The Behavior of a Mirror

A mirror is a reflective surface where all of the energy is reflected back into the space. A feature of mirrors is that we don’t perceive them as surfaces, but rather as windows. We don’t perceive the mirror itself, we perceive the space whose image has been reflected, including three-dimensional depth in the perceived reflected space. The better the mirror is, the better the reflected image. If we degrade the mirror, by the addition of a tint, for instance, or by smudges on the surface, or cracks in the surface, the resulting image is similarly degraded and the “mirror as surface” becomes increasingly apparent.

So it is for reflected sounds, particularly from loudspeakers.

What About Comb Filtering?

One part of the mythology regarding early reflections is that they generate interference patterns with the direct sound, resulting in comb filtering and timbral degradation. Interestingly, this problem is a severe one for microphones, but not so for ears. For reasons related to our integration of that volley of sound artifacts, we don’t perceive comb filtering of phase-locked sounds that arrive from significantly different directions. Further, what little comb-filtering we perceive is diminished as the volley of early reflections in the playback room becomes richer. So, in a highly reflective room, comb filtering will be essentially inaudible, while it is generally quite audible even between the speakers themselves in an anechoic chamber, where only two artifacts exist.

So, comb filtering problems exist primarily in the domain of microphones and recording, and not significantly in the domain of loudspeakers and playback, except possibly under highly damped or anechoic conditions. Consider again the volley of sound artifacts that constitute the perceptual construct we call “a sound.” We identify members of the volley via their related spectra and phase-locked qualities. These are spread out over time. When we emit recordings of these volleys from loudspeakers into a playback room, if the early reflections from the playback room walls are similar in spectra, particularly at high frequencies, the ear accepts that these are additional information regarding the recording, NOT the playback room. And it is the high frequencies that especially help us form really solid and palpable phantom images.

Happily, such an illusion supports the reverberant information from the recording as well. This means that the early reflections in the playback room carry not only the direct sound and early reflections from the recorded space but also the reverberance to the listener, in an expanded and enriched way. Spaciousness is enhanced, depth is enhanced, images are enhanced, envelopment is enhanced. It’s a win/win/win/win kind of situation!

Why Don’t We Make Our Rooms Totally Reflective?

In theory, this all sounds terrific. So why don’t we make our playback rooms totally reflective? Why, we could just set up a room of totally reflective acoustic surfaces (how about polished marble?). A veritable auditory fun-house!

The problem, of course, is what happens AFTER the early reflections have occurred, AFTER 50 milliseconds have gone by. At that point in time, reverberance starts.

Reverberance is the part of the playback sound event that carries perceptually audible information about the playback room. It commences at 50 milliseconds and can run on for many seconds in a large reverberant room. In small rooms, it usually doesn’t go on for more than a second. Interestingly, the reverberance that exists between 50 and 150 milliseconds especially tends to interfere with clarity and intelligibility of sound.

This is why we need to absorb sound in a playback room. We need to shorten the sound decay to a point where the reverberance of the playback room never gets a chance to build up.

Happily, it turns out that it is extremely cheap and easy to make a small room highly reflective at almost all frequencies for a brief period (50-100 ms.) and highly absorbent after that. Without going into actual design details (I’ve actually worked out a topology based on this thinking I call a Moulton Room), we simply make the end of the room behind the loudspeakers highly absorbent at all frequencies, and the other walls highly reflective. All the energy from the speakers propagates along the length of the room and back and is then absorbed. All early reflections are broadband, and all reverberance is absorbed. It really is almost that straightforward!

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