Masking Masking: a process in which the threshold of one sound
(signal) is raised by the presentation of another sound (masker).
Masking represents the difference in decibels (dB) between a signal
threshold without a masker present and the threshold with a masker
present. Signal: pure tone, masker: noise In everyday life,
signal=conversation, masker=music, or signal=music,
masker=conversation
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Masker-signal relationships and sound level Music and a quiet
fan: If the noise of the fan is progressively increasing and the
level of the music is constant, the fan will mask the music. A pure
tone signal and a noise: If the noise is progressively increasing
and the level of the pure-tone is constant, the noise will mask the
pure-tone. To make the signal audible in the presence of this noise
is to increase the signal level. The lowest level of the noise
rendering the signal inaudible is 0 dB effective masking level or
kneepoint. Threshold for a signal in the presence of a masker is
effective masking of the masker for that signal.
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Masking of tones by other tones A threshold shift: the
numerical difference in dB between a signal threshold in quiet and
the threshold obtained with a masker present. The most efficient
masker for a pure tone is another pure tone of the same
frequency-cannot differentiate the two pure-tones. The closer the
frequency of the masker is to the frequency of the signal, the more
efficiently it will mask the signal Upward spread of masking: The
masking effect of a tone is greater above the masking frequency
than below it (when the level of a masker tone is 60 dB and above)
because of the shape of the wave envelope on the basilar
membrane.
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Masking of tones by narrow noise bands (1) Narrow-band noise is
noise restricting in its frequency range. High-pass filters: allow
only frequencies above a certain frequency to pass. Low-pass
filters: allow only frequencies below a certain frequency to pass.
Band-pass filters: allow frequencies between two points to pass
through. Cutoff frequency: frequencies selected as the pass points
Rolloff: quantified in dB/octave.
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Masking of tones by narrow noise bands (2) The critical band:
range of frequencies around a test frequency contributing to the
masking of that frequency. When a tone is masked by noise, only
those frequency components in noise that lie in the narrow range
around the test frequency are responsible for the masking process.
Outside this critical band-nothing to the masking process. When a
tone is barely masked by noise, the energy content of the noise
that falls within the critical band is equal to the energy of the
tone.
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Masking of tones by narrow noise bands (3) Narrow-band noise:
approximately equally distributed energy but restricted to a narrow
frequency region. White noise: energy approximately equally
distributed over a very wide frequency range. Narrow-band noise is
a more efficient masker than white noise because the masking effect
of noise on a tone is related only to the energy falling within the
critical band of frequencies. 0 dB effective masking point is
energy falling in the critical band of noise equaling that of the
signals.
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Masking of tones by narrow noise bands (4) Depending on the
auditory stimulus For pure tones, narrow-band noise is the
preferred masker. For speech stimuli, a broader band of frequencies
is necessary for masking purposes because speech is a broad-band
signal.
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Effective masking (EM) Level per cycle (LPC): OASPL(Overall
sound pressure level of noise)- 10(log 10 BW) where BW is the
bandwidth of the noise EM (Effective masking): LPC + 10(log 10 CB)
where CB is critical band. Threshold for a 1000 Hz tone in the
presence of this noise: EM-audiometric zero at 1000 Hz
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Special Cases of Masking (1) Simultaneous masking: the signal
and masker are presented together Forward masking: masking of a
signal by a masker presented and terminated immediately before the
presentation of the signal Backward masking: masking of a signal by
a masker presented immediately after the presentation of the signal
No consistent effect when the time separation exceeds 50 msec. The
closer in time the signals are, the greater the effect.
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Special Cases of Masking (2) Masking level difference (MLD):
Binaural or monaural In phase or out of phase Quantified by
determining the amount of reduction in masking provided by a
particular condition referenced to the condition that provides
maximum masking. The MLD is most readily assessed at levels of
effective masking of 40 to 50 dB with low- frequency stimuli
(200-500 Hz).
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Masking in Clinical Audiology (1) Cross-hearing: unwanted
transmission of audible sound from one ear to the other. Crossover:
Amount of sound energy reaching the non-test cochlea by bone
conduction. Cross-hearing occurs when crossover exceeds the bone
conduction threshold of the non-test cochlea. By air conduction,
crossover is at a level 40 to 60 dB less than the level of
presentation in the test- ear whereas crossover is 0 to 10 dB by
bone conduction Cross-skull attenuation: differences between the
level of presentation in the test ear and crossover to the non-test
ear.
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Masking in Clinical Audiology (2) Interaural attenuation (IA):
difference between the level of presentation in the test ear and
crossover to the non-test ear. 40 dB for air conduction 90-100 dB
for insert earphones 0 dB for bone conduction The problem of
cross-hearing can be solved by properly masking noise in the better
(non-test) ear