Page 181 - The Master Handbook Of Acoustics

156 CHAPTER SEVEN

There are many ways of generating artificial reverberation, but the
challenge is finding that method which mimics actual music halls and
does not introduce colorations (frequency-response aberrations) into
the signal. Historically, a dedicated reverberation room has been
employed by the larger organizations. The program is played into this
room, picked up by a microphone, and the reverberated signal mixed
back into the original in the amount to achieve the desired effect.
Small reverberation rooms are afflicted with serious coloration prob-
lems because of widely spaced modes. Large rooms are expensive.
Even though the three-dimensional reverberation room approach has
certain desirable qualities, the problems outweigh the advantages, and
they are now a thing of the past.
Spring reverberators have been widely used in semiprofessional
recording because of their modest cost. In this form of reverberator, the
signal is coupled to one end of a spring, the sound traveling down
the spring being picked up at the other end. Because of quality prob-
lems, spring reverberators are also rapidly passing from the scene.
The reverberation plate, such as the Gotham Audio Corporation’s
EMT-140, has been a professional standard for many years. It, too, is
slipping into oblivion because of the more favorable cost/performance
ratio of the newer digital devices.

Artificial Reverberation: The Future

An understanding of the digital reverberators now dominating the
field can be achieved best by studying the basic principles of the old
abandoned methods. What audiophile has not fed a signal from the
playback head of his or her magnetic tape recorder back into the record
head, and was enthralled by the repetitive “echo” effect? The secret
lies in the delay resulting from the travel of the tape from one head to
the other. Delay is the secret ingredient of every form of reverberation
device. The delays associated with the return of successive echoes in
a space is the secret ingredient of the natural reverberation.
This principle is illustrated in the simple signal flow schematic of
Fig. 7-18. The incoming signal is delayed, and a portion of the delayed
signal is fed back and mixed with the incoming signal, the mixture
being delayed again, and so on.
Schroeder has found that approximately 1,000 echoes per second
are required to avoid the flutter effect that dominates the above tape-