Second-order modulation detection thresholds for pure-tone and narrow-bandnoise carriers

Modulation perception has typically been characterized by measuring detecti on thresholds for sinusoidally amplitude-modulated (SAM) signals. This stud y uses multicomponent modulations. "Second-order" temporal modulation trans fer functions (TMTFs) measure detection thresholds for a sinusoidal modulat ion of the modulation waveform of a SAM signal [Lorenzi et al., J. Acoust. Soc. Am. 110, 1030-2038 (2001)]. The SAM signal therefore acts as a "carrie r" stimulus of frequency f(m), and sinusoidal modulation of the SAM signal' s modulation depth (at rate f(m)') generates two additional components in t he modulation spectrum at f(m)-f(m)' and f(m)+f(m)'. There is no spectral e nergy at the envelope beat frequency f(m)' in the modulation spectrum of th e "physical" stimulus. In the present study, second-order TMTFs were measur ed for three listeners when f(m) was 16, 64, and 256 Hz. The carrier was ei ther a 5-kHz pure tone or a narrow-band noise with center frequency and ban dwidth of 5 kHz and 2 Hz, respectively. The narrow-band noise carrier was u sed to prevent listeners from detecting spectral energy at the beat frequen cy f(m)' in the "internal" stimuli's modulation spectrum. The results show that, for the 5-kHz pure-tone carrier, second-order TMTFs are nearly low pa ss in shape; the overall sensitivity and cutoff frequency measured on these second-order TMTFs increase when f(m) increases from 16 to 256 Hz. For the 2-Hz-wide narrow-band noise carrier, second-order TMTFs are nearly flat in shape for f(m) = 16 and 64 Hz, and they show a high-pass segment for f(m) = 256 Hz. These results suggest that detection of spectral energy at the en velope beat frequency contributes in part to the detection of second-order modulation. This is consistent with the idea that nonlinear mechanisms in t he auditory pathway produce an audible distortion component at the envelope beat frequency in the internal modulation spectrum of the sounds. (C) 2001 Acoustical Society of America.