Measuring the threshold for speech reception by adaptive variation of the signal bandwidth. I. Normal-hearing listeners

An adaptive test has been developed to determine the minimum bandwidth of s peech that a listener needs to reach 50% intelligibility. Measuring this sp eech-reception bandwidth threshold (SRBT), in addition to the more common s peech-reception threshold (SRT) in noise, may be useful in investigating th e factors underlying impaired suprathreshold speech perception. Speech was bandpass filtered (center frequency: 1 kHz) and complementary bandstop filt ered noise was added. To obtain reference values, the SRBT was measured in 12 normal-hearing listeners at four sound-pressure levels, in combination w ith three overall spectral tilts. Plotting SRBT as a function of sound-pres sure level resulted in U-shaped curves. The most narrow SRBT (1.4 octave) w as obtained at an A-weighted sound-pressure level of 55 dB. The required ba ndwidth increases with increasing level, probably due to upward spread of m asking. At a lower level (40 dBA) listeners also need a broader band, becau se parts of the speech signal will be below threshold. The SII (Speech Inte lligibility Index) model reasonably predicts the data, although it seems to underestimate upward spread of masking. (C) 1999 Acoustical Society of Ame rica. [S0001-4966(99)02305-X].