The properties making for
good hearing and freedom from noise have not been widely understood by many,
but modern building techniques directly influence acoustics. This paper is a comprehensive overview of
sound waves, absorbent materials, and how both play an important role in the
decisions made in acoustical architecture.
It focuses on the understanding of how and why acoustical architectures
are constructed the way they are. The
different places that are discussed include theaters, opera houses, musical
venues, and churches with acoustical architecture. When one of these sites is built there is careful planning on how
it will be constructed and what it will be constructed of. They must look at the questions surrounding
the acoustical presentation that will take place in the venue, such as what
events will take place there, what type of acoustics are most important, and
how many people is the space meant for?
They might choose a gypusm plaster dome, a smaller area with thin walls
and floors, or an open area with two thick walls; it all depends on the
circumstances or the purpose of the event.
All these important choices revolve around the understanding of what
makes good acoustics. First off, to
understand acoustical architecture one must fully grasp the concept of sound
waves, such as important questions concerning reflection, diffraction,
refraction, reverberation, and echoes.
Just as we have eyes for the detection of light and color, we also
are equipped with ears for the detection of sound. We seldom take the time to ponder the characteristics and
behaviors of sound and the mechanisms by which sounds are produced and
detected. Sound is a wave, which is
created by vibrating objects and distributed through a medium from one location
to another (World Book). A wave
can be described as a disturbance that travels through a medium, transporting
energy from one location to another location.
The medium is simply the material through which the disturbance is
moving; it can be thought of as a series of interacting particles. The example of a slinky wave is often used
to illustrate the nature of a wave. The
disturbance is typically created within the slinky by the back and forth
movement of the first coil of the slinky.
The first coil becomes disturbed and begins to push or pull on the
second coil; this push or pull on the second coil will displace the second coil
from its equilibrium position. As the
second coil becomes displaced, it begins to push or pull on the third coil; the
push or pull on the third coil displaces it from its equilibrium position. This process continues in consecutive
fashion, each individual particle acting to displace the adjacent particle;
subsequently the disturbance travels through the slinky. As the disturbance moves from coil to coil,
the energy which was originally introduced into the first coil is transported
along the medium from one location to another (World Book).
A sound wave is similar in
nature to a slinky wave for a variety of reasons. First, there is a medium, which carries the disturbance from one
location to another. Typically, this
medium is air; though it could be any material such as water or steel. The medium is simply a series of
interconnected and interacting particles.
Second, there is an original source of the wave, some vibrating object
capable of disturbing the first particle of the medium. “The vibrating object, which creates the
disturbance, could be the vocal chord of a person, the vibrating string of a
guitar or violin, the vibrating tunes of a tuning fork, or the vibrating
diaphragm of a speaker” (World Book).
Third, the sound wave is transported from one location to another by
means of the particle interaction. “If
the sound wave is moving through air, then as one air particle is displaced
from its equilibrium position, it exerts a push or pull on its nearest
neighbors, causing them to be displaced from their equilibrium position” (World
Book). This particle interaction
continues throughout the entire medium, with each particle interacting and
causing a disturbance of its nearest neighbors.
Like any wave, a sound wave
doesn't just stop when it reaches the end of the medium or when it encounters
an obstacle in its path. Rather, a
sound wave will undergo certain behaviors when it encounters the end of the
medium or an obstacle. Possible
behaviors include reflection off the obstacle, diffraction around the obstacle,
and transmission (accompanied by refraction) into the obstacle (Encyclopedia
Britannica). When a wave reaches
the boundary between one medium and another medium, a portion of the wave
undergoes reflection and a portion of the wave undergoes transmission across
the boundary. “The reflected wave may
or may not undergo a phase change (be inverted) depending on the relative
densities of the two media” (Encyclopedia Britannica). The amount of reflection is dependent upon
the dissimilarity of the two media. For
this reason, acoustically minded builders of auditoriums and concert halls
avoid the use of hard, smooth materials in the construction of their inside
halls. A hard material such as concrete is as dissimilar as can be to the air
through which the sound moves; subsequently, most of the sound wave is
reflected by the walls and little is absorbed.
