Low‐Frequency Noise and Methods for Calculating Speech Intelligibility

Speech intelligibility was tested in a low‐frequency masking noise. Various conditions of speech and noise filtering below 1200 cps were employed in order to test the relative effects of direct low‐frequency masking and upward spread of masking to higher frequency regions. Tests were carried out at three noise spectrum levels which corresponded to overall unfiltered levels of 85, 105, and 115 db. The results showed significant decreases in intelligibility as the noise energy was admitted progressively between 300 and 20 cps. Further, the speech frequencies between 20 and 300 cps provided no significant contribution to intelligibility under any noise condition tested. It is, therefore, concluded that the low frequency energy in the noise produced upward spread of masking. This interpretation is reinforced by the fact that spread of masking was greater at higher noise levels and also at lower speech‐to‐noise ratios. Implications for calculating the effects of noise interference upon intelligibility are discussed.