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发信人: nlpir (regent), 信区: Art
标 题: Secret of Architectural Acoustics Revealed [ZZ]
发信站: 荔园晨风BBS站 (Fri Nov 22 12:50:43 2002), 站内信件
The Secret of Architectural Acoustics Revealed
Silence is Golden
Background Noise
Have you ever noticed that your radio seems awfully loud when you stop
your car after listening on the highway? Or have you ever felt that
palpable relief when the air conditioner shuts off? We don't notice
background noise---but background noise determines what we can hear
and understand in the foreground.
Some times we don't want to hear everything. Imagine if you could hear
and understand every conversation at your office. It would be terribly
distracting.
But when we do want to hear every little thing---at a religious service,
in an important meeting, at a play, or at a concert---background
noise is critical.
During a lecture or sermon, any audible sound not made by the speaker is
noise; during a performance, *any audible sound not created by a
performer is noise*.
There are, of course, many aspects to excellent acoustical design.
However, in any space intended for listening, strict control of noise is
fundamental. I call these spaces "critical-listening space."
Sources of noise include traffic, airplanes, machinery, plumbing,
lights, and people in other spaces. In a space for listening, the
worst offender is usually the heating, ventilating, and air-conditioning
system.
The (undamaged) human ear is so sensitive that we can detect sounds that
displace the eardrum by roughly the diameter of a hydrogen molecule.1
This means that background noise determines the softest sound that a
performer or speaker can effectively utilize. Even in spaces that most
people would consider quiet, the background noise level can be twenty to
thirty decibels above the threshold of hearing. You don't notice this,
but the performer has lost twenty to thirty decibels of dynamic
range!
This is illustrated in the graph below, with HVAC noise criteria
curves for comparison.2
NC-40 is considered acceptable for such noncritical spaces as lobbies
and corridors. NC-30 would be acceptable for a motel room. NC-20 is
often given as acceptable for churches or drama theatres.3 But look at
how much area there is between the threshold of hearing curve4 and
NC-20. There is an awful lot of audible sound being covered up by an
HVAC system at NC-20.
Speech Intelligibility
When a theatre is truly quiet, an actor can use his entire dynamic
range, from a shout to a whisper, and still be clearly understood. Since
the quiet moments in a drama are often the most electrifying, strict
control of background noise is essential.
The ancient Greek theatres are known for their almost magical
acoustics for speech. One can hear a drachma drop from the farthest
seats in the theatre at Epidaurus. How can this be so? The answer is
that no audible sound covers up the sound of the coin striking
stone--and so it is heard.
Music
Similarly for music, a silent background allows a performer to exploit
his entire dynamic range. The loud climaxes of a musical performance can
be wonderfully stirring, but the quiet moments set off these
climactic moments and give them their power. Without the quiet moments,
music is all on the same dull level. Furthermore, some of the most
intense, magical moments in music are the softest. These moments are
only possible when the hall in which they occur is truly quiet.
Recording engineers understand this. They commonly turn off all
mechanical systems and most lights while recording.
Speech and Music in the Same Space
Many spaces are used for both speech and music, the best example being
church sanctuaries. For such mixed use, a silent background is
particularly important. This is because reverberation (the persistence
of sound in space) is necessary for music. Without reverberation,
music sounds flat and dull. But reverberation can interfere with
speech intelligibility by prolonging the sounds of speech, smearing them
in time.
A good speaker corrects for this effect by speaking slowly and clearly,
working with reverberation to enhance the sound of his voice. However,
reverberation amplifies background noise, creating a double
difficulty for speech. Contrary to common belief, speech can work
quite well in a properly designed reverberant space, but only if
background noise is minimized.
Audience Noise
Audience noise is beyond the direct control of the architect or
acoustical consultant. However, research shows that audience members are
significantly quieter when background noise levels are very low. In the
City of Birmingham Symphony Hall, Birmingham, England---a hall with
exceptionally low background noise---audience members are so attentive
during quiet music passages that they hold their breath to listen.
