Simultaneous masking occurs when a sound is made inaudible by a noise or unwanted sound of the same duration as the original sound. For example, a powerful spike at 1 kHz will tend to mask out a lower-level tone at 1.1 kHz. Also, two sine tones at 440 and 450 Hz can be perceived clearly when separated. They cannot be perceived clearly when presented simultaneously.
Critical bandwidth If two sounds of two different frequencies are played at the same time, two separate sounds can often be heard rather than a
combination tone. The ability to hear frequencies separately is known as
frequency resolution or
frequency selectivity. When signals are perceived as a combination tone, they are said to reside in the same
critical bandwidth. 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. This individual coding only occurs if the frequency components are different enough in frequency, otherwise they are in the same critical band and are coded at the same place and are perceived as one sound instead of two. The filters that distinguish one sound from another are called
auditory filters, listening channels or
critical bandwidths. Frequency resolution occurs on the basilar membrane due to the listener choosing a filter which is centered over the frequency they expect to hear, the signal frequency. A sharply tuned filter has good frequency resolution as it allows the center frequencies through but not other frequencies (Pickles 1982). Damage to the cochlea and the outer hair cells in the cochlea can impair the ability to tell sounds apart (Moore 1986). This explains why someone with a hearing loss due to cochlea damage would have more difficulty than a normal hearing person in distinguishing between different consonants in speech. Masking illustrates the limits of frequency selectivity. If a signal is masked by a masker with a different frequency to the signal, then the
auditory system was unable to distinguish between the two frequencies. By experimenting with conditions where one sound can mask a previously heard signal, the frequency selectivity of the auditory system can be tested.
Similar frequencies How effective the masker is at raising the threshold of the signal depends on the frequency of the signal and the frequency of the masker. The graphs in Figure B are a series of masking patterns, also known as masking
audiograms. Each graph shows the amount of masking produced at each masker frequency shown at the top corner, 250, 500, 1000 and 2000 Hz. For example, in the first graph the masker is presented at a frequency of 250 Hz at the same time as the signal. The amount the masker increases the threshold of the signal is plotted and this is repeated for different signal frequencies, shown on the X axis. The frequency of the masker is kept constant. The masking effect is shown in each graph at various masker sound levels. Varying intensity levels can also have an effect on masking. The lower end of the filter becomes flatter with increasing decibel level, whereas the higher end becomes slightly steeper. Changes in slope of the high frequency side of the filter with intensity are less consistent than they are at low frequencies. At the medium frequencies (1–4 kHz) the slope increases as intensity increases, but at the low frequencies there is no clear inclination with level and the filters at high center frequencies show a small decrease in slope with increasing level. The sharpness of the filter depends on the input level and not the output level to the filter. The lower side of the auditory filter also broadens with increasing level. These observations are illustrated in Figure H. ==Temporal masking==