When working with an acoustician in the design or
renovation of a hall it is helpful for all to have an understanding
of the basic concepts in auditorium acoustic design. You can't really
design a hall just by knowing the basics of auditorium design. The
acoustician maintains an arsenal of trade secrets and insider techniques,
reserved to managing sound once it's been launched from the loudspeaker.
But by understanding the basics, you can at least keep track of
what and why various things are being done. You can explain things
to fellow parishioners. Also, long before you even start, you have
to find someone to help with the design work. Understanding auditorium
design helps you interview candidates and recognize who is not able
to answer straightforward questions. Finally understanding auditorium
acoustics will help give you second thoughts when you find there
is a problem with understanding sound from a loudspeaker. Your speaker
was probably installed by perfectly competent sound contractor and
is still working just fine. If it's loud enough to hear and you
still can't understand it, then your problem isn't loud speakers,
Besides keeping the rain out, the next most important
thing an auditorium must do is to provide a place where speech can
be clearly understood. This means a good auditorium will have a
good intelligibility rating. The set of minimum acoustic requirements
that are met by a working auditorium starts with the direct sound
from the speaker being loud enough, that means it replicates conversational
sound levels. The background noise in the hall has to be fairly
quiet. The hall acoustics should be fairly free from echoes and
other types of late reflections. And finally, the hall acoustic
is not very reverberant at all. Here we take a look at each of these
factors as they apply to the three basic types of auditoriums that
have evolved over the last century.
Reviewing the Basics of Auditorium Design
Auditorium design begins with the loudspeaker and
how it plays sound into the hall. It ends with how the hall returns
reflections of the sound back to the audience. The speakers should
produce a sound level at about 65 dB,A everywhere in the seating
area. It should have at least 20 dB of "head room" so
that short lived bursts of sound up to 85 dB,A can be replicated
without any hint of speaker or electronic distortion. Electronic
distortion must be avoided. Distortion of the signal is one of the
fastest ways to cause people to lose their understanding of the
sound. In addition, the loudspeaker system should sound similar
no matter where a person is seated. This is achieved when the speaker
system is tested and confirmed to provide a fairly flat frequency
response curve for every seat in the house.
The natural noise floor of the hall itself should
be at least 20 to 30 dB quieter than the speaker level heard at
the seats throughout the entire frequency range for speech. Generally
the ambient noise levels for an empty good auditorium would be about
25 to 30 dB,A. This noise is the sound of the hall when everything
is turned on, lights, air conditioning and even the sound system.
The only thing that isn't happening is that someone isn't talking
into the mic. Only a fairly good sound meter can measure sound levels
this low. Then we open the doors and the audience moves into the
hall. The background noise levels rise up to about 35 dB,A because
of the breathing and other rustling that people naturally do. It
is a well-established fact of human behavior that in a group, we
collectively manage to make just a little more noise than the background
noise level. That's why we act quietly in a library and noisily
in a packed diner. (See Figure D)
Fig-D Noise levels,
sound levels and head room
Early reflections are very helpful but not necessary
for achieving good intelligibility in a quiet hall. But many halls
are quite perfect and that's when early reflections become a good
little helper, especially for those of us who have begun to lose
some hearing efficiency. A bright and clear sounding auditorium
will be providing something like 30 separate early reflections,
each arriving well within the first 1/40th second following the
initial impact of the direct signal. Some of these will be strong
and some will be weak but the overall summed power of all the early
reflections should be in the range of 60 dB,A to provide good speech
reinforcement. (See Figure C & E)
Fig-C Sound level vs
time for an acoustic event. Both direct and early
reflections help with understanding.
Fig-E Left: Direct
signals leave the speakers and impact the audience.
Right: Indirect signals are
those that bounce off nearby surfaces into the audience.
The next group of reflections is to be avoided.
They are those that arrive at the listener's ear within a time delay
following the direct signal between 1/40th second and 1/4 second.
These reflections should be relatively indistinct and quiet, about
15 dB below the speech signal range. The late reflections and echoes
are the most problematic part on auditorium acoustics. Some designers
simple eliminate all late reflections by absorbing them. Other more
tricky designers like to cover the surfaces of the hall with sound
scattering devices to disburse late reflections, breaking them up
into a plethora of fairly quiet reflections that do not obscure
the perception and understanding of speech.
Following this should be a low level rise and fall
of reverberation, again in the range of about 15 dB below the direct
and early reflected signals. The overall reverberation should fall
away and become inaudible within 1 second following the initial
direct signal. (See Figure F & G)
Fig-F Late reflections,
echoes and reverberations obscure understanding
the direct plus early reflections.
Fig-G Left: Diffuse
late reflections to raise reverberation level. Absorb late
reflections to reduce reverberation level
Right: Reverberation is residual
sound that has lost all sense of direction.
Starting in about 1985 a new concept was being introduced
(by SynAudCon) to a few of the top sound contractors in the country.
