Let's start with the basics. The architect designs
a great looking and comfortable auditorium. The sound contractor
installs a great looking sound system. The people attend the grand
opening and are impressed with what they see, but they have gathered
for more than a dazzling display of architecture, lighting, electronics,
carpets, glass, surface textures and paint. They have come to be
in an auditorium, a place to hear and, moreover, a place to listen
to and learn from the lecture or, as the case may be, the sermon.
The outer beauty of an auditorium is recognized by how it looks,
but the inner more lasting beauty of the auditorium is truly known
by how it sounds. And with this we mark the beginning of our journey
into auditorium acoustics.
Lots of sound, but little is heard
sound wave starts at the loud speaker, which is suspended high overhead
in the front of the hall. Seated way below, are the many people
who came to hear that sound. The greater the size of the audience,
the farther from the speaker they have to sit. An audience of 1,000
people would occupy about 8,000 square feet of floor space. A member
of that audience typically might be seated some 50 feet away from
the loudspeaker. The sound wave emitted by the loudspeaker spreads
out in the shape of an expanding quarter sphere. By the time this
wave reaches the audience, it has expanded out to a radius of about
50 feet. It has spread out over a quarter sphere surface area of
7,850 square feet or about 1.13 million square inches.
Each ear of a person collects about one square inch
of sound, funneling it down into the eardrum. A person in the audience
of an auditorium collects about two square inches of the sound wave,
that's just about 0.00017 percent of the total sound emitted by
the central cluster loud speaker. This tiny fraction of sound is
called the "direct sound" because it goes directly from
the loud speaker to the listeners' ears. (Figure-1).
If 1,000 people are in the audience, their combined
ears collect only 0.17 percent of the direct sound emitted by the
loud speaker. The rest of the sound, the other 99.83 percent of
the sound, is called "indirect sound". What happens to
all this indirect sound is what auditorium acoustics is all about.
If the indirect sound is neglected or mishandled, the auditorium
will sound bad, and if it is well handled, the auditorium will sound
To recap, auditorium design or renovation can be
understood to involve three consecutive areas of expertise. The
architect designs a building that is attractive, comfortable and
allows people to see what is going on. The sound contractor supplies
a sound system to the auditorium that makes a direct sound loud
enough so people can hear what is going on. However, nearly all
of the sound generated by the sound equipment misses its intended
target, the ears of the people. Picking up and handling the stray
sound is the responsibility of the acoustical engineer. How it is
collected and processed makes all the difference between a good-
and a bad-sounding auditorium.
Noise blocks our ability to hear
auditorium is meant for understanding speech. It should be kept
quiet so the people can hear and understand what is being said.
Speech heard in the auditorium should be comfortably loud and crystal
clear. Noise destroys sonic clarity. We have all seen photos taken
in a fog. The general clatter in a hall from air conditioning to
feet shuffling through the aisles can create a continuous din, a
noise floor that seems to white out, haze over and block out the
detain in an otherwise perfectly fine presentation. We have also
seen photos taken on a clear day but blurred by a moving camera.
Similarly, noise such as echo and reverberation can act as a blurring
agent that makes it difficult to even make out what sound is actually
there. These are the two kinds of noise. The fairly continuous din
of extraneous noise is called "background" noise. The
echo and reverberation of sound emitted from the loudspeaker is
called "acoustic" noise. (Figure-2).
The ability to hear and understand depends on the
"signal to noise" ratio. We want as much direct signal
as is comfortable to receive and as little noise as possible. It
would seem that if the noise is a little too loud then simply turning
up the volume of the direct signal, the loudspeaker, should solve
the problem. This doesn't work very well. Loud sound is uncomfortable.
Loud sound does improve the fairly constant background noise to
signal ratio. Loud sound does not improve the acoustic noise to
signal ratio because the loudness of the acoustic noise depends
directly on the loudness of the loudspeaker.
Also there is a context factor. People expect to
hear a conversational style lecture at conversational sound levels
(60 dB-A), a quiet voice at quiet voice levels (40 dB-A) and a raised
voice at a raised voice level (70 dB-A). Cranking up the sound throws
the presentation off, whereby the loudness is out of context with
the lecturer's action --it's simply not natural.
