Oscillations, Waves, and Interactions - GWDG

62 A. Kohlrausch and S. van de Par
Figure 17. Thresholds
for N0S0 (open symbols)
and N0Sπ (closed
symbols) are shown for
masker-signal phases of
0 degree (cricles) and
for phase values of 90
degrees (squares) as a
function of center frequency.
Thresholds were
measured at a masker
bandwidth of 25 Hz and
are shown for four subjects
in the lower four
panels. Average results
for the four subjects
are shown in the upper
panel. Reused with permission
from Ref. [40].
Copyright 1998, Acoustical
Society of America.
to changes in IID seems to be independent of center frequency and amounts to
about 1 dB [49]. The sensitivity to ITDs contained in the finestructure of the signal
decreases strongly above 1 kHz and is basically absent above 2 kHz [50,51].
4.2 Transposed stimuli
Transposed stimuli were introduced in the early 1990’s to investigate the reasons
why the BMLD for sinusoidal stimuli in broadband noise maskers decreased at high
frequencies [52–54]. For the N0Sπ condition obtained with Gaussian noise, the BMLD
is about 15 dB at 500 Hz, while it is reduced to 2 to 3 dB at 2 kHz. Two mechanisms
were thought of to contribute to this reduction (see, e. g., Ref. [55]).
1. One contribution could come from the fact that auditory filters become wider at
higher frequencies. Therefore, the rate of fluctuations of interaural differences
at the output of an auditory filter will increase at higher frequencies. If one
assumes that the auditory system is limited in its ability to follow those rapid
changes, such an increased rate should have negative effects on the BMLD.
2. Another reason could lie in the loss of phase locking in the neural system at
higher frequencies. This does imply that at frequencies above about 1.5 kHz,
information about the finestructure of acoustic waveforms gets gradually lost.
In consequence, at high frequencies the binaural system has only access to
interaural differences in the envelope.