Sound Localization Experiments
USD Internet Psychology Laboratory
Background. The sound localization experiment is designed to provide some insight in to how the brain detects and processes sound location information. There are two classes of information, or "cues", that the auditory system utilizes to localize sounds. These cues are referred to as interaural (i.e., between the ear) differences. First, sound localization is made possible by the interaural difference in the time-of-arrival (Delta T) of a sound wave. If a sound signal originates from a point 90 degrees to the right of straight-ahead, then the right ear will receive the signal several hundred microseconds before the left ear. The brain is capable of discriminating very small interaural differences in the time-of-arrival. The second cue involves the interaural difference in intensity (Delta I). The same signal as above will arrive at the right ear with a higher level of intensity than at the left ear.
To further complicate the matter, each of these cues operates more effectively at different sound frequencies. For example, if a 1 KHz signal is presented at the same location as above, we are more likely to rely on Delta T as our primary localization cue. In contrast, we rely on the Delta I cue to localize sounds at high frequency. All of these cues are directly related to properties of the human head, location and shape of the ear, and the sound waves themselves. For example, high frequency sounds possess a shorter wavelength than low frequency sounds. This high frequency sound wave has a tendency to be blocked by the head when a signal is located off center. Therefore, a sound shadow has been cast and an interaural difference in intensity will be introduced as a result.
System Requirements. This experiment requires the use of stereo headphones and a Sound Blaster compatible sound card.
General Procedure. Both the Delta I, and Delta T studies will be conducted using a pair of headphones designed to simulate a sound localization experience. However, two different types of stimuli will be presented for each of the experiments.
Delta I. The Delta I experiment involves testing your ability to localize sounds based on an interaural difference in intensity. On the screen you will see a model of a human head with lines radiating out in a "protractor" type manner. To the extreme left is 0, and to the extreme right, is 180 (units of degrees). We would suggest that you close your eyes to enhance the localization experience. Your job is to enter the perceived location of each sound stimulus presented.
Delta T. The Delta T experiment involves testing your ability to localize sounds based on an interaural difference in the time of arrival. The same model applies in this experiment as in the Delta I experiment. This time you will be presented with a pair of ‘tones’ as the stimulus. The first tone is called the reference tone. It originates from exactly 90 degrees in front of you. The second is the comparison tone. Its position will vary on every trial. Your job is to enter the perceived location of the 2nd (comparison) tone on each trial.
Lab Report
1. What is the independent variable?
2. What is the dependent variable?
3. What was the psychophysical method used in this experiment?
4. Plot your data for both Delta I and Delta T.
5. Describe your perceived sound localization as a function of the actual change in sound intensity (Delta I). Describe the relationship between your perceived sound localization as a function of the actual difference in sound arrival (Delta T).
6. Discuss the differences or similarities between these two functions (Delta I vs Delta T).
End of Sound Localization Experiments.