Interaural Time Difference in Water

E.M. 2010-06-15 MEDS5377 Sound Localization of Low-Frequency Tones via Interaural Time Difference (ITD) in Water

Introduction: Sound localization is closely tied to binaural hearing in humans, especially when localizing sound in the azimuth (horizontal) plane. If there is a sound stimulus exactly 0º in front of a person, the sound reaches both ears at the same time. If you place a sound stimulus at an angle to the right 45º, the sound will reach the right ear first, and then the left ear afterwards; this is an interaural time difference (or ITD). However, ITD predominates as a localization method with low-frequencies. For high-frequency stimuli, sound localization is generally accomplished via interaural level difference (ILD). This is because with high frequency stimuli, the head produces a shadow that decreases the intensity of the stimulus in the more distant ear (or a level difference).

Objective and Hypothesis: The objective of this experiment is to determine how being submersed in water affects human capabilities for sound localization. We will test the hypothesis that being underwater will worsen a person’s ability to localize a low-frequency sound (for which ITD dominates) on a horizontal plane because of the consequent changes in the Interaural Time Difference (ITD) that will result from the speed of sound in water being faster than in air. Furthermore, with training, humans can better their performance in sound localization underwater.

Experimental Design: This hypothesis should be tested on a group of individuals with a well developed auditory system and no signs of developed hearing loss anywhere between the ages of twenty and forty, none of which should have spent considerable amounts of time in the water. A minimum of 100 subjects should be used. There should be two sets of testing: both set up the same, only with one being above water (Dry Test) and the other one underneath (Wet Test). For the Wet Test, there should be two sub-groups: those who can SCUBA (Group 1) and those who cannot and thus will use a snorkel mask (Group 2). The first set of testing should be used as a control to see how good the subjects are at localizing a sound to begin with; the second set of testing should be used to compare success rates. The Dry Test should be done in an anechoic chamber. Thirteen different speakers should be set up pointing towards the subject at 1.5-meters away from the location where the subject should be sitting, all at the same height but different angles (0º, ± 15º, ± 30º, ± 45º, ± 60º, ± 75º, and ± 90º), all capable of administering the same intensity and pure tone of 800-Hz with a duration of 50ms. Subjects should not be allowed to move their heads for this experiment, but rather indicate which speaker (or rather at which angle) the sound is coming from when a stimulus is played. Let there be a 70-ms time period between each stimulus. The same set up of thirteen speakers at the same set angles and distance should be set-up for the Wet Test in the ocean. Using an ocean (or very large pool) is preferable because it will minimize reflective noise. Subjects should similarly not be allowed to move their heads, but keep as stationary as possible with SCUBA gear (Group 1) or snorkel mask (Group 2). Have the same tone, intensity and duration of stimulus for all tests, and ask each subject which angle the stimulus came from. E.M. 2010-06-15 MEDS5377 Expected Results: If my hypothesis is correct, all the subjects will perform better in the Dry Test as opposed to the Wet Test, however Group 1 will also perform better than Group 2 in the Wet Test. This would make sense because our brain has accommodated to the interaural time difference between ears in air, but being trained in sound localization underwater should also play a role in the development of our auditory system. If my hypothesis is incorrect, then the presence of water does not affect sound localization, and either the brain accommodates for the ITD in water or there is another system that is used for sound localization.

References: 1. E. A. Macpherson and J. C. Middlebrooks, “Listener weighting of cues for lateral angle: the duplex theory of sound localization revisited,” J. Acoust. Soc. Amer., vol. 111, no. 5, pp. 2219–2236, May 2002. 2. Yost, William A. Fundamentals of Hearing: an Introduction. San Diego: Academic, 2007. Print. E.M. 2010-06-15 MEDS5377

Grade A

Very good premise and hypothesis. There is actually a guy doing this at the submarine base in Groton, CT. You should talk to him. He sometimes comes up to UCONN to talk to Bernstein and I have heard him talk at the Binaural Bash in Boston. It turns out that the Navy divers who are the subjects really do not like having their head fixed under water.

STUDENT CRITIQUE I think that overall this is an interesting idea for a project proposal that incorporates a lot of what we have been learning in class and applies it in a new direction. One aspect where additions could be made would be the introduction. What type of work has already been done with examining ITDs and differences in conduction medium? It would be interesting to see more theoretical ideas of how changes in temperature, water disturbance and the presence of other objects would effect this study. This also may be a possible confound when examining the data since the conditions of the ocean/body were not specified in this proposal and could vary between trials.

Background and objections- I would have also liked elaboration on the aspects of training that can be done to better someone’s ITD detection and how that works biologically with in the ear. It would be great to have some additional sources to read for more background information.

In terms of data collection, I would love to also see more elaboration. Elimination of hand movement was mentioned but other aspects of how the data would be received by the experimenter and collected was not. How will the subject be able to indicate sounds that come from “behind” their line of sight with out adding waves/causing a disturbance in the water?

Additionally what type of tone will be played- will the tone be played to be a SAM tone? Ease of detect ability of this tone versus others could make for another interesting side study. Is there prior literature that suggests 800 Hz, and if so would you be interested in doing a few different pitches, modulation frequencies, our loudness of auditory stimulus?

Making these changes will make your study more comprehensive and give you more information and data to analysis and compare afterward.