Mitigation recommendations included:
- attenuate radiated compressor noise by erecting a
sound curtain barrier wall,
- control reflected noise from the ceiling by adding a roof to the sound curtain wall,
- reduce reflected noise by adding acoustical absorption to the adjacent
concrete wall and ceiling surfaces,
- construct a dedicated room to enclose the two compressors from concrete block or
stud and gypsum board, and
- relocate nearby employees to a location further away from the
The client elected to implement recommendations incrementally. After the implementation
of recommendations b and c the noise complaints ceased. Sound level
validations were made. eNoise Control found that a 13 dBA reduction had been attained.
attenuation design, sound level measurements, and the effectiveness of
the mitigation will be discussed.
A growing light industrial manufacturer added a second air compressor
when there assembly areas need grew. The new air compressor was located
out of the way of facility traffic in the corner next to the existing
air compressor. The air compressors were the vertical tank type with a 5
horsepower electric motor mounted on top of the tank. The assembly area
was approximately 30 feet away from the location of the air compressors.
Management received numerous complaints from employees in the assembly
area after installing the second air compressor. Although the noise
levels in the assembly area did not exceed
OSHA guidelines, management
was concerned with worker moral and reductions in productivity. This
case study report accounts the experience.
eNoise Control analyzed
the noise sources, and provided environmental
recommendations. Our Acoustical Consultant made several recommendations to the
manufacturer to mitigate the noise, some of which were implemented. The
manufacturer decided to implement the recommendations incrementally.
After two recommendations were implemented the complaints ceased, and
eNoise Control returned to the site to conduct validation
The manufacturer's first concern was to lower the overall noise of the
compressors and provide a safe working environment for their employees.
The manufacturers other concerns were; (1.) ensuring the noise mitigation
items would not impede the proper functioning of the equipment, (2.)
ease of accessibility to the equipment for maintenance and repair, (3.)
that the noise mitigation solution be movable and re-usable, if at a future date, they move the equipment, (4) overall cost
of noise control solution. The design criterion was to
decrease the noise levels
by 10 dBA at 3 feet.
eNoise Control viewed the air compressors and their layout in
relation to the adjacent walls, ceiling structure and the assembly
area. The air compressors cycled on and off due to the variable demands
of the air tools in the assembly area. When work was idle each
compressor would run for a few seconds independently to recharge its
tank. This would occur at irregular intervals. When production was at
full speed, both compressors would run simultaneously to keep up with
the air demands. Unger Technologies conducted
sound level measurements
at both of the described conditions. Measurements were made with a Casella 1/3 octave integrated spectrum analyzer, with an ANSI Type 1
precision 1/2" microphone and preamplifier. Measurement results were
analyzed and compared to the criteria. Unger Technologies determined the
amount of noise attenuation required to meet criteria, and developed
recommendations to provide the prescribed amount of mitigation to the
air compressors. Unger Technologies returned to the site to conduct
observations and make validation measurements.
Two equipment elements; the compressor and the motor, generate the noise
produced by the air compressor. Unger Technologies was unable to measure
Sound measurements were taken both near-field (within 5' of the
compressors) and far-field (30' away at the assembly area). eNoise Control
compared the compressor noise "finger print" near-field using a 1/3
octave band sound level meter and compared this signature to the far-field
readings in the assembly area. We determined that the major noise
source was being generated from the motor and radiated noise from the
compressor tank. The objectionable compressor noise was determined to be
mid-frequency noise at 400-500 Hz range. We presented our findings to
eNoise Control recommended mitigation efforts at or near the source
in order to maximize the attenuation efforts. The primary intent of the
recommendations was to decrease the decibel levels without limiting
access to the equipment. Recommendations included the following in order
Attenuate radiated compressor noise by erecting a
sound curtain barrier wall constructed of a 1” fiber glass absorber, bonded to a 1 lb. per sq. inch loaded vinyl noise barrier, suspended
from a double track system with floor mounted uprights positioned to
form a noise barrier on the two open sides of the corner of the
facility where the compressor is located. This suggestion could
decrease the compressors radiated noise by 10 - 13 dBA.
Control reflected noise from the ceiling by adding a
roof to the sound curtain wall constructed of modular rigid sound
curtain and plywood panels and wrapped with a limp mass barrier
valence blocking the openings in the sound curtain wall system
created by the track and trolleys the curtain is suspended from.
Reduce reflected noise by adding acoustical
absorption to the adjacent concrete wall and ceiling surfaces using
a durable vinyl faced 2” thick sound curtain absorber panels with
fiberglass fill, or melamine acoustic foam, on all interior concrete
wall surfaces within the curtain enclosure.
Construct a dedicated room to enclose the two
compressors - constructed of concrete block or stud and gypsum board.
Implementation of this recommendation should reduce the noise by 20 – 25 dBA.
- Relocate nearby employees to a location further away from the
compressors. Moving the noise receivers to a different area of
the plant would eliminate the noise problem for these employees
The manufacturer decided to implement the first two
recommendations; to enclose the Compressors with a sound curtain wall on
two sides and install a roof over the walls. Validation measurements
indicated 13 dBA of attenuation was achieved at 3 ft. outside of the
enclosure and a 14 dBA reduction at the location of the nearby assembly
workers. Management opted not to build a dedicated room for the
compressors because of its permanence if they decided to relocate the
compressors at a later date. Also, management did not elect to move the
assembly workers because of space limitations and disruption of material
flow within the facility. No absorption was applied to the interior
walls of the enclosure. The noise complaints from the employees ceased,
and management had achieved its noise reduction goals.