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BOUNCE
Aside from structure
born sound, sound travels by three paths; 1) direct, 2)
reflected, and 3) diffracted. The reflected
paths that sound travels by are the bounces that sound waves take when they
impact hard surfaces. If that surface is soft, it responds to that impact and
absorbs the sound or a portion of it. Throw a ball (a tennis ball, not a
bowling ball) against a brick wall. Now throw the ball against a pillow. Get
the idea?
When sound reflects from the interior surfaces of a space,
two things happen:
1) The space gets louder, and
2) the sound becomes distorted.
Both of these
simultaneous occurrences are usually not desirable.
ABSORPTION
Sound is a form of energy and, you
can't get rid of energy. Energy is either dissipated because it spreads out or
is converted into another form of energy or matter. Acoustically absorptive
materials actually convert sound energy into mechanical energy - wiggles. When
sound waves (fluctuations in air pressure) impact a material that is
acoustically absorbent, that material responds - moves. The ability of a
material to absorb sound in this manner is measured in a test that provides a
NRC (Noise Reduction Coefficient). This is the simple mathematical average of
absorption at four frequencies: 250, 500, 1000 and, 2000 Hertz (Hz) also called
cycles per second. These four frequencies are the ones we hear the best and are
generally those most dominant in the sounds created in the real world.
So, a material with a NRC of .80 absorbs an average of 80% of the sound
that impacts it. Whether this is good or bad depends on the acoustical need.
Certain surfaces should reflect sound. A band shell or the angled ceilings and
forward sidewalls of an auditorium are supposed to reflect and project sound
away from the stage or podium. The rear wall, on the other hand, should absorb
sound.
DEALING WITH REFLECTED
NOISE
ARCHITECTURAL ACOUSTICS:
Modifying sound levels to make them compatible with the intended use of the
space. One size does not fit all. Most of the spaces that the facility manager,
architect or engineer deals with should be quieted. There are also spaces where
it does not much matter one way or the other.
The following are
examples of space where excessive noise may be a problem and how to deal with
it:
OPEN PLAN OFFICE SPACE - Because there
are no full height barriers, there are many reflected paths that sound can take
to travel between work stations and other adjacent space. An expansive
discussion of these types of spaces can be found at
OPEN PLAN OFFICE ACOUSTICS. Perimeter walls
and storage (files, shelving, etc.) that provide reflective surfaces should be
treated with acoustical wall panels.
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Acoustical panels on filing add a unique
functional design element. |
Before - block walls in office space reflect
sound. |
After - acoustical panel appliqués provide for a "softer"
look while reducing noise reflections. |
LARGE OPEN AREAS - Spaces such as cafeterias, atriums,
multi-purpose space, etc. also tend to be a problem. They are just too loud and
hence, unpleasant. This may or may not interfere with the use of the space - at
least some of the time. The noise from these spaces can spill over to adjacent
areas where quietness is important.
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Acoustical wall panels reduce noise levels in common
space to add utility for dining and other functions while prevent it from
spilling over into adjacent workspace. |
Again, the
ceiling presents the best opportunity to reduce noise levels.
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| Custom panel array corrects the problems caused by a
hard surface ceiling. |
Custom pattern of acoustical ceiling panels enhances
design while reducing noise levels and correcting distortion. |
Where a
suspended ceiling is not practical or in keeping with the design of the space,
suspended banners or baffles may be employed. Acoustical panels may be surface
mounted to hard ceilings. Perimeter walls should also be treated with
absorptive panels.
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Array of acoustical panels provide
for the performance of a highly absorptive suspended ceiling without the
expense of adjusting lighting, sprinklers or air diffusers. |
CLOSED OFFICE SPACE - Sound (noise to everyone else but
the people that created it) that enters or is created within an office is made
louder if it can bounce around. Acoustical wall treatments can serve to correct
this and may provide a cost efficient alternative to creating a tighter
envelope of walls and ceilings. There are also corollary advantages (looks
good, makes speaker phone conversations more intelligible).
OTHER
NUISANCES:
Copy
rooms, with or without doors (which are never closed anyway), broadcast sound
out into office space or corridors leading to office space. Acoustical wall
treatments reduce the noise level in the copy room and should be installed on
the on the corridor walls outside these rooms which act as conduits for sound.
Mechanical rooms, equipment rooms, etc., even when doors are tightly
sealed and other violations of their envelope are blocked, often disturb
adjacent space. Absorbing, thus reducing, noise levels in these rooms with
absorptive industrial grade (cheaper) finishes can reduce the noise level by as
much as 50% (10dBa). This is a lot easier than adding to the mass of the walls
and ceilings of such space.
REVERBERATION
When sound bounces
around it not only gets louder, it also becomes distorted. Announcements in the
airport, an arena, or other similar space are good examples of what kind of
problems excessive reverberation can cause. The "old railroad station effect"
if you will.
This is caused by sound bouncing repeatedly off of
ceilings, walls, floors and whatever else there is that doesn't absorb it.
While you are hearing the first syllable of a word, a specific note of music or
other sound, you are also hearing that which preceded it. They overlap - echo.
Sometimes many times before they become inaudible. You hear a lot, but
understand little. In large space the effect is obvious, in smaller space, less
so.
The measurement of reverberation in a space is called reverberation
time (RT). It is the amount of time it takes for a sound to diminish (decay) 60
decibels. The longer the time, the more distortion. In certain environments,
such as concert halls and recital rooms, a certain degree of reverberation is
desirable.
In many instances, degrees of reverberation that are not
perceivable to the occupant of the room do cause problems with recorded or
tele-transmitted sound. The section on VIDEO
CONFERENCING - TELE CONFERENCING addresses this in more detail.
The
introduction of absorptive materials will also provide for an acceptable level
of RT for any space - large or small. Again, the function of the space dictates
what the RT should be.
In some instances the use of acoustically
reflective surfaces is essential. Auditoriums, for example, are designed to
reflect and project the sound from the stage or podium to the audience. The
section on AUDITORIUMS, MEETING AND
TRAINING ROOMS addresses these issues.
When absorptive materials are
introduced into a space, that space gets quieter and the "echoiness"
diminishes. Calculating the correct amount of treatment and its placement can
be a little more complicated than would be apparent to the casual observer.
Qualified suppliers or consultants can provide acoustical design support -use
them.
Specific tutorials with
graphics and/or photos can be accessed by clicking on the following
topics:
 OPEN PLAN ACOUSTICS
OVERALL NOISE LEVELS
VIDEO CONFERENCING – TELE
CONFERENCING
CALL CENTERS
NOISE ISOLATION
HVAC NOISE
AUDITORIUMS, MEETING AND TRAINING
ROOMS
CORPORATE DINING FACILITIES
Topics in other
sections address acoustical issues in facilities such as medical, fitness, and
dining. |
