Audible circuit noise in hearing aid amplifiers

Abstract

Audible electronic circuit noise generated within a hearing aid is distracting to a listener in quiet situations and, if the noise level is high enough, may cause listener irritation and rejection of the hearing aid. Thus for hearing aid specification and fitting purposes, it is useful to know the acoustic levels at which this internal noise may become audible and also at which it may become objectionable. For hearing aid amplifier circuit specification and design purposes, it is useful to know the same levels in electrical terms. This paper reports on a study that used an amplifier with no acoustic input and a hearing aid receiver output to simulate internally generated hearing aid circuit noise. Results are reported for testing eight subjects with high-frequency hearing loss for the perceived acoustic and electrical levels at which internal circuit noise became both audible and objectionable.

The Causes and Effects of Distortion and Internal Noise in Hearing Aids

“…This graph shows that the noise has become audible where the hearing aid noise level is such that the 1/3-octave band at 1000 Hz has reached the level of the audiometric loss. Agnew (1997b) also measured the level of internal hearing aid noise that was perceived to be objectionable by eight listeners with moderate sensorineural hearing impairments. The descriptor "objectionable" was defined for the purposes of this study as the level of noise that the subject felt would be unpleasant to listen to for an extended period of time.…”

Measurement of hearing aid internal noise

“…The audibility of a HA's internal noise is dependent on the circuit noise amplitude spectrum, low-level gain, and the listener's auditory thresholds ͑Killion, 1976; Agnew, 1997;Thompson, 2003͒. The loudness or annoyance of such noise may be dependent upon the spectral shape of the noise and, thus, cannot be described by a single value ͑Hellman and Zwicker, 1987͒.…”

Hearing Aid Processing Changes Tone Burst Onset: Effect on Cortical Auditory Evoked Potentials in Individuals With Normal Audiometric Thresholds

“…The input level used in Billings et al (2011) was 40 dB SPL, whereas it was 60 dB SPL in the current study. Assuming that the microphone is the dominant source of internal noise in the hearing aid (Agnew, 1997;Thompson et al, 2002), the SNR at the input of the amplifier will be lower when the level of the tone burst/stimulus is lower. At any instant, the gain applied to the constant noise floor and other input stimuli will be the same because they occur simultaneously, and the gain in such cases is determined only by the higher level stimulus (Wolfe, Thompson, Swim, Wood, & Schafer, 2007).…”

“…Expansion, a processing stage in which gain decreases as the input level decreases, mainly catered to minimize internal noise of the hearing aid (Bray & Ghent, 2001), may or may not be activated during the ISI. Third, the use of a hearing aid imposes a noise floor on the signal, and nonlinear signal processing can raise the level of the noise floor (Agnew, 1997;Lewis, Goodman, & Bentler, 2010;Thompson et al, 2002).…”