ULTRASONICS

. MAY

1981

neoprene with the cork in place. Within the experimental uncertainties, no effect was found. For normal incidence, the same procedure was clearly inappropriate. Advantage was taken of the broad spectrum of the nominal 5 MHz transducer (Fig. lc) to use a broadband reflection technique, the spectrum reflected by the sample being compared to that from a notional perfect reflector. In practice this was achieved by measurement of the reflection from the Perspex block and applying a correction (multiplying by 2.6925) based on plane wave assumptions. The apparatus is shown schematically in Fig. 6. Results and discussion Reproducibility

(

IOcm

/

Plan view of tank: a - for transmission measurements; Fig. 4 b - for reflection measurements at other than normal incidence

On the basis of a fourfold repetition of measurements with several different configurations, the reproducibility appeared to be within f 5%. Although this appears to be a rather large uncertainty, the majority is considered to be due to the rather poorly controlled nature of the structure and properties of the materials used. It appears that most of the materials that are effective as ultrasonic absorbers are generally difficult to machine to a precision that is commensurate with a small fraction of a wavelength. For the present investigation this level of reproducibility was considered adequate if not ideal. The potential errors that arise in the coefficients from this irreproducibility can be calculated approximately in the following manner: Each coefficient, c, (i.e. the reflection coefficient or the transmission coefficient) is calculated from the square of

b---l Sampb

TlQl-U&M

I

1

IX- Y recorderj

-----------SSpectrum

Block diagram for transmission Fig. 5 measurements

and non-normal

analyser reflection

described for transmission coefficient measurement. The hydrophone was positioned to receive the maximum reflected signal (with the transmitter and hydrophone acoustic axes intersecting at the surface of the sample) in order to remove the effect of the hydrophone directionality. The transducer - sample separation was the same as that used for the transmission measurements and the reference (no sample) data recorded with the hydrophone aligned directly with the transmitter at a distance equal to the total path length (via reflection at the front surface) used for the reflection measurements. Although no such effects were observed at 3.5 and 4.5 MHz, when the transmitter was driven at frequencies of 2.5 MHz or below the hydrophone appeared to be picking up direct side-lobe radiation. For neoprene and paraffin wax, the reflected signal was sufficiently strong that the direct signal was negligible. To obtain reliable data for the car mat (which reflected very little of the incident energy - Fig. 11) a small piece of 6 mm thick cork tile was interposed between the hydrophone and the transmitter (Fig. 4b). The lack of effect of the acoustic disturbance on the measurements was checked by repeating the measurements on

ULTRASONICS.

MAY

1981

T/s sepamtlon

a

Experimental arrangements for transducer response and Fig. 6 normal reflection measurements: a -plan-view of tank; b - schematic circuitry

129

the ratio of two voltage measurements,

~1and b:

c = (a/b)’

If there is an equal potential percentage error, e, in each of the voltage measurements, then the uncertainty in c can be written approximately as: c f AC =f$

(1?e)4

z

EI (1 + 4e) provided e < 1.

Using a value of e of 5% we find that the figures of imprecision may be as great as 20%, which is neither inconsistent with the results obtained, nor enough to affect the main conclusions arising from the results reported. Transmission coefficient measurements

The results of the measurements of the transmission coefficient at normal incidence for rubber car mat, paraffin wax and neoprene are shown as a function of frequency in Fig. 7, while Figs 8, 9 and 10 show, respectively, the transmission coefficient for different angles of incidence on to rubber car mat, paraffin wax and plastic door mat. Before discussing the results on the other materials those on the door mat will be considered. It is clear from Fig. 10 that, apart from decreasing transmission at higher angles of incidence, the transmission results appear to be very erratic. This was adduced to be caused by the structure

FrequencyCMHz3 Frequency dependence of the transmission Fig. 8 rubber car mat at various angles of incidence

coefficient

of

1

Frequency CMHzl Fig. 9

)I

i

0

1 Fmency

Fig, 7 normal

130

Transmission incidence

coefficients

CMHrJ

as functions

of frequency

for

As Fig. 8, for paraffin

wax

of the mat and is confirmed by the data in Table 3 which shows the maximum and minimum values of the transmission coefficient measured by normal incidence as the sample moved perpendicular to the sound beam. A movement of only + 1 cm was needed to achieve this variation, which showed no definite trends, but may have been related to the way in which the mat was constructed, (Fig. 3). Movements as great as + 1 cm failed on the other samples to produce any such effect. Although the significance of this effect is likely to have been enhanced by the use of a small receiver (the hydrophone), it was felt that an important criterion for tank lining materials is the maximum energy which may be transmitted, and the use of large area receivers is likely to give unrealistically high values of the insertion loss for inhomogeneous materials.26 The high values of transmission recorded at some frequencies, and

ULTRASONICS.

