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RE/P Files: A Special Emphasis On Reverberation

From the archives of the late, great Recording Engineer/Producer (RE/P) magazine comes a wealth of knowledge about reverb. (July/August 1979 issue)

By Jim Cunningham September 30, 2015

The cover of the July/August 1979 issue of Recording Engineer/Producer (RE/P) magazine.

Artificial reverberation, or echo as it is often called, is one of the most useful effects available to the recording engineer. Recently there has been an explosion of reverb devices available in the marketplace, and to compete for your dollars, they all must claim to be the best if not the ultimate machine of all time.

Actually, each has strong and weak points, so the purpose of this article will be to investigate the four basic types and see how they perform. The four types are: 1. Acoustic, 2, Plate, 3. Spring, and 4. Digital.

Each of these occasionally has been compared to natural reverb, so a look at some of the natural characteristics should be worthwhile.

Music is most often performed in a room large enough to accommodate musicians and an audience, so assume our space is 240,000 cubic feet or 100’ x 40’ x 60’. The recommended reverb time for such a space is 1.6 seconds at 500 Hz. Under average humidity conditions, the reverb time at 10,000 Hz, however, is likely to be no greater than .8 sec.

One important aspect of reverberation is the room modes or number of resonances per Hz the room exhibits, as this influences the coloration of the sound. For the reverb decay to be smooth, the room boundaries must be designed so that all resonances decay at the same rate.

Although this room exhibits the staggering number of ten thousand resonances per Hz in the critical mid-frequencies, only about three are necessary as far as human hearing is concerned. Another characteristic of this room is the density of reflections.

There are many thousands of reflections in the first second, although once again, only about 1,000 are necessary. This is shown in Figure 1, each line representing one reflection from a boundary surface; notice how they decrease in intensity as they increase in number.

Figure 1 shows another interesting aspect of natural reverberation; there are very few reflections in the early period — just enough to add some “air” around the direct sound but not enough to clutter it. Note that the reflections are random with respect to time and intensity and the time difference between reflections is usually not more than 20 milliseconds.

Figure 1

One more characteristic: each reflection may bounce around as many as 50 times before becoming inaudible, traveling an average distance of 38.7’.

In doing this the higher frequencies will suffer more loss than the lows which accounts for the shorter reverberation time at high frequencies.

Before anyone gets the impression that all this is a recommendation for live “in concert” recording — or giant echo chambers — it is not. But perhaps the preceding will serve as a base of reality from which we can assess the various artificial reverberation systems.

The Acoustic Chamber
The acoustic chamber is the oldest type of artificial reverb; ideally, it is a room of at least 2,000 cubic feet whose walls are non- parallel to avoid flutter echoes and “hard” or reflective enough to have an absorption characteristic of .03 Sabines, easily obtainable with a few coats of verathane on plaster, the ratio of length, width, and height should not be integral multiples or the frequency response of the room will be unnecessarily irregular.

All of this should give a reverb time of about 3.5 seconds at 500 Hz. With an average temperature and humidity condition, the maximum reverb time at 10 kHz will be 1.25 seconds. For slightly longer times at 10 kHz, water vapor could be inserted.

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