The optimal noise-rejection threshold for normal and impaired hearing

Abstract

Binary masking represents a powerful tool for increasing speech intelligibility in noise. An essential aspect involves the local criterion (LC), which defines the signal-to-noise ratio below which time-frequency units are discarded. But binary masking is a victim of its own success in one regard—it produces ceiling sentence intelligibility across a broad range of LC values, making the exact optimal LC value difficult to determine. Further, the optimal value for hearing-impaired (HI) listeners is largely unknown. In the current study, the optimal LC was determined in normal-hearing (NH) and HI listeners using speech materials less likely to produce ceiling effects. The CID W22 words were mixed with noise consisting of recordings from a busy hospital cafeteria, then subjected to ideal binary masking. LC values ranged from -20 to + 5 dB relative to the overall SNR of -8 dB. NH subjects were tested at 65 dBA and HI subjects were tested at 65 dBA plus NAL-RP hearing-aid gains. Preliminary results suggest that the optimal LC is similar for NH and HI listeners. Additional conditions involving different speech materials and noise types suggest that the optimal LC can vary as a function of speech and/or noise type. [Work supported by NIH.]

An ideal quantized mask to increase intelligibility and quality of speech in noise

“…LC was set to À15 dB in order to be 5 dB below the overall SNR. This relationship between LC and SNR has been considered near optimal (e.g., Brungart et al, 2006;Kjems et al, 2009;Vasko et al, 2018), and has also been employed by us previously (Healy et al, 2013;Healy et al, 2014). To create the IBMprocessed signals, the mask was applied to the speech-plusnoise mixture by multiplying each mixture T-F unit by the value of the IBM for that unit.…”