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Nuts&Bolts
The Elements of
Acoustics
If I may paraphrase Mark Twain,
who is surely, with Shakespeare, the
most paraphrased author in English
literature, everyone talks about
acoustics but no one does anything about
it. I would go a little further. When most
people do talk about acoustics, they get
rather a worried expression, as though
they dread running out of conversation
just a few words out.
My goal in this article is not to tell
you all about acoustics. I did so quite a
few years ago in a series of articles (UHF
No. 30-36 inclusively). There are, what
is more, some very good books on
acoustics, though some of them are, it
must be admitted, written for graduates
of schools of architecture. It may, nonetheless,
be useful to speak of the basic
questions of acoustics, if only to make
further reading less of a slog.
I must begin by saying that acoustics
is the study of sound as it affects human
hearing. This is the way that an architect,
or an experimental psychologist,
would understand the word. To audiophiles,
however, acoustics has a more
restricted sense: it is the behavior of the
environment (i.e. the room) as it affects
reproduced sound. I shall of course use
this second definition.
I shall also warn you that you are not
likely to find, in this rather brief article,
definitive answers to all questions about
even this limited aspect of acoustics. If
I am able to evoke the right questions, I
shall perhaps have placed you on the path
toward some answers.
Finally, I shall assume that anyone
reading this publication needs no explanation
of such concepts as frequencies,
or why they are measured in Hertz.
The anechoic environment
Many audiophiles know this term
from the phrase “anechoic chamber.”
Such a chamber is a working tool for
acousticians, and it is, quite obviously, a
22 ULTRA HIGH FIDELITY Magazine
by Paul Bergman
room with no echoes. Even so, I should
caution you that both the term and the
concept are inexact.
Better than the word echo is the word
reverberation. Both echoes and reverberation
are the result of sounds that
are reflected from structural surfaces.
However, an echo is a reverberation that
arrives long enough after the original
sound that it is perceived by the human
brain as a separate sound. This happens
in cathedrals, but not in homes, even if
the room is very large.
A normal-sized room can never
be totally anechoic. One can reduce
reverberation a great deal by lining the
room with elaborate wideband sound
traps, and by making a floating floor
of a material that resembles fishnet.
The intention is that there should be
no reflective surfaces close to either the
device under test (a loudspeaker, say) or
the test microphone. From this description
it is easy to see that an anechoic
chamber cannot be made small. For
that reason, it cannot be inexpensive to
build, and indeed it is outside the reach
of all but specialized university faculties,
government laboratories, and a very few
equipment manufacturers. To the best
of my knowledge, no audio magazine has
yet joined that select list.
For those left off the list, there does
exist an environment that is nearly
anechoic: the outdoors. There are of
course no walls or ceiling to reflect
sound outside. The ground is reflective,
certainly, but some are less reflective
than others. A ploughed field is probably
Acoustics is the
science of… Yes,
of what? Let us
introduce you to the
basic concepts.
as good as you can get, unless you are a
parachutist.
There is of course the question of
why you would want to have access to
an anechoic chamber. It almost certainly
wouldn’t be to listen to music,
which would seem thin, distant and
unnatural, compared to what we are
used to hearing indoors. An anechoic
chamber is of use mainly for technical
measurements. Only in such a chamber
can we measure what is coming from a
loudspeaker, and exclude effects contributed
by the room. If a conventional
room is large, one can actually use a
measurement method known as FFT
(Fast Fourier Transform), which can do
its work before the first reflected sound
has time to arrive at the microphone.
For reasons that are probably clear,
this is classed among “quasi-anechoic,”
methods, which do not, however, work
in small rooms. I stress this last point for
the benefit of certain audio magazines (I
am not referring to UHF) which attempt
to use FFT in standard living rooms.
One could well ask whether anechoic
measurements are actually useful. They
can certainly allow you to predict how
a loudspeaker would behave if the room
acoustics were removed from the equation.
Removing the room is not possible,
however, nor (as you may have deduced)
would it be desirable. You want a “good”
acoustical environment, whatever that
may be, rather than no acoustical environment.
Reverberation
In a true anechoic environment,
there would be no reverberation at all.
Sound would travel out from the source
and continue out, never returning. As
an observer in that situation, you would
hear only the sound that is aimed directly
at your ears. All other sounds would be
lost.
In an actual room, sound bounces off
boundary surfaces (walls, floor, ceiling)
and remains in the room. You would
expect that a sound source in a room
would sound louder than it would in
anechoic chamber or out of doors. That
is certainly the case, though it is important
to understand the characteristics of
the reflected sound. It is well known that,
in audio design, one avoids materials