Oscillations, Waves, and Interactions - GWDG

54 A. Kohlrausch and S. van de Par
with dashed lines), however, show a clear dependence of frequency offset and thresholds
are generally higher than the low-noise noise thresholds, in line with the idea
that the inherent fluctuations in the Gaussian masker prohibit the detection of the
modulations introduced through the addition of the sinusoidal target signal. At larger
frequency offsets, however, the modulations introduced by the target signal become
higher in rate, and thresholds are relatively low. Note that the frequency offsets are
considerably lower than peripheral filter bandwidths. Thus the patterns of thresholds
observed in Fig. 10 are not likely to be influenced by peripheral filtering. For the
10-Hz maskers (circles), also for the low-noise noise (solid lines) there is a dependence
of frequency offset suggesting that there are inherent fluctuations in the low-noise
noise masker that influence masking at small frequency offsets. Note that for small
offsets the low-noise noise thresholds are considerably lower than the Gaussian noise
thresholds (circles with dashed lines). Again the dependence on frequency offset that
is observed here is a reflection of the processing of temporal envelope fluctuations
and not of spectral resolution.
3.1.4 Outlook
We have seen that the use of low-noise noise as masker does lead to different thresholds
compared to Gaussian noise. This supports the idea that temporal fluctuations
in Gaussian noise are a significant factor in auditory masking. Thus, low-noise noise
may be an interesting stimulus also in the future to study the contribution of envelope
fluctuations to masking.
For measurement techniques low-noise noise may also be of value because it is a
signal that couples a low crest factor with a continuous spectrum. Although in hearing
experiments, the bandwidth of low-noise noise is usually limited to at most that
of one critical bandwidth to prevent that peripheral filtering reintroduces fluctuations
in the envelope, for physical measurements this restriction may not exist and
wideband low-noise noise may be used to put maximum wideband power in a system
that somehow is restricted in its maximum amplitude.
3.2 Multiplied noise
While the generation of low-noise noise required intensive use of digital computers,
the noise described in this section, multiplied noise, made its appearance as psychoacoustic
stimulus already in the analog period [37]. It was a very convenient way to
generate bandpass noises with tunable center frequency (see below) and these noises
were therefore quite useful in spectral masking experiments in which noise maskers
with variable center frequencies and steep spectral cutoffs were required [38,39]. The
reason to discuss this stimulus in the context of this chapter is, however, based on its
envelope properties, which allowed to test specific ideas about monaural and binaural
hearing.
3.2.1 Definition
Multiplied noise, sometimes also called multiplication, or regular zero-crossing noise,
is generated by directly multiplying a lowpass noise having a relatively low cutoff