Virginia Tech, Blacksburg, VA
Ft. Defiance High
Ft. Defiance Gym
Emerald Hill Elementary Gym
The following are some of the most commonly asked questions we get on
Architectural Acoustics. The
FAQ link on our Links-Educational Sites page will take you to a broader FAQ file on acoustics.
What does a sound absorbing material do?
How do walls block sound?
Why is my church fellowship hall so noisy?
Why do the footsteps of my neighbor upstairs sound like an
Why is my child's classroom so noisy they cannot hear
My child's classroom is quiet, but they still cannot understand
the teacher. Why?
We just had a new sound system installed, but there are
dead spots in the room, and places where words are not understood. What
does a sound absorbing material do?
The term "acoustical material" is usually applied to a material that absorbs
a majority of the sound impinging upon it. These are used to prevent undesirable
sound reflections. Such reflections could result in clearly perceived
echoes, slightly delayed echoes that reduce the clarity of speech, increased
noise levels, or excessively long reverberation.
Sound absorbers are usually porous materials that turn sound into heat.
These absorb primarily the higher frequencies unless the material is very
thick. At thickness of one to two inches is practical to absorb mid-frequencies
well. Examples of these materials are fiberglass, mineral fiber, open-cell
foams (not Styrofoam), shredded wood fiber, un-painted porous concrete
block, heavy drapery in folds, and carpet. Vibrating panels and tuned
cavities are also used to absorb low frequency or bass sound. A gypsum
or wood-panel wall with an air cavity, or a suspended ceiling can be a
good bass absorber. Some fiberglass wall panels have high-density surface
layers that improve bass absorption. Special concrete blocks with slots
to their cavities are the most common cavity absorber.
Our ears expect most rooms will have more absorption at higher frequencies.
However, if the difference between the absorption at high and low frequencies
is too much, the room will sound "boomy." This often occurs with thin
porous materials applied to hard surfaces. One must carefully select materials
to balance the absorption. This is especially important in rooms that
will be used for music, or where very natural sounding speech is desired.
Most, but not all, suspended acoustical ceilings are better balanced than
most wall treatments. They achieve good bass absorption acting as a panel
with an airspace above. One way of balancing a room is to use a combination
of materials with opposite characteristics.
The performance of an acoustical material is measured by the Sabine absorption
coefficient. This will vary with frequency and with the thickness and
mounting of the material. Thicker porous materials and those mounted with
an airspace behind them will have more absorption at low frequencies.
A widely publicized rating is the mis-named "Noise Reduction Coefficient"
or NRC. This is the average absorption at 250, 500, 1000, and 2000 Hz.
It can be very misleading. Two materials can have very different characteristics
producing very different results but a similar NRC rating. NRC results
also can be influenced by tuning systems to be absorptive only at the
frequencies in the average, and not in the broad range in between.
Most sound absorbing materials are not good sound blockers. However, the
addition of a sound absorbing material inside a cavity of a sound-blocking
wall can improve its sound blockage ability. Back to Top
do walls block sound?
Most solid walls with no open leaks will provide 40 to 50 dB of blockage
for middle frequency sound. This is somewhat amazing if you consider that
40 dB amounts to blocking 99.99% of the sound, and 50 dB amounts to blocking
99.999%. With special care, blockage up to 60 dB or more can be achieved.
However, success beyond 50 dB depends on careful construction, and is
often limited by flanking or sound leakage by paths around the wall, door
or window. A small leak can have a major effect on a wall with otherwise
Higher frequency sound is easier to block than low frequency or bass sound.
The bass sound has longer wavelengths. The wall looks thinner to the low-frequency
sound. Walls are rated in their ability to block speech or speech-like
sound by the Sound Transmission Class (STC). Their blockage of bass sound
will be less than the STC. The wall with the higher STC may actually be
a poorer blocker of bass sound.
The major factor that influences the blockage of the structure is its
weight. However, the efficiency of the material used can be improved by
separating it into isolated layers. The more separation space provided,
the greater the increase in blockage. For instance, suppose you have a
layer of material that by itself blocks 20 dB. If you double the thickness
of the layer, the blockage is improved by about 5 dB at most frequencies.
However, if you use two layers well separated from each other, the blockage
can be increased 20 dB or more at some frequencies. The problem is in
establishing good separation, especially for lower frequencies. The improvement
for bass sound may be only about 5 dB. To achieve the best improvement
with isolated layers, the cavity between layers must contain sound absorptive
material, and the connections between layers should be flexible. Light
gauge steel studs provide better isolation than wood studs. Resilient
channels can be added to one side of wood studs to increase the flexibility.
Two sets of studs can provide even better results even if wood.
