ASHRAE Journal - November 2013 - 73

Column RefRigeRation applications
Andy Pearson

When is a Sound a Noise?
and trombones of the marching band from the other
side of the park, but not the trumpets, although they
Neil Diamond famously sang about "a
may be just as loud to the audience at the bandstand.
beautiful noise" and the Psalms exhort us
High-pitched noise escapes more readily through small
to "make a joyful noise to the Lord," but in
openings in walls. It has a shorter wavelength than the
most cases involving fridge plants the noise low notes, so passes through smaller openings without
so much distortion. Leaving a small window open can be
is neither beautiful nor joyful.
as bad as leaving the whole door open if the noise pitch is
Noise and vibration (next month's topic) are caused by
high. We are most sensitive to noises at and slightly above
movement. Strictly speaking, the former is movement
the pitch of normal speech. Higher notes can be very
of molecules in air (or sometimes some other fluid), and
annoying; lower notes are generally more tolerable even if
vibration is movement in solids. I like to think of the genthey carry more energy than the high squeal.
eral audible movement of molecules as "sound;" it only
There are many things that fridge plant designers can
becomes "noise" when someone objects to
do to minimize the risk that the sound of
it. To support this, I note that we talk about Not everyone appreciates Neil's latest CD. their machinery becomes a noise. If the
"sound attenuation," but refer to "noise
equipment is hidden from view it is less
pollution." However, I fully accept that the
likely to attract complaints-and making the
terms are subjective and are often used
sound travel a longer path to the listener by
interchangeably.
erecting a screen has an additional benefit.
Good management of sound, to prevent
If a screen is used it should preferably be
it turning into noise, is as much about
close to the source of the noise; it will be
psychology as it is about physics. If the sound of a refrig- much less effective if it is halfway between source and
eration system blends in with other background sounds
listener. It should also be as tall as it can be; the further
then it is less likely to be noticed. However, if the plant
the noise has to travel (compared with "line of sight")
suddenly switches on and makes an audible difference,
the better. If noise is tonal then pay special attention to
it might attract complaints even if the sound levels are
the treatment of the tone, even if it is officially within
below the specified level-and in some cases even if they specification. If variable speed equipment is used and
are below the general background level. This is particuswitches to a different mode at a set time, then make the
larly true if the sound is very "tonal"; in other words, if it change gradually rather than in a single step.
is a pure note rather than a general wash.
In one case a neighbor complained because of noise at 11
The noise from road vehicles, trains or other random
p.m. It turned out that this was condenser fans stepping
movements, even speech and music, tends to cover a
down from full speed to a lower "night-time" setting to
wide range of pitch, but the noise from refrigeration
reduce the noise, in compliance with the local regulations.
plant is often close to a single note, and is typically right
It was the sudden change that made the fans noticeable,
in the most sensitive part of our hearing. A tone against
although they were quieter (and within specification)
a broad range of background noise is likely to be noticed after the change. Once the variable speed controller was
even if the background is up to four times louder, and an set to reduce speed gradually from 10:45 p.m. onwards,
intermittent noise can be even more irritating.
the neighbor was happy again. However, that was lucky
The pitch of a noise-is it a high soprano or a low bass
because once a sound has been noticed it is usually treated
note-also has an effect on how it is perceived. Low
as a noise, and it is difficult to get it unnoticed again. The
pitched noises will travel far through the air without
answer to the title's question is "When it annoys!"
being scattered, but high pitches are more readily lost
among the background. This is why you hear the tubas
Andy Pearson, Ph.D., C.Eng., is group engineering director at Star Refrigeration in Glasgow, U.K.
By Andy PeArson, Ph.d., C.eng., MeMBer AshrAe

N ovem ber 2013

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