The Effect Interaural Intensity Or Magnitude Environmental Sciences
The current study examined the effect interaural intensity or magnitude played in the perception of sound localisation. The study comprised of 128 students from both sexes and various age groups. Each participant, using headphones, was presented with differentiating levels of stimulus within a 180 degree azimuth plane, from which they nominated the possible origin of the stimulus. Stimulus intensity in each ear ranged from 0 dB to 52 dB. Results indicated, that when presented with different intensity levels, participants accurately localised the sound as originating from the left or right azimuth plane. However, it was found that the standard deviation obtained for each mean presented a debateable question regarding a trade off between correctly choosing the side the sound came from compared to the true concrete angle the sound localised at.
According to Westen, Burton and Kowalski (2006), when a person hears a sound two major components of the sound wave interact to enable us to hear. The first component, frequency, represented by hertz (Hz), is the number of cycles that the sound waves fluctuate every second. Higher number of cycles or Hz results in a high pitch while a low number results in low pitch. The second component, called amplitude or intensity, is the loudness of the sound, and is represented in decibels (dB). Westen et al. (2006) state that with 0 dB being an absolute point for being able to hear a sound wave, everyday communication with other members of society is undertaken around 60dB.
A point of discussion in previous studies regarding sound frequency and amplitude is the notion of sound localisation. Rayleigh (1909) noted that when sound frequency travels from a direction that is not directly in front or behind a person, the head acts like an obstacle to the sound wave's path. He stated when sound contacts one ear prior to the second ear, this results in the person localising the origin of the sound to the side with first ear contact. This phenomenon results in what is known as a 'sound shadow' effect. This effect is a result of the sound frequency emitted having to bend around the shape of the head in order to reach the shadowed ear (Middlebrooks & Green, 1991; Sabin, Macpherson & Middlebrooks, 2005; Westen et al., 2006).
They further stated that the difference in the pitch of the frequency causes a person to differentiate the location of the incoming sound. It has been observed that when a high pitch sound originates, the sound waves have difficulty refracting around the head, causing a delay in time for the sound to be received by the shadowed ear compared to the initial ear. This difference in sound arrival times is referred to as interaural disparities of time (ITD) (Sabin et al., 2005; Ungan & Ozmen, 1996). However, as stated by Ungan and Ozmen (1996), ITD only plays a part of what enables sound localisation to occur. They state that differences in intensity level or amplitude (IID) also affect the processing of sound localisation. The combination of these two cues for sound localisation has come to be known as the Duplex Theory of Sound Localisation.
Middlebrooks & Green (1991) stated that previous studies which yielded beneficial results concentrated on sound localisation being a result of either an interaural level difference or an interaural timing difference. However some studies deviated from the familiar belief in the principles of ITD and IID, concentrating their efforts on the results being due to brain activity. Studies including the works of Okamoto, Nakagawa, Fujisaka and Tonoike (2005) discovered that upon localisation of a sound, the right hemisphere showed more signs of active involvement and stimulation in identifying the correct origin in the azimuth plane.
Studies have also shown that the central nervous system plays a role in processing interaural differences. It has been found that upon initial steps to localise the origin of a sound, the subject's specific spatial cortical neurons were triggered, determining where sounds originated from in auditory space (Recanzone & Beckerman, 2004; Sabin et al., 2005; Ungan & Ozmen, 1996).
However, it has also been reported by Sabin et al. (2005) that even with the assistance of specific spatial neurons and duplex theory of sound localisation, participants in trials still incorrectly identified the location of sounds. They found as dB levels decreased, participants had trouble distinguishing where the sounds originated from. Hebrank and Wright (1975, as cited in Sabin et al., 2005) found similar results whereby participants made an increasing number of errors localising sounds when the intensity level of sounds were decreased.
The following study was developed in order to further understand the mechanisms of sound localisation. In particular, the study focuses on interaural intensity disparities and investigates the hypothesis that using a dichotic presentation of stimuli through headphones, participants would be able to correctly identify where the illusionary sounds originated from in the azimuth plane.
Participants comprised of 128 second-year university students who attended various PYB204 tutorial classes. The experiment was not restricted in gender or age, and therefore a mix of both males and females across varying age groups was utilised. A tutor was also present within each study to administer instructions, as well as to record the results of each participant.
Computer software measuring auditory localisation was made available via computers in the tutorial rooms. Each student used headphones in order to identify the localisation of the sounds they heard from the program. The headphones also aided as a control measure to limit students from contaminating other trials. A record slip similar to that provided at Appendix 1 was also used to record each students result.
The study incorporated a dichotic repeated measure design which consisted of a stimulus randomly being presented repeatedly 35 times. The study was structured in a 1 x 7 full factorial level design as illustrated in Figure 1. The cat meowing noise simulation is represented at level 1, while the different combinations of amplitude presented in each ear is represented by level 2.
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