Defining "early"

I define short, or early, time delays as the reiterations of sound events that follow within the first twentieth of a second, or 50 milliseconds, of the original sound. They are usually caused by room reflections, and are closely allied to the idea of ambience, the sonic signature or character of a room. Generally, such reflections have an amplitude within 10 decibels of the original sound and, in conventional reverberant rooms, there are at least 6 - 10 of them for each sound event, all arriving at our ears from different directions.

Odd as it seems, a sound is not usually single event, but an amalgam of at least seven events (those early reflections plus the direct sound). A normal rectangular room of medium size (12 x 15 x 8 feet, for instance) has a floor, a ceiling and four walls (unless, of course, it is Spinal Tap’s motel room just before checkout). Therefore, any sound made in that room will travel to the listener via a direct path and at least six reflected paths. The original sound will take about 10 milliseconds to travel to the listener and the longest reflection path that represents only one reflection will probably take about 30 milliseconds. Therefore, all of those first reflections will get to the listener over a period of about 20 milliseconds, from different directions.

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	Seven-path model of early reflections in a room. All of these sound-paths are perceived by the listener as part of a single sound, until some of the paths get so long that they begin to be heard as reverb or an echo.

Now then, we don’t perceive these reflections as such. Our auditory system integrates this family of early delays with the original sound, so that what we consciously hear appears to be a single sound that is actually an amalgam of direct sound and early reflections. This compound sound has a much richer timbre and a more interesting character than the single direct sound alone. Musicians have known this for years, which is why playing out of doors (where there are no early reflections) is not very popular.

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