Current State of Sound Field Audiometry

Sound field audiometry involves using test stimuli delivered through speakers instead of earphones.  An Audiologist will do this when testing difficult to test patients (infants).

When testing is performed in the sound field as opposed to calibrated earphones, the interactions between the room acoustics, test stimuli and psychoacoustic perceptions of the listener become critical.  The current standard for audiometers is ANSI 3.6-1996.  This standard does not include recommendations for calibrating stimuli used in sound field testing.  ANSI 3.6-1996 includes an appendix (not part of the official standard) which addresses sound field calibration and includes suggestions regarding the acoustic characteristics of the room, location of the speaker(s), appropriate reference level for a speech signal in the sound field, and the frequency response of the sound field.   

The lack of a standard is of particular concern because research indicates that differences in speaker placement, sound field stimuli, and calibration methods can significantly affect threshold measurements (Stream and Dirks, 1974; Barry and Resnick, 1978,; Orchik and Rintelmann, 1978; Stephens and Rintelmann, 1978; Morgan et al, 1979; Wilber, 1979; 1985; Dillon and Walker, 1980, 1981, 1982a,b; Walker and Dillon, 1983; Walker et al, 1984; Cox and McCormick, 1987). 

Rochlin 1993 conducted a survey of 942 ASHA certified Audiologists.  The survey was in the form of a questionnaire that examines their practices regarding sound field testing.  More specifically, to describe and analyze sound field stimuli, sound field test conditions, and sound field calibration procedures currently employed by audiologists nationwide and to investigate the extent of agreement between current practice and recommended practice.

The survey revealed that 70% of respondents used sound field testing for testing infants, toddlers, and uncooperative clients who refuse earphones, as well as for performing hearing aid evaluations. 

Forty-two different audiometers were reported by the respondents.  Staab and Rintelmann (1972) reported that different manufacturers vary considerably in frequency deviations, modulation rates, and waveforms of warble tones (refer to sound field calibration using warble tones).

The sound field calibration techniques used by respondents was also examined. Most of them (74%) indicated that an equipment repair personnel was responsible for performing the calibration.  This includes such people as calibration technicians, specialists, or instrumentation and calibration service providers.  Approximately 10% of respondents said they were responsible for calibrating the equipment themselves. 

The following table indicates the norms and reference values upon which the respondents based their calibration. 

Reference Values Used for Electroacoustic Calibration

N = 240

(taken form Rochlin, 1993)     

The above values from Rochlin 1993 are interesting and at the same time disturbing.  What these figures indicate is that less than a third of respondents who calibrate electroacoustically, know and use normative values substantiated in the literature. 

The Rochlin 1993 survey also inquired about whether the respondents felt that their sound field methods and practice matched to industry acceptable practices.  An acceptable practice was any of the following four criteria.

1.   Stimuli delivered through speakers to obtain frequency dependent thresholds should be warble tones or narrow bands of noise, but not pure tones (Moran et al, 1979; Wilber, 1979, 1985; Byrne and Dillon, 1981; Goldberg, 1981; Dillon and Walker, 1982b; Walker and Dillon, 1983; Walker et al, 1984; Arlinger and Jerlvall, 1987; ASHA, 1991)

2.   Sound field stimuli calibration methods should include electroacoustic measurements, with or without biologic calibration (Streams and Dirks, 1974; Morgan et al, 1979; Walker et al, 1984; Cox and McCormick, 1987; ASHA, 1991).

3.   Electroacoustic measurements obtained should be compared to one of the sets of normative values reported in the literature (Streams and Dirks, 1974; Morgan et al, 1979; Walker et al, 1984; Cox and McCormick, 1987; ASHA, 1991).

4.   Sound field should be calibrated at least 4 times annually (ASHA 1989; Wilber, 1985).

Of these four designated criteria, only 7% or respondents indicated that they met all four.     

The results of this survey clearly show a lack of standardization among sound field test rooms, a variety of stimuli used for sound field testing, and inconsistent calibration methods.  Audiologist need to be aware of this weakness since this inconsistencies may cause them to make incorrect assumptions about any of the following

a.     The extent of hearing loss

b.     Need for amplification

c.     Improvement from hearing aid 

d.     Improvement or decrease in hearing over time

e.      Improvement in hearing after surgery

f.       Eligibility for special education  

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