Reflection of sound waves
off of surfaces can lead to one of two phenomenons - an echo or a
reverberation. A reverberation often
occurs in a small room with height, width, and length dimensions of
approximately 17 meters or less (Encyclopedia Britannica). Why the magical 17 meters? The effect of a particular sound wave upon
the brain endures for more than a tiny fraction of a second; the human brain
keeps a sound in memory for up to 0.1 seconds (Encyclopedia Britannica). If a reflected sound wave reaches the ear
within 0.1 seconds of the initial sound, then it seems to the person that the
sound is prolonged. The reception of
multiple reflections off of walls and ceilings within 0.1 seconds of each other
causes reverberations - the prolonging of a sound. Since sound waves travel at about 340 m/s at room temperature, it
will take approximately 0.1 s for a sound to travel the length of a 17 meter
room and back, thus causing a reverberation (Encyclopedia Britannica). This is why reverberations are common in rooms
with dimensions of approximately 17 meters or less. Many individuals have observed reverberations when talking in an
empty room, when honking the horn while driving through a highway tunnel or
underpass, or when singing in the shower. In auditoriums and concert halls,
reverberations occasionally occur and lead to the displeasing garbling of a
sound.
But reflection of sound
waves in auditoriums and concert halls do not always lead to displeasing
results, especially if the reflections are designed right. Smooth walls have a tendency to direct sound
waves in a specific direction.
Subsequently the use of smooth walls in an auditorium will cause
spectators to receive a large amount of sound from one location along the wall;
there would be only one possible path by which sound waves could travel from
the speakers to the listener (Bagenal 85).
The auditorium would not seem to be as lively and full of sound. Rough walls tend to diffuse sound,
reflecting it in a variety of directions (Bagenal 17). This allows a spectator to perceive sounds
from every part of the room, making it seem lively and full. For this reason, auditorium and concert hall
designers prefer construction materials that are rough rather than smooth.
Reflection of sound waves
also lead to echoes. Echoes are
different than reverberations. “Echoes
occur when a reflected sound wave reaches the ear more than 0.1 seconds after
the original sound wave was heard. If
the elapsed time between the arrival of the two sound waves is more than 0.1
seconds, then the sensation of the first sound will have died out” (Encyclopedia
Britannica). In this case, the
arrival of the second sound wave will be perceived as a second sound rather
than the prolonging of the first sound.
There will be an echo instead of a reverberation. Reflection of sound waves off of a surface
is also effected by the shape of that surface.
Flat or plane surfaces reflect sound waves in such a way that the angle
at which the wave approaches the surface equals the angle at which the wave
leaves the surface (Encyclopedia Britannica). Reflection of sound waves off of curved surfaces leads to a more
interesting phenomenon. Curved surfaces
with a parabolic shape have the habit of focusing sound waves to a point. “Sound waves reflecting off of parabolic
surfaces concentrate all their energy to a single point in space; at that
point, the sound is amplified” (World Book). Many individuals have seen an exhibit that utilizes a
parabolic-shaped disk to collect a large amount of sound and focus it at a
focal point. If you place your ear at
the focal point, you can hear even the faintest whisper of a friend standing
across the room. There is an example of
such an exhibit at N.C. State University.
Parabolic-shaped satellite disks use this same principle of reflection
to gather large amounts of electromagnetic waves and focus it at a point where
the receptor is located.
Reverberation is the
interval of time during which a sound continues after the source has
ceased. “This is particularly noticeable
in an empty room with hard smooth surfaces and no drapery or upholstery. The effect of it is that sounds tend to
persist, so that if succession of sounds is produced they overlap and become
confused” (Bagenal 16). An example that
might occur in such an acoustical space is given in Planning for Good
Acoustics:
If, when listening to a speaker, we hear at a given instant not only the syllable just pronounced, but a series of earlier syllables as well, the resulting confusion will make it extremely difficult to distinguish what is being said. If the time, during which the sound once produced persists, is large, then the building will be extremely difficult to speak in. A moment’s reflection will show us that the state of affairs here is indicated easily becomes ‘confusion worse confounded’. Finding that he is not being heard the speaker ‘raises his voice’; the louder the sounds the longer they persist and therefore the greater is the number of overlapping syllables. One possible remedy: he must speak slowly and not too loud. ( Bagenal 16)
Diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. The diffraction of water waves has the ability to travel around corners, around obstacles and through openings. “The amount of diffraction, the sharpness of the bending, increases with increasing wavelength and decreases with decreasing wavelength” (Encyclopedia Britannica). In fact, when the wavelength of the waves are smaller than the obstacle or opening, no noticeable diffraction occurs (Encyclopedia Britannica). Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms. A small obstruction such as a thin column causes practically no sound shadow and has no appreciable effect on the spread of the waves. In short, “the effectiveness of an obstacle depends on its size as compared with the wave length of the disturbance” (Bagenal 8).
