Chapter 6: Masking
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
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.
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.
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.
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.
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.
Masking of tones by narrow noise bands (4) Depending on the auditory stimulus (1) For pure tones, narrow-band noise is the preferred masker. (2) For speech stimuli, a broader band of frequencies is necessary for masking purposes because speech is a broad-band signal.
Effective masking (EM) Level per cycle (LPC): OASPL(Overall sound pressure level of noise)- 10(log10BW) where BW is the bandwidth of the noise EM (Effective masking): LPC + 10(log10CB) where CB is critical band. Threshold for a 1000 Hz tone in the presence of this noise: EM-audiometric zero at 1000 Hz
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.
Special Cases of Masking (2) Masking level difference (MLD): (1) Binaural or monaural (2) In phase or out of phase (3) Quantified by determining the amount of reduction in masking provided by a particular condition referenced to the condition that provides maximum masking. (4) The MLD is most readily assessed at levels of effective masking of 40 to 50 db with lowfrequency stimuli (200-500 Hz).
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.
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. (1) 40 db for air conduction (2) 90-100 db for insert earphones (3) 0 db for bone conduction The problem of cross-hearing can be solved by properly masking noise in the better (non-test) ear