Auditory masking

[1] Let us say that for a given individual, the sound of a cat scratching a post in an otherwise quiet environment is first audible at a level of 10 dB SPL.

However, in the presence of a masking noise (for example, a vacuum cleaner that is running simultaneously) that same individual cannot detect the sound of the cat scratching unless the level of the scratching sound is at least 26 dB SPL.

The amount of masking will vary depending on the characteristics of both the target signal and the masker, and will also be specific to an individual listener.

This effect is thought to occur due to filtering within the cochlea, the hearing organ in the inner ear.

A complex sound is split into different frequency components and these components cause a peak in the pattern of vibration at a specific place on the cilia inside the basilar membrane within the cochlea.

These components are then coded independently on the auditory nerve which transmits sound information to the brain.

By experimenting with conditions where one sound can mask a previously heard signal, the frequency selectivity of the auditory system can be tested.

Each graph shows the amount of masking produced at each masker frequency shown at the top corner, 250, 500, 1000 and 2000 Hz.

[1] This phenomenon is called on-frequency masking and occurs because the masker and signal are within the same auditory filter (Figure C).

[5]The masking pattern changes depending on the frequency of the masker and the intensity (Figure B).

For low levels on the 1000 Hz graph, such as the 20–40 dB range, the curve is relatively parallel.

This shows that there is a spread of the masking effect upward in frequency as the intensity of the masker is increased.

[1] Harvey Fletcher carried out an experiment to discover how much of a band of noise contributes to the masking of a tone.

[5] These observations are illustrated in Figure H. Temporal masking or non-simultaneous masking occurs when a sudden stimulus sound makes inaudible other sounds which are present immediately preceding or following the stimulus.

This can be in the absence of, or in addition to, another effect and is due to interactions within the central nervous system between the separate neural inputs obtained from the masker and the signal.

[1] Experiments have been carried out to see the different masking effects when using a masker which is either in the form of a narrow band noise or a sinusoidal tone.

If the frequency difference is small then the sound is perceived as a periodic change in the loudness of a single tone.

The influence of beats can be reduced by using a narrowband noise rather than a sinusoidal tone for either signal or masker.

Hence they have a greater effect on psychoacoustic tuning curves than quadratic difference tones.

This happens at relatively high levels hence have a lesser effect on psychoacoustic tuning curves.

This “off frequency” filter reduces the level of the masker more than the signal at the output level of the filter, which means they can hear the signal more clearly hence causing an improvement of auditory performance.

Auditory masking is exploited to perform data compression for sound signals (MP3).

Figure A – adapted from Gelfand (2004) [ 1 ]
Figure B – Adapted from Ehmer
Figure C – Adapted from Gelfand 2004 [ 1 ]
Figure D – Adapted from Gelfand 2004 [ 1 ]
Figure E – adapted from Moore 1998 [ 5 ]
Figure F – adapted from Moore 1998 [ 5 ]
Figure G – adapted from a diagram by Gelfand [ 1 ]
Figure H – adapted from Moore 1998 [ 5 ]
figure I – ipsilateral simultaneous masking