Sound localisation is the process of determining the location of a particular source of sound, and this is achieved through the use of binaural and monaural cues. To investigate what factors affect the ability to localise sound, one hundred and eighty six psychology students underwent an experiment where subjects indicated what direction they heard a set of keys coming from when the keys were shaken above the participant’s head on three different positions of the midline. The four conditions in this study included head movement, no head movement, normal pinnae and distorted pinnae. As hypothesised, sound localisation improved significantly when the head was mobilized rather than immobilised, in conjunction to the pinnae being normal rather than distorted. These findings suggest that sound localisation is most successful when the head is mobile and the pinnae is normal.
The Effects of Pinnae and Head Movement in Sound Localisation
The brain is equipped with the ability to perceive hearing, meaning it is able to recognise differences in intensity, spectrum and timing in order to correctly detect auditory stimuli. It is important to be able to locate sounds in the space surrounding us, and this is achieved through using the pinnae and head movement. Pinnae effects and head movement assist in directional auditory localisation (Muller, B.S., & Bovet, P. 1999) and spatial hearing (Denise Van Barneveld, Floor Binkhorst, A. John Van Opstal 2011). The pinnae is the outer structure of the ear which catches and guides sounds into the inner ear, and this study will assess the differences of sound localisation if the pinnae is distorted and undistorted. The use of head movement in this study is to investigate whether sound localisation improves when the head is mobilized or immobilised. The role in sound localisation is to detect the direction of the auditory stimuli coming from in front, behind, above or below the head (Martijn J H Agterberg; Myrthe K S Hol; Ad F M Snik; Marc van Wanrooij; A John Van Opstal), and this is done through the use of different cues, including binaural and monaural cues (Marc. M. Van. Wanrooij, A. John Van Opsal (2006). Binaural cues use interaural time difference (ITD) and interaural intensity difference (IID). ITD refers to the length of time that it takes for a noise to reach both ears, for example, if a noise originates from one particular side of the head, the noise will take longer to reach the opposite ear. This determines what direction sound is coming from. IID refers to the intensity of the auditory stimuli that is being heard, and this will help determine what direction the sound is coming from, similar to ITD. These compare sound on both ears through the recognition of sound pressure and time difference (Denise Van Barneveld, Floor Binkhorst, A. John Van Opstal 2011).
Monaural cues are different to binaural cues as they only use one ear rather than both to localise sound.
This study was based on Muller and Bovet’s article Role of Pinnae and Head Movements in Localising Pure Tones (1999), which investigated the role of the pinnae and head movement in sound localisation, as well as sound frequency. In their study, they discovered that head movement as well as normal pinnae and low frequency tones resulted in improved sound localisation. The study that the psychology students underwent did not include frequency tones to make matters simpler and to truly understand the function of pinnae and head movements in the role of sound localisation.
To hypothesize, the accuracy of sound localisation will be greater when the pinnae is not distorted rather when the pinnae is distorted, and it is further hypothesized that the accuracy of sound localisation will be greater when the head is mobilised rather than immobilised.
There are two independent variables in this study, and they each have two stages. These stages are normal pinnae and head immobilization, normal pinnae and head mobilization, distorted pinnae and head immobilization and distorted pinnae and head mobilization. The dependent variable in this study is the correct sound localisation.
Method
The aim of this study was to discover what factors contribute to correct sound localisation in regards to pinnae and head movement.
Participants
There was a total of 186 participants involved in this study. These participants were university students and were located at three different ACU campuses across Australia, 24.2% of which from the Brisbane campus, 34.9% from the Melbourne campus and 39.8% from the Strathfield campus. Of these students, 34.9% were male and 65.1% female.
Materials Used
Materials used in this study involved four materials; a classroom chair for the participant to sit on, a set of keys that the students supplied themselves, a sheet of paper that indicated to the person shaking the keys above the participant’s head in what order to shake the keys, and a writing utensil.
Procedure
Participants merged off into randomly selected groups of three, one then sat on a chair and closed their eyes whilst another dangled a set of keys approximately one meter above their head, and the keys were skaken for two seconds at a time. The sitting participant indicated whether they heard the keys from above, in front or behind their head. The other student documented the answers and compared these answers to the correct location the keys were dangled from. Participants completed the task within 15 minutes.
