Noise Control Concepts – Questions and Answers
Acoustical Concepts – Used in Noise Control
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 shown below 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. Other weighting scales (C and B) are also shown. 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?
| 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 |
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!
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.