Daily Number of
Maximum Allowable
Exposures
SPL
1
105 dB
4
100 dB
8
95 dB
Variations to this table should be based on more detailed information and
BUMED should be consulted.
c.
Noise Control. Little detailed guidance can be given in the
control of gas flow noise in hyperbaric chambers. Because of the hazards of
fire, selection of sound absorbing materials become a problem. However,
three noise reducing techniques that can be used aggressively are summarized
briefly:
(1) Minimize Air Flow Rate. Acoustic power in an air-flow system
varies as the fifth power of the flow rate. For example, if an air-moving
system delivers just 10 percent more air flow than required, the overall
sound level can be expected to increase by 60 percent. Therefore, it is
necessary to minimize the excess air flow in a pneumatic system if the sound
level is to be kept within acceptable limits.
(2) Minimize Resistance to Air Flow. Air flowing around or against
surfaces produces turbulence, and turbulence is a potential source of sound,
The turbulence energy content determines the magnitude of the sound.
Turbulence can be minimized by:
(a)
Using short, streamline flow paths.
(b)
Using large cross sections.
(c)
Minimizing abrupt discontinuities and changes in flow
path.
(d)
Eliminating flow path obstructions.
(e)
Keeping boundary surfaces smooth.
(f)
Using turning vanes.
(3) Minimize Line-of-Sight Transmission. The radiative sound path
from the noise source to the surrounding air should be made as indirect as
possible, and barriers in the line-of-sight path should be as massive as
practical. This approach could include the construction of a plenum chamber
exterior to the hyperbaric chamber for particularly high air flows. The
designer should also investigate available mufflers, which can reduce noise
levels appreciably. Caution should be exercised however, as muffler fillers
cannot be either flammable or toxic.
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