Design
The experiment involved two independent variables, and the participant was instructed for the first part of the experiment to have normal pinnae and head immobilisation to locate the sound of the keys. They were then instructed to distort their pinnae by folding the top part of the ear down and leave the head immobilised. They then indicated where they located the sound of the keys. The next part of the experiment was to have normal pinnae, but now the participant had free head movement, and the process of locating the sound of the keys repeated. The last part of the experiment was to distort the pinnae and have free head movement while locating the sound of the keys.
Results
The mean number of correct sound localisations was calculated for each of the four conditions. These means are presented in Figure 1. As can be seen in the Figure 1, the mean number of correct sound localisations in the two normal pinnae conditions were significantly higher than the number recalled in the two distorted pinnae conditions (F(1,185)=430.05.55, p<.001). Although the difference was not as large, is can be seen in Figure 1 that the mean number correct sound localisations made in the two head movement conditions were significantly greater than those of the two no head movement conditions (F(1,185)=40.61, p<.001).
Figure 1. The mean number of correct sound localisations under the normal and distorted pinnae, and the head movement and no head movement conditions.
Discussion
The purpose of this experiment was to understand sound localisation and it’s connection to the pinnae and head movement, and through the experimentation of independent variables it was discovered that the results matched up with the hypothesis, as sound localisation was at it’s greatest when the pinnae was normal rather than distorted and the head was mobile rather than immobile.
These findings were predicted because the pinnae is structured in a specific way to detect and localise sound to the best of a human’s ability and head movement is a natural reflex when a sound is detected. Using both of these simultaneously help to localise sound to the participant’s greatest ability.
According to the results, head immobilisation does not effect sound localisation dramatically, but distorted pinnae makes a significant impact on sound localisation, making it much poorer.
The limitations of this study included the fact that participants supplied their own keys, meaning each group’s keys would have a different sound. Some keys could have made a louder or quieter noise than others or a noise of a higher or lower frequency. Another thing that could have affected the results is that the length the keys were held to the participant’s were approximate, as were the amount of time the keys were shaken for.
In future research, all participants should use the same set of keys as well as having exact measurements metres and time.
Conclusion
In conclusion, this study revealed that the pinnae and head movement affects sound localisation, and distorting the pinnae and having an immobilised head results in poorer quality of sound localisation, therefore having normal pinnae and mobile head movement results in more accurate sound localisation.
References
Muller, B. S., & Bovet, P. (1999 September) Role of pinnae and head movements in localizing pure tones. 58 (3), 170-179
Denise Van Barneveld, Floor Binkhorst, A. John Van Opstal (2011 Spetember) Absence of compensation for vestibular-evoked passive head rotations in human sound localization. 34 (7), 1149-1160
Marc. M. Van. Wanrooij, A. John Van Opsal (2006) Sound Localization Under Perturbed Binaural Hearing. 97 (1), 715-726
Martijn J. H. Agterberg, Myrthe K. S. Hol, Ad F. M. Anik, Marc Van Wanrooij, A. John Van Opstal (2012) Contribution of monaural and binaural cues to sound localisation in listeners with acquired unilateral conductive hearing loss: improved directional hearing with a bone-conduction device. 286 (1-2) 9-18
References: Muller, B. S., & Bovet, P. (1999 September) Role of pinnae and head movements in localizing pure tones. 58 (3), 170-179 Denise Van Barneveld, Floor Binkhorst, A. John Van Opstal (2011 Spetember) Absence of compensation for vestibular-evoked passive head rotations in human sound localization. 34 (7), 1149-1160 Marc. M. Van. Wanrooij, A. John Van Opsal (2006) Sound Localization Under Perturbed Binaural Hearing. 97 (1), 715-726 Martijn J. H. Agterberg, Myrthe K. S. Hol, Ad F. M. Anik, Marc Van Wanrooij, A. John Van Opstal (2012) Contribution of monaural and binaural cues to sound localisation in listeners with acquired unilateral conductive hearing loss: improved directional hearing with a bone-conduction device. 286 (1-2) 9-18
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