Directional Hearing Aids (2001)

When asking individuals with hearing impairment to identify the situations for which the most communication difficulty is encountered, listening in noise seems to be nearly universally mentioned. In fact, I would venture that if queried, almost no patient would refuse the offer of improved levels for sounds of interest in relation to the level of background noise. Improving the signal-to-noise ratio (SNR) for listeners with hearing impairment has long been a goal of various amplification schemes. The reasons for attempting to improve the SNR delivered to hearing aid wearers are obvious given clear evidence of reduced speech recognition with increasingly less favorable SNRs. Moore (1989) suggested that individuals with normal hearing require a SNR of at least +6 dB for satisfactory communication. Unfortunately, data overwhelmingly show that individuals with sensorineural hearing loss generally require even more favorable SNRs for satisfactory communication than listeners with normal hearing (Carhart and Tillman, 1970; Cooper and Cutts, 1971; Dirks et al, 1982; Groen, 1969; Killion, 1997; Plomp, 1976; Schum, 1996; Sutter, 1985). Children with hearing impairment appear to be even more negatively effected by poor SNR (Boothroyd et al., 1996; Crandell, 1993; Crandell and Smaldino, 2000; Finitzo-Hieber and Tillman, 1978), leading the American Speech-Language-Hearing-Association (Bess et al., 1996), and others (Berg, 1993; Bistafa and Bradley, 2000; Blair, 1990; Smaldino and Crandell, 1995), to recommend SNRs of at least +15 to +30 dB in educational settings. Unfortunately, most classrooms have SNRs between −6 and +6 dB, making learning in such environments difficult (Bess et al., 1984; Crandell and Smaldino, 2000).

Results from past investigations offer clear evidence that listening in poor SNRs is a significant problem for listeners with sensorineural hearing loss. Listening problems in noisy environments can be devastating, and lead some listeners with hearing loss to avoid difficult listening situations, resulting in withdrawal and greater isolation, potentially impacting their overall quality of life in a negative way (Jackson, 1997; McCay, 1996; Mulrow et al., 1990).

Hearing aids represent a common rehabilitation method for listeners with sensorineural hearing loss. Hearing aids using standard (omnidirectional) microphones, while effective at increasing audibility for speech and other sounds, are largely ineffective in improving inadequate SNR conditions whether they use analog (Killion and Villchur, 1993; Killion, 1997; Plomp, 1978; Tyler and Kuk, 1989; Van Tasell, 1993; Verschuure et al., 1999) or digital (Ricketts and Dahr, 1999; Walden et al., 2000) techniques. In fact in some cases, SNR may be made worse by some hearing aid styles that have single omnidirectional microphones (Beck, 1983; Ricketts, 2000a). Listeners wearing hearing aids that have standard microphones are likely to require more favorable SNRs than listeners with normal hearing to accommodate the noise-related problems associated with their poorer hearing thresholds. This difficulty is highlighted in Figure 1 in which the aided sentence reception threshold as a function of pure tone average (PTA) hearing threshold for 70 listeners collected as part of two different investigations have been plotted (Ricketts et al., 2001; Ricketts, unpublished data). The regression line through these data and the linear regression of the unaided data are also plotted. These data were all collected in a single listening environment designed to emulate a noisy restaurant, and the speech recognition results from these studies were averaged across three or four hearing aid models of the same style to obtain a more stable representation of performance. The test hearing aids included analog, digitally programmable analog, and digital signal processing (DSP) instruments. The SNR necessary for these listeners to achieve 50% correct sentence recognition performance was measured using two ten-sentence blocks of the Hearing In Noise Test (HINT; Nilsson et al., 1992; Nilsson et al., 1994).



Figure 1.

Mean aided HINT sentence reception thresholds, as a function of PTA hearing threshold, for 70 listeners. The predicted, aided, linear regression of these data is also plotted. For comparison purposes the linear regression of these same subjects’ ...



Although there is considerable variance present in the data shown in Figure 1, as reflected by the moderate correlation (r = 0.59), two trends are apparent. First, in good agreement with previous data, listeners with greater hearing loss require more positive SNRs for equivalent sentence recognition performance (i.e., 50%). Second, and more pertinent to the current discussion, the relationship between SNR and hearing loss is remarkably similar across the aided and unaided listening conditions. Specifically, the average HINT performance for aided and unaided conditions is nearly identical for listeners with better hearing, suggesting that these hearing aids provided no SNR benefit over the unaided ear. For listeners with greater hearing loss, HINT performance is greater for the aided, as compared to the unaided condition; however, this is likely due to increased audibility for these listeners, rather than an improvement in SNR. These results are also in good agreement with surveys that indicate that listeners with hearing impairment commonly experience problems in noise, even when fit with amplification. For example, Kochkin (2000a) reported that 25% of individuals who own hearing aids but do not wear them cite poor performance in background noise as the major reason for hearing aid rejection.