Sound and Oscillation

# Hearing persistence

Hearing persistence is the minimum time of arrival necessary for the distinction of two or more sound waves by our auditory system.

What is auditory persistence?

When two sounds fall on our ear with a very small time difference, it is impossible to distinguish the sound sources that produce these sounds. The name of this phenomenon is hearing persistence or auditory persistence , and it is related to the shortest time interval between the arrival of two sound waves to our auditory system.

How does auditory persistence work?

Imagine that two speakers can emit different sound waves, but that take exactly the same time to reach your ears. In that case, you would not be able to distinguish the sound emitted by each of the speakers.

It turns out that if the time between two incident sound waves is less than 100 ms (100 milliseconds), or 0.1 s, our auditory system is not able to clearly distinguish the sound sources. The sensation of hearing a sound takes some time for our brain to process, and that time is the time of auditory persistence .

What are the consequences of auditory persistence?

It is thanks to this characteristic of the auditory system that we are able to hear the echoes. Echoes are nothing more than reflected sounds that take longer than 0.1s to reach our ears.

It is also through this definition that we can calculate the minimum distance between an obstacle and a sound source for the echo to be produced. Considering the speed of sound in air to be about 340 m/s and the minimum auditory persistence time (0.1 s), we can use the average velocity equation to calculate the distance sound must travel before it is possible to distinguish the primary sound from the sound. reflected sound:

v = ΔS
Δt

340 = ΔS
0.1

Cross-multiplying the terms, we find:

ΔS = 340×0.1 = 34 m

The result indicates that a sound wave must propagate a minimum distance of 34 m so that we can hear the echo from a sound source, but since the wave must be directed towards an object, be reflected and then return to our ears, the shortest distance between sound source and a reflecting object is approximately 17 m.

When the reflected sound arrives with a time difference of less than 0.1s, the ear perceives the secondary sound as an extension of the original sound. This phenomenon is called reverberation.