Sound installations were soon going to be judged by a new set of
performance criteria, the intelligibility spec and contractors had
to bone up to be able to do jobs that would meet this new, higher
level of performance. . Prior to this, sound systems only had to
meet loudness specifications. Now, loudness was not enough, understandability
was the final judge of a system. This was a major step forward in
the evolution towards good sound. The only problem was that providing
good loudspeakers was no longer good enough. Bad hall acoustics
could easily ruin any good sound system. The top sound contractors
in the country had to learn about acoustics. Intelligibility meters
soon become available and have become more affordable over time.
Nearly any good sound system was required to achieve a score of
80 to 85% in every seat in the house when measured with intelligibility
Auditorium Acoustic Design
Today's auditorium is generally quite different
than those built early in the last century. Both types use loudspeakers
but that's where the similarity ends. We will consider both types
here and the transition auditoriums built within the last few decades.
We start with the traditional auditorium, made out of heavy rock
blocks or pour in place concrete walls with ceilings made out of
wood or concrete beams. We end up simplifying construction to reduce
costs. Today, concrete block or tilt-up concrete walls are used
to outline the space and roofs are made out of corrugated metal
supported by exposed metal trusses. The shell of today's auditorium
is built not much different from an industrial space.
Fundamentally the old world auditorium is a "what
you see is what you get" type of building. The interior surfaces
of the building are what manage the sound. The seating, the height
and the interior architecture all work in unison to produce the
required intelligible acoustic condition for reasonable listening.
The reflecting surfaces of the hall provides for some early reflections
but not much. The sheer volume of the hall helps to avoid generating
late reflections and the multifaceted ceiling and upper wall surfaces
further act to diffuse the late reflections. The audience provides
the acoustic materials that act to control the reverb time.
Architects are flocking to the new design trend
in auditorium design and it's very different from the classical
auditorium. These new spaces are large concrete boxes that have
been decked out with sculpted wooden, plastic, metal, sheetrock
and sometimes even glass panels. The hall is full of big, curved
panels that are suspended off the walls and again high overhead.
The new look and sound in auditorium, church and music hall design
is one of acoustic clouds, lots of acoustic clouds hanging in midair,
below a completely blacked out high bay ceiling. Although efficient
to build and outfit, to the traditionalist, these halls, sporting
their marching arrays of flying sound panels seem a little strained,
possibly too technical, if not somewhat contrived. The acoustic
clouds however are intended to adjust the signal to noise ratio
in a direct and effective way. They provide for early reflections,
diffuse and weaken the late reflections and regulate the reverb
level and decay rate. The audience provides some acoustic absorption
and the rest is located way up out of sight, behind the acoustic
In between these two styles we find built in the
recent past, large sweeping rooms with padded seats and carpet,
topped off with the largest expanse of an acoustic tile ceiling
one could ever imagine. This type of hall is also a large concrete
box but its interior surface has been built out to create a very
dead hall. Its design seems directly opposite to that of the classic
concrete and marble auditorium. There are no early reflections designed
into the space, certainly no late reflections and as well, no reverberation.
The only heard sound in the hall is the direct sound from the loudspeakers.
Built on the supposition that if reverberation is bad for speech,
then an acoustically dead space must be good for speech. These spaces
are so big and so dead that the audience suffers from sensory depravation.
Distributed sound systems have to be installed in the acoustic ceiling
in an effort to help inject life back into the space. One thing
is for sure about these spaces. Because they are so acoustically
dead, they work great for making TV shows.
The Classic Auditorium
This is the type of auditorium that was widely built
during the WPA years to help bring relief to the grips of the great
depression in the early 1930's. The features of this hall are well
documented in older architectural and acoustic design books. It
is tall, a little longer than wide and has balcony seats running
on the two sidewalls and the back wall. The ceiling has a deeply
coffered design and the sidewalls are lined with pillars or rounded
pilasters. This hall uses a central speaker cluster elevated high
over the proscenium of the raised stage.
This hall is a classic example of minimalist design.
It employs an elevated, central speaker position to provide fairly
uniform sound levels to every seat in the audience. The room is
a high volume hall, with ceilings 40 to 50 feet high over the main
floor. From the viewpoint of the speaker cluster, the main floor,
two sidewalls and rear wall is nearly completely absorptive when
the hall is fully occupied. The only remaining surface that the
speakers can see is the ceiling and it appears to be a very diffusive
surface acting to scatter sound in all directions. (See Figure H)
Fig-H Classic auditorium
design. Elevated central speaker, balconies, coffered
ceiling and stage curtain.
