DETECTION OF TONES IN LOW-NOISE NOISE - FURTHER EVIDENCE FOR THE ROLEOF ENVELOPE FLUCTUATIONS

This paper investigates the role of envelope fluctuations in simultane ous masking conditions. Thresholds for tones in noise with a flat temp oral envelope (low-noise noise, LNN) were compared with those in Gauss ian noise. All measurements were performed with a running-noise presen tation of 500-ms maskers. The sinusoidal signal was spectrally and tem porally centered in the masker. The main findings were: (a) The 5.5-dB threshold difference between 100-Hz-wide Gaussian and LNN maskers at 1 kHz that was previously observed using frozen noise (cf. Hartmann an d Pumplin [J. Acoust. Soc. Am. 83, 2277-2289 (1988)]) is also apparant for running noise, although thresholds are generally higher in the la tter condition. (b) The threshold difference between Gaussian and LNN maskers at 1 kHz reaches a maximum of 9.4 dB at a masker bandwidth of 25 Hz, while at 10 kHz, the difference reaches a maximum of 15 dB at b andwidths of 50 and 100 Hz. For a 100-Hz-wide masker presented at diff erent center frequencies, there is no advantage for LNN maskers below 1 kHz. Towards higher frequencies, the difference between the two nois es increases and reaches about 15 dB at 10 kHz. (c) At 1 kHz with a 10 0-Hz bandwidth, decreasing the signal duration from 500 to 20 ms incre ases the threshold difference to 7.6 dB. (d) Thresholds in a dichotic condition, in which the masker is in phase and the signal is out of ph ase, lie within 2 dB for the two noise types, and are nearly constant for masker bandwidths between 5 and 100 Hz. It is argued that the prim ary detection cue in LNN is not an increase in energy, but rather an i ncrease in envelope fluctuations due to the addition of the signal. Th is hypotheses is supported by simulations with an auditory-filterbank model. The simulations further suggest that, for a large LNN advantage , it is not sufficient that the LNN envelope is hat at the output of t he on-frequency filter. In addition, it is crucial that off-frequency filters also yield a flat temporal envelope.