And Now, Some Questions

Obviously, it is not the gas pressure variations that we are interested in. It is, rather, a complex range of information and emotions that is CARRIED BY SUCH PRESSURE VARIATIONS that is what we are really interested in transmitting and perceiving.

It is useful to ask some questions about that information, such as:

• What are the features that allow us to distinguish one sound from another?
• How do we distinguish between the sounds of sources and sounds of reflections?
• How do we extract the sense of multiple pitches from a single complex wave?
• Why don’t we do this for all complex waves?
• How come we don’t get hopelessly confused by all the reflections from the environment? Come to think of it, how come we barely even notice them?
• How can we recognize Samantha’s voice on the telephone, when it has been band-limited to a very small percentage of her original sound?
• What makes it possible for us to “hear” that a room has been freshly painted, or that we’ve added a sofa?

The fact that all these things occur so naturally and effortlessly in our perception obscures the complexity of the underlying system. When we make recordings, the mechanical ears that we use, microphones, don’t have that same complex underlying system. As a result, they lose a lot. When we play the resulting audio signals back through loudspeakers, they lose even more. Physically speaking, it is not a pretty picture! That it works at all is miraculous.

If we’re gonna be good, really good, at our craft of recording engineering it behooves us to get a handle on how us humans perceive this stuff, and then to do what we can to make the sounds our loudspeakers put out as useful, informative and entertaining as possible for the human auditory systems possessed by our clients and their fans.

This involves some hard thinking. Most of the operation of the hearing system is concealed from us (that’s why I call it pre-conscious). So we have to work on our ability to infer what is going on by observing the relationship between what we perceive and what we know by physical measurement is happening. A lot of it, when we get into it, is pretty spooky.

Hard thinking is mainly a process of asking hard questions. For instance, the process of neural transmission from the ear to the brain isn’t instantaneous – in fact it takes something like 5 – 10 milliseconds! So we’re always perceiving a delayed version of what happened. Given that that is so, how can musicians play together? And how come we don’t notice the delay?

Another puzzler from the same dismal swamp: we don’t perceive the early reflections of a sound source in a small room for up to about 40 milliseconds – this is part of what is called the Precedence Effect. How come? Is this part of what we call masking, where one sound artifact conceals another? And speaking of masking, did you know that under certain circumstances a sound can be masked by another sound that comes AFTER it? How can this be?

A BIG question from the realm of stereophony has to do with the phantom image. How come there is one? Why don’t we get phantom images from two violins playing the same note? Why does this only seem to happen with loudspeakers? Zork-11 doesn’t get it!

How can we hear chords? Why don’t we hear overtones as chords? Why don’t we hear barometric changes (they’re gas pressure-variations too, you know)? How come reverb doesn’t confuse the hell out of us? How can we actually like the stuff? Why isn’t an anechoic chamber the best place to play music?

As we begin to pull the answers to these questions together, using what we know about the origins of the hearing mechanism and what it needed to do to help us survive in a Darwinian world of natural selection, we can begin to build up a little bit more robust and sensible understanding of what is really going on with our hearing, and how to use that knowledge for fun and profit.

Next month, we’ll take a hard look at what I call the Audio Window, the physical ranges within which we perceive sound. We’ll also consider how analog and digital audio are fitted to those ranges. Ain’t science fun?

Thanks for listening.