Sound Field Measurement Tutorial

Audiologic Evaluation Committee, Working Group On Sound Field Calibration

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• Langue: English

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Sound Field Measurement Tutorial
Audiologic Evaluation Committee, Working Group on Sound Field Calibration
Reference this material as: American Speech-Language-Hearing Association. (1991). Sound Field
Measurement Tutorial [Relevant Paper]. Available from www.asha.org/policy.
Index terms: acoustics, assessment
DOI: 10.1044/policy.RP1991-00025
© Copyright 1991 American Speech-Language-Hearing Association. All rights reserved.
Disclaimer: The American Speech-Language-Hearing Association disclaims any liability to any party for the accuracy, completeness, or
availability of these documents, or for any damages arising out of the use of the documents and any information they contain.Sound Field Measurement Tutorial Relevant Paper
About This This tutorial was prepared by the American Speech-Language-Hearing
Document Association (ASHA) working Group on Sound Field Calibration of the Committee
on Audiologic Evaluation and accepted for publication by the ASHA Executive
Board (EB 119-90). The Working Group members are Laura Ann Wilber, chair;
William Melnick; Donald E. Morgan; and Patricia G. Stelmachowicz. The
Committee on Audiologic Evaluation members are Sandra Gordon-Salant, chair;
S. Joseph Barry; Evelyn Cherow, ex officio; Thomas A. Frank; Gregg D. Givens;
Michael Gorga; Susan W. Jerger; Sharon A. Lesner; and Robert Margolis. Teris
K. Schery, 1988–1990 vice president for clinical affairs, was monitoring vice
president. The contributions of Lucille Beck and Jo Williams are acknowledged.
This tutorial is not intended to be a standard; rather its purpose is to provide an
overview of issues related to sound field measurement and suggestions for clinical
practice.
****
I. Introduction Auditory measurements in a sound field are routine procedures in most clinical
audiological settings. For infants, young children and difficult-to-test individuals
who will not tolerate earphones, there are few alternatives to sound field
measurements for the determination of threshold levels. Sound field measurements
also are used to evaluate the real ear characteristics of amplification systems, either
with behavioral methods or probe tube measures of insertion gain. Although there
are many problems associated with sound field measures, they remain an integral
part of many clinical assessments. Unfortunately, at present there is no standard
or official guideline for sound field measurement.
Testing in the sound field requires an awareness of the limitations imposed by the
characteristics of the room, the background noise levels, properties of the
loudspeaker, unavoidable movement of the listener, the type of stimuli, and a
variety of other factors. In some instances it may not be possible to alleviate or
even minimize the effects of these variables. The above variables in the acoustic
environment may interact unpredictably to affect the reliability of both threshold
and supra-threshold sound field measures. From a practical standpoint, clinicians
often may be forced to set priorities and compromise accuracy in order to achieve
a particular goal. The specific application (e.g., sound field threshold estimates in
infants, supra-threshold hearing aid measures) may dictate the decision.
The purpose of this tutorial is to provide an overview of the problems encountered
with auditory measurements conducted in the sound field and to furnish
suggestions that will significantly reduce or eliminate these problems. Many of the
issues addressed in this document are also applicable to probe tube microphone
measures of hearing aid gain and output. However, the additional complexities
related to stimulus selection and calibration of probe tube measures prohibit a
detailed treatment of probe tube measurement within the scope of this paper. The
issues that are addressed are related to equipment, sound field environment,
selection of test stimuli, calibration of the sound field, and equivalent threshold
sound pressure levels (ETSPLs). Finally, this tutorial includes a glossary of terms
found in the text, and which often are used in acoustic measurements conducted
in the sound field.
1Sound Field Measurement Tutorial Relevant Paper
II. Equipment The equipment used in sound field measurements consists of a stimulus generator,
loudspeakers, and calibration equipment. The stimulus generator is usually an
audiometer and will not be discussed here. The problems related to loudspeakers
and sound field measurement procedures will be discussed below.
A. Loudspeakers
Testing in the sound field requires use of a loudspeaker to transduce an electrical
