Auditory masking

Hearing Auditory system Auditory cortex Auditory evoked potentials Acoustic nerve Ear Psychophysics Acoustics Audiology Audiometers Audiometry Auditory acuity Auditory discrimination Auditory localization Auditory masking Auditory stimulation Auditory thresholds Auditory perception Auditory displays Auditory feedback Auditory hallucinations Loudness perception Music perception Pitch perception Speech perception Other aspects Auditory displays Listening (interpersonal) Clinical issues Ear disorders Hearing disorders Hearing impairment Deaf Cochlear implants Hearing aids Tinnitus This box: view• talk• edit In psychoacoustics, Auditory masking occurs when the auditory perception of one sound is affected by the presence of another sound.[1] Auditory masking in the frequency domain is known as simultaneous masking, frequency masking or spectral masking. Auditory masking in the time domain is known as temporal masking or non-simultaneous masking. Contents 1 Masked threshold 2 Simultaneous masking 3 Effect of frequency on masking patterns 3.1 Critical bandwidth 3.2 Similar frequencies 3.3 Lower frequencies 3.4 Effects of intensity 4 Temporal masking 5 Other masking conditions 6 Effects of different stimulus types 7 Ipsilateral, contralateral and central masking 8 Mechanisms of masking 9 Off frequency listening 10 Non-simultaneous masking 11 Sound masking Systems 12 Ver también 13 Referencias & Bibliografía 14 Textos clave 14.1 Libros 14.2 Papeles 15 Material adicional 15.1 Libros 15.2 Papeles 16 External links Masked threshold The unmasked threshold is the quietest level of the signal which can be perceived without a masking signal present. The 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 unmasked thresholds. Gelfand provides a basic example.[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. Sin embargo, in the presence of a masking noise (por ejemplo, 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. We would say that the unmasked threshold for that individual for the target sound (es decir,, the cat scratching) es 10 dB SPL, while the masked threshold is 26 dB SPL. The amount of masking is simply the difference between these two thresholds: 16 dB. 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. While the person in the example above was able to detect the cat scratching at 26 dB SPL, another person may not be able to hear the cat scratching while the vacuum was on until the sound level of the cat scratching was increased to 30 dB SPL (thereby making the amount of masking for the second listener 20 dB).  Simultaneous masking Simultaneous masking occurs when a sound is made inaudible by a noise or unwanted sound of the same duration as the original sound.[2] Por ejemplo, a powerful spike at 1 kHz will tend to mask out a lower-level tone at 1.1 kHz. Además, two sine tones at 440 and 450 Hz can be perceived clearly when separated. They cannot be perceived clearly when presented simultaneously. A not masked threshold is the quietest level of the signal which can be perceived in quiet. Masked thresholds are the quietest level of the signal perceived when presented in noise. The amount of masking is the difference between the masked and not masked thresholds (Gelfand 2004). For example if the masked threshold is 35dB and the not masked threshold is 20dB the amount of masking would be 15dB. This is illustrated in figure A. File:Masker increased threshold.svg figure A Adapted from a diagram by Gelfand (2004) The basic masking test involves the not masked thresholds being measured on a subject. Then the masking noise is introduced at a fixed sound level and the signal is presented at the same time. The level of the signal is varied until the new threshold is measured. This is the masked threshold (Gelfand 2004).  The phenomenon of masking is often used to investigate the auditory system’s ability to separate the components of a complex sound. For example 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. Frequency resolution 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 (Moore 1986).  These filters are called auditory filters or listening channels and it is thought that they line up along the basilar membrane, overlapping. Frequency resolution is said to occur on the basilar membrane due to the listener choosing a filter which is centred over the frequency they want 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 cause reduced sharpness of tuning (Moore 1986). This explains why someone with a hearing loss due to cochlea damage would have more problems than a normal hearing person with frequency selectivity. This would cause them, por ejemplo, to have difficulties distinguishing between different consonants in speech (Moore 1995).  Masking illustrates the limits of frequency selectivity even in a normal hearing person. 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. Therefore by carrying out an experiment to see the conditions which are necessary for one sound to mask a previously heard signal, the frequency selectivity of the auditory system can be investigated (Moore 1998).  Effect of frequency on masking patterns 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, otherwise known as masking audiograms adapted from findings by Ehmer (Gelfand 2004). Each graph shows the amount of masking produced at each masker frequency shown at the top corner, 250, 500, 1000 and 2000Hz. Por ejemplo, in the first graph the masker is presented at a frequency of 250Hz 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. File:Maskingpatterns sp11.jpg figure B Adapted from Ehmer, Illustrating changes in masking patterns for different masker frequencies and intensities Figure B shows along the Y axis the amount of masking- so how much the not masked threshold in quiet is raised to get the masked threshold in noise. The X axis shows the frequency of the signal. You can see that the greatest masking is at the centre frequency, when the masker and the signal are the same frequency, and this decreases as the signal 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 auditory system can not distinguish between them and so the signal is masked (Gelfand 2004).  