MAY 1981

I ?

rypical enw b

farneoprene dab

2.0

25

5c Frequency CMHz3

Fig. 10

As Fig. 8, for plastic door mat

the variability, indicated that for the present study values of the reflection coefficient were not worth recording, although this does not necessarily imply that the door mat may not be a useful material, as is discussed in the next section. For the samples of the other materials, it can be seen that the transmission coefficient decreases rapidly with increasing frequency (Fig. 7), and that the decrease is more rapid for smaller angles of incidence. The very low value of transmission of neoprene above 1.5 MHz precluded measurements of varying incidence transmission with the present experimental arrangement.

T-1

Rubber car mot

O.CW

06 1.5

errOr bor

f forwxdala

Paraffin wx

--x 2.0

25

Frequency Fig. 11 normal incidence

30

3.5 . 4.0 Frequency CMW

dependence

of the reflection

45

A 5.0

,. 5.5

coefficients

for

45

50

coefficients

of

\ Reflection coefficient measurements

\

The reflection coefficient results at normal incidence are shown in Fig. 11, while the results for different angles of incidence for paraffin wax and neoprene are shown in Figs 12 and 13 respectively. Care must be taken in interpreting the normal incidence curves from Fig. 11 which are included on Figs 12 and 13 with the curves for nonnormal incidence. The normal incidence results are likely to be underestimated because of the large size of the receiver used, as discussed in the previous section. Bearing in mind the error bars shown, it can be seen that little frequency dependence is observed over the range of frequencies of the measurements, and in terms of the gross sampling performed in the frequency domain.

4 \ \

Normal incidence transmission coefficient maximum Table 3. and minimum values for the plastic door mat sample Normal

incidence

transmission

coefficient

Frequency

[MHz1

Maximum

Minimum

0.95

0.404

0.066

1.50

0.107

0.001

2.40

0.071

0.002

3.50

0.030

0.003

4.50

0.355

0.023

ULTRASONICS.

20

MAY

1981

Fig. 12 paraffin

25

3.0 3.5 4.0 Frequency CMHrI

Frequency dependence of the reflection wax for various angles of incidence

131

Apart from a more detailed investigation (concerned with higher frequencies, and the effects of surface roughness), two problems remain for security in practical situations. The first is that almost without exception, the source of

the most significant reverberations in an open tank is the liquid surface.’ If transducers, targets, etc are to be manipulated in the tank by mechanisms above the tank, as is the customary arrangement, a solid piece of paraffin wax cannot be used as an absorber. It is conceivable that droplets of plastic or wax floating on the surface could be used, but this remains to be investigated.

005

t

L

OT5

0.04

Typcal error bci

Fig. 13

40

-

45I

50 I

Frequency [MHz1

As Fig. 12, for neoprene

An approximate measurement of the characteristic impedance of the paraffin wax was performed with a result of 0.8 times the impedance of water, giving a figure of 0.03 for the plane wave reflection coefficient at a step impedance change. This is the same order of magnitude as the measured values (Fig. 11). The low value of reflection coefficient for the car mat (Fig. 11) - below the measuring threshold of the apparatus precluded angular measurements at varying angles of incidence. It is clearly important to identify the mechanism involved in this result, that is, whether the ultrasound is very effectively scattered or whether the structure of the mat acts as a gradual impedance change causing most of the energy to travel into the mat to be absorbed (or reflected by structures behind it!). The relatively high values of the transmission coefficients (Fig. 7) suggest the latter procedure, with fairly low absorption at low frequencies. Comparison of Figs 11 and 7 suggests that if the reflection characteristics are in fact independent of frequency, that is, the primary mechanism of reflection is a step impedance change, then the attenuation defined as (1 - reflection coefficient - transmission coefficient ) appears to decrease with decreasing frequency. The figure of about 90% for the attenuation of neoprene obtained from the data in Figs 7 and 11 compares well the 92% quoted by Wells.” Conclusions From the discussion given above it is clear that with care useful measurements can be made on materials used as absorbers in ultrasonic tanks. It appears that a material that reflects (or scatters) little of the energy incident upon it and has a high attenuation is to be recommended. Of the materials tested in this study paraffin wax and neoprene have good characteristics. The attenuating properties can of course be increased by increasing the thickness of the material used. The low reflection coefficient of paraffin wax, its relatively high attenuation and the ease of moulding or working it, suggest that it could fulfil most of the requirements that may be made.

