On the near non-existence of "pure" energetic masking release for speech

Stone et al. [(2012). J. Acoust. Soc. Am. 132, 317-326] showed that a masker constructed to produce a near-constant envelope at the output of each auditory filter reduced speech intelligibility less than maskers of the same mean level with fluctuating envelopes, produced by 100% sinusoidal amplitude modulation (SAM) at 8 Hz. Here, this effect was explored for a range of SAM rates from 1 to 81 Hz. Speech was filtered into 28 channels. A sinusoidal masker centered on each channel was added to the channel signal. The maskers were either unmodulated or had 100% SAM. In most conditions, even-numbered channels were presented to one ear and odd-numbered channels to the other. The signal-to-masker ratio was adapted to measure the Speech Reception Threshold (SRT) corresponding to 50% correct. The fluctuating masker benefit (FMB), the difference in SRT between the SAM and unmodulated masker, was negative for all SAM frequencies except 1 Hz. Due to the different slopes of the psychometric functions, when SRTs were inferred for more realistic performance levels, 74% or more, FMB was zero or negative for all SAM rates. It is concluded that a positive FMB, when it occurs, is a release from modulation and not energetic masking.