For good intelligibility, there will be at least
20 dB between the quietest parts of the desirable signal and the
background noise --since the more quiet parts of speech are easily
in the 40 dB-A range and less. Background noise levels in a good
auditorium will be as low as 20 dB-A. Also, there should be at least
10 dB between the signal and the acoustic background noise.
Background noise is all the sounds one hears when
the lecturer is not saying something. There are three types of background
noise. Just sit and listen in most any meeting space and you can
distinguish operational system noise, intruding outside noise and
self-generated audience noise. In order to achieve a strong signal
to background noise, the background noise has to be reduced to as
quiet as possible.
operating system includes all things that operate in order to accommodate
the occupancy of the auditorium. Here we have the noise from systems
that handle electricity, water and air in the building. They include
the hum of lighting ballasts, the hiss and hum of the sound system,
overhead circulation fan motor hum, video projector fans, air-conditioning
noise and water pipes, both fresh water and wastewater. (Figure-3).
Other systems sounds include the more intermittent operation of
a dishwasher, garbage disposal, refrigerators, space heaters, toilet,
faucet shut-off, watering, water hammer and thermal expansion and
utility devices such as the copy machine, coke machine and cold
water drinking fountain. The air-conditioning system is usually
a strong contributor of noise, piping turbulent air and fan noise
into the room through every air supply and return opening.
Intruding outside noise is conducted into the auditorium
through the windows, doors, room and walls of the building. Traffic
noise penetrates inside, in from the street and down from overhead
planes. Parking-lot noise --driving, starting, door slamming and
sidewalk conversations --contribute to intruding noise. Rain and
wind can cause noise by hammering and scraping on the building.
Outdoor stationary equipment such as heat exchangers and sprinklers
cause noise. Even HVAC units mounted on the rooftop generate noise
that can come in through the roof and upper windows, let alone shake
the structural beams of the building. Activities in other parts
of the building get into the auditorium by passing directly through
the walls but also by simply traveling in the air, down corridors,
under doors and through air-conditioning ductwork, room to room.
Self-generated audience noise also raises the noise
within the auditorium. Here we have the rustle of paper, books,
coats and clothes, shoe scuffing, candy wrappers, kids talking and
parents hushing, coughing and sneezing, baby shouts, whining and
eventually crying. Noise is generated even when people breathe and
when they make little noises of agreement and appreciation and whisper
to each other. To illustrate, a person trying to stand absolutely
still, breathing as shallowly as possible still generates enough
noise to register 20 dB-A at a distance of 10 feet.
There is a symbiotic effect of background noise.
Some noise begets more noise. The quiet of a library provides testimony
to this effect. It starts quiet and stays quiet all day. When the
background noise is at a raised level, people feel that they too
can make a little noise and no one will notice. But multiple this
by 1,000 people and we have a significant increase in people-generated
noise. This then results in a further raised noise floor and, once
again, it seems easy for people to make just a little more noise.
This spiraling effect can create a very noisy auditorium, full of
disruption and inattention. An auditorium whose background noise
level starts in the low 20 dB-A range stays quiet when the audience
Acoustic noise, echoes and reverberation
Sound expands away from the loud speaker. Most of
what is created is not directly heard but goes past the audience
and begins reflecting around the hall. If a reflection is strong
and we know where it comes from, it is called an echo. If we hear
many reflections at one time from seemingly no special direction,
it is called reverberation.
Generally, any echo is bad. In addition, and to
put it simply, loud reverberation is bad. But quiet reverberation
can be interesting, if it is in limited doses. Both reverberation
and echoes degrade the perception of timing in the material being
presented. Strong echoes are disorienting to the timing aspect of
speech or music, like trying to be coordinated in a disco strobe
dance floor. It is not unusual for echoes to bother the performer
more than anyone else in the auditorium. Echoes usually bounce off
the back wall of the auditorium and because the person on stage
is farthest from the back wall, the echo for the performer is the
most delayed. And it is most important that the performer does not
suffer disorientation due to echoes. We cannot forget Pavarotti
walking off the stage of a large hall filled with people because
the echo was so strong that he couldn't sing --but certainly, as
we understand acoustics, we can understand and forgive.
