NBSIR 76-1110

The Acoustic Pressure Field

Alongside a Manikin's Head

with a View Towards In Situ

Hearing Aid Tests

Edwin D. Burnett and George F. Kuhn

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August 1976

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 76-1110

THE ACOUSTIC PRESSURE FIELD

ALONGSIDE A MANIKIN'S HEAD

WITH A VIEW TOWARDS IN SITU

HEARING AID TESTS

Edwin D. Burnett and George F. Kuhn

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

August 1 976

U S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

THE ACOUSTIC PRESSURE FIELD ALONGSIDE A MANIKIN'S HEAD

WITH A VIEW TOWARDS IN SITU HEARING AID TESTS*

Edwin D. Burnett and George F. Kuhn

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

To simulate actual usage conditions, the frequency response of hearing aids was measured on the head of a manikin over the frequency range

of 0.2 to 8 kHz. The acoustic pressure around the head can vary rapidly

as a function of frequency and location. In order to compare and interpret the hearing aid response at various frequencies and locations on the

head, it is necessary to precisely know the pressure variations. The

amplitude and phase of the acoustic pressure were measured in increments

ranging from 2 mm to 5 mm alongside a manikin's head with frontal sound

incidence. The acoustic driver was located in front of the manikin at a

distance of 1.0 m from the ear canal axis. The test frequencies were the

octave band center frequencies from 0.5 kHz to 4.0 kHz and the third

octave band center frequencies from 4.0 kHz to 8.0 kHz. It will also be

shown that pink noise of 6% and 29% bandwidth at 6.3 kHz and 8.0 kHz has

a smoothing effect on the acoustic pressure variation with location.

*As presented at the 91st meeting of the Acoustical Society of America,

Statler-Hilton Hotel, Washington, D.C., April 7, 1976.
*
NOTATION

a = radius of the sphere

D (ka) me -i6 m

(ka)

dh^2 ) (ka)

i —jj—t— where h^ 2 '(ka) is the spherical Hankel function d(ka) m

of the second kind.

i = /=T

k = acoustic wave number

m = summation index

p^

= incident pressure on the sphere’s surface

P q

= free field plane wave pressure

p^

= scattered pressure on the sphere’s surface

= m-th order Legendre Polynomial of the first kind

r = radial distance from the center of the sphere

6 = angle betweep the incident wave field and a point on the

sphere’s surface

INTRODUCTION

In the past, the gain, the frequency response, the saturation level

and the distortion in hearing aids have been measured between 0.2 and

5.0 kHz in a (approximate) free field using a 2-cm 3 coupler (ANSI, 1975;

IEC, 1958). It can be expected that in the near future the useful frequency range of hearing aids will be extended to approximately 8 kHz.

Furthermore, it is well known that the head and/or the torso causes the

sound pressure level around the head, where hearing aids are typically

placed, to differ considerably with position and with frequency, from the

free-field pressure level (Wiener, 1947a, 1947b; Rschevkin, 1963; Burkhard

and Sachs, 1975; Kuhn, 1976). In order to test and compare the performance of different hearing aids (see for example, ANSI, 1975 and Veterans

Administration, 1975), under conditions which resemble those that are

actually encountered in practice, it is desirable to account for the diffraction effects of the wearer's head and torso. An electroacoustic manikin has recently been developed (Burkhard and Sachs, 1975) which dimension

ally represents the median sized person. Using frontally incident sound,

Madaffari (1974) measured the acoustic pressure as a function of frequency

on the side of this manikin's head at 30 points, spaced 2 cm apart on a

rectangular grid. These pressure measurements do show severe pressure

minima and maxima both as a function of frequency and location. As expected from theoretical predictions (Wiener, 1947b; Rschevkin, 1963) the

pressure level behaves smoothly with position at low frequencies but becomes less and less well behaved as the frequency is increased above 2 kHz

Since the head is not perfectly spherical or spheroidal and since it

has several protrusions and indentations such as the eyes, nose, mouth
and pinnae, analytical predictions of pressure at the surface of the head

can only be approximate. The purpose of this investigation, therefore, is

to measure the acoustic pressure alongside this manikin's head at 2-5 mm

intervals for fixed center frequencies. Pressure measurements were made

with discrete tones at preselected frequencies. Since the pressure level,

at high frequencies, changes rapidly with position and since sharp pressure minima exist, the microphone location is critical. Therefore, at the

upper frequencies pressure measurements were also made with 6% and 29%

bandwidth pink noise (around those same center frequencies) in order to

investigate the spatial "smoothing effect" that noise would have on the

pressure maxima and minima alongside the head.

