Oscillations, Waves, and Interactions - GWDG

58 A. Kohlrausch and S. van de Par
types. Higher values of the threshold indicate a stronger effect of the masker on the
audibility of the signal.
The data for the sinusoidal masker (circles, continuous line) indicate a highly
nonlinear growth of masking. When the masker level is changed by 12 dB from 66 to
78 dB SPL, the threshold increases by 30.3 dB, corresponding to a slope of 2.5 dB/dB.
The sinusoidal masker produces more masking than any of the other maskers. From
all noise maskers, the least masking is obtained for the 20-Hz-wide multiplied noise
masker (open triangles), while the strongest masking effect is seen for the 100-Hzwide
Gaussian masker. These data show thus the effect of two stimulus parameters
on spectral masking: The more envelope minima occur in the masker, the lower
the masked threshold (compare the three open symbols in Fig. 13). And, given a
specific envelope distribution, faster fluctuations in the envelope, i. e., an increased
bandwidth, lead to higher thresholds (compare open and filled symbols of the same
type in Fig. 13).
Van der Heijden and Kohlrausch [45] emphasized in their conclusion the relevance
of noise statistics for the interpretation of experimental data: “In summary, our data
show that, particularly with high masker levels, attention should be paid to the exact
nature of narrow-band stimuli used to mask a target at a frequency above the
masker frequency [...] . These differences are by no means restricted to extremely
slowly fluctuating maskers. On the contrary, masker fluctuations are relevant under
many common experimental conditions, such as the ‘low-frequency tail’ of the psychoacoustic
tuning curves. Unfortunately, in many published papers the influence of
the type of noise used as masker (particularly multiplication noise versus Gaussian
noise) has generally been neglected” ([45], p. 1806).
The envelope spectra of different noise types, shown in Fig. 12, suggested a critical
test of the concept of modulation filter banks, which was developed in the Ph. D.
thesis by Torsten Dau [36,46,47]. This concept provided a new view on modulation
detection, and emphasized the role of the intrinsic envelope fluctuations of the carrier
on the ability to detect sinusoidal amplitude modulations applied to this carrier.
While for noise carriers, modulation detection is limited by intrinsic modulations of
the carrier, for sinusoidal carriers, the limitation in detecting amplitude modulations
comes from intrinsic properties of the auditory system [48]. An important element in
this concept are modulation filters: a certain range of modulation frequencies falls
into the same modulation filter, and modulation detection is possible if the applied
modulation is sufficiently strong compared to the intrinsic modulations within the
modulation filter centered on the test modulation.
In such a framework, one would expect very different modulation detection thresholds
for noise carriers with different envelope spectra. For example, the modulation
spectrum of low-noise noise has very little energy at low modulation frequencies and
therefore, modulation detection should be much better at these modulation rates
than, e. g., for Gaussian or also for multiplied noise. Dau et al. [41] tested this
expectation by measuring modulation detection for the three noise types shown in
Fig. 12.
In the left panel of Fig. 14, experimentally determined modulation thresholds are
shown for the three noise types, all with a bandwidth of 50 Hz. First of all, we see
that Gaussian and multiplied noises have the highest thresholds at low modulation