Environmental Applications
Community Reaction To Noise
Listed below are some of the key factors
which can reduce the community tolerance
level for noise in environmental applications.
- Where there are exceptionally low background
ambient noise levels.
- A noticeable fluctuation in sound level
which would call attention to the source.
- Pure tones or discrete frequency sounds
regardless of the overall intensity.
- Elevated noise sources such as vents,
stacks, outdoor cooling towers and other
clearly visible noise sources.
- Any noises that disturb or interfere with
sleep, communication or recreation.
- ntermittent, impulsive or startling noises.
- Low frequency sound which causes vibrations
in windows, walls and other parts of
building structures.
- Distracting noise sources, such as breaking
of glass at a bottling plant.
- Any changes in noise patterns.
Predicting Acoustical Barrier
Wall Performance
The nomogram at right can be used to describe
acoustical barrier sound attenuation. Transmission loss or
sound blocking through a freestanding partition or barrier
wall will be determined in part by the acoustical properties
of the barrier. The second factor affecting barrier
wall performance is spillover noise following the diffracted
path as illustrated in the figure at right. Sound waves
will have a tendency to bend or diffract over the top and
around the sides of a barrier wall especially in the lower
frequencies. In the higher frequencies sound waves
diffract less and are much more directional in nature. The
shielding effect of the acoustical barrier and resultant
noise shadow area beyond it are determined by the
geometric relationship between the source, the receiver
and the barrier height.
How To Use
The Nomogram
In the figure at right, distances A, B and D should be
determined as follows. Distance A is from the point noise
source (not the height) of the equipment to the top of the
acoustical barrier. Distance B is from the top of the barrier
to the receiver position (figure ear/head level). Distance
D is from the source to the receiver (straight line). In the
example at right the path length difference (A+B-D)
equals 2 ft. Plotting a straight line from the path length
difference through the frequency of noise in question on
line F (1000 Hz) intersects the dB line at 16 in the example.
Thus the estimated attenuation for this application
would be 16 dB. Please note that the nomogram does not
take into consideration the contribution from reflective
surfaces. To be conservative in applications where reflective
surfaces are present it is recommended that the final
dB figure be discounted 20% to 25%. As the angle (€)
between the direct and diffracted paths increases, so does
the noise reduction.
Predicting Community Reaction To Noise
- Plot octave band sound pressure levels on
Figure 3 at each frequency 63Hz to
8000Hz.
- Determine the value of N where the plotted
data intersects the highest curve.
- Determine the sum total of all correction
factors that apply as outlined in Figure 1.
The sum equals value CF. These factors will
influence the composite noise rating N1.
- Calculate the composite noise rating N1
from the formula N1 = N - CF.
- Refer to Figure 2 for predicted community
response based on the calculated composite
rating N1.
- When dealing with sensitive community
noise issues it may be necessary to
contract the services of an acoustical
consultant.
Sound Propagation Outdoors
Sound propagation is affected by changes in
atmospheric conditions. Temperature variations
will influence sound wave propagation in the
direction of cooler air. Above left shows the
shadow zone created as sound waves bend
toward cooler air at higher altitudes. When this
occurs, a noise source may be visible at a
distance but quieter than expected. The other
extreme shown above right occurs when air is
cooler closer to the ground such as at night or
over calm ground. If the ground surface is
reflective, sound waves will continue to
bounce and hop, traveling much farther than
otherwise expected.
Wind directions and currents also affect
sound propagation outdoors. Noise sources
emitting sound in the direction of wind
travel (downwind) will tend to propagate
farther than expected as shown above right.
Conversely, sound emitting in the direction
against the wind (upwind) will travel less
than expected because of the shadow zone
created as illustrated above left. This
phenomenon when combined with temperature
fluctuations can explain the common
occurrence of aircraft noise fading in and
out of hearing range while the plane is
moving toward the listener.
Treating Pure Tones And Fundamental Harmonics
The above example plotted for an induced
fan air system shows a frequency spectrum
with spikes at the fan fundamental or blade
passage frequency and decreasing spikes at
each harmonic or whole number multiple.
Most types of rotating equipment such as
compressors, engines, blowers and fans
generate these pure tone spikes that are
elevated above the other frequencies. The
tones and harmonics are related to the rotational
speed of the equipment and the
number of blades, lobes or other driving
components. In the example above, the fan
tone is a function of the RPM divided by 60
times the number of blades on the fan wheel.
For applications such as co-generation (boiler
induced draft), dust collectors, scrubber
systems, incinerators, etc. the ventilation fan
generates its fundamental tone in the 100 to
300 Hz frequency range. This low frequency
noise warrants special treatment with tuned
silencer designs. Standard packed silencers
provide overall A scale reductions but can
miss the offending fan tone which is usually
the source of neighborhood complaints in the
first place.
|