Change in Force, Pressure or Speed produce sound. There are two types of sound - Structureborne and Air-borne. Both are caused by vibration.
Structure-borne sound is vibration transmitted through the earth, water, buildings, etc. It is generally referred to as "shock or vibration." Airborne noise results when vibration sets air molecules into motion - creating "waves" of compression and expansion.
Figure 1 - Sound Traveling Through Air (available in full paper)
These changes in pressure are detected by the eardrum and translated into the motion of tiny frequency-sensitive "hairs" within the inner ear. This stimulation is conveyed by the nervous system and interpreted by the brain as sound.1[
The human er has an incredible dynamic range, from the faintest rustle of leaves (a pressure differential of 20µPa) to the sound of a nearby jet engine (a pressure differential over 100,000,000µPa) - representing one trillion times more energy. To make it a bit easier to work with, this pressure differential has been depicted in logarithmic form, using the term "decibel" - or dB. The lowest threshold of hearing is set at zero decibels, and the "threshold of pain" is 140dB. Every increase of 3 decibels is equivalent to a doubling of the energy.
The average human ear responds to frequencies from approximately 20Hz (cycles-per-second) to nearly 20,000Hz. Dogs and bats can hear frequencies much higher than this.
The human ear is very frequency-sensitive, responding far better to sounds in a frequency range from 500–4000Hz. Research in the twenties determined that the threshold of hearing varies markedly - improving (dropping) as the frequency of sound rises from 20Hz., and peaking at about 2000–4000Hz., before rolling off at higher frequencies. As the intensity of sound increase, the human ear response becomes more linear. This data was incorporated into the development of "weighting curves" for sound measuring instrumentation.
Figure 2 - Weighting Curves (available in full paper)
Later research confirmed that the "A" curve most closely represents not only how the human responds to low levels of sound, but defines the ear's propensity for damage from exposure to high noise levels. This is why the "A" weighting is used for industrial noise measurements, where hearing loss is the major issue, and for community noise work where perception and annoyance are the major issues. Sound pressure levels, taken at a given point, are generally referred to in dBA.
The duration of vibration determines whether a sound is considered continuous or impulsive. Noise exposure is the product of the sound pressure level and the duration of exposure. The shorter the duration of the sound, the lower the perceived intensity, and the less hearing damage.
Consider the difference between a pure tone (such as a flute) and a complex tone (such as that produced by a violin). The amount of harmonic content creates "timbre," and may contribute to the "annoyance" factor of noise. It does not affect the potential for hearing loss, however.
Noise … is defined as "unwanted sound." It's all around us! It's a product of an industrialized society!