Diffraction of sound by a rigid sphere has been. treated by many

authors (for example, Firestone, 1930; Wiener, 1947a; Morse, 1948; and

Rschevkin, 1963) and a derivation is therefore not repeated here. The

formulation by Rschevkin (1963) for the total pressure on the surface of

a rigid sphere is particularly useful. His solution for the total pressure, that is, the incident pressure plus the scattered pressure, normalized to the incident free-field pressure, is

THEORY

( 1 )

r=a

D (ka)e i6®

(ka) m

wherein the harmonic frequency dependence has been suppressed.

2
Morse (1948) generated a set of tables (and small and large argument

functional approximations) which allow Eq. (1) to be readily evaluated.

Equation 1 will be used to evaluate the head surface pressures up to 2.0

kHz (see section on Results).

Spherical coordinates (r,Q,<f>) were used to derive Eq . (1). However,

the measurements were made in a cartesian coordinate system (x,y,z).

Therefore, to compare theory and experiment, the experimental results

around the side of the head were transformed to a spherical coordinate

system with <J> = 0°, using the expression

0° = 360' distance from the head’s median plane to the measurement point . head’s circumference in the measurement plane

EXPERIMENT

The pressure measurements were made on the manikin in the upright

q position in a 500 m free volume anechoic room as shown in Fig. 1. The

mouth of the acoustic driver shown on the left is 49 mm in diameter and

is 1.0 m* from the ear canal axis in the horixontal plane. A "1/4-inch"

microphone, on the axis of the mouth of the driver, was used as a feedback microphone to maintain the same pressure at all frequencies at this

feedback microphone. (This feedback microphone was not used for measurements with noise.) A "1/2-inch" microphone with a 9.3 cm long, 2-mm internal diameter probe, filled with damping material, was used to measure

the pressure alongside the head. The probe microphone is shown in Fig. 2

*The 1.0 m distance is typical of conversational speech and is being con- sidered for hearing aid testing (Burkhard, 1976). These and other pres- sure measurements were also made at a source to ,ear canal-axis distance

of 3.5 m and will be the subject of a future paper.

3
Since the pressures around the head were normalized to the free field

incident pressures at any one frequency, the microphone probe does not

need to be calibrated. However the probe microphone must be stable.

Initially, some foam was wrapped around the microphone and preamplifier

but later removed since it had no effect on the pressure near the head

surface. The microphone probe was mounted on x-y-z coordinate mechanical

slides which can be adjusted to a resolution of 0.01 mm. The slides themselves were mounted behind the manikin, in its acoustic shadow, to minimize the effect of the scattered pressures on the measurements (see Fig. 3).

The free field (incident) pressure was measured with the microphone

probe at a point vertically above the ear canal axis on contour 2 (see

Fig. 4), with the manikin removed. The measured pressures were normalized

to this free field pressure and converted to pressure levels (dB)

.

The shape of the head along the contours (but on the opposite side

of the head), shown in Fig. 4, was mapped out by noting the appropriate

x-y-z coordinate of the microphone probe when it just came in contact with

the head surface. The contours shown in Fig. 4 are typical of hearing aid

microphone locations. Contours 1, 2, and 3 are spaced 5 mm apart as are

contours 4, 5 and 6. Contour 4 was chosen so that the microphone probe

just touches the outside perimeter of the pinna. The measurement positions

along contours 1 through 6 were spaced exactly 2 mm, 4 mm, and 7 mm from

the head surface. However, for the sake of brevity, this presentation

will report only the results measured 4 mm from the head surface and

along contours 2 and 4.* The probe was moved in 4 mm increments along

*The measurements of the pressure amplitude and phase for the other con- tours will be described in a future paper.

4
contour 2 over a total distance of 10 cm. Since contour 4 follows the

shape of the head and the microphone probe is set in a cartesian coordinate system, it is extremely complicated to move along this contour at

exactly 4-mm intervals. Therefore, the increments were chosen to be 4 mm

in either the horizontal plane or 4 mm in the vertical plane depending on

whether the contour lay primarily in the horizontal or vertical plane,

respectively.

The test frequencies were the octave band center frequencies from

0.5 kHz to 4.0 kHz and the third octave band center frequencies from 4.0

kHz to 8.0 kHz. The measurements were repeated at all test frequencies

using 6% (of the center frequency) bandwidth pink noise. Additional

measurements were made with 29% (of the center frequency) bandwidth pink

noise at 6.3 kHz and 8.0 kHz.

The measurements were made with the equipment shown in Fig. 5 with

the exception that the "1/4- inch" feedback microphone was not used for

the measurements using noise.

