Oscillations, Waves, and Interactions - GWDG

50 A. Kohlrausch and S. van de Par
Figure 7. Illustration of the low-noise noise generation. The top panel shows the power
spectrum of the Gaussian noise at the start of the iterative process, the middle panel the
spectrum of the low-noise noise after one iteration, the lower panel the spectrum of the
low-noise noise after 10 iterations.
ponents in the passband causes the flat envelope that is seen in the lower panel of
Fig. 6. Due to this careful arrangement of phase and amplitude values throughout
the noise spectrum, any modification of this spectral ordering will affect the flatness
of the temporal envelope. In Fig. 8, the low-noise noise signal of Fig. 6, which was
centered at 500 Hz, and had a bandwidth of 100 Hz, is shown after being filtered with
a 78-Hz-wide gamma-tone filter centered at 500 Hz. As can be seen, the degree of envelope
fluctuation has increased considerably. Since the gamma-tone filter used here
is a reasonable first-order approximation of auditory peripheral filtering, this figure
demonstrates that the properties that are present in the external stimulus should not
be taken to be representative for the manner in which the stimulus is represented
within the auditory system.
3.1.3 Role in hearing research and perceptual insights
As discussed before, Gaussian noise is a frequently used signal to serve as a masking
stimulus in experiments investigating auditory masking. Early experiments of
Fletcher [32] used noise signals of various bandwidths to determine detection thresholds
of sinusoidal signals centered in the bandpass noise maskers. In these experiments
it was found that only the masker energy that was spectrally close to the
sinusoidal target signal contributed to the masking effect of the noise. This led to
the concept of the critical band which indicates the spectral range that contributes
to the masking effect on the sinusoidal signal. The integrated intensity of the masker
within this range determines the masked threshold.