Influence of advanced hearing aid technology on choice of signal for probe microphone measures.

Modern probe microphone systems offer a wide range of measurement options including a multiplicity of test signals. The signal of choice for any particular measurement is largely determined by the purpose in performing the measurement. For example, while a pure tone sweep would be fine for measuring a real-ear-to-coupler difference, the patient's own vocalizations would probably serve better for troubleshooting occlusion-related complaints. In this commentary, it assumed that the goal of the probe microphone measurement is to ensure speech audibility in fitting one of the current digital hearing aids, and the influence of advanced signal processing in choosing signal type for this purpose is discussed. Important considerations include compression characteristics in the hearing aid, the fitting rationale employed, and any special signal processing in the hearing aid (e.g. noise reduction). Modern hearing aids, and digital hearing aids in particular, can provide input-level dependent amplification over a wide range of input levels. This implies that in verifying a fitting, it is important to choose a prescriptive method intended for fitting nonlinear amplification as well as to test the response of the hearing aid at different input levels. Concerning signal type, it should be borne in mind that non-speech signals will tend to underestimate gain for real speech when hearing aids are operating in a nonlinear fashion, with the greatest discrepancies occurring for swept pure tones. In a study comparing input signal types for obtaining frequency responses of hearing aids with AGC circuitry, Preves et al (1989) found lower output levels with swept pure tones than with broadband noise. They asserted that frequency response curves obtained using broadband noise inputs would be more representative of how "real world" sounds are processed by the hearing aid. Stelmachowicz et al (1996) used hearing aid gain for continuous discourse as the basis for comparison with different input signal types. Like Preves and his colleagues, they found lower hearing aid gain for swept pure tones than for broadband signals, particularly when the hearing aids were set to provide non-linear amplification. Compared to hearing aid gain for real speech, the gain observed for swept pure tones varied by up to 14 dB. Another aspect of compression systems in modern hearing aids is that they often have two or more compression bands in which the compression characteristics can differ. Depending on the number of bands and degree of overlap between bands, dramatic differences can be observed in measurements performed with pure tone sweeps versus broadband test signals (Edwards et al, 1998). Note that some prescriptive methods for nonlinear amplification define target gain values for a specified signal type. For example, the insertion gain targets generated by GN ReSound's propri-