RESULTS

The sound pressure levels along contour 2 at 0.5, 1.0, 2.0, and 4.0

kHz, normalized to the free field sound pressure level are shown in Fig. 6.

It can be seen that the pressure levels near the front of the head are

greater than those at the back of the head. The pressure is smooth and

well-behaved with position at frequencies < 2.0 kHz. At 4 kHz the pressure levels range from approximately + 6 dB to - 2 dB from the front to

the back of contour 2.

Theoretical pressure levels on the surface of a rigid sphere are

shown in Fig. 7, along with measured results, for frequencies of 0.5, 1.0,

5
and 2.0 kHz. The theoretical pressure predictions for a sphere lie within

1.5 dB of the measured results. At 2.0 kHz, however, the theoretical

level predictions are lower than the measured data T Burkhard and Sachs (1975) show that the pressure at the ear canal

is increased by approximately 2.5 dB by the reflection from the torso/

shoulder. As can be seen from Fig. 7, if a correction of a nominal + 2.5

dB is applied to the predicted pressure levels along the side of the head,

then the resulting pressure levels are in good agreement with the measurements .

The pressure levels alongside the head at 5.0, 6.3, and 8.0 kHz are

shown in Fig. 8. As the frequency is increased, the number of relative

pressure minima and maxima also increase, i.e. the pressure field becomes

less and less well-behaved with increased frequency. At frequencies of

5.0 kHz or less the pressure level is greater near the front than the

back. However, above 5.0 kHz, at 6.3 and 8.0 kHz, the pressure levels

change rapidly with position forming sharp minima and maxima. Also, the

pressure maxima near the middle and the rear of the head have the same

magnitude as the pressure levels near the front. Such sharp pressure

minima and maxima make the hearing aid microphone location on the head

critical

.

The pressure levels along the outside perimeter of the pinna, on

contour 4, are shown in Figs. 9 and 10. Again, the pressure is wellbehaved and smooth at frequencies of 4 kHz and less, with a maximum spread

of approximately 6 dB, as shown in Fig. 9. However, a sharp pressure

minimum is formed, which is probably a shadow from the pinna, at frequencies above 5 kHz. Thus, near the pressure minimum, a change in sound

6
/

pressure level of 15-16 dB occurs over a distance of less than 2 cm.

These minima, behind the pinna, are much steeper and more severe than

those alongside the head.

The pressure field becomes smoother if the signal is not a discrete

tone, but a band of noise. The sound pressure level alongside the head

using 6% bandwidth and 29% bandwidth pink noise is compared to the discrete frequency pressure levels at 6.3 kHz in Fig. 11. The pressureversus-distance curve becomes increasingly smoother as the bandwidth is

increased. Thus the change in the overall pressure level relative to

the discrete frequency change is reduced by approximately 2 dB and 5 dB

with 6% and 29% bandwidth noise, respectively. In particular, the nulls

become less sharp between the - 2 cm and the - 3 cm positions. Thus the

microphone location becomes less critical as the bandwidth of the noise

signal is increased.

The effect of the signal bandwidth on the maximum and average change

in sound pressure level resulting from a 5-mm change in the hearing aid

microphone location is summarized in Table 1. Column 1 of Table 1 refers

to the "vertical difference" in sound pressure level in going from contour 1 to contour 2 or 3. Column 2 refers to the "longitudinal difference"

which represents the change in sound pressure level along contour 2 when

the microphone position changes by 5 mm. The third column refers to the

lateral difference in the sound pressure level when the microphone is

moved from the position 2 mm from the head surface to 7 mm from the head

surface, normal to the head surface.

7
I

CONCLUSIONS

The spatial variation of the sound pressure level along the head of

a manikin is smooth and decreases monotonically , within ± 1 dB, from tfye

j front to the back of the head for frequencies < 2 kHz. For frequencies

between 4 and 8 kHz the spatial pressure variation is less well-behaved

and oscillates by as much as 14 dB from the front to the back of the head

over the 10-cm range of contour 2.

The pressure variation immediately behind the pinna is smooth at low

frequencies. However, a sharp null in the spatial pressure variation is

formed at frequencies of 5.0 to 8.0 kHz. Thus, hearing aid microphones

behind the pinna would be exposed to severe level differences from position to position and frequency to frequency since the null shifts position

with frequency. The pressure measurements at such a location are therefore unreliable unless particular care is used in the microphone placement.

It has also been shown that it may be useful to test hearing aids

with random noise of finite bandwidth (see Fig. 11 and Table 1), particularly above 5 kHz, where the sharp minima and maxima, formed under discrete tone excitation, are smoothed out and less severe. However, this

smoothness of the spatial pressure variation is at the expense of some

frequency resolution due to the bandwidth of the noise.