Archivo:Auditoryfiltermaskersignal1.svg figure C Showing on frequency masking where the frequency of the signal is within the frequency band of the masker File:Off frequency mask diff freq1.svg figure D- showing off frequency masking where the frequency of the signal is outside the frequency band of the masker Off-frequency masking is when the signal and masker are at different frequencies (figure D.) The amount the masker raises the threshold of the signal is much less in off frequency masking. You can see from figure E however, that it does have some masking effect because some of the masker overlaps into the auditory filter of the signal (Moore 1998).  Archivo:Maskersameauditoryfilter1.svg figure E Showing the amount of masker which shares the same auditory filter/listening channel as the signal 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. File:Onandofffreqlistening1.svg figure F Showing the relationship between the level of the masker and the masking threshold for both on and off frequency masking This is because only a certain amount of the masker overlaps into the auditory filter (see Auditory filters) of the signal, therefore more masker is needed to mask the signal (Moore 1998).  You can also see from figure B that the masking pattern changes depending on the frequency of the masker and the intensity. You can observe from the 1000Hz graph that for low levels e.g. 20-40 dB the curve is relatively symmetrical. As the masker intensity increases the curves become wider with greater masking particularly to signals at a frequency higher than the masker (Gelfand 2004). This shows that there is a spread of the masking effect upward in frequency as the intensity of the masker is increased. The curve is much shallower in the high frequencies than in the low frequencies and this is termed upward spread of masking. This means that a sound (masker) masks high frequency signals much better than low frequency signals (Gelfand 2004).  You can also observe from figure B 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). This is due to particular patterns of activity on the basilar membrane. As mentioned before, masking experiments reveal information about the frequency selectivity of the ear and the listening channels/auditory filters which are used to distinguish between one frequency and another. Fletcher carried out an experiment to discover how much of a band of noise contributes to the masking of a tone. He carried out a masking experiment whereby 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 maximum masking effect and energy outside this critical band does not have an influence on the masking effect. 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 which is within this auditory filter effectively masks the tone but the rest of the masker which is outside the filter has no effect (figure G.) Archivo:Maskercriticalbandwidth1.svg figure G Showing the amount of masker which contributes to the masking of the tone signal - known as critical bandwidth. Energy outside the auditory filter does not contribute to the masking of the tone signal. Adapted from a diagram by Gelfand (2004) 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. 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 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.[3] 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.[4] 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.[5] Similar frequencies File:Maskingpatterns sp11.jpg Figure B - Adapted from Ehmer 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. Por ejemplo, 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. File:Auditoryfiltermaskersignal1.svg Figure C - Adapted from Gelfand 2004[1] Archivo:Off frequency mask diff freq1.svg Figure D- Adapted from Gelfand 2004[1] 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.[1] 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 cannot distinguish between them and they are perceived as one sound with the quieter sound masked by the louder one (Figure D).  Archivo:Maskersameauditoryfilter1.svg Figure E - adapted from Moore 1998[5] 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)[5] Archivo:Onandofffreqlistening1.svg Figure F - adapted from Moore 1998[5] 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.[5] Lower frequencies 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. As the masker intensity increases the curves separate, especially for signals at a frequency higher than the masker. This shows that there is a spread of the masking effect upward in frequency as the intensity of the masker is increased. The curve is much shallower in the high frequencies than in the low frequencies. This flattening is called upward spread of masking and is why an interfering sound masks high frequency signals much better than low frequency signals.[1] 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.