Tuesday, December 07, 2004

Perceiving Acoustic Source Orientation in Three-Dimensional Space

Experiment conducted by John G. Neuhoff from the College of Wooster – Department of Psychology

Many studies have been done that indicate listeners can identify the where a sound is coming from, yet there have been relatively few studies that show that we as listeners can decipher which direction the sound is projecting from a given source. This experiment tries to prove just that – whether or not the human ear can perceive which direction the sound is projecting without visual clues. Assuming that the loudspeaker (refer to drawing) will not move other than in a 360 degree rotation pattern, listeners are asked which direction the speaker is pointing in relation to themselves. Obviously, Neuhoff, the conductor of the experiment, wanted to remove the ability of the listener to watch the speaker as it rotated, so he decided to blindfold all of the listeners. Essentially what they were measuring was the ability of the auditory system to spatially take over for the visual system. So many studies have been done identifying the sound source because it is the auditory system that initializes the visual system when you hear something. This is the localization part. By the time that the projection comes into play, the visual system has already taken over. You see what is making the sound and then which direction the sound is projecting. This experiment attempts to eliminate the visual system to see if the auditory and spatial systems can take over for the visual system. The auditory system is very unused to identify the source of sound without this orientation that the visual system allows and this experiment was designed to show how well it can adapt.

The subjects for all of the parts of the experiment were 18 to 25-year-old undergraduate students. They all said that they had normal hearing.

The experiment they designed tested the listeners on their accuracy for determining the facing angle of the loudspeaker. In this experiment, facing angle (point at the HELPFUL HINTS sheet) can be defined as the direction that the loudspeaker is facing in relation to the listener. In this experiment, they hoped to measure two main variables. The first was how much the distance from the loudspeaker affected the listener’s ability to gauge the facing angle of the loudspeaker. The second thing that they were trying to measure was the ability of the listener to identify the facing angle of the loudspeaker with either a constant sound as the loudspeaker rotated or only having the loudspeaker sound at the start and finish of the rotation.

The first variable had to do with this part of the experiment (point to the first row on the EXPERIMENT sheet). The listeners were placed at two different differences away from the speaker (point at the EXPERIMENTAL SETTING sheet). The first group of listeners was seated at .91 meters away from the loudspeaker while the second group was seated twice as far away at 1.82 meters from the loudspeaker. Their findings were not surprising. They found that listeners were much better at identifying the facing angle when they were closer to the loudspeaker. The only hard part from there was explaining why this occurred. Neuhoff proposed that this was because of the interaural level differences, also called ILDs. These are usually stronger as the sound source gets closer to the listener. This combination of the facing angle and how indirectly or directly the sound is reaching us is how Neuhoff explained the fact that the people that were closer were more accurate in their estimates of the facing angle. The second part – the indirect versus direct measure of sound – is believed to be caused by changing the ratio of direct sound to reflected sound. For example, if you had speakers pointed directly at you, very little of the sound that you would hear would be bouncing off of the wall behind the speaker, however if the speaker was faced 180˚ away from you, the majority of the sound that you would hear would be first reflected off of the wall before coming to you. Neuhoff believes that it is this synthesis of the ILDs and the ratio of direct-to-indirect sound that enables us to tell what angle the sound is coming from.

Then, they split this previously described experiment addressing the distance at which the listener is sitting, into two more separate experiments (point at EXPERIMENT sheet). This experiment measured the ability of the listener to identify the facing angle when the sound source was constant or not. The first part of this subdivision of the experiment measured the listener’s ability to guess the facing angle when given dynamic rotation cues while the second section of the experiment used only static directional cues. Dynamic rotation cues, which were part of the first experiment, means that the loudspeaker was sounding the entire time while it was rotating. Using static directional cues, like in the second section of the experiment, means that loudspeaker sounded only after it had already been rotated. The speaker was sounded at the beginning and at the end of the rotation only. As you would probably be able to guess, the listeners were better able to identify the angle of the loudspeaker with the constant sounding of the dynamic cues, especially when the speaker passed directly in front of the listener at some point.

The experiment also found another interesting finding, however they did not expect to measure when they first designed the experiment. Usually the error were no more than 60 degrees, however, they found that the number of reversals spiked around 180 degrees. For this experiment, a reversal (point at the HELPFUL HINTS sheet) means that the listener made an error of over 165 degrees. The interesting part of this finding is that it was the highest when the speaker was facing 180 degrees away from the listener. The most common mistake made by listeners was that the speaker was facing directly at them. This was interesting because common sense would tell us that the position 180 degrees – as indirect a sound as you can get from the speaker – was often mistaken for the speaker pointing straight at the listeners. Neuhoff hypothesized that this may be due to the lack of a direct sound coming from a specific direction, either left or right. Essentially, having the loudspeaker facing directly at you is 100 percent direct sound whereas having the loudspeaker faced 180 degrees away from the listener would be 100 percent indirect sound.


ILDs (interaural level differences) the inequity between the intensities of sound entering each of the ears. In theory, this would help your mind figure out where the sound is coming from by this degree of inequality between your two ears.

I don’t know where my last paragraph disappeared to (perhaps it took an early holiday break…however, it is more probable I accidentally erased it ☺ ), but here is a new one for all of you to enjoy.

Throughout the experiment, Neuhoff manipulated two different variables. The first was the distance between the listener and the loudspeaker. He found that the listener was much more accurate in their predictions about which direction the loudspeaker was facing when the subject was closer. He attributed this to the IDLs and the ratio of direct to indirect sound that the listener hears. Secondly, he changed the loudspeaker setting to sounding constantly or sounding in the stopped position only. His results were not surprising; he found that giving the listener to hear the speaker as it rotated really aided them in identifying the facing angle of the loudspeaker. This was especially true when the loudspeaker passed directly in front of the listener. This experiment was successful at measuring the auditory system's ability to identify the projection angle of a particular noise in the absence of a visual system.