Speech intelligibility for target and masker with different spectra

The speech intelligibility index (SII) calculation is based on the assumption that the effective range of signal-to-noise ratio (SNR) regarding speech intelligibility is [− 15 dB; +15 dB]. In a specific frequency band, speech intelligibility would remain constant by varying the SNRs above + 15 dB or below − 15 dB. These assumptions were tested in four experiments measuring speech reception thresholds (SRTs) with a speech target and speech-spectrum noise, while attenuating target or noise above or below 1400 Hz, with different levels of attenuation in order to test different SNRs in the two bands. SRT varied linearly with attenuation at low-attenuation levels and an asymptote was reached for high-attenuation levels. However, this asymptote was reached (intelligibility was not influenced by further attenuation) for different attenuation levels across experiments. The − 15-dB SII limit was confirmed for high-pass filtered targets, whereas for low-pass filtered targets, intelligibility was further impaired by decreasing the SNR below − 15 dB (until − 37 dB) in the high-frequency band. For high-pass and low-pass filtered noises, speech intelligibility kept improving when increasing the SNR in the rejected band beyond + 15 dB (up to 43 dB). Before reaching the asymptote, a 10-dB increase of SNR obtained by filtering the noise resulted in a larger decrease of SRT than a corresponding 10-dB decrease of SNR obtained by filtering the target (the slopes SRT/attenuation were different depending on which source was filtered). These results question the use of the SNR range and the importance function adopted by the SII when considering sharply filtered signals

Effect of Harmonicity on the Detection of a Signal in a Complex Masker and on Spatial Release from Masking

The amount of masking of sounds from one source (signals) by sounds from a competing source (maskers) heavily depends on the sound characteristics of the masker and the signal and on their relative spatial location. Numerous studies investigated the ability to detect a signal in a speech or a noise masker or the effect of spatial separation of signal and masker on the amount of masking, but there is a lack of studies investigating the combined effects of many cues on the masking as is typical for natural listening situations. The current study using free-field listening systematically evaluates the combined effects of harmonicity and inharmonicity cues in multi-tone maskers and cues resulting from spatial separation of target signal and masker on the detection of a pure tone in a multi-tone or a noise masker. A linear binaural processing model was implemented to predict the masked thresholds in order to estimate whether the observed thresholds can be accounted for by energetic masking in the auditory periphery or whether other effects are involved. Thresholds were determined for combinations of two target frequencies (1 and 8 kHz), two spatial configurations (masker and target either co-located or spatially separated by 90 degrees azimuth), and five different masker types (four complex multi-tone stimuli, one noise masker). A spatial separation of target and masker resulted in a release from masking for all masker types. The amount of masking significantly depended on the masker type and frequency range. The various harmonic and inharmonic relations between target and masker or between components of the masker resulted in a complex pattern of increased or decreased masked thresholds in comparison to the predicted energetic masking. The results indicate that harmonicity cues affect the detectability of a tonal target in a complex masker

Phase discrimination ability in Mongolian gerbils provides evidence for possible processing mechanism of mistuning detection.

Compared to humans, Mongolian gerbils (Meriones unguiculatus) are much more sensitive at detecting mistuning of frequency components of a harmonic complex (Klinge and Klump. J Acoust Soc Am 128:280-290, 2010). One processing mechanism suggested to result in the high sensitivity involves evaluating the phase shift that gradually develops between the mistuned and the remaining components in the same or separate auditory filters. To investigate if this processing mechanism may explain the observed sensitivity, we determined the gerbils' thresholds to detect a constant phase shift in a component of a harmonic complex that is introduced without a frequency shift. The gerbils' detection thresholds for constant phase shifts were considerably lower for a high-frequency component (6,400 Hz) than for a low-frequency component (400 Hz) of a 200-Hz harmonic complex and increased with decreasing stimulus duration. Compared to the phase shifts calculated from the mistuning detection thresholds, the detection thresholds for constant phase shifts were similar to those for gradual phase shifts for the low-frequency harmonic but considerably lower for the high-frequency harmonic. A simulation of the processing of harmonic complexes by the gerbil's peripheral auditory filters when components are phase shifted shows waveform changes comparable to those assessed for mistuning detection Klinge and Klump (J Acoust Soc Am 128:280-290, 2010) and provides evidence that detection of the gradual phase shifts may underlie mistuning detection