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Clean Room Environments
Noise Control for
Clean Room Environments
By Larry Hansen, Aeroacoustic Engineering Consultants, LLC,
Reprinted from HYBRID CIRCUIT TECHNOLOGY, 11/84, Copyright
1984, Lake Publishing Corp.
Libertyville, Illinois, USA
As if the control of industrial noise was not difficult
enough, clean room environments impose sanitation constraints
diametrically opposed to the basic construction of noise control materials
and systems. Historically, the generic properties of acoustic materials
were permutable combinations of fibrous batts, perforated metals, and
porous substrates. Of course, any material which would generate
particulate matter, harbor micro-organisms, or create stagnant air pockets
was the bane of a contamination control system. The net result was a
relegation of noise control to a very low priority and clean room
personnel had to live with an unacceptable or marginal acoustic
With so much attention focused on the need for aseptic
acoustics, a number of products emerged which specifically addressed the
peculiar needs of clean room environments. Amazingly, many of these
products and systems not only exhibit superior noise control capabilities,
but also enhance the ability of all clean-air
components in the facility to achieve and maintain the desired level of
The solution to any problem begins with the
identification of its basic factors; clean room noise is no different. The
acoustic elements may be reduced to source, path, and perceiver.
Noise can reach a listener's ears via several routes.
The most obvious route for noise sources within a workspace is the direct
path, a straight line through the air from source to perceiver. The
acoustically hard surfaces found in clean rooms make reflections from the
ways, ceiling, floor, or any solid object contribute as much or more to
the acoustic ambiance than the direct path. The persistence of sound in an
enclosed space, as a result of multiple reflections after the sound is
stopped, is called reverberation
This phenomenon is illustrated in figure 1.
Also, when acoustic waves are propagated through solids
and air, they may travel a direct route through floors and walls and
arrive at the perceiver after re-radiation, as shown in figure 2.
Generally, the most cost-efficient and dramatic means
of reducing noise levels is to reduce the acoustical output of the source
itself. This approach generally will require major modifications, tighter
quality control, tighter tolerances on moving parts, more sophisticated
balancing or rotational mechanisms, and sometimes a total redesign of the
technique utilized to perform the task or tasks for which the equipment is
intended. Vibrating surfaces will cause compression and rarefaction of the
air which is perceived as sound, and any of the above mentioned
modifications to a
Figure 1, clean room reverberation compounds noise problems with
sound source are aimed toward reducing the vibration of
any component to its lowest possible level. Normally, these modifications
are not within the capability of the user and must be left to the
At the other end of the noise control path is the
perceiver. The most expedient method of controlling noise exposure at the
perceiver's end usually means removing the affected per-son from the sound
field. When this cannot be done, the alternative is to have the person
wear personal protective devices. This process is actually considered a
control on the path of the noise.
A dramatic reduction of source noise may be achieved
with total enclosures and partial barriers. Common industrial acoustic
materials are unsuitable for sanatized environments due to their
construction of fibrous material and perforated facings. Assembled
structures can promote the growth of micro-organisms and cause a
genuine hardship for the cleaning crew. Once assembled,
these enclosures, typically eight pounds per square foot of wall and
ceiling, are difficult if not impossible to move for equipment
Acousticians and material engineers have collaborated
to develop noise control products from light weight plastics with
acoustical resonance qualities. Environmentally stable, these systems are
chemically resistant and may be repeatedly washed or steam-cleaned for
sanitation purposes. Formed from transparent, rigid plastic sheets, there
are no cracks or crevices for particulate accumulation. A system may be
made hermetically tight for total mist and fume containment. A sterile
environment may be maintained within the sound enclosure for cost
effective-ness. Then the entire exterior area would not be subjected to
clean room standards.
Larger structures may require an integral support
frame. The use of aluminum rather than steel in the construction of these
frames is recommended for two important reasons. First, treated aluminum
does not give off the numerous oxide compounds associated with steel.
Second, aluminum is considerably lighter, and much easier to handle and
install, resulting in overall cost savings and portability of the system.
However, the elements should be treated with an iridite finish before they
are painted to arrest the growth of the single oxide associated with
Combining the above features, a system may be developed
which not only reduces source noise but also may improve the sanitary
atmosphere surrounding a process. A clean room compatible design featuring
portability for time-sharing among noise sources is pictured in figure 3.
Many times a clean room will
Figure 2, typical paths of noise propagation and re-propagation
Figure 3, complete portability allows the noise containment system
to be time-shared or quickly moved to meet changing production needs
while providing a clear view of the process.
