In the hearing experiment, there is a question asking which side the sound comes from.
If you haven't done the interesting experiment, please click the link to participate.
（Spoiler）
The sound signals on both channels of the audio files are exactly the same, except that one channel has one millisecond (0.001 sec) delay from the other. Most people think that the sound is only on the leading channel, but in fact, there are sounds on both sides.
People use Interaural Time Difference (ITD) to determine the sound's position. Assume that the room temperature is 25 degrees Celsius and the speed of sound is 343 meters per second. Since the time difference between the left and right channels of these two audio files is 0.001s, this is equivalent to a distance of 34 cm in space, which actually exceeds the distance between the ears of a person.
In fact, humans do not only use ITD to determine a sound source's position, but also use Interaural Level Difference (ILD), especially the sound with higher frequency. Because the sound with higher frequency has a shorter wavelength, it has more possibility to be blocked by obstacles [1]. Even our own head is an obstacle. Of course, there are more complicated mechanisms needed to determine the height and front-back position of a sound source. In fact, this experiment does not have enough information to precisely determine the sound source's position, but ITD alone is enough to produce an illusion here.
Once the ears receive a sound signal, the Anterior Belt will perceive "what"– the content of the sound, and the Caudal Belt will perceive "where"– the position of the sound in space [2]. What we perceive is the signal that our brain has sorted out. The position and content of the sound we perceive may not be facts. The perception even varies with our habits and associations.
In the previous workshop event, Glissando Illusion and Chromatic Illusion were the examples to discuss the spatial sense of a sound. Most participants felt that the sound would move up and down, left and right, or back and forth with the change of pitch. Some people described their feelings in more detail. For example, some described the sound moving or extending, condensing, etc., in a certain curve or speed in a specific direction. These audio files have only pure pitch information, but are surprisingly enough for us to have spatial associations. There is no complex coding that contains spatial information applied in those fun technologies such as virtual reality or movie soundtracks.
It is so intriguing that many people always associate the high pitch to the right and above in space, and the low pitch to the left and below. This finding is so consistent with our habits and the common rules in the music industry, such as keyboard instruments, orchestral configuration and DAW tools. Are we born to act this way, or do we associate like this after getting used to these tools?
In addition, a participant of my auditory questionnaire revealed that he is a dancer, so he is also very interested in music. Unfortunately, I don’t know if he has attended my workshop. I really hoped to discuss it with him that day. Are there any specific rules for dancing matching with music? Perhaps the above-mentioned association of sound and space also influences the expression of a dancer's body. This may play an important role in the matching of dance and music.
[1] Kyriakakis, Chris. Fundamental and Technological Limitations of Immersive Audio Systems. Proceedings of the IEEE, vol.86, no.5, May 1998.
[2] Deutsch, Diana. Musical Illusions and Phantom Words (p. 33). Oxford University Press. (2019)