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Nuts&Bolts
to have a shorter reverberation time than
a large one. A concert hall can have an
average RT 60
between two and three
seconds, and it will sound fine. However
the sound can bounce perhaps a dozen
times during that period. To obtain the
same number of bounces, our “typical”
listening room would need an RT 60
of
perhaps a quarter of a second. This is
extremely difficult to achieve, especially
at low frequencies.
How sound travels
You may have seen articles on acoustics
showing sound travelling the way
light rays do, like this.
24 ULTRA HIGH FIDELITY Magazine
which model is “right,” but which one is
more useful in predicting the way sound
will behave.
This change in transmission mode is
important in the understanding of listening
room acoustics. High frequencies
do tend to be directional, which is why
speakers must sometimes be “toed in,”
so that the listener is on axis with the
tweeters of both speakers. As for the
bass, its omnidirectional nature is highly
significant. Low-pitched sounds radiating
toward the back of a loudspeaker will
be concentrated and beamed forward by
the room boundaries. That means, for
one thing, that a speaker will seem to
have much more bass in a normal room
than it would in an anechoic setting.
a comb filter effect, because the resulting
frequency response, instead of being a
flat line, resembles the teeth of a comb.
The graph above is the result of a
delay between direct and reflected sound
of half a millisecond.
Of course, one solution is to keep the
speakers well away from walls, especially
side walls. It goes without saying that is
easier to accomplish in a large room than
a small room. Good nearby absorption
of higher to medium frequencies is also
very helpful.
Early reflections
In any room short of an anechoic
chamber, there will be surfaces that
reflect sound. This is not only inevitable
It seems evident that, if sound actually
travels the way light does, it is highly
directional. When we discuss reflections
from nearby surfaces (as I shall, below),
we do assume that sound travels in this
fashion.
However you may have seen diagrams
which show sound behaving in a
different fashion, rather like ripples in a
pond:
but desirable, since our brain “expects”
an ambient sound field rather than sound
coming exclusively from a single source.
On the other hand, it is well known that
“early reflections” can cause confusion in
the sound field.
This problem is mainly evident at
medium to high frequencies, frequencies
at which the sound wave tends to
behave the way light rays do. An early
reflection is from a surface that is very
Live End, Dead End
The concept is directly related to the
early reflections problem, and came out
of the research of the late acoustician
Richard C. Heyser, along with Don
and Carolyn Davis of Syn-Aud-Con.
Not everyone realizes that “Live End
Dead End” (LEDE) is a registered
trade mark. Even professional acoustic
designers often use it generically.
The LEDE concept is based on
close to the loudspeaker, close enough avoidance of early reflections (by making
that the bounced sound can easily be the end with the speakers as absorbent
confused with the direct sound. as possible) with a highly reflective surface
Such a reflected sound is likely to be
behind the listener. The research
quite loud, losing little energy from its of Heyser and his colleagues confirmed
first bounce. Because the wavelength is that we don’t notice reflections from
It is evident that the behavior of
sound cannot be the same in the second
short, the two waves (direct and reflected)
may arrive at the listener’s ear partly or
behind us unless they come far later
than the direct sound (50 to 100 mS).
case, since clearly it now spread out in
entirely out of phase. If they are exactly
Because the two opposing walls are not
all directions, no longer travelling A disc as a out we of phase, love this means to that use… one wave
both reflective, standing waves (which I
ray of light would. Which model is cor- will compress the air at the very spot
shall discuss next) cannot be sustained.
rect?
Frederick Fennell where is the perhaps other the wave greatest rarefies wind it. The band two leader There of all time. is much And he
more than that to
The surprising answer is never that sounded they will better of course than cancel. he does However on this remarkable that is not disc. LEDE, Available certainly, on CD
but it is the element
both are. High frequency sounds (with HDCD do all. encoding) If they cancel and LP. out at one place in the
that is most often borrowed by profes-
travel in straight lines, not unlike light room, for instance at your left ear, they
sional and amateur acousticians alike.
rays. Lower frequencies are more like
the ripples in water, which are more
omnidirectional in nature. To be sure,
there is no sudden transition between
one behavior mode and the other.
Medium frequencies can be thought of
as behaving a little like both the straight
line and the ripple. The question is not
may actually add at another place, such as
your right ear. To make things worse, the
two waves may cancel at one frequency,
but add at another frequency.
It is easy to imagine the result. Frequency
response is ragged, rising and
falling with position and frequency. This
ragged response is often referred to as
Standing waves
In contrast to early reflections, which
affect mainly higher frequencies, standing
waves affect lower frequencies, say
from 300 Hz down, and higher than that
in smaller rooms.
At low frequencies, you may recall,