Catherine Mayo

Evaluating the intelligibility benefit of speech modifications in known noise conditions

The use of live and recorded speech is widespread in applications where correct message reception is important. Furthermore, the deployment of synthetic speech in such applications is growing. Modifications to natural and synthetic speech have therefore been proposed which aim at improving intelligibility in noise. The current study compares the benefits of speech modification algorithms in a large-scale speech intelligibility evaluation and quantifies the equivalent intensity change, defined as the amount in decibels that unmodified speech would need to be adjusted by in order to achieve the same intelligibility as modified speech. Listeners identified keywords in phonetically-balanced sentences representing ten different types of speech: plain and Lombard speech, five types of modified speech, and three forms of synthetic speech. Sentences were masked by either a stationary or a competing speech masker. Modification methods varied in the manner and degree to which they exploited estimates of the masking noise. The best-performing modifications led to equivalent intensity changes of around 5dB in moderate and high noise levels for the stationary masker, and 3-4dB in the presence of competing speech. These gains exceed those produced by Lombard speech. Synthetic speech in noise was always less intelligible than plain natural speech, but modified synthetic speech reduced this deficit by a significant amount.

The contribution of durational and spectral changes to the Lombard speech intelligibility benefit

Speech produced in the presence of noise (Lombard speech) is typically more intelligible than speech produced in quiet (plain speech) when presented at the same signal-to-noise ratio, but the factors responsible for the Lombard intelligibility benefit remain poorly understood. Previous studies have demonstrated a clear effect of spectral differences between the two speech styles and a lack of effect of fundamental frequency differences. The current study investigates a possible role for durational differences alongside spectral changes. Listeners identified keywords in sentences manipulated to possess either durational or spectral characteristics of plain or Lombard speech. Durational modifications were produced using linear or nonlinear time warping, while spectral changes were applied at the global utterance level or to individual time frames. Modifications were made to both plain and Lombard speech. No beneficial effects of durational increases were observed in any condition. Lombard sentences spoken at a speech rate substantially slower than their plain counterparts also failed to reveal a durational benefit. Spectral changes to plain speech resulted in large intelligibility gains, although not to the level of Lombard speech. These outcomes suggest that the durational increases seen in Lombard speech have little or no role in the Lombard intelligibility benefit.

Flexibility of acoustic cue weighting in children’s speech perception

Nittrouer and colleagues [Nittrouer, J. Phonetics 20, 1–32 (1992); Nittrouer and Miller, J Acoust. Soc. Am. 101, 2253–2265 (1997); Nittrouer et al., Percept. Psychophys. 62 (2000)] have found that in identifying certain syllable contrasts, young children make more use of syllable–internal formant transitions (relative to other available acoustic cues) than do older children and adults. The evidence for this change in the degree to which listeners weight, or use, certain cues comes predominantly from studies of fricative contrasts (e.g., /sV/–/∫V/, /sV/–/stV/, /Vs/–/V∫/). The current study tests the flexibility of children’s weighting of acoustic cues by examining cue weighting across a wider range of phonetic contexts. In particular, this study attempts to determine whether children’s focus of perceptual attention can be led away from transitions in contexts where such cues are relatively less salient. Additionally, the study tests children’s ability to identify phonemes in an extreme situation, in the co...