The Consequences of ‘Leaky’ Enclosures
David C. Byrne, National Institute for Occupational Safety and Health

Pittsburgh, Pennsylvania

From an engineering perspective, an
ideal industrial noise control solution
focuses directly on the actual source of
the noise. Eliminating the noise-generat-
ing mechanism altogether obviates the
need for other noise control treatments or
hearing protection devices. However, in
cases where attenuating the source is not
feasible, engineering controls must be
oriented toward blocking the path that
the sound waves travel toward employ-
ees. Acoustical enclosures are commonly
used as sound path treatments to contain
the noise from a machine; alternatively,
a control room/booth or equipment
operator's cab may be used to isolate the
worker from the noise.

Anyone who has successfully used
acoustical enclosures knows that the de-
sign, procurement, and installation pro-
cess is deceptively simple. All too often
first-time efforts fail to account for many
of the constraints that can render the en-
closure essentially useless. Things that
cannot be overlooked include: providing
convenient worker access (physical and
visual); allowing for proper machine op-
eration/product flow; and supplying
fresh air or preventing undue heat or con-
taminate build-up inside the enclosure.
All of these factors must be given careful
consideration, otherwise the enclosure
will not perform adequately.

Enclosures that come as “standard
equipment” or are an integral part of the
machine are usually accepted by workers
and usually work quite well. On the other
hand, employees may reject the addition
of a retrofitted enclosure, and intention-
ally defeat its purpose if it hinders pro-
duction or interferes with normal work
processes in any way. Enclosures are
commonly dismantled and inadvertently
(or perhaps purposely) reassembled in-
correctly, leading to a severe degrada-
tion in their acoustical properties. Inter-
lock switches may be intentionally
disabled to allow the equipment to oper-
ate without all doors and access hatches
being securely closed. Sometimes main-
tenance personnel don't even make an at-
tempt to put the enclosure panels back
together at all.

Perhaps the most important consider-
ation that is overlooked when consider-
ing a machine enclosure or operator
booth/cab is the adverse effects of a
‘leaky’ enclosure, i.e., an enclosure with
a large number of openings or unsealed
penetrations. These openings are usually
referred to as “flanking paths,” which are
defined as any route by which sound
travels from one side of a barrier to an-
other, other than the sound that goes di-
rectly through the wall or barrier itself.

There is a very rapid deterioration in the
amount of achievable noise reduction if
even only small holes are present in an
otherwise solid enclosure. This effect is
not necessarily intuitively obvious and
has the tendency to be overlooked; how-
ever, it will become readily apparent af-
ter installation when the actual attenua-
tion doesn’t come anywhere near what
was expected. Unfortunately, there are
numerous examples of “home-made”
equipment cabs/enclosures that are es-
sentially worthless, although some come
close to being effective but could be much
better if there weren't so many openings
left in them.

Figure 1 illustrates the effect different
sizes of openings have on the actual
amount of attenuation provided by an
enclosure. The straight line depicts the
condition where all walls are solid and
there are no openings. Obviously, in this
case the potential transmission loss of the
enclosure equals the actual transmission
loss achieved. Four ‘leaky’ conditions are
also shown, representing the effect of a
very small (0.1%) opening to a 20% open-
ing. The arrows on this graph point out
an example where a mere 0.1% opening
degrades the attenuation of an enclosure
by 10 dB. This graph also illustrates the
fact that an enclosure wall with a low
transmission loss value to begin with will
be less affected by unsealed openings
than a barrier with a high transmission
loss.

10
Potential Transmission Loss, dB

Figure 1. Effect of leaks in an acoustical enclo-
sure. Example - the transmission loss of a wall
with a potential TL of 39 dB will be reduced to
29 dB with a leak equal to only 0.1% of the
total area of the enclosure.

Figure 2 shows an equipment operator
cab that has many potential problems
from an acoustical point of view. The
door seals are either damaged or missing.
Although it is difficult to observe from a
photograph, there is a slight opening
around each lever, gauge, and foot pedal
whereby sound can leak in. Also, with-
out any internal absorption in the cab the
interior becomes a reverberant environ-

Figure 2. Equipment operator cab with acous-
tical problems.

ment, which contributes to the noise
problem.

Figure 3 shows an example of a well-
designed operator cab. The doors and
windows close securely. Also, a flexible
barrier material or caulking was used to
seal around all controls, allowing ad-
equate movement of levers while main-
taining the overall integrity of the enclo-
sure. In other situations, equipment
manufacturers employ electrical controls
rather than mechanical ones to minimize
the number of moving parts that must
pass through the enclosure wall into the
cab.

Whether purchasing an off-the-shelf
enclosure or custom building one, it must

Figure 3. A well-designed operator cab


seal absolutely air-tight to provide maxi-
mum effectiv: ;. One method to en-
sure that this will be achieved is to imag-
ine that the enclosure will be used to hold
water. Remember that any place where
water can flow out represents a potential
sound leak. If the enclosure passes this
imaginary leak test, then it is more likely
to deliver the amount of attenuation it
was designed to provide. Also, a “light
test” may be used where someone sits
inside a darkened enclosure and looks for
light entering around doors or other
openings. Any glimmer of light indicates
a gap that needs to be sealed. A dollar bill
or other slip of paper is often used to
check the effectiveness of a door or win-
dow seal. If the bill is inserted between
the door and the seal as it is closed, it
should not be able to be removed if the
door gasket is adequate.

Of course, for production reasons it is
not always practical to completely seal an
equipment enclosure. When it is neces-
sary to have openings for product flow or
ventilation, a silencer, sound trap, or an
acoustically treated chute should be
used. A common mistake is to neglect
items such as conveyors, which must be
enclosed for at least a portion of their
length immediately before entering and
after exiting the enclosure. Therefore,

some creative design work may be neces-
sary to minimize any open areas and
maintain acoustical integrity of the enclo-
sure. This is a common situation encoun-
tered in industrial noise control; i.e., no
single approach is applicable in all situ-
ations, and the most practical solution
combines the exact science of acoustics

with all production, maintenance, and
economic constraints.

The Effect of Not Wearing
Your Hearing Protector

HPO with labeled NAR of 30 dB
HPO with labeled NAR of 25 dB
HPO with labeled NAR of 20 48
HPO with labeled NAR of 15 dB

0 10  % 4 8 60

‘Time HPD Not Worn, min

Effective NRR when an HPD is not worn for
the full 8-hour shift

It is important to be aware of the
drastic reduction in the amount of pro-
tection afforded by an earplug/earmuff

minutes during an 8-hour workshift.
The accompanying graph illustrates
this effect for Hearing Protection De-
vices (HPDs) with four different Noise
Reduction Ratings (NRRs). Assuming
workers are exposed to a constant
level of hazardous noise throughout
the workday, there is a surprising drop
in the effective NRR provided by any
particular HPD after only 10 minutes
of non-use. Further, if the protector is
not used for 60 minutes per shift (i.e.,
it is worn for seven of eight hours)
then the effective NRR of nearly all
HPDs is reduced to nine decibels or
less, regardless of the labeled NRR.
Therefore, it is not sufficient to wear
your hearing protection most of the
time — you must wear it all of the time
to adequately protect yourself from
noise-induced hearing loss.

if it is removed for even just a few