that it occupied when it was worn by each subject. The effect of each subject's body on each hearing aid's response at each l/3-octave frequency was deter-.
BODY-BAFFLE AND REAL-EAR EFFECTS IN THE SELECTION OF HEARING AIDS FOR DEAF CHILDREN
Norman P. Erber Central Institute for the Deaf, St. Louis, Missouri
A difference between expected and obtained results in hearing-aid selection often can be explained on a physical basis alone. For example, the frequency response of a hearing aid typically is obtained by suspending the instrument in a free sound field and measuring the acoustic output of the receiver in a standard 2-cc coupler. However, w h e n a body-type hearing aid is used by a deaf child, it normally is worn on his chest, and its receiver is coupled to an earmold in his ear canal. Under these conditions, the acoustic input to the hearing-aid microphone and the acoustic output of the receiver will differ from that measured in the laboratory. Because the dynamic range of sensitivity of profoundly deaf children typically is small, a neglected 5-10 dB of amplification can be critical in the selection of hearing aids. A clinical procedure is proposed by which the output of a hearing aid can be matched more accurately to the sensory capacities of an impaired ear. This method defines both thresholds and hearing-aid output in terms of sound pressures generated in a 2-cc coupler and considers only the specific body-baffle and real-ear effects that are produced by the patient himself.
I n most h e a r i n g clinics, the audiologist selects a h e a r i n g aid for a deaf c h i l d by c o m p a r i n g the electroacoustic characteristics of several i n s t r u m e n t s w i t h the pure-tone thresholds of the child. T h i s p r o c e d u r e is e m p l o y e d because of the difficulty i n o b t a i n i n g speech r e c e p t i o n d a t a f r o m y o u n g h e a r i n g - i m p a i r e d c h i b dren. Occasionally, contradictions occur between the e s t i m a t e d a m o u n t of a m p l i f i c a t i o n r e q u i r e d a n d that w h i c h actually is f o u n d to be c o m f o r t a b l e a n d useful to the child. T h e s e discrepancies can occur even w h e n b o t h the characteristics of the h e a r i n g aid a n d the thresholds of the c h i l d are m e a s u r e d carefully. O f t e n the difference between the results expected f r o m a h e a r i n g a i d a n d those a c t u a r y o b t a i n e d has a s i m p l e physical basis. For e x a m p l e , the u s u a l m e t h o d of o b t a i n i n g a h e a r i n g aid's f r e q u e n c y response is to suspend the instrum e n t i n a free s o u n d field a n d m e a s u r e the acoustic o u t p u t of its receiver in a s t a n d a r d 2-cc coupler. T h e results of this p r o c e d u r e can be d u p l i c a t e d in a n y electroacoustics laboratory. W h e n a p a t i e n t uses a body-type h e a r i n g aid, though, he usually wears it on his chest i n a m o d e r a t e l y r e v e r b e r a n t s o u n d field, a n d its receiver is c o u p l e d to a n e a r m o l d in his ear canal. Both the acoustic i n p u t to the h e a r i n g - a i d m i c r o p h o n e a n d the acoustic o u t p u t of the receiver will 224
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ERBER: BODY-BAFFLEAND REAL-EAR EFFECTS
225
differ from the same measurements taken in the laboratory. Errors in matching instruments to hearing-impaired children can occur if one overlooks the distinctions between laboratory procedure and everyday use. B O D Y - B A F F L E EFFECTS
When a hearing aid is worn on the chest, its frequency response is not the same as that measured when the aid is suspended in space, because the human body absorbs and reflects sound in a complex way. Previous studies (see Nichols et al., 1947), in general, have shown that placing a hearing aid on the chest increases the sound pressure at the microphone of the instrument about 2-6 dB in the frequency range 200-800 Hz, sharply decreases the sound pressure about 5-15 dB in the range 1000-2500 Hz, and causes negligible variation in the response for frequencies above about 3000 Hz. Research also has indicated that: the acoustic output of a hearing aid is reduced in the high frequencies when clothing covers it (Carlisle and Mundel, 1944); the decrease in level that is characteristic of the midfrequencies is not so prominent when the aid is worn in a shirt pocket rather than in the center of the chest (Hanson, 1944; Nichols et al., 1947); the size of this decrement in level also is reduced when the aid is raised even slightly from the surface of the body (Nichols et al., 1947); body-baffle effects diminish when the aid is worn in a highly reverberant sound field (Nichols et al., 1947) ; the effect of the human body can be simulated only with elaborately constructed baffles (Carlisle and Mundel, 1944; Hanson, 1944); and the angle of sound incidence must be considered (Byrne, 1972). Recently, the effects of other physical variables on hearing-aid performance were investigated in our laboratory. In this experiment, each of 10 adults (age range 20-31 years: five males, five females) and 10 children (age range 4-9 years: five males, five females) wore each of four hearing aids at different times outside their clothing at center-chest position. Two of the instruments contained topmounted microphones, while the other two employed front-mounted microphones. T h e aids chosen for comparison represented the four most common models currently in use by deaf pupils at Central Institute for the Deaf. T h e measurements were made in an anechoic chamber. Each subject stood 9 ft from a loudspeaker that was mounted 5 ft above the floor. T h e acoustic input signal was wide-band noise with a spectrum level of 60 dB SPL at the hearing-aid microphone. T h e acoustic output generated by the receiver of each aid in a standard 2-cc coupler was measured in 1/3-octave steps with Brfiel and Kjaer analysis and recording equipment. Later, comparable free-field measurements were made while each instrument was suspended at the exact position that it occupied when it was worn by each subject. T h e effect of each subject's body on each hearing aid's response at each l/3-octave frequency was determined by subtracting its acoustic output under free-field conditions from that obtained when the aid was worn on the body. Our findings confirm the general relation of the body-baffle effect to fire-
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226
JOURNAL OF SPEECH A N D HEARING DISORDERS
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XXXVIII, 2
quency as described by earlier investigators, but the results demonstrate notable differences between the effects of adults and children. For example, regardless of the type of aid worn, the mean body-baffle effect is about 1-5 dB greater for adults than for children in the frequency ranges 200-400 and 1000-1500 Hz (Figure 1). The orientation of the microphone within the hearing-aid case also was found to be an important variable. When the subjects wore hearing aids with topmounted microphones, the mean body-baffle effect was about 2 dB greater at most frequencies than when they wore instruments with front-mounted microphones (Figure 1). Also, the narrow dip in sound pressure that is characteristic
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