Page 182 - The Master Handbook Of Acoustics

157
REVERBERATION

Analog Delay Analog
in out

FIGURE 7-18
The simplest artificial reverberation device. The technique of utilizing the time delay
between the record and playback heads of a magnetic tape recorder is of this type.

recorder experiment and to sound natural to the ear. With a 40-msec
delay, only 1/0.04 = 25 echoes are produced each second, a far cry from
the 1,000 per second desired. One solution is to arrange many of the
simple reverberators of Fig. 7-18 in parallel. Four such simple rever-
berators, arranged in parallel as in Fig. 7-19, produce only 4 × 25 = 100
echoes per second. It would require 40 such reverberators in parallel to
achieve the required echo density, and this is impractical.
One approach to producing the necessary echo density, and simulta-
neously, a flat frequency response is illustrated in Fig. 7-20. Here numer-
ous delays feed back on themselves, combining to feed other delays,
which in turn recirculate back to the first delay. The + signs in Fig. 7-20
indicate mixing (addition), and the × signs indicate gain (multiplication).
Remember that multiplying by a fraction less than unity gives a gain of
less than unity—in other words, attenuation. The digital reverberator of
Fig. 7-20 only suggests how greater echo density along with good fre-
quency response might be achieved. Actually, the better digital reverber-
ators in use today are far more complicated than this. 13 The resulting
artificial reverberation available today from the top of the line has far
higher echo density, flatter frequency response, and a more natural
sound than the best of the old mechanical devices. The less expensive
digital units also show great improvement over those of the past.

Arrival Time Gap

There is one characteristic of natural reverberation in music halls
that was revealed by Beranek’s careful study of music halls around