Refraction of waves involves
a change in the direction of waves as they pass from one medium to
another. Refraction, or bending of the
path of the waves, is accompanied by a change in speed and wavelength of the
waves (World Book). So if the
medium and its properties are changed, the speed of the waves is changed. Thus waves passing from one medium to another
will undergo refraction. Refraction of
sound waves is most evident in situations in which the sound wave passes
through a medium with gradually varying properties. For example, sound waves are known to refract when traveling over
water (World Book). Even though the sound wave is not exactly changing
media, it is traveling through a medium with varying properties; thus the wave
will encounter refraction and change its direction. “Since water has a moderating effect upon the temperature of air,
the air directly above the water tends to be cooler than the air far above the
water” (World Book). Sound waves
travel slower in cooler air than they do in warmer air. For this reason, the temperature in an
architectural design is important.
Now that the concept of
sound waves is understood we are able to take this information and apply it to
the knowledge of true architectural decisions.
Architectural designs often include a variety of absorbent materials
used for numerous acoustical purposes.
These materials absorb and reflect sounds in different manners, which
entails for the architect to consider what materials to use when building. Absorbents that are often used include
glass, the audience, brick, plaster, marble, fiber slabs, flooring, and natural
stone.
The coefficient table of
absorbent materials developed by Sabine gives architects a true scale of
comparison. The Sabine tables are very
helpful to architects but two factors must also be considered (Bagenal 237-8):
1. Since all materials absorb differently at different pitches, coefficients are required over the whole range of the musical scale.
2.
The
fixing and finishing of the materials behind all building surfaces influence
rigidity and therefore affects absorbing power. The samples tested in the laboratory must then be erected and
fitted according to common specification and must be in large size in order to
get a relation between surface and wavelength corresponding to actual
conditions in a room.
The greater the coefficient the more the sound is absorbed, for example an open window has a coefficient of 1.0 (C4), meaning the sound is totally absorbed. Glass has a coefficient of .027 (for C4) in the Sabine tables. It absorbs largely by transmitting sound through it and the transmission varies with the thickness of the glass. It also varies with the size of the pane and with the pitch of the note. An audience also plays as an absorbent. There role in absorbing is considerable between low and high notes but not between middle notes and high notes (Bagenal 240). An audience was found to have a coefficient of .96 for pitch C4. “An audience helps explain also the difference in energy condition of a room when full and the same room when empty. It also explains the long reverberation common in churches where the seating area is generally small compared to the height of the building and the reflecting power of walls and vaults” (Bagenal 240). The absorbent material of brick has a coefficient of .032 (C4) for an 18-inch wall of hard brick in cement (Bagenal 243). The absorption is due mostly to the porosity of the brick. High notes are absorbed slightly more than low notes with bricks (Bagenal 243). Plaster is often applied on type of an existing material to help absorb sound. Its coefficient for a C4 pitch is .25. Although plaster comes in a variety of styles and materials so this value can change. Plasters used for absorbing must be left undistempered because otherwise it will block the pores of the material and destroy its absorbing value (Bagenal 245). “Absorption of plaster on brick is less than that of plaster on wood lath. Plaster on brick is mainly reflecting, but the plaster on lath is slightly resonant. Therefore plaster on lath is useful for music rooms and its resonance is affected by the distance part the studs” (Bagenal 246). The coefficient for such materials as marble is .01 (C4). These materials use in acoustic design is for reflecting not absorbing purposes. Special materials used for absorbing purposes on walls and ceilings are slabs made of woody fiber compressed and perforated. These slabs have the advantage of being light and can be made to adhere to ceiling surface without the use of battens. They are made in a variety of thicknesses thus having different coefficients of absorption. Maximum efficiency use of the slabs is with the perforations facing outward (Bagenal 248). All architectural designs include flooring often made of concrete. “But a concrete floor is negligible as an absorbent and can be allowed a coefficient of .015 (C4) absorbing. Therefore large areas of monumental flooring not covered by seats tend to increase reverberation” (Bagenal 252). Just recently it was found that some natural stone was found to have an appreciable sound absorption (Bagenal 265). A stone, known as Cottesloe stone, found in Australia is now being used in buildings because of its very high porosity (Bagenal 265). “Water can readily be poured through it, showing that the air cavities inter-connect. It is being used in interior work in slabs 2 inches thick” (Bagenal 265). Sabine also found that if any absorbing material is placed at a position in a room, where sound is concentrated upon it, it is obvious that its efficiency will be increased. Now that an architect has an idea of how well different materials absorb sound, they are able to make final choices concerning the structure and acoustics of a building.