Examples of Spaces with Low Background Noise
Meyerson Symphony Hall, Dallas
Domain Forget, Charlevoix Quebec
New Jersey Performing Arts Center, Newark, NJ
Clemens Theatre, Christopher Dock Mennonite School, Lansdale, PA
Gymnasiums, Restaurants, Cafeterias
Listening may not be the primary function in your space. However, people
are always hearing, and the acoustics of these spaces can almost always
be improved by lowering background noise (as well as other measures).
I was recently in a gymnasium, for instance, with horrible screeching
noise from the lights. The poor gym teacher who has to work in that
space must have a perpetual headache.
The Common Condition
In the vast majority of places where I listen---churches, theatres,
lecture halls, recital halls, concert halls---background noise imposes a
haze in front of the sound. This noise itself goes unnoticed by most
people. Instead, they notice that their experience is diminished: the
tone color of the violins is dull; the sound lacks clarity; they can't
quite understand the words.
The usual response to the ubiquitous blanketing of desired sound by
background noise is to turn up the amplification. Amplification often
adds fifteen to twenty decibels above the background noise to the
level of the speaker. Wouldn't it be better to reveal the speaker by
removing twenty to thirty decibels of background noise?
Furthermore, lower background noise makes the job of the sound system
vastly easier.
See and Hear for Yourself
The effect of background noise can be compared to looking through a
dirty window; one doesn't notice the dirt on the window, one simply
can't make out the view. Clean the window for a striking improvement
in clarity. And so it is when background noise is controlled.
You can hear this for your self by the following simple experiment.
You need two other people to help.
Two people stand on opposite ends of the room.
The third person is on hand to manage the noisemakers: fans, ventilation
systems, lights, dimmer racks, air-conditioning, etc.
Turn on all noisemakers. In actual practice not all these devices may be
on at the same time, but for the sake of demonstration turn them on
to create the greatest contrast.
Converse with the person across the room. Note the effort necessary to
make yourself understood. Note the effort necessary to understand.
Turn off all the noisemakers at once.
Listen.
Proceed with your conversation.
Note the change in effort necessary to understand and be understood.
Even in rooms that have other acoustical difficulties, the improvement
should be clear and palpable.
What Is To Be Done?
When designing a new critical-listening space, consider the location
of noise producing machinery such as air-handlers, and the design of a
silent mechanical ventilation system from the very beginning.
Few people realize that it is possible to supply cool air to a room
without creating any noise. It is. However, since this is not the
usual practice, such a silent system must be considered from the
beginning of design.
An architect once called me to help with the design of a high school
auditorium. I discovered that it was too late in the design to move
two large air-handlers from their location on the roof of the
auditorium. They might just as well have been put on stage! I did my
best to help out, but nothing I could recommend for the inside of the
room will cancel out the deleterious effect of those two huge
noisemakers.
In the case of historic renovation, noise control may be the only option
available for improving the acoustics of a space. In some cases, merely
quieting a noisy mechanical ventilation system will affect a drastic
improvement in the acoustics.
Conversely, many a perfectly lovely space has been ruined acoustically
by loud new ventilation systems. When planning the renovation of a
worship or performance space, make noise control the first
consideration.
Noise control is fundamentally important to the success of any
building for listening. It can make the difference between excellent
sound and the usual mediocrity. This is especially true on a tight
budget. Better to save money by leaving out the seats! After all,
these buildings are often meant to last for more than a hundred years.
Conclusion
Since the level of background noise determines what we can hear in a
space, it determines the level of acoustical excellence. This is truly
the secret of great acoustics. Awareness of this secret in the early
stages of design brings excellent acoustics for your new
critical-listening space within your grasp.
---------------------------------------------------------------------
1 Stanley A. Gelfand, Hearing, Marcel Dekker, 1990, p.325.
2 Leo Beranek, Noise and Vibration Control, revised edition,
(Institute of Noise Control Engineering, 1988), p.569.
3 1995 ASHRAE Applications Handbook, p.43.5.
4 Calculated from William A. Yost and Mead C. Killian, "Hearing
Thresholds," Encyclopedia of Acoustics, ed. Malcom J. Crocker, (John
Wiley & Sons, Inc, 1997), p.1549.
5 J.P. Newton & A. W. James, 'Audience noise-how low can you get?,'
Proceedings of the Institute of Acoustics, Vol. 14 Part 2, pp. 65 -70
(1992).
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