Reverberation is the ongoing part of sound in a
large hall that gradually decays away, a totally chaotic lingering
presence of a previous direct sound, a sonic afterglow, a remembrance.
Loud reverberation upsets the timing of sequential sonic events
by blurring everything together. It is especially detrimental to
speech and music in small hard-surfaced rooms. However, it can also
be great personal fun, as in singing in the shower, but, in this
case, the singer and the listener are one and there are no concerns
for improving the communication.
Quiet reverberation can contribute to the feeling
that a larger-than-life experience is taking place. It adds a dramatic
flair of importance to speech. It is an essential accompanist to
acoustic music sources as orchestra, ensemble, choir and organ.
Reverberation generally ruins the presentation of modern electronic
There are three aspects of reverberation to be understood.
Onset time delay is the time between the direct signal is heard
and the reverberation begins to be heard. The second is how loud
the reverberation becomes. The third is how long the reverberation
lasts or can be heard; the "reverb time" is officially
the number of seconds it takes for sound to die down a full 60 dB.
Reverberation in an auditorium that is used for speech, lectures
and talks should have one-third second onset time delay, be at least
10 dB-A quieter than the direct signal and have a reverb time die
out within 1.25 seconds.
If reverberation builds up too quickly it competes
with the clear perception of the sequence of sounds that make up
speech. A short reverberation onset time will fill the essential
quiet moment that exists between and delineates sequential sonic
events. The introduction of each new sound is blurred by the upwelling
presence of the old sound. Speaking more slowly can help this situation,
but forced slow speaking is a stopgap measure at best. Acoustically
slurred speech is very difficult to understand. The time delay for
the onset of reverberation should be about one-third of a second.
Background noise is best if kept at least 20dB below the sound levels
of speech. People speak at a rate of about three separate sounds
per second. Some languages speak more slowly and others more quickly;
auditorium acoustics have to be designed for the kind of speech
that takes place in them.
loudness of the reverberation is important, at least 10 dB-A below
the level of direct speech will create reasonably clear speech.
(Figure-4). A reverb level of 10 dB-A below the direct signal is
very desirable. The loudness of the reverb changes the feeling of
the auditorium. A warm, cozy, personal chatauqua style auditorium
will have a large difference between direct and reverb levels, as
much as 18 dB-A. A cold, impersonal, more political rally sounding
auditorium will have a lower difference, possible as little as 5
Finally the length of time the reverberation remains
audible is to be adjusted. Generally large rooms for speech are
allowed reverb times of 1.5 seconds. Smaller auditoriums and more
intimate sounding rooms should have reverb times as low as 0.9 seconds.
The personal, conversational chatauqua style auditorium, growing
popular in the world of broadcast TV church worship, will have reverb
times as low as 0.7 seconds.
These three reverb factors are generally the same
for auditoriums used for speech, plays and modern music. More traditional
music tends to sound better with longer reverb onset delay times,
louder reverb levels and longer reverb decay times. Some auditoriums
are built to support a varied venue, from speech and plays to operas
and symphonies. They have adjustable acoustics: reflecting and absorbing
panels that are moved, exposed or hidden to independently adjust
the three factors of reverberation.
An old saying: Look before you leap
The auditorium is a purpose-built hall, built for
audition, listening. Before it can be designed, the architect has
to understand what the large hall is to be used for. Before the
sound contractor can specify the sound system, the purpose of the
hall must be understood. Before the acoustical engineer can bring
a voice to the auditorium, the feeling and style of presentations
intended for the hall has to be understood. Voicing the auditorium
means deciding what to do with 99 percent of the sound, generated
by the loudspeakers but not directly heard by the audience. A bright
and beautiful looking auditorium will attract people. But the quiet,
good-sounding auditorium will keep them coming back.
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