All solid materials will have a weak point at a "critical" frequency.
This frequency is lower and thus more of a problem for thick, stiff materials.
The critical frequency of a thin, limp sheet of lead is very high. The
critical frequency of many common materials is in the frequency range
of speech. The effect of the critical frequency can be reduced by adding
a vibration damping effect. An example is the laminating layer in safety
glass. Another way is to use several layers of thinner material to raise
the critical frequency. Using layers of different thicknesses assures
that one layer will be strong at the critical frequency of another.
Structureborne sound, such as footsteps on a floor above, constitutes
a more difficult problem. The structure itself becomes the noise source,
rather than a blocker of the noise. Other examples of structureborne sound
are plumbing and mechanical equipment that is not isolated from the structure.
Sometimes very loud airborne sound can excite a structure (such as a side
wall or roof) and become structureborne around a good wall.
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is my church fellowship hall so noisy?
The church fellowship hall that is overly reverberant and noisy is the most
common single problem we face. These rooms are usually used for social or
dining functions when many people are speaking at once. Other times they
may be used for recreational activities by young people who are loud. Sometimes,
they include gymnasium facilities. Unfortunately, they are often built with
all hard surfaces.
These rooms should be treated with a significant amount of sound absorptive
materials to control sound reflections and noise. A suspended acoustical
ceiling is often the most economical solution in new construction. If the
ceiling is low, it should be highly absorptive. If it is high, it needs
to be only moderately absorptive. However, some wall treatment is often
needed in combination with high ceilings. In some existing rooms it is difficult
to treat the ceiling. Wall treatment alone can help, though there will not
usually be an ideal result. Wall treatments and materials added to the ceiling
after initial construction are usually more expensive than ceiling treatments
installed in initial construction.
There are a variety of materials available to treat existing rooms. They
satisfy differing needs for durability, appearance, cost, and acoustics
that result from different room uses, budgets, goals, and existing architecture.
These include materials applied to or hung from the ceiling and materials
added to walls. There are other options available for use in initial design
that are not easily used later. We can even have a wood ceiling that is
absorptive if that is desired. Carpet on the floor can be part of the solution,
but is rarely a complete solution or a necessary part of the solution. Back
do the footsteps of my neighbor upstairs sound like an elephant?
Many if not most new apartments and condominiums are built using wood
frame construction. Wood structures are inherently flexible. Floors can
easily flex and vibrate at low frequencies. There is no known way to eliminate
the characteristic "thump" of such floors. Carpet or special materials
under hard floor surfaces can eliminate "tapping" sounds. Careful construction
can eliminate squeaks and pops. However, a degree of the "Thump" will
always be with us. Back to Top
is my child's classroom so noisy they cannot hear the teacher?
A number of factors have influenced the recent design of school ventilation
systems to make them very noisy. Fans should never be located in a classroom
or over the ceiling of a classroom, but they commonly are. Many classrooms
are too noisy even for adults with good hearing and language comprehension.
Young children, anyone with hearing difficulty, and people who are listening
to a second language need quieter conditions. The key is a well-designed,
quiet ventilation system. Back to Top
child's classroom is quiet, but they still cannot understand the teacher.
Even with quiet ventilation systems, it is difficult to understand in
many classrooms because they are too reverberant. The extremes are rooms
with high and hard ceilings. These were common many years ago. Unfortunately,
there has been a recent return to this practice by some who believe that
high ceilings with skylights are the key to a good classroom. Classrooms
should be designed with low, highly absorptive ceilings. Otherwise, significant
amounts of absorptive wall treatment should be used. Recent research indicates
that young children, people with hearing difficulties, and those listening
to a second language need less reverberation than has traditionally been
considered acceptable. Back to Top
just had a new sound system installed, but there are dead spots in the
room, and places where words are not understood. What is happening?
Unfortunately, many firms that install sound systems do not fully understand
how the system can interact with the room, or even how multiple loudspeakers
can interact with one another. Sound from the loudspeakers could be reflecting
from the walls and reaching your ears with a long delay after the initial
sound has reached you directly from the loudspeaker. If you have multiple
loudspeakers that are not close together, you could be hearing the sound
from the different loudspeakers with a time delay. Sound travels faster
as an electronic signal in a wire than as in a wire than it does through
the air. Thus, if there are extra speakers near the rear of the room,
the sound from these must be delayed electronically so you hear it at
the same time you hear sound from the speakers near the front of the room.
Another problem can occur because of interaction between the sound coming
from two speakers very close together as in a cluster. At certain locations,
sounds at certain frequencies from the two speakers will combine in a
way that they cancel each other. We commonly find these problems and recommend
that clients choose sound-system contractors and designers very carefully
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