Noise is pollution...,
...an unwanted byproduct of almost any mechanical activity.
We can be thankful that most mechanical processes are relatively inefficient
noise generators: mechanical/acoustical efficiencies range from 1 part in 1,000
(high speed air flow) to 1 part in 10,000,000 (mechanical equipment with heavy
casings).
Noise can be controlled at the source, along the path, and at the
receiver.
Source noise control aims to reduce sound power
through improved mechanical design. The input power and/or the
mechanical/acoustical efficiency are reduced. Path noise control aims to reduce sound energy
by re-directing or absorbing the sound after it is released into the
environment. Tools for this task include barriers, walls, enclosures,
sound absorbing materials, mufflers, etc. -
Receiver noise control aims to limit the
disturbance at the reception point. Typical applications here include
operator enclosures and hearing protection.
Where feasible, Nelson Acoustical Engineering
recommends noise control at the SOURCE.
Projects
A large manufacturing plant with 100+ acres under one roof required comprehensive
noise control recommendations to reach the corporate goal of 85 dBA
time-weighted average (TWA) throughout the plant. Individual noise
sources were located, measured and ranked. Employee locations were also
identified. A mathematical model of noise emission and propagation in
the space permits sound level estimates. Combined with installed cost
estimates, a priority ranking is assigned for each noise source.
 The graphic
at right shows a noise model generated from actual noise emission
data. The diamonds indicate the location of noise sources, and the
circled numbers indicate the number of employees stationed at that location.
Double-click to see a full-size image.
A major manufacturer of Internet hardware desired to develop its own
vibration testing capability, and chose a location within a mixed-use building
also housing office employees and other vibration-sensitive measurement
equipment. The vibration test equipment includes high-amplitude shakers and seismic shakers.
Extensive noise control efforts were recommended for the walls and ceiling
of the vibration laboratory to minimize interference with work activities in
nearby spaces.
A model of the foundation and structure was developed to estimate vibration amplitudes throughout the
structure. Amplitudes were compared to criteria related to sensitive
structures, comfort of office personnel, and audible noise in developing
recommendations for an aggressive foundation design.
 The graphic
at right shows a segment of the special "disco" ceiling
design used in this project. Double-click to see a full-size image.
NASA Glenn Research Center has historically
performed research on aerospace propulsion systems.
Along the way it has been necessary to design some very unique and
very large gas flow experiments. The Design Guide provides guidelines
for low-noise gas flow equipment design and an Excel™ workbook for
estimating noise emission from gas flow equipment. Please note that the
criteria used in this workbook are optimized for use at NASA GRC.
 The graphic
at right depicts a screen shot from the
workbook. Double-click to see a full-size image.
Many manufacturing operations are in the
habit of using compressed air to blow water, dust, and debris off of
workpieces. Unfortunately, compressed air is very noisy. Low-noise
nozzles exist that reduce the levels by 10 dB(A) or better, but in some cases
the amount of air is so prodigious that even the ganged low-noise nozzles are
too loud. In this case, a method was developed for the client that
demonstrates how to design blower-based systems that deliver the same airflow
as the present system with significantly reduced noise.
Example Equations from
Engineering Study:
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