Elements of Effective Hearing Aid Performance ( 2004 )

High-performance hearing aids require flat insertion gain (before processing) and low distortion, so that when signal processing is introduced the pre-processed response of the aid does not bias the processing. High-performance hearing aids need to process the signal to compensate for the patient's perceptive distortions (primarily accentuated high-frequency hearing loss and recruitment), and this processing must be adjusted to the hearing of the individual patient. The goal of the processing (compression combined with frequency-response shaping) is to fit the speech band into the patient's residual dynamic range of hearing, amplifying each element of speech to the same relative level in the patient's dynamic range of hearing as the unamplified element has in the normal dynamic range of hearing. Introduction Effective performance in sound reproducing equipment refers to the accuracy with which the reproduced sound imitates the original live sound. Effective performance in hearing aids involves more than fidelity to the original sound: it implies a restoration of normal hearing to the extent possible to a patient with impaired hearing. To the hearing-impaired listener the fidelity of a hearing aid is not fidelity to the input sound but fidelity to the normal perception of that sound. Restoration toward normal hearing involves three elements in the design and adjustment of the hearing aid: 1) The hearing aid must not introduce unwanted changes to the signal reaching the eardrum; 2) the signal needs to be processed to compensate for perceptive distortions created by the hearing impairment; and 3) the processing needs to be adjusted to the hearing of the individual patient. Real-ear Frequency Response of the Hearing Aid To measure the frequency response of an amplifier, the input of the amplifier is swept over the frequency spectrum with a sine-wave signal, and with the amplifier output properly loaded, the output signal is recorded. If the graph of output amplitude versus frequency is identical or almost identical to the graph of input amplitude versus frequency, the amplifier is said to have flat frequency response. Determining the effective frequency response of a hearing aid, which is to say the frequency response of the aid when it is in the ear, is a more complex procedure. The common way to measure hearing-aid response is with the output of the aid connected to a 2-cc coupler. This is a legitimate method, but the desired result is not flat response, for two reasons. First, the acoustic load presented to the hearing aid by the 2-cc coupler is not an accurate simulation of the load presented by a real ear: the coupler does not match the increase of canal impedance at higher frequencies (and consequent increase of hearing-aid response) created by the increased stiffness of the eardrum as the frequency goes up. Second, the physical presence of a hearing aid or earmold in the ear introduces unwanted changes to the transmission path of the ear; these changes affect the signal reaching the eardrum and must be counted as part of the response of the hearing aid. Neutral frequency response in a hearing aid is not flat frequency response but flat insertion gain. Insertion gain is the difference the operating hearing aid makes to the gain of the open ear. It is the aided minus the unaided gain of the ear. When a hearing aid has flat insertion gain over its frequency range, the sound pressures it produces at the eardrum for a given sound stimulus are no different, except in amplitude, from the pressures that would be produced at the eardrum by the same stimulus at the open, unaided ear. The sound-transmission path of the unaided ear has anything but flat frequency response. The most important influence on the frequency response of this path is the air-column resonance of the ear canal. The average ear has a primary canal resonance at about 2700 Hz, which creates a broad peak in the frequency response of the ear's transmission path of approximately 15 dB. The physical presence of a hearing aid or earmold that closes off the ear canal reduces the working length of the canal, typically by about half, which doubles the frequency of canal resonance. Closing off the canal also changes the mode of resonance from quarter-wave to half-wave, doubling the resonance frequency a second time. A third effect of the physical presence of the hearing aid is that the edge correction, which increases the effective length of an open canal, is lost. The net effect of these changes is to raise the resonance frequency of the canal to the 13-kHz region, well beyond the range of the typical hearing aid. When a new component is inserted into an electronic or acoustical system it is necessary to take into account not only the frequency response of the component itself, but the possible effect of that component on the frequency response of the rest of the system. The physical presence of a hearing aid in the ear removes