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Noise Control Concepts

What is noise?
Noise is unwanted sound which may be hazardous to health, interfere with speech and verbal communications or is otherwise disturbing, irritating or annoying.
What is sound?
Sound is defined as any pressure variation in air, water or other fluid medium which may be detected by the human ear.
What are the characteristics of sound?
The two most important characteristics which must be known in order to evaluate the sound or noise are it's amplitude and frequency. The amplitude or height of the sound wave from peak to valley determines the loudness or intensity. The wave length determines the frequency, pitch or tone of the sound.
What are wavelengths?
Sound wavelengths are the linear measurement of one full cycle of displacement where the motion of air molecules is first compressed and then rarefield or expanded. The wavelength is determined by the ratio of the speed of sound to the frequency.
How are these characteristics expressed?

The frequency of sound is expressed in wavelengths per second or cycles per second (CPS). It is more commonly referred to as Hertz. Low frequency noise is 250 Hertz (Hz) and below. High frequency noise is 2000 Hz and above. Mid-frequency noise falls between 250 and 2000 Hz.

The amplitude of sound is expressed in decibels (dB). This is a logarithmic compressed scale dealing in powers of 10 where small increments in dB correspond to large changes in acoustic energy.

What are octave bands?

Standardized octave bands are groups of frequencies named by the center frequency where the upper limit is always twice the lower limit of the range. Test data for performance of acoustical materials is standardized for easy comparison at the center frequencies. Equipment noise levels and measurement devices (dB meters) also follow the preferred octave bands.

What Is the difference between dB and dBA?

dB sound pressure levels are unweighted. dBA levels are "A" weighted according to the weighting curves to approximate the way the human ear hears. For example, a 100 dB level at 100 Hz will be perceived to have a loudness equal to only 80 dB at 1000 Hz. The dBA scale is based on a child's hearing and was originally documented based on actual hearing tests to characterize the human ear's relative response to noise.

Is hearing loss permanent?

Yes! Permanent hearing loss occurs when the tiny hair cells in the cochlea (inner ear) are damaged or destroyed. A healthy cochlea contains approximately 40 thousand hair cells which are necessary to transmit sound vibrations to the brain. Exposure to excessive noise levels will damage the hair cells resulting in permanent, irreversible hearing loss.

Is a 5 dB change significant?

Yes! The pressure associated with the loudest known sound is more than one billion times that associated with the faintest sound. Such a large range is unmanageable for measurement purposes. Using a logarithmic scale compresses the range to between 0 and 200 dB. At right, various sound level changes are referenced to relative loudness and acoustic energy loss. A 5 dB change is more than a 50% change in acoustic energy!

Sound Level Change Acoustic Energy Loss Relative Loudness
0 dB 0 Reference
-3 dB 50% Perceptible Change
-10 dB 90% Half as Loud
-20 dB 99% 1/4 as Loud
-30 dB 99.9% 1/8 as Loud
-40 dB 99.99% 1/16 as Loud
Is sound power the same as sound pressure?

No! While both sound power levels (Lw) and sound pressure levels (Lp) are both expressed in decibels, the referenced standards for each are different. More importantly, the sound power level is the total acoustic energy output of a noise source independent of environment. Sound pressure levels are dependent on environmental factors such as the distance from the source, the presence of reflective surfaces and other characteristics of the room/building/area hosting the source. Actual sound pressure levels will always be higher than sound power levels.

What is tonal noise?

Tonal noise is commonly referred to as discrete frequency noise and is characterized by spectral tones that are pure tone in nature. Pure tones are wave forms that occur at a single frequency. Tonal noise is generated by rotating equipment at a predictable frequency relating to the rotational speed of the shaft and the number of compressor vanes, fan blades, engine pistons, gear teeth, etc. The fundamental tone (F) may also manifest itself at progressively lower intensity levels at integer harmonic multiples (2F, 3F, etc.). Tolerance levels for tonal noise are generally at a lower threshold.

What is impulse noise?

Impulse noise is a short duration transient acoustic event characterized by a sudden rise or spike in sound pressure followed by a uniform or oscillatory decay (depends on type of source equipment) lasting less than 1/2 second. Impulse noise usually exhibits a distinct spectral signature across the frequency range without the presence of discrete tones. Examples of impulse noise include gunshots, pulse cleaning systems, punch presses, etc.

What is the audible range?

At birth, the audible frequency range is 20 Hz to 20,000 Hz. Generally speaking the average audible range in humans is from 30 Hz to 17,000 Hz. Sound pressure wave forms below and above this range are described as infrasonic and ultrasonic. Infrasonic sound is experienced as a flutter while ultrasonic sound produces no sensation of hearing.

What is diffraction?

Diffraction of sound is “bending” of the pressure wave around objects, obstacles and walls. Diffraction is greatest with low frequency sound or where the wavelength is large compared to the object it strikes. As illustrated above, diffraction of sound results in a less pronounced acoustic shadow zone.

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