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A new miniature wide-range electret microphone with a flat frequency response is available to the audiology clinic to measure sound pressure in the patient's ear canal. This article describes the clinical application of this new microphone for the selection and monitoring of hearing aid response. Preliminary results of the relationship between ear canal measurements and functional gain are also provided. This new clinical technique can provide the audiologist with quick objective data on the response of a hearing aid while it is being worn by the patient.

Results comparing hearing aid performance utilizing probe microphones with performance measured in a commercial hearing aid test box using an HA-2 coupler are presented. Sixteen hearing-impaired subjects (23 ears) were divided into two groups. Group I utilized unvented earmolds with short canals and large bores; group II utilized unvented earmolds with long canals and small bores. Results revealed that large individual differences across frequencies as well as within and between groups preclude predicting how a hearing aid will perform on a real ear from test box measurements.

An individual head, auricle, and ear canal can alter signals from a hearing aid other sources of acoustic stimulation compared to artificial couplers. The most realistic method for quantifying or verifying certain parameters of acoustic simulation delivered to the ear by a hearing aid is to measure the sound pressure level of a signal in the ear canal of an individual while wearing an aid. Probe microphone measurements have been conducted and reported for the past 33 years. Until now, such microphone measurements have used a hollow tube inserted into the ear canal which leads to a transducer outside the canal. This paper describes our preliminary clinical application of a new wide range, flat response, miniature electret microphone, measuring 4 x 5 x 2 mm for measurement of insertion gain in a patient's ear canal. We believe the information from these recordings can be useful in developing objective clinical procedures for the selection and monitoring of wearable amplification for hearing-impaired patients.

Real ear SPL (RE/SSPL90) produced by four different hearing aids was compared to the standard SSPL90 measurements for the same hearing aids on a 2-cc coupler. The effects of venting, location, and reliability for RE/SSPL90 were also studied. As expected, the real ear output level was greater than the coupler level. No systematic influence of earmold venting was found. An earmold that occludes an ear canal decreases the output in the high-frequency range so that only a small difference remains between RE/SSPL90 and 2-cc coupler levels above 2000 Hz. The difference seems to be affected slightly by the output impedance of the hearing aid receiver system. Individual variation for the RE/SSPL90 is as much as 10 dB. Finally, the saturated output levels measured were found to be insensitive to changes in actual microphone input levels caused by different procedures for calibrating ambient SPL. The RE/SSPL90 method is easy to use and reliability seems quite adequate for the typical clinical setting.

The acoustical problems involved in matching the saturation sound pressure level for a 90 dB input signal (SSPL90) of a hearing aid to individual discomfort level were investigated. The real ear SPL (RE/SSPL90) produced by a supra-aural earphone used when measuring uncomfortable loudness (UCL), and RE/SSPL90 produced by three different hearing aids at 90 dB SPL input, were measured for nine subjects, using a miniature microphone technique, and compared to the corresponding coupler levels used when matching hearing aid maximum output to UCL. It was found that a hearing aid often gives about 5 dB, and sometimes 10 dB, higher RE/SPLs than the earphone, if the hearing aid output levels, as measured in a 2-cc coupler (IEC126), are equal to the earphone output levels as measured in a 6-cc coupler (NBS9A). It is recommended that a safety margin of at least 5 dB be used in the preliminary fitting when matching hearing aid SSPL90 to the patient's UCL, converted to dB SPL.

Recent research suggests that persons with sensorineural hearing impairment should derive extra benefit from amplification that eliminates or greatly reduces low frequencies, i.e. frequencies below 1 500 or 2 000 Hz. Such amplification seems to reduce the detrimental effects of the upward spread of masking on speech intelligibility, especially when listening in noisy environments. Also, current research indicates that extended high-frequency amplification, between 4 000 and 6 500 Hz is especially beneficial for optimal speech intelligibility. 9 patients with a relatively flat, moderate to severe sensorineural hearing loss were evaluated in the clinic and for daily listening situations while wearing conventional broad-frequency hearing aids with an upper range of about 4 500 Hz. They were also evaluated under these same circumstances with a hearing aid that reduces low frequencies and extends the high frequencies to nearly 6 500 Hz. Results indicate that these patients performed better and perferred the hearing aid that extends the high and reduces the low frequencies, particularly in noisy places. As a group, they did not prefer this type of amplification in quite listening situations.