The first venue that will be
explored in this realm of acoustical architectural is theaters and opera
houses. Theaters and opera are
industries of the art. The art of the
theater is an art of speech within a building.
Therefore the acoustical conditions are vital to a shows success. In a theater it is important to have room
for a large audience. “Although an
audience does not generally suspect it, in every large theater there is an
inertia of sound to be overcome, reverberation to be controlled, and tonal
adjustments to be made” (Bagenal 164).
The actor has to generally deal without the aid of a back wall or
orchestral floor to be used as reflectors.
The actor has one reliable reflector, the stage floor. Therefore it is crucial that the design of
the theater help make up for these lost components. The parts of the proscenium most necessary as reflectors for
sound are those portions of the walls that extend from the floor up to the
level of the actor’s head and also the ceiling slay above (Bagenal 165). Those at head level are often the only
reflectors able to reinforce sound where it is most needed. “For this reason boxes, exit doors, and
drapery on this plane are harmful.
Therefore exit doors are better placed forward on side walls, and if
proscenium boxes are necessary they should be raised at least 8 or 10 feet
above stage level and the reflecting surfaces below should not be broken by
deep recesses” (Bagenal 165). The
ceiling is best kept flat or if curved should have a radius at least twice the
height of the theater. Many commercial
theaters carry tradition by using a flat or cambered ceiling. The “Covent Gardens” is an example of such
tradition. A bad innovation took place
in the 19th century with the introduction of the dome. Domes must be avoided in order to prevent
both echoes and reverberation (Bagenal 166).
Although marginal coves in the rear seats above and below the gallery
may be useful in reinforcing sound because they direct sound at the angle they
are reflected. Therefore using these
coves helps transfer sound to a wider range of the audience. Theaters in large cities must also take into
the account traffic noises. Many of
these theaters prevent direct penetration of sound by avoiding placing large
windows and vents on the noisy sides (Bagenal 169). All theaters include an audience and deciding how they are
arranged is important. The best style
is the fan-shape (Bagenal 173). “This
shape places the great body of audience at the right distance from the stage
and provides that the walls themselves shall be useful reflectors” (Bagenal
173).
Theaters share many
characteristics with opera houses, except the singing component involved. Since music is the major requirement, tone
must be considered. Therefore resonance
and reverberation must be deliberately designed, and allow for the best
acoustical sound for that particular audience.
Most opera houses were built without acoustical architecture being truly
considered. Opera houses have two
sources of sound and one cannot drown out the other. In Florence, Italy the “Massimi Opera House” experienced the
problem of too long reverberation (Bagenal 175). Therefore velarium had to be hung from the ceiling. At the “Grand Opera House” in Paris
reverberation is too short and the tone becomes dead (Bagenal 176). “In New York the ceiling of a large opera
house had recently to be lowered at a great expense” (Bagenal 176). These large, musically adapted buildings
have an architectural design of their own, but until recently many of them had
architectural problems, acoustically speaking.
Many venues have music as
their primary requirement when being designed.
In such venues an audience requires sufficient loudness and also
distinctness. “It is natural to sustain
a choral note for an appreciable time and the same is true of an organ
note. For this reason tone production
by voice and organ are naturally suited to the long reverberation conditions of
a church and are suited to a comparatively slow tempo. On the other hand percussion instruments do
not sustain and build up tone; instead they cause a sudden burst of energy, and
in such cases the decay or reverberation period of a sound may be several times
longer than the time taken to initiate it” (Bagenal 87). A good musical concert hall requires a
longer reverberation than a theater. An
ideal musical building might include a plan with curved ends, wood surfaces as
resonators, flat ceiling with coved margins, and boxes at each end of the hall
at high level (Bagenal 103). “The
curved end at the back of the building having the right radius will diffuse
sound harmlessly and prevent a clap-back.
