Intelligibility of a target voice improves when its fundamental frequency (F0) differs from that of a masking voice, but it remains unclear how this masking release (MR) depends on the two relative F0s. Three experiments measured speech reception thresholds (SRTs) for a target voice against different maskers. Experiment 1 evaluated the influence of target F0 itself. SRTs against white noise were elevated by at least 2 dB for a monotonized target voice compared with the unprocessed voice, but SRTs differed little for F0s between 50 and 150 Hz. In experiments 2 and 3, a MR occurred when there was a steady difference in F0 between the target voice and a stationary speech-shaped harmonic complex or a babble. However, this MR was considerably larger when the F0 of the masker was 11 semitones above the target F0 than when it was 11 semitones below. In contrast, for a fixed masker F0, the MR was similar whether the target F0 was above or below. The dependency of these MRs on the masker F0 suggests that a spectral mechanism such as glimpsing in between resolved masker partials may account for an important part of this phenomenon.

Speech recognition in a complex masker usually benefits from masker harmonicity, but there are several factors at work. The present study focused on two of them, glimpsing spectrally in between masker partials and periodicity within individual frequency channels. Using both a theoretical and an experimental approach, it is demonstrated that when inharmonic complexes are generated by jittering partials from their harmonic positions, there are better opportunities for spectral glimpsing in inharmonic than in harmonic maskers, and this difference is enhanced as fundamental frequency (F0) increases. As a result, measurements of masking level difference between the two maskers can be reduced, particularly at higher F0s. Using inharmonic maskers that offer similar glimpsing opportunity to harmonic maskers, it was found that the masking level difference between the two maskers varied little with F0, was influenced by periodicity of the first four partials, and could occur in low-, mid-, or high-frequency regions. Overall, the present results suggested that both spectral glimpsing and periodicity contribute to speech recognition under masking by harmonic complexes, and these effects seem independent from one another.