Sound (not noise) may be defined as any pressure variation that the human ear can detect. The number of pressure variations per second is called the frequency of the sound. This is measured in Hertz(Hz). The frequency of a sound creates its distinctive tone. Combinations of these tones make up the everyday sounds we hear, be it traffic noise or music. There are low frequencies such as thunder or high frequencies such as a whistle. The normal hearing range for a healthy young person is about 20 Hz (low tone) to 20,000 Hz (high tone).
Another main quantity used to describe sound is the size of the sound pressure level (SPL) fluctuation on the human ear. It is the increase or decrease of sound pressure that allow us to hear things as getting louder or softer in volume. Sound pressure is measured in decibels (dB). This is a scale used to measure how much sound pressure there is at given time and place. Because the dB scale is logarithmic, an increase of 6 dB represents a doubling of the sound pressure level. However, an increase of 10 dB is required before the perceived sound appears to be twice as loud.
In terms of sound pressure level, audible sounds range from 0 dB (silence) to 130 dB (pain). Some examples of dB measurements are: a library / 35 dB, a business office / 65 dB, boulevard traffic / 90 dB, and a jet take-off / 125 dB.
Sound is recorded and measured through a weighting network. There are three different internationally standardized weightings: "A", "B" and "C". The following dB test data was collected using "A" weighting (dBA). Weightings "B" and "C" were not used because they do not correlate well with subjective tests.
Noise may be defined as any unwanted sound, regardless of decibels (dB) or frequency of the sound.
ETC's Sensor Sound Suppression Hoods & Skirts
ETC offers optional sound suppression kits, Sssh and Ssss, for their dimmer racks. These are referred to as "w/ baffle" and "w/ skirt" in the dB test data. The sound emanating from a dimmer rack comes from the cooling fan that maintains a safe operating temperature. The fan noise is noticeably suppressed by the reduction certain frequencies that the human ear is particularly sensitive to. This noisy zone is between 2000 Hz and 5000 Hz. These are higher frequencies that many find annoying. Frequencies in and around this area appear to "stick out" while the lower frequencies blend in with ambient room sounds.
NOTE: Although the sound suppression kits reduce the overall dB level, in some cases certain lower frequencies may increase. This is due to a slight increase in air back pressure caused by the baffling effect. In essence there may be less noisy high frequencies accompanied by more warm low frequencies. This frequency shift changes the sound of the fan into something more pleasant to the ear, hence a reduction in noise.
Cumulative Noise Level, dbA @ 1 meter = 58.4dbA without SSSH hood, 58.4dbA with SSSH hood
Cumulative Noise Level, dbA @ 1 meter = 51.1dbA without SSSH hood, 50.9dbA with SSSH hood.
Cumulative Noise Level, dbA @ 1 meter = 54.1dbA without SSSH hood, 51.6dbA with SSSH hood.