Oscillations, Waves, and Interactions - GWDG

60 A. Kohlrausch and S. van de Par
Figure 15. The vector representation
of an N0Sπ stimulus. The
masker M and signal S are added in
the left ear with zero phase difference,
resulting in the vector L, and
subtracted in the right ear, resulting
in the vector R. Only interaural
intensity differences (IIDs) are
present in the resulting binaural
stimulus. Reused with permission
from Ref. [40]. Copyright 1998,
Acoustical Society of America.
Many of the headphone-based binaural experiments have used the paradigm of
Binaural Masking Level Differences (BMLDs). BMLDs are indicators of a detection
advantage in a specific binaural condition, relative to a reference condition without
any interaural differences. Depending on signal parameters, this detection advantage
can amount to more than 20 dB and it is commonly thought that this aspect of
spatial hearing contributes to our ability to communicate in acoustically adverse
environments (sometimes indicated with the term ‘cocktail-party effect’).
In this section, two signals types are described which both were introduced to
evaluate specific model predictions about binaural unmasking.
4.1 Multiplied noise
As mentioned in the previous section, multiplied noise is a stochastic signal which has
regular zero crossings. By adding a sinusoidal test signal with a frequency equal to
the noise’s center frequency, one has control over the relative phase between masker
and signal, in a similar way as by adding two sinusoids with the same frequency. In
the context of binaural experiments, this property can be extended to so-called N0Sπ
conditions, in which the Noise masker has no interaural difference while the Signal
has an interaural phase difference of π. When a Gaussian noise is used as masker
in such a condition, the addition of the sinusoidal signal introduces random fluctuations
in both Interaural Time Differences (ITDs) and Interaural Intensity Differences
(IIDs). With such a masker, it is not possible to asses the individual contributions
of ITDs and IIDs to the process of binaural unmasking. In the following, we will
describe how this is possible by using multiplied noise and sinusoidal signals with a
fixed phase relation to the masker’s waveform.
4.1.1 Acoustic properties and perceptual insights
In Fig. 15, the multiplied-noise masker M and a signal S are shown in vector representation.
The two panels together show the right and the left signal for the condition
N0Sπ. In the left panel, the angle between M and S is zero and, since the signal