132

The second question that arises is that although measurements of the type mentioned above are valuable in the first stages of building an enclosure for use with continuous waves, they represent only indirect evidence for the quality of the actual reverberation level encountered. It would be of considerable advantage to develop the reverberation time measurement used in audio acoustics for use at MHz frequencies. Measurements have been reported at 100 kHz,27 and with calibrated miniature ultrasonic hydrophones’ it should be possible to provide relatively inexpensive instrumentation to assess any given enclosure. References 1

2 3

4 5

6

I

8

9

10

11 12 13 14 15 16 17 18

Chivers, RC., Some practical considerations in the measurement of the output of medical ultrasonic devices, J. Med. Eng. Tech. (in press) Hill, C.R, Calibration of ultrasonic beams for bio-medical applications, Phys. Med. Biol. 15 (1970) 241-248 Zieniuk, J.K., Chivers, R.C., Measurement of ultrasonic exposure with radiation force and thermal methods, Ultrasonics 14 (1976) 161-172 Chivers, RC., Bosselaar, L., Filmore, P.R, On the effective area in diffraction correction, J. Acoust. Sec. Am. (in press) Walton, A.J.P.B., Chivers, RC., The piezoelectric hydrophone for ultrasonic output assessment. In The evaluation and calibration of ultrasonic transducers, Ed. M. Silk, IPC Science and Technology Press (1978) 96-105 Chivers, RC., Lewin, P.A., A comparison of different designs of miniature hydrophone probes. Proc. Ultrasonics International 79, Graz, IPC Science and Technology Press (1979) 434-446 Farmery, M.J., Whittingham, T.A., A portable radiation force balance for use with diagnostic ultrasonic equipment, Ultrasound Med. Biol. 3 (1978) 373-379 Holzhemer, J.F., Taenzer, J.C., Havlice, J.F., Ramsey, S.D., Green, P.S., A facility for the investigation of the bioeffects of ultrasound. In Ultrasound in Medicine, vol. 3B, Edited D.N. White, (1977) 1995-1998 Wells, P.N.T., Bullen, MA. Follett, D.H., Freundlich, HF., Angel1 James, J., The dosimetry of small ultrasonic beams, Ultrasonics 1, (1963) 106-110 Wells, P.N.T., Bullen, M.A., Freundlich, H.F., Milliwatt ultrasonic radiometry, Kftrasonics 2, (lY64) 124-128 Miller, E.B., Eitzen, DC., Ultrasonic transducer characterization at the N.B.S., IEEE Trans. Sonics. Ultrasonics SU26 (1979) 28-37 Newell, J.A., A radiation pressure balance for the absolute measurement of ultrasonic power. Phys. Med. Biol. 8 (1963) 215-221 Kossoff, G., Balance techniques for the measurement of very low ultrasonic power outputs, J. Acoust. Sot. Am. 38 (1965) 880-881 Weinstein, M.S., Some design considerations for high frequency anechoic tanks, J. Acoust. Sot. Am. 25 (1953) 101-105 Toulis, W.J., Simple anechoic tank for underwater sound, J. Acoust. Sot. Am. 27 (1955) 1221-1222 Tamarkin, P., Eby, R.K., Tank wall lining for underwater sound use, J. Acoust. Sot. Am. 27 (1955) 692-698 Bruel, P.V., Sound insulation and room acoustics, Chapman Hall, London (195 1) 22 Zwikker, C., Kosten, C.W., Sound absorbing materials, Elsevier, Amsterdam (1949) 88

ULTRASONICS.

MAY 1981

19

20 21 22 23

Cullurn, D.J.W., The practical applications of acoustic principles, E. and F.N. Spon, London (1949) 152 Kuttruff, If., Room Acoustics, 2nd ed, Ch 5, Applied Science Publishers, London (1979) Worpe, E., Private Communication (1975) Bobber, RJ., Underwater electroacoustic measurements, Naval Research Laboratory, Washington D.C. (1970) B&tow, F.G., Chivers, R.C., Sluman, E.E., A simple ultrasonic transducer manipulator, Ultrasonics 18 (1980) 134

Power Sources If your work involves paramount.

ultrasonic

24

25 26 27

Breed, W.L., Donnelly, J.D., Kinsler, L.E., Investigation and application of reverberation measurements of acoustic power in water J. Acoust. Sot. Am. 35 (1963) 1621-1625 Wells, P.N.T., Physical principles of ultrasonic diagnosis, Academic Press, London, (1969) Marcus, P.W., Carstensen, E.L., Problems with absorption measurements of inhomogeneous solids,J. Acoust. Sot. Am. 58 (1975) 1334-1335 Bj@rn@,L, Kjeldgaard, M. A wide frequency band anechoic water tank, Acustica 32, (1975) 103-109

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ULTRASONICS.

MAY

1981

133