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 (A) between the direct and diffracted paths increases, so does the noise reduction.

Predicting Community Reaction To Noise

1. Plot octave band sound pressure levels on Figure 3 at each frequency 63Hz to 8000Hz.
2. Determine the value of N where the plotted data intersects the highest curve.
3. 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.
4. Calculate the composite noise rating N1 from the formula N1 = N - CF.
5. Refer to Figure 2 for predicted community response based on the calculated composite rating N1.
6. 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.