The Audio Window

Apology & Intro

Some months back, I promised the readers of Recording (that’s you!) that I would write a series of articles about how we hear. Unfortunately, some personal issues and other work interfered with my good intentions. Nick was very understanding. Happily, I’ve now got the series firmly scheduled, and so, with any luck, you can check out my wacky views on hearing on a monthly basis until next June or so.

In my first article, back in May, I discussed how improbable our hearing mechanism and usage would seem to a newly arrived alien (Zork-11 from Betelgeuse IV, I called him). I posed some perplexing questions, and wondered aloud about what’s REALLY going on. At the end of that article, I promised the next article would be about the so-called “Audio Window.” And so here we are.

Before we delve into the actual sensory mechanism of the ear, we need to talk a little about the medium in which we sense sound, air. We need to consider briefly the range of magnitudes of various physical characteristics of that air that result in our perception of sound. We need to talk about the nature of air itself, and how it supports sound.

This air we live in is a compressible gas. This means that the relative density of the air is quite variable, and the number of “air” molecules in any given space, or volume of air, may change. Such density changes as a function of temperature (cold air is denser than warm air) and altitude (“low” air is denser than “high” air). Air density can also be changed as a function of what we call displacement, or the insertion of some other material into some given air space.

When we displace air suddenly, it is called “excitation.” The result is that the density of the air around the point of excitation changes. Further, such density tends to expand away from said point of excitation. This physical process is what leads to the notion of “sound.” We “excite” the air molecules at some point in space, displacing them. They in turn displace other molecules. A wave of density flowing away from the original point of excitation reaches our ears some distance away, some time later. Voila. We perceive a sound. That’s how it happens, plain and simple.

Now, there are some limits to this.

To begin with, the medium (air) has certain limits to its density. At the minimum, there are no molecules present. This is called a vacuum, and it happens in outer space (air has weight and is therefore attracted to the surface of the earth) or in an enclosed space from which we have extracted all air, by means of a suction pump. In a vacuum, there can be no displacement (because there are no molecules). Therefore, there can be no sound. Period.

At the other end of things, when we make air more dense by compressing it, at some point it changes state from a gas into a liquid. Such a liquid is not viable for humans, so the issue of how sound might be transmitted in liquid oxygen is pretty much irrelevant for most studio work.

Meanwhile, the speed at which waves of air pressure can travel is constant (for any given temperature and altitude, or density). This is called the “speed of sound” and it is approximately 1130 feet per second at sea level and 70° F.

These qualities serve to describe something about the medium of air itself. For our purposes, it is useful to consider the range of magnitudes within which “sound” as perceived by humans exists. I call this set of ranges “The Audio Window.” I use it a lot in teaching.

This “window” is an interesting way to consider sound. In a general way, it frames the three most important “dimensions” within which sound exists and the physical limits of the audibility of sound for us humans. I have also found it is useful to consider what lies “outside” the dimensions of the window – the physical behaviors of air that are similar to sound behavior but that aren’t audible.

The three dimensions that we will consider here are frequency, amplitude and time. I usually show them on three axes: horizontal (or X) axis for frequency, vertical (or Y) axis for amplitude, and front-back (or Z) axis for time.

	Three axes of the audio window, representing Frequency, amplitude and time.

Two of these “dimensions” are related – frequency and time. We consider them separately because of the way our hearing works. “A sound” consists of a group of frequencies occurring over a given time period usually greater than 50 milliseconds. Because the array of frequencies plays such an important part in our perception of timbre, it is useful to consider them independently from the time dimension itself, where we can consider the relationship of events and their component parts (i.e. spectral, dynamic and spatial envelopes).

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