But it has a disadvantage, the diffusion occurs after it has passed
through a focal point, and this focal point may cause complaints in the seats
where it occurs. The curved end
therefore improves platform conditions, but not necessarily audience
conditions” (Bagenal 103). The wood
surfaces would be used as an attempt of an amplifier. The flat ceiling in a musical hall is a non-harmful fashion that
helps diffuse sound with the help of coves.
The effect of tiers of boxes with draped openings is used to reduce
reverberation and prevent long reflected paths. “To sum up, the good acoustics of a musical style building rests
on its short reverberation combined with a resonant wood structure” (Bagenal
183). Although there are many musical
venues that are open air such as outdoor pavilions. Outdoor pavilions and other open area venues still follow the
same principles (Bagenal 192):
1.
Sound
should be intensified at the source by reflectors
2.
Sound
should have a clear passage from speakers to listeners
3.
No
sound should be returned upon the stage or front seats leaving no
reverberation.
The voice
needs much help in an outdoor venue therefore two main reflectors must be
strictly adhered to, back wall of the stage and orchestra floor (Bagenal
192). “The stage should be narrow and
the back wall should be of stone and have only shallow breaks upon it” (Bagenal
192). The performer in an open-air
venue should be taught how to use their reflecting surfaces, such as “turning
towards the wing or the back wall and using it to deliver tone outwards”
(Bagenal 194). All musical venues
require a close look at reflection in order to be able to reach the whole
audience aurally.
The architectural design of
churches is very imperative to the acoustical quality of sound. First off, a church must provide for the
chanting of the Mass and also for the sermon.
These are two conflicting sets of requirements because one is music
while the other is the speaking voice.
This is where the architect must make their first major decision: Is the
church’s main focus on the sermon or the choral music? If the focus is put on the sermon then the
cheapest way to keep down reverberation is to limit volume to a minimum
(Bagenal 211). In order to minimize
volume, the sidewalls and panels of the vault should be rendered with acoustic
plaster or laid in acoustic tiles (Bagenal 211). “If the church wants more absorbing power then the remainder of
the wall surfaces can be rendered in a sand-faced lime plaster finished with
wood and left undistempered” (Bagenal 211).
If the church design is small and there is excessive reverberation then
there are such remedies as curtains, cord matting, and pew cushions. Another inexpensive style to reduce
reverberation in churches is filling panels of roofing with absorbent
fiberboard undistempered (Bagenal 214).
If the main focus of a
church is the choral music then reverberation is necessary and ought to be
provided. When designing for such a
church, the size of the congregation must be considered. Often hard plasters and/or plain brick
surfaces are used to deliberately lengthen reverberation (Bagenal 245). Also the placement of the choir is
necessary. Singers must be able to hear
themselves as well as be able to make themselves heard; therefore the higher
the choir is placed the better (Bagenal 215).
“A church has large wall areas of masonry or brick compared to the wood
and drapery of a concert hall or opera house, so that over-tones are relatively
more active, tone is sharper and brass more powerful. This tone difference is able to throw out popular concert-room
soloists when they come to sing in a cathedral” (Bagenal 220). One key element that an architect would
consider when building a church is that choir music is generally more articulate
than orchestral. “The reverberation with
a full congregation and festival choir in a church should be 2.5 seconds. This is short enough to give distinctness to
instruments and yet allow a full choral tone to voices” (Bagenal 224). Churches contain a difficult choice in
deciding what their acoustical need should be focused towards, but with a
knowledge of acoustical architecture a church can still be structured with
success.
Whether a
structure is built to resonate sound or to absorb it, the design of almost any
building has a form of acoustical architecture to it. The decision process of acoustical architecture includes so many
essential elements, such as what materials to use, the range of sound to be
obtained, and other important questions involving acoustics. Architects not only understand the structure
to acoustics, but they know how sound waves travel. Sound is effected heavily in buildings because of how the waves
are absorbed, reflected, reverberated, diffracted, and/or refracted. After mastering and understanding the world
of sound and how it correlates to architectural designs a successful plan can
be created.
Bagenal, Hope. Planning for Good Acoustics. London: Methuen & Co. Limited, 1931.
“Acoustics.” The New Encyclopedia of Britannica, 1987 ed.
“Acoustics.” World Book, 1978 ed.
“Sound Waves.” The New Encyclopedia of Britannica, 1987 ed.
“Sound Waves” World Book, 1978 ed.