Figure 4, major source of fan and blower noise
Figure 5, all air movers generate a modulated air stream
exhibit an unacceptable sound pressure level even
before the first piece of processing equipment is brought on line. For
industrial usage, filtered airflow velocities of 40 feet per minute or
higher are frequently used to minimize the effects of lateral dispersion
of particulate and matter. This means large air moving devices, and large
air movers mean big noise. Major noise components associated with air
sys-terns and air stream sources are identified in figures 4 and 5,
Noise generated from dynamic un-balance and
repropagated shell noise can be reduced using conventional vibration
isolators and acoustic materials, since the equipment is usually outside
the clean air loop. However, the noise modulated air stream is a different
situation. Air noise cannot be ex posed to sound absorbing, fibrous
materials due to the inherent danger of particulate entrainment and
subsequent clogged HEPA filters and room contamination. Fortunately, air
noise can be handled effectively in an aseptic manner.
Since air duct interiors are safe from physical abuse,
faced or encapsulated absorbers make excellent linings in sound traps.
Linings are more acoustically efficient if located at transition points
along the duct, for instance, at a right angle bend or drop.
An effective alternative is a broadly tuned, Helmholtz
resonator at the in-let, and discharge of the fan or blower. A direct
antithesis of the Helmholtz phenomenon is blowing across the neck of a
partially filled bottle to produce a tone. If noise
modulated air is passed by a properly tuned cavity,
sound is actually absorbed at a specific frequency. When a number of
differently tuned resonators are ganged together, a broader band of noise
is absorbed. Since there is no fibrous packing in this silencer, there is
little chance of contamination.
An augmentation of the duct treatment is a plenum
chamber as illustrated in figure 6. In some installations, especially
where more than one fan or blower is used, a plenum enclosure may be a
utilitarian choice for sound suppression. A plenum's acoustic performance
will be greatly increased (by approximately 10 dB in some frequencies) if
the interior surfaces are lined with encapsulated or faced absorbers. If
duct linings or silencers are incorporated with a lined plenum chamber,
ventilation noise, for virtually any size air system, would be
The perceived noise in the workspace is the last
consideration. Acoustically hard surfaces in a clean room will resemble a
reverberation chamber, creating spatial sound problems even if the area is
noise-free. Sustained sound levels garble verbal communications and
support an un-comfortable environment.
An acoustic ceiling, thick carpeting, and wall
coverings ordinary would solve the problem. However, this solution would
be unacceptable for a clean room. New advances in film technology have
produced. Encapsulating materials which are mechanically rugged and nearly
transparent to a sound wave. Encapsulated absorbers may be incorporated as
ceiling panels as in
Figure 6, multiple blower systems held
in check by sound absorbing plenum.
figure 7, free hanging baffles, or sealed to walls out
of the way.
Encapsulated products are most frequently pressed into
service as component parts of a ceiling system. An example of the noise
reductions afforded by such a system is evident in table 1. Note the
example is for a single frequency. To calculate the anticipated dBA
reduction for the space under study, the computation would have to be
repeated for each octave frequency of concern, dBA weighting factored in,
and logarithmically sum-med.
Reverberation control in closely packed machine
environments can be an effective noise control measure. In spaces far from
TABLE 1 - CEILING SYSTEM NOISE
|Acoustic Control Example:
Encapsulated ceiling panels:
Class 10,000 Manufacturing Area: 70’ long x 120’ wide x 12’ high:
||Plaster on Concrete
|Windows & Lights
|Production Equipment &
A1 = 391.69
Addition of acoustic Ceiling Panels to entire ceiling area, except
light fixture area
|Windows & Lights
Equipment & Working surfaces
Minus wall area lost from installation
|Reduction in Decibel = 10 log10 (A2/A1)
This example = 10 log10 (7958.01/391.69)
= 13.08 dB
The addition of acoustic Ceiling Panels creates a 13.08 Decibel
Figure 7, encapsulated acoustic ceiling panels can enhance clean
room light levels while providing years of trouble free service.
sound source, approximately 20 feet or further, the
addition of absorbent materials can reduce the reverberant build-up by 10
dB or more. Often these remote areas are highly populated and will afford
sufficient noise reduction without further engineering controls or
personal protection. It should be noted, however, that usually little
significant reduction is achieved within 10 feet of the source, which
commonly includes the operator.
Aseptics and acoustics need not be mutually exclusive
pursuits. Noise control systems can function in clean room environments
while providing many years of trouble-free service, creating a pleasant
working atmosphere conducive to maximum efficiency.
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