8
REFERENCES

ANSI (1975), S3. 22, "Draft Proposal for Specification of Hearing Aid

Characteristics," American National Standards Institute, New York.

Burkhard, M. D. and Sachs, R. M. (1975), "Anthropometric manikin for

acoustic research," J. Acoust. Soc. Am. 5_8, pp. 214-222.

Burkhard, M. D. (1976), personal communication.

Firestone , F. A. (1930) , "The phase difference and amplitude ratio at the

ears due to a source of pure tone," J. Acoust. Soc. Am. 2^, pp. 260-268.

IEC (1959) , "Recommended methods for measurements of the electro-acoustical

characteristics of hearing aids," Publication 118, International

Electrotechnical Commission, Geneva, Switzerland.

Kuhn, G. F. (1976), "An objective sound localization model in the azimuthal

plane," Seminar at the National Bureau of Standards, Washington, D.C.,

April 29, 1976 (to be published).

Morse, P. M. (1948), 2nd ed. Vibration and Sound , McGraw-Hill Book Co.,

Inc. , New York, pp. 448 and 450.

Rschevkin, S. N. (1963). The Theory of Sound , Pergamon Press Ltd., Oxford,

pp. 362-363.

Veterans Administration (1975), "Hearing Aid Performance Measurement Data

and Hearing Aid Selection Procedures," U.S. Government Printing Office,

Wash., D.C. 20402, VA Report No. IB 11-48.

Wiener, F. M. (1947a), "On the diffraction of a progressive sound wave by

the human head," J. Acoust. Soc. Am. 19_, pp. 143-146.

Wiener, F. M. (1947b), "Sound diffraction by rigid spheres and circular

cylinders," J. Acoust. Soc. Am. 19^ pp. 444-451.

9
TABLE 1

DIFFERENCES IN AMPLITUDE CAUSED BY 0.5-CM VARIATIONS

IN MEASUREMENT POSITION

Direction

Frequency

Hz

Vertical

difference

dB

Longitudinal

difference

dB

Lateral

dif f erence

dB

Ave Max Ave Max Ave Max

< 2000 < 0.5 < 0.5 < 0.5 0.5 <0.5 0.5

4000 1.0 1.6 0.5 2.4 <0.5 1.5

4000

6% BW

noise

0.7 1.3 0.4 2.2

5000 1.1 2.3 0.7 1.9 <0.5 1.5

5000

6% BW

noise

0.5 1.4 0.7 1.6

6300 1.1 2.1 1.2 3.2 <0.5 1.5

6300

6% BW

noise

0.6 1.9 0.9 2.1

6300

29% BW

noise

0.4 0.8 0.5 1.5

8000 1.2 2.5 1.6 4.0 0.5 1.5

8000

6% BW

noise

0.6 1.8 1.2 3.2

8000

29% BW

noise

0.5 1.4 0.6 1.6

10

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NBS-1 1 4 A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

2J6S//1 it,' /no

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTIT LE

The Acoustic Pressure Field Alongside a Manikin's Head

with a View Towards In Situ Hearing Aid Tests*

5. Publication Date

6. Performing Organization Code

200.03

7. AUTHOR(S)

Edwin D. Burnett and George F. Kuhn

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No. 2003416

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Dept, of Medicine & Surgery

Veterans Administration

810 Vermont Avenue, N.W.

Washington, D.C. 20420

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

*As presented at the 91st meeting of the Acoustical Society of America,

Statler-Hilton Hotel, Washington, D.C., April 7, 1976.

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

To simulate actual usage conditions, the frequency response of hearing aids was measured on the head of a manikin over the frequency range of 0.2 to 8 kHz. The

acoustic pressure around the head can vary rapidly as a function of frequency and

location. In order to compare and interpret the hearing aid response at various

frequencies and locations on the head, it is necessary to precisely know the pressure

variations. The amplitude and phase of the acoustic pressure were measured in

increments ranging from 2 mm to 5 mm alongside a manikin's head with frontal sound

incidence. The acoustic driver was located in front of the manikin at a distance of

1.0 m from the ear canal axis. The test frequencies were the octave band center

frequencies from 0.5 kHz to 4.0 kHz and the third octave band center frequencies from

4.0 kHz to 8.0 kHz. It will also be shown that pink noise of 6% and 29% bandwidth at

6.3 kHz and 8.0 kHz has a smoothing effect on the acoustic pressure variation with

location

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Audiometry; diffraction; head diffraction; hearing aids; instruments for hearing;

manikin

.

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