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 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 rms pressure of the collection of waves which make up the sound. The relationship of sound intensity to pressure is discussed below. 