Range Cues


When it comes to localizing a source, we are best at estimating azimuth, next best at estimating elevation, and worst at estimating range. In a similar fashion, the cues for azimuth are quite well understood, the cues for elevation are less well understood, and the cues for range are least well understood. The following cues for range are frequently mentioned:
The physical basis for the loudness cue obviously stems from the fact that the captured sound energy coming directly from the source falls off inversely with the square of range. Thus, as a constant-energy source approaches a listener, the loudness will increase. It is equally obvious that the received energy is proportional to the energy emitted by the source, and that there cannot be a one-to-one relation between loudness and range. Just playing a sound at a low volume level will not, in itself, make it seem to be far away. To use loudness as a cue to range, we must also know something about the characteristics of the source. In the case of human speech, each of us knows from experience the different quality of sound associated with whispering, normal talking, and shouting, no matter what the sound level. The combination of loudness and knowledge of the source provides useful information for range judgments.

Motion parallax refers to the fact that if a listener translates his or her head, the change in azimuth will be range dependent. For sources that are very close, a small shift causes a large change in azimuth, while for sources that are distant there is esentially no azimuth change.

In addition, as a sound source gets very close to the head, the ILD will increase. This increase becomes noticeable for ranges under about one meter. An extreme case is when there is an insect buzzing in one ear, or when someone is whispering in one ear. In general, sounds that are heard in only one ear are threatening and are uncomfortable to listen to. It is particularly important to keep this in mind when designing HCI systems for headphone listening. As we will see, to get the listener to think that the sound is on one side, it is not at all necessary to have all of the sound in that ear and nothing in the other ear.

The final cue listed is the ratio of direct to reverberant sound. As we mentioned above, the energy received directly from a sound source drops of inversely with the square of the range. However, in ordinary rooms, the sound is reflected and scattered many times from environmental surfaces, and the reverberant energy reaching the ears does not change much with the distance from the source to the listener. Thus, the ratio of direct to reverberant energy is a major cue for range. At close ranges, the ratio is very large, while at long ranges it is quite small. Fortunately, this is a relatively easy and effective cue to manipulate for HCI applications.

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