signal to an acoustical stimulus. Problems not typically encountered with
earphones may be introduced when using loudspeakers, primarily due to the
interactions among the transducer (loudspeaker) characteristics, the acoustic
environment (test room), and the positioning of the listener in the sound field. The
interactions among these variables affect the spectral characteristics of the acoustic
energy reaching the ear of the listener. There exist no American National Standards
Institute (ANSI) or International Electrotechnical Commission (IEC) standards
defining minimum characteristics to be met by loudspeakers used in audiologic
testing; therefore, it is important to review some of the factors affecting
loudspeaker performance.
The ideal loudspeaker for audiologic testing should possess the following general
characteristics: (a) broad bandwidth (minimally 100–10,000 Hz); (b) constant
output as a function of frequency (“smooth” frequency response); (c) low
distortion; (d) capability of accurately transducing transient as well as steady-state
signals; (e) uniform radiation pattern in the sound field; and (f) high electroacoustic
efficiency (high acoustic output with low voltage input).
No single loudspeaker has been designed that can satisfy all of the above
requirements. Of the many loudspeakers available, the most widely used is the
direct radiator type. Within the frequency and intensity requirements for audiologic
testing in the sound field, such loudspeaker systems provide the most efficient
transduction of currently employed acoustic signals. However, to attain the desired
bandwidth and constancy of frequency response at test locations in the sound field,
it is typically necessary to utilize two or more loudspeakers coupled with a
crossover network, dividing the overall frequency range so that each loudspeaker
operates within its optimum frequency band. Unfortunately, such loudspeaker
arrays are particularly vulnerable to sharp changes in output at the frequencies
coinciding with the crossover regions. Loudspeaker systems used in sound field
testing must be evaluated in the environment in which they will be used to
determine the effects of crossover networks on the frequency response of the
system.
The frequency response of a loudspeaker may vary greatly in both the overall
frequency range and the relative levels within the overall range. Figure 1 includes
the frequency response of a typical supra-aural earphone and two hypothetical
frequency responses of loudspeaker systems having a nominal frequency range of
200 to 10,000 Hz. Transduction of very narrow band signals probably would not
be affected by the variations in frequency response shown for the loudspeakers.
However, when complex signals are transduced, the overall sound pressure level
(SPL) and the relative contribution of any frequency band in the signal will be
affected by the specific frequency response differences across the two systems.
For example, if a broad band signal such as speech is transduced through the two
loudspeakers the overall SPL might be identical from each speaker, but the relative
2Sound Field Measurement Tutorial Relevant Paper
Figure 1. Frequency response characteristics from: (a) typical supra-aural earphone, (b) (c) two typical loudspeakers
(reprinted with permission from Dirks, Stream, & Wilson, 1972).
contribution of low-frequency and high-frequency components could be
significantly different between the two loudspeakers. The difference in relative
contribution between the high- and low-frequency bands could influence hearing
test results when individuals with sharply sloping hearing loss configurations are
evaluated. It has been suggested that loudspeakers used for the transduction of
speech vary no more than ±5 dB across the frequency range from 200 to 6000 Hz
(Dirks, Stream, & Wilson, 1972).
The loudspeakers in Figure 1 also provide examples of irregularities in frequency
response which would affect narrow-band signals (but not pure tones) transduced
into the sound field. When a frequency modulated (FM) signal (see section on
Stimuli) is transduced, the deviations in the frequency response of the loudspeaker
interact with the FM signal deviations to introduce amplitude modulation into the
acoustic signal. Such amplitude modulations can be avoided by ascertaining that
the loudspeaker frequency response is constant within the frequency modulation
range of any FM signal to be delivered to the loud speaker. When frequency-by-
frequency differences in loudspeaker responses are encountered, it may be possible
to adjust the level in the sound field using narrow band equalizers to overcome
inadequacies in the loudspeaker system.
When a signal is transduced through a loudspeaker, a near field and a far