The influence of selected masking noises on Lombard and sidetone amplification effects.
The present experiments were designed to test the notion that the Lombard and sidetone amplification effects are related to the masking ability of a presented background noise. In Experiment 1, eight normal-hearing college students spoke in quiet and while listening to five frequency bands of noise which were equated for intensity but differed in ability to interfere with speech intelligibility. Vocal intensity increased as the frequency band, and thus the masking ability of the noise increased. In Experiment 2, eight additional students spoke in quiet and while listening to four of the frequency bands of noise used in Experiment 1. The noises were equated for loudness rather than intensity. Again, vocal intensity increased as the masking ability of the noise increased. In Experiment 3, 20 additional students spoke while listening to 0 and 20 dB of sidetone amplification mixed with three of the noise bands from Experiment 1. The noises were equated for intensity and mixed with sidetone. The sidetone amplification effect increased as the masking ability of the noise increased. The results of these experiments indicate that interference with speech intelligibility is directly related to elicitation of the Lombard and sidetone amplification effects. The loudness or intensity of a noise has little control over vocal intensity independent of the contribution to masking of speech.