Clifton Effect and Precedence Demonstrations

(see AUDIO FILES for information on the *.wav files)

(The effects can only be heard if the sounds are presented to two loudspeakers. The loudspeakers should be placed at about +45 degrees (right) and -45 degrees (left), where 0 degrees is straight ahead. The listener should be about 3 feet from the loudspeakers. The exact configuration is probably not important, as long as the loudspeakers are not too close together or too far apart and the listener either too close or too far away).

The Precedence Demonstration is the classic procedure for demonstrating the Precedence Effect (see references below) which states that it is the first acoustic information that arrives at a listener that determines the location of a sound.

Figure 1 describes the three stimulus conditions for the Precedence Demonstration. A click or transient sound is presented to one loudspeaker and the same click is presented to the other loudspeaker a few milliseconds later. The first click is considered the source sound and the delayed click the echo sound. Three time separations between the source and echo clicks are used: first 32 ms, then 12 ms, and finally 2 ms. The first change (from a delay of 32 to 12 ms) occurs about one-third of the way through the click train and the second change (from a delay of 12 to 2 ms) two-thirds of the way through. The second change (from a delay of 12 to 2 ms), is usually more difficult to detect. The click pairs are presented at a rate of about three times per second.

You should first perceive two click sounds that appear as a sort of "gallop" when the echo is delayed 32 ms, then the clicks should fuse into one click sound and the perceived location should be at the loudspeaker presenting the lead click (when the delay is 12 ms), and finally the image should move toward a location midway between the two loudspeakers and the click sound may appear to get slightly louder when the echo delay is 2 ms.

When the delay between the source and echo is between about 5 and 20 ms (depending on the acoustics of the room) there is a fusion of the two clicks or a suppression of the location information provided by the echo. As such, the first or source click takes precedence in determining the location of the sound. This is the Precedence Effect. When the delay is very short then there is complete fusion ("summation" see the Blauert reference below), and a "phantom" image occurs between the two loudspeakers as would happen if the clicks were presented simultaneously to the two loudspeakers. When the delay is very long fusion no longer occurs and each click is perceived as a separate sound source. Echo suppression is an advantageous property of auditory processing since we usually want to know about the source of the sound not about its echoes.

In 1987 Rachael Clifton (see references below) introduced a change to the classical Precedence Demonstration. This change is described in Figure 2. Half way into the click train, the source and echo clicks are reversed. That is, after several click-pair presentations as was presented for the Precedence Demonstration (12 ms delay for the echo), the loudspeaker that had delivered the first or source click now delivers the delayed or echo click and vice a versa for the loudspeaker that delivered the echo click (it now delivers the first or source click). In the Clifton Effect, an unexpected percept occurrs immediately after the switch. In the Clifton demonstration the two clicks are always seperated by 12 ms and the click pairs are presented at a rate of about 3 per second.

Most listeners perceive the source of the click for the first half of the click train as being at or near the location of the lead loudspeaker (that is, the Precedence Effect is working as shown in the Precedence Demonstration above). Immediately after the switch, for 3 or more click-pair presentations, two clicks are perceived often coming from the location of each loudspeaker. Then after 4 or 5 click-pair presentations after the switch a single click is again perceived and its location is now at the loudspeaker presenting the lead click, which is the opposite loudspeaker from that which produced the lead click for the first half of the click train. Listen carefully for a sound coming from both loudspeakers. Most listeners tend to hear two clicks at the beginning of the click train, then fusion occurs, then after the switch two clicks are heard, and then finally fusion reoccurs but at the location of the other loudspeaker.That is, after the switch there is a temporary cessation or breakdown of the Precedence Effect; fusion and/or echo suppression stops. This breakdown occurred because of the prior exposure you had during the first half of the click train. Thus, your experience with the source and echo clicks somehow influenced your perception after the source and echo clicks were switched. Eventually however, echo suppression reoccurs and the Precedence Effect works as before. This demonstration suggests that echo processing might depend on our listening experience in the reflective spaces that we are currently in.

Suggested References:

Blauert, J., Spatial Hearing. Cambridge, MA: MIT Press, 1983.

Clifton, R. K., Breakdown of echo suppression in the precedence effect. Journal of the Acoustical Society of America, 82(5), 1834-1835, 1987.

Yost, William A., New Developments in the Study of Spatial Hearing, Audiology Today, 6, 9-12, 1994.

Yost, William A. and Guzman, Sandra J. Sound Source Processing: Is There an Echo in Here?, Current Directions in Psychological Sciences 5(4), 125-131, 1996