Some sources quote 120 dB as the pain threshold and define the audible sound frequency range as ending at about 20,000 Hz were the threshold of hearing and the threshold of pain meet. This tolerance does not make them immune to the damage that loud sounds can produce. Generally, younger persons are more tolerant of loud sounds than older persons because their protective mechanisms are more effective. A nominal figure for the threshold of pain is 130 decibels, but that which may be considered painful for one may be welcomed as entertainment by others. The nominal dynamic range of human hearing is from the standard threshold of hearing to the threshold of pain. The threshold of hearing varies with frequency, as illustrated by the measured hearing curves. The actual average threshold of hearing at 1000 Hz is more like 2.5 x 10^-12 watts/cm^2 or about 4 decibels, but zero decibels is a convenient reference. This value has wide acceptance as a nominal standard threshold and corresponds to 0 decibels. Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity: Where V 1 and V 2 are the measured voltage amplitudes. Since common microphones such as dynamic microphones produce a voltage which is proportional to the sound pressure, then changes in sound intensity incident on the microphone can be calculated from DI(dB) = 20 log 10(V 2/V 1) The factor of 20 comes from the fact that the logarithm of the square of a quantity is equal to 2 x the logarithm of the quantity. The power carried by a traveling wave is proportional to the square of the amplitude. The pressure P here is to be understood as the amplitude of the pressure wave. The standard threshold of hearing can be stated in terms of pressure and the sound intensity in decibels can be expressed in terms of the sound pressure: Because of the great sensitivity of human hearing, the threshold of hearing corresponds to a pressure variation less than a billionth of atmospheric pressure. Since audible sound consists of pressure waves, one of the ways to quantify the sound is to state the amount of pressure variation relative to atmospheric pressure caused by the sound. To assess sound loudness, as distinct from an objective intensity measurement, the sensitivity of the ear must be factored in. The most common approach to sound intensity measurement is to use the decibel scale:ĭecibels measure the ratio of a given intensity I to the threshold of hearing intensity, so that this threshold takes the value 0 decibels (0 dB). Another more common way to express sound intensity is the decibel scale. It is equivalent to the average power per unit area. The intensity of a sound wave is measured as the rate at which it transports energy per unit area. Many sound intensity measurements are made relative to a standard threshold of hearing intensity : This scale is referred to as the decibel scale. The basic units are watts/m^2 or watts/cm^2. The usual context is the measurement of sound intensity in the air at a listener's location. Sound intensity is defined as the sound power per unit area.
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