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.
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.
The tables at left show the additive effect for adding equal and unequal decibel levels. Unless the two levels differ by 10 or more dB there will always be some increase to the higher level. Frequency levels can also be added together in a similar fashion to get overall dB levels.
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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.