It is instructive to run through the basic acoustic
calculations for the large classic auditorium design. Here are some
basic ratios. Each seated person occupies about 7 square feet of
floor space and provides about 3 square feet of sound absorptive
surface. A hall that is 200 feet wide and 300 feet long will provide
about 7,000 seats on the main floor. The rear balcony section will
be about 50 feet deep and provide about 1250 seats. The sidewall
balconies will be about 30' deep and provide 1000 seats each. The
total seating of the hall is 9250 seats. Occupied hall calculations
are based on the hall being 2/3rds full, just over 6000 people.
The audience provides about 18,500 square feet of
absorption distributed over the floor, side and back walls. The
hall has a volume of 3 million cubic feet. Empty it will have a
reverb time of about 7 seconds. This means in all its complexity,
it has a physical acoustic surface equivalent of about 21,400 square
feet. The hall has a floor and ceiling surface area of 60,000 sqft,
surface area each. The walls have a surface area of 50,000 sqft.
The average absorption coefficient of the surface of the hall is
already about 12.5%, including atmospheric absorption effects.
When the audience arrives, they bring into the hall
their additional component of sound absorption, bringing the total
absorption up to about 40,000 square feet. The reverb time for the
hall will now be about 3 3/4 seconds, far from the "required
1 second". To be able to meet the desirable 1 second reverb
time it would take a total of 150,000 square feet of absorption
in the hall. This means that nearly 100% of the 170,000 square feet
of total surface area of the hall would have to be covered with
sound absorption. This was not possible in the early days of marble
surfaced halls. But auditorium designers didn't stop trying. They
invented "acoustical plaster", a surface that looked hard
but wasn't. There were numerous formulas for this material but over
time the art of acoustic plaster, the formulations and the skilled
people who applied it by hand have all disappeared. Still, the hall
acoustics could not come close to accommodating a 1 second reverb
The floor, sidewalls and rear wall present 100,000
sqft of surface area that intercept the sound from the speakers.
With the sound absorption of the people added to the natural absorption
of the hall surfaces, these 3 surfaces present about 31,000 sqft
of absorption facing the loudspeaker. This means that 31% of the
incident sound is absorbed during the first reflection. Sound reflecting
off these 3 surfaces is reduced in strength by about 2 dB. If we
assume that the sound reflects back across the room, traveling an
average of 225 feet and taking about 1/5 second. Sound is further
attenuated by the natural surface absorption of about 10% upon this
reflection. As a result, by the time sound makes one round trip
in the hall from the proceneum wall into the audience and back,
it has been attenuated down to 62% of it's original strength, about
-2.5 dB all in one round trip time period of 1/10th second. After
one second, there will be 20 such round trips and the overall sound
will have dropped by about 25 dB in strength. The hall will have
a reverb time of about 2.4 seconds. Adding stage curtains that show
even when pulled back will drop the reverb time to about 1.8 seconds.
This is a reasonable reverb time for a large hall.
But there is more than "reverb time" to
hall acoustics. There are the good early reflections and the bad
late reflections. The early reflections need to be cultivated. The
late reflections need to be weeded out. The balcony facing is sculpted
to provide early reflections back down to the main floor. The back
wall of the balcony and ceiling is sculpted to provide early reflections
into the balcony seats. (See Figure A)
Fig-A Classic auditorium
is shaped to enhance presence of early reflections.
The late reflections are mainly dealt with by combining
two features. It begins with having a heavily coffered ceiling.
Any sound that is heading upwards eventually hits the coffered ceiling,
only to be splintered into a cascade of tiny and off angled reflections.
The second factor is the height of the ceiling. A 70' ceiling with
a loudspeaker mounted some 35' off the floor starts splashing sound
back onto the main floor at about 40 ms after the direct signal
has passed through the audience. The coffering of the ceiling breaks
this reflection up into dozens of low-level reflections with a variety
of additional time delays. The high coffered ceiling acts to diffuse
and randomize the only possible late reflections in the hall. Other
sounds that are traveling upward that went over the heads of the
balcony seating continues upwards after the wall bounce and are
also intercepted by the deeply coffered ceiling scattering grid
ceiling. The low level of time-delayed backfill continues until
it is overwhelmed with the rise and decay of the reverberant part
of the hall sound. (See Figure B)
Fig-B Classic auditorium
is shaped to convert harmful late reflections into
helpful early reflections and reverberation.
The classic auditorium of the early 1900's was almost,
nearly a perfectly balanced system of people, space, speakers and
surfaces. It was a symphony in sound and architecture.
Oldies but Goodies, Gone Forever
The old auditoriums, built in the early part of
the 20th century could be designed and built to sound pretty good.
They were giant, expensive hand built civic halls. But over the
wear and tear of time, they began to look run down, worn out and
shabby. After WWII, a new product took the architectural and building
world by storm, acoustic ceiling tiles. During the same time a new
form of civic pride dictated "off with the old and on with
the new" and this kind of new meant concrete block walls and
acoustic tile ceilings, and a whole new way to build auditoriums.
The era of fine sounding old traditional civic auditoriums ended
in a bang, sounded by the wrecking ball.
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