an acoustical 2700-Hz, 15-dB response peak from the signal reaching the eardrum. Removing this peak has the same effect as introducing an electronic 15-dB response dip into the hearing-aid circuit. That original 15-db peak is part of the natural sound that we hear in speech and music, and the hearing aid needs to make up for the loss. The 2-cc coupler frequency response that corresponds to flat insertion gain of a hearing aid in an average ear has been measured and plotted by Killion and Revit (1993). A family of these "CORFIG" curves (Coupler Response for Flat Insertion Gain) is shown in Fig. 1. Each curve represents the coupler response that predicts flat insertion gain for a particular type of hearing aid -BTE, ITE, etc. Figure 1. CORFIG curves (coupler response for flat insertion gain). Solid line is for a BTE hearing aid; dashed line for a full-concha ITE; dotted line for a small ITC; and bottom dashed line for a deep canal CIC (from Killion Revit, 1993). The 2700-Hz peak in the CORFIG curve that makes up for the loss of the 15-dB ear canal-resonance peak is only about 5 dB, because the 2-cc coupler underestimates the real-ear high-frequency response of the hearing aid, as described above and as shown in Fig. 2. At 2700 Hz the 2-cc coupler underestimates real-ear response by about 10 dB. Figure 2. Comparison between eardrum and 2-cc sound pressure produced by a hearing aid (from Killion & Revit, 1993). To illustrate what serious errors can be made when the CORFIG curves are not taken into account, Fig. 3 shows the 2-cc coupler frequencyresponse curves published by a hearing-aid manufacturer for two of its former models, an ITE and a BTE. The ITE curve was a good approximation of its CORFIG curve; the BTE curve was not. Yet in spite of the large differences in insertion gain between these two models, the manufacturer prescribed the same fitting for each. Whatever processing was used to program the BTE aid, the processing was biased by the inadvertent changes in BTE insertion gain. The difference between the 2-cc coupler responses of the two models is shown at the bottom of Fig. 3. Figure 3. Comparison between 2-cc coupler response curves of ITE and BTE models of the same hearing-aid manufacturer (not current models). The same fitting program was prescribed for each. Hearing aids with flat insertion gain and low distortion at all useful levels could be described as "high fidelity" for normal listeners. Sounds of the outside world would hardly be changed by such aids. Hearing aids are not designed for normal listeners, but experimental data (Killion, 2003) demonstrate that hearing-impaired listeners show the same preference for flat insertion gain and low distortion (aside from processing) that normal listeners do. Compensation for Perceptive Distortions The second design requirement of a high-performance hearing aid is that the hearing aid compensate for the perceptive distortions of the hearing-impaired patient. There are many kinds of perceptive distortion. Some patients, for example, particularly those with severe-to-profound hearing impairment, lose hearing sensitivity to changes of frequency, either for sequential sounds or for simultaneous sounds such as occur in speech formants. But the great majority of persons with non-conductive hearing impairment suffer primarily from two perceptive distortions that affect speech understanding: the change of hearing loss with frequency -usually increased hearing loss at higher frequencies -and the change of hearing loss with sound level, a phenomenon called recruitment. Fortunately these distortions can be compensated, in varying degree, by the gain characteristics of the hearing aid. Steinberg and Gardner (1937), who first analyzed recruitment, called it "variable hearing loss," referring to the variation of hearing loss with sound intensity. A person with recruitment -and this would be almost anyone with a non-conductive impairment -hears intense sounds at near-normal or even normal loudness, which means that little or no amplification is needed at high sound levels. As sound intensity is decreased, however, the loss gets progressively worse, and more and more gain is required from the hearing aid. The degree of recruitment usually varies with frequency, typically increasing at higher frequencies. Recruitment makes weak elements of speech inaudible, and has a direct effect on both aided and unaided speech intelligibility. If the proper amount of hearing aid gain is provided to bring the higher-level elements of speech to comfortable loudness for a hearing-impaired person with recruitment, the weaker consonants (like the fricatives, which make a major contribution to speech understanding) will be underamplified, and will be either inaudible or too soft to identify. If the gain is then increased to bring the consonants to a comfortable loudness, the high-level elements of speech may become uncomfortably loud, and other loud sounds, e.g., the sound of a slammed door, can be painful. Th