Masking can be simultaneous or non simultaneous.
See also sound masking
The "not masked threshold" is defined as the quietest level of the signal which can be perceived without any masking present, and a "masked threshold" is the quietest level of the signal perceived when combined with a specific masking noise. The amount of masking is the difference between the masked and not masked thresholds.
The basic masking test involves the not masked thresholds being measured on a subject, and then introducing the masking noise simultaneously. The level of the signal is varied until the new threshold is measured.
One sound, for example, a cat scratching a post, is played gradually louder until it is barely audible. If it is first audible at 10 decibels, that is the not masked threshold. While holding the original sound at the same level, another sound is introduced. For our example, a vacuum cleaner is run at the same time.
If the cat scratching cannot be heard if the vacuum cleaner sound is greater than 26 decibels, this is the masked threshold (Gelfand 2004). . The difference between the two sound levels is the amount of masking, in this case 16 decibels. This value is specific to the two sounds used and to the listener, a different person might be able to hear the cat with 30 decibels of vacuum cleaner, and the amount of masking would be 20 decibels.
The phenomenon of masking is often used to investigate the auditory system’s ability to separate the components of a complex sound. If two sounds of two different frequencies (pitches) are played at the same time, two separate sounds can often be heard rather than a combination tone. This is otherwise known as frequency resolution or frequency selectivity. This 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 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 coded at the same place and are perceived as one sound instead of two (Moore 1986).
The filters that discriminate one sound from two are called auditory filters or listening channels and it is thought that they line up along the basilar membrane. 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 centre 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 (Moore 1995).
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 (Moore 1998).
Figure B shows along the Y axis the amount of masking. The greatest masking is when the masker and the signal are the same frequency and this decreases as the signal frequency moves further away from the masker frequency (Gelfand 2004). This phenomenon is called on-frequency masking and occurs because the masker and signal are within the same auditory filter (figure C). This means that the listener can not distinguish between them and they are perceived as one sound with the quieter sound masked by the louder one (figure D).
The amount the masker raises the threshold of the signal is much less in off frequency masking, but it does have some masking effect because some of the masker overlaps into the auditory filter of the signal (figure E), (Moore 1998).Off frequency masking requires the level of the masker to be greater in order to have a masking effect; this is shown in figure F. This is because only a certain amount of the masker overlaps into the auditory filter of the signal and more masker is needed to cover the signal (Moore 1998).
Figure B also shows that as the masker frequency increases, the masking patterns become increasingly compressed. This demonstrates that high frequency maskers are only effective over a narrow range of frequencies, close to the masker frequency. Low frequency maskers on the other hand are effective over a wide frequency range (Gelfand 2004).
Fletcher carried out an experiment to discover how much of a band of noise contributes to the masking of a tone. In the experiment, a fixed tone signal had various bandwidths of noise centred on it. The masked threshold was recorded for each bandwidth. His research showed that there is a critical bandwidth of noise which causes the maximum masking effect and energy outside that band does not affect the masking. This can be explained by the auditory system having an auditory filter which is centred over the frequency of the tone. The bandwidth of the masker that is within this auditory filter effectively masks the tone but the masker outside of the filter has no effect (figure G.)
This is used in MP3 files to reduce the size of audio files. Parts of the signals which are outside the critical bandwidth are cut out leaving only the parts of the signals which are perceived by the listener (Sellars 2000). Another application of auditory masking in everyday situations is the cocktail party effect.
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 (Moore 1998). 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 (1-4kHz) the slope increases as intensity increases, but at the low frequencies there is no clear inclination with level and the filters at high centre frequencies show a small decrease in slope with increasing level (Moore 1998). 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 (Moore 1998). These observations are illustrated in figure H.
Ipsilateral masking ("same side") is not the only condition where masking takes place. Another situation where masking occurs is called contralateral ("other side") simultaneous masking. In this case, the instance where the signal might be audible in one ear but is deliberately taken away by applying a masker to the other ear.
The last situation where masking occurs central masking. This refers to the case where a masker causes a threshold elevation. 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 (Gelfand 2004).
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.
When a sinusoidal signal and a sinusoidal masker (tone) are presented simultaneously the envelope of the combined stimulus fluctuates in a regular pattern described as beats. The difference between the frequencies of the two sounds equals the rate that the fluctuations occur. If the frequency difference is small then the sound is perceived as a periodic change in the loudness of a single tone. If the beats are fast then this can be described as a sensation of roughness. When there is a large frequency separation, the two components are heard as separate tones without roughness or beats. Beats can be a cue to the presence of a signal even when the signal itself is not audible. The influence of beats can be reduced by using a narrowband noise rather than a sinusoidal tone for either signal or masker. (Moore 1986)
There are many different mechanisms of masking, one being suppression. This is when there is a reduction of a response to a signal due to the presence of another. This happens because the original neural activity caused by the first signal is reduced by the neural activity of the other sound (Oxenham et al 1998).
Addition is the adding of several maskers to result in an increased final masker threshold greater than the original maskers (Lincoln 1998).
Combination tones are products of a signal/s and a masker/s. This happens when the two sounds interact causing new sound, which can be more audible than the original signal. This is caused by the non linear distortion that happens in the ear (Moore 1986).
For example, the combination tone of two maskers can be a better masker than the two original maskers alone (Moore 1986).
The sounds interact in many ways depending on the difference in frequency between the two sounds. The most important two are cubic difference tones and quadratic difference tones (Moore 1986).
Cubic difference tones are calculated by the sum
F1 – F2
(F1 being the first frequency, F2 the second) These are audible most of the time and especially when the level of the original tone is low. Hence they have a greater effect on psychoacoustic tuning curves than quadratic difference tones.
Quadratic difference tones are the result of
F2 – F1
This happens at relatively high levels hence have a lesser effect on psychoacoustic tuning curves (Moore 1986).
Combination tones can interact with primary tones resulting in secondary combination tones due to being like their original primary tones in nature, stimulus like. An example of this is
3F1 – 2F2
Secondary combination tones are again similar to the combination tones of the primary tone (Moore 1986).
Off frequency listening is when a listener chooses a filter just lower than the signal frequency to improve their auditory performance. 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 (Moore 2004).
The effect of auditory masking is used in Sound masking systems. These are audio systems that broadcast White noise for the purpose of hiding an unwanted sound. The unwanted noise may be intermittent sounds from machinery, people or other sources. Usually, this sound is filtered to provide the best effect of hiding the unwanted noise.