Discuss human binaural sound localisation and lateralisation and related binaural phenomena including binaural masking level differences. How are these phenomena dependant on the frequency composition of the auditory stimuli?

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Discuss human binaural sound localisation and lateralisation and related binaural phenomena including binaural masking level differences. How are these phenomena dependant on the frequency composition of the auditory stimuli?

        Sound localisation is the ability to identify the location from which a sound is emanating (Goldstine, 2002) which serves many important purposes such as determining which direction to avoid or where to look in order to identify the source. Lateralization refers to the ‘apparent location of a sound source within the head’ (Moore, 1989) which is normally manipulated by using headphones to control the effect of other cues such as interaural differences. Lateralization has been described as the laboratory version of localization because of the ease at which it can be investigated and manipulated to investigate participants’ perception of distance and direction. Binaural sound localisation refers more specifically to the process of discriminating the origins of sound using information obtained by both ears, particularly by the comparison between these two sets of information.

The method of identifying the origin of the sound using this is dependant on the type of sound, for example pure tones which consist of a single pitch or frequency or complex sounds which are more complicated. For pure tones these comparisons can be between the interaural time difference (ITD) which is the amount of time it takes to reach one ear being compared with the amount of time it takes to reach the other, or the interaural intensity difference (IID) which depends on the difference in intensity of the sound. ITD can help with sound localisation because if the origin of a sound is more to the left of the person for example, it will reach the left ear before the right with the amount of time between them indicating the angle. The difference will be at the maximum (about 1 millisecond) when directly to the left or right and will be zero if the sound is to the front or back of the person.

        IIDs occur because of the sound waves being diffracted by the head and other parts of the body which creates a ‘sound shadow’ on the far side of the head (Handel 1993). The size of this shadow is dependent on the wavelength, as long wavelengths will easily diffract around the head to reach the other ear, but those with short wavelengths won’t and so will therefore create a larger shadow. This is because low frequency (and therefore long wavelength) waves are capable of bending around the head, but shorter ones are blocked. This happens if the wavelength is less than or equal to the size of the head, which is around 17.5 cm in diameter and will occur for frequencies of higher than 1500 Hz (Begault 1994).

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        However effective these cues can be, they are not without their limitations. If the frequency is less than 1500 Hz, IIDs become unreliable and ambiguous leaving ITD as the main source of information. Additionally, if the wavelengths are shorter than the size of the head, having a frequency of around 750 Hz or less, these too become ambiguous as it is difficult to ascertain how many periods of the wave have passed. Because of this, it has been thought that ITDs are used to detect the location of sounds producing low frequency waves and IIDs used for high frequency ones, ...

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