[1] Archivo:Maskercriticalbandwidth1.svg Figure G - adapted from a diagram by Gelfand[1] Harvey Fletcher carried out an experiment to discover how much of a band of noise contributes to the masking of a tone. en el experimento, a fixed tone signal had various bandwidths of noise centered 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 centered 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 represented with reduced precision. The parts of the signals which are perceived by the listener are reproduced with higher fidelity.[6] Effects of intensity File:OutputlevelMoore.svg Figure H - adapted from Moore 1998[5] 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.[5] These observations are illustrated in Figure H. Temporal masking 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. Masking which obscures a sound immediately preceding the masker is called backward masking or pre-masking and masking which obscures a sound immediately following the masker is called forward masking or post-masking.[5] Temporal masking's effectiveness attenuates exponentially from the onset and offset of the masker, with the onset attenuation lasting approximately 20 ms and the offset attenuation lasting approximately 100 ms. Similar to simultaneous masking, temporal masking reveals the frequency analysis performed by the auditory system; forward masking thresholds for complex harmonic tones (p. ej.., a sawtooth probe with a fundamental frequency of 500 Hz) exhibit threshold peaks (es decir,, high masking levels) for frequency bands centered on the first several harmonics. En realidad, auditory bandwidths measured from forward masking thresholds are narrower and more accurate than those measured using simultaneous masking. Temporal masking should not be confused with the ear's acoustic reflex, an involuntary response in the middle ear that is activated to protect the ear's delicate structures from loud sounds. Other masking conditions File:Ipsisimmasking.png figure I - ipsilateral simultaneous masking Ipsilateral ("same side") masking is not the only condition where masking takes place. Another situation where masking occurs is called contralateral ("other side") simultaneous masking. En este caso, 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 is called 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.[1] Effects of different stimulus types 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 (tono) are presented simultaneously the envelope of the combined stimulus fluctuates in a regular pattern described as beats. The fluctuations occur at a rated defined by the difference between the frequencies of the two sounds. 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.[3] Ipsilateral, contralateral and central masking Masking can be carried out in several different conditions. One of these being Ipsilateral, simultaneous masking which refers to the instance where masker and maskee are both delivered to the test ear at the same time. This can be both on-frequency or off-frequency (Gelfand 2004).  Archivo:Ipsisimmasking.png figure I Demonstrating ipsilateral simultaneous masking Another condition of masking is contralateral simultaneous masking. This condition of masking refers to the instance where the signal might be audible in the non-test ear (through transcranial conduction) but is deliberately obliterated by applying a masker to the non-test ear. The last condition of masking is central masking. This refers to the case where a masker causes a threshold elevation (makes a previously heard signal inaudible) in the absence of, or additional to, any ipsilateral, contralateral or cross-masking effect. It is due to interactions within the central nervous system between the separate neural inputs derived from the masker and the signal (Gelfand 2004).  Mechanisms of masking 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.[7] Combination tones are products of a signal and a masker. 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. Por ejemplo, the combination tone of two maskers can be a better masker than the two original maskers alone.[5] 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.[5] 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.[5] 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.[5] Off frequency listening 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).  Non-simultaneous masking Main article: Temporal masking Non simultaneous masking is when the signal and masker are not presented at the same time. This can be split into forward masking and backward masking. Forward masking is when the masker is presented first and the signal follows it. Backward masking is when the signal precedes the masker (Moore 1998).  Sound masking Systems 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. Normalmente, this sound is filtered to provide the best effect of hiding the unwanted noise. See also Auditory stimulation Cocktail party effect Masking Simultaneous masking Spectral mask Temporal masking References & Bibliography Key texts Books Gelfand, S.A. (2004) Oído- An Introduction to Psychological and Physiological Acoustics 4th Ed. Nueva York, Marcel Dekker Moore, B.C.J. (2004) An Introduction to the Psychology of Hearing, 5ª Ed. Londres, Elsevier Academic Press Moore, B.C.J. (1986) Frequency Selectivity in Hearing, Londres, Academic Press Moore, B.C.J. (1995) Perceptual Consequences of Cochlear Damage, Oxford, Oxford University Press Moore, B.C.J. (1998) Cochlear Hearing Loss, Londres, Whurr Publishers Ltd Pickles, J.O. (1982) An Introduction to the Physiology of Hearing, Londres, Academic Press Sellars, P. (2000) Behind the Mask, Cambridge, Sound on Sound, Disponible desde: HTTP://www.soundonsound.com/sos/may00/articles/mp3.htm [Accessed on 26th February 2007] Papers Abdala, C., & Folsom, R. C. (1995). 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