About Time and Also About Its Relationship to Frequency

The time axis is quite peculiar, and its implications for perception of sound are poorly understood in general. Nonetheless, it is an extraordinarily important dimension for us humans, and a rudimentary understanding of it is necessary if we are going to understand much at all about how we really hear.

The shortest sound we can hear is an impulse about 20 microseconds long. This is related, of course, to the upper limit of frequencies, about 20,000 Hz. We can, theoretically, hear sounds that are indefinitely long. However, from a perceptual standpoint they cease to be perceived as events and instead become “a continuum.” I sort of arbitrarily put that longer limit at about ten minutes, though most of the time it is a good bit shorter. In a practical sense, a musical note that goes on for thirty seconds is about as long as we can stand before we begin to get really bored! Meanwhile, the sound events (like musical notes and spoken words) that we work with in audio are generally between 50 milliseconds (1/20th of a second) and five seconds long.

There are two other primary things to know about the time dimension. The first is that there is a fundamental neurological boundary for humans at about 50 milliseconds. Events occurring more quickly than that are perceived as a single or continuous event, not as a series of events. Events occurring less quickly are perceived as separate individual events. This holds true for vision as well (hence our 30 fps rate for film). So, as I noted earlier, a 5 Hz. square wave will be heard as a series of clicks, while a 50 Hz. square wave will be heard as a continuous tone.

The other time phenomenon worth keeping in mind is our integration time. When we perceive “a sound,” we integrate all of the versions of that sound that occur within the first 50 milliseconds of the sound into a single holistic perception. So, we sort of average all versions of a sound that occur during that period to produce our conscious perception of the sound source. We will discuss this at considerable length in later articles.

Finally, we have to keep in mind that frequencies are a subset of the time dimension. Individual pitched (periodic) sounds consist of an array of frequencies, as I mentioned above. The lowest such frequencies (fundamentals) usually fall within a four octave range from about 60 Hz. to 1 kHz. All frequencies above 1 kHz. can be regarded as harmonics that enable us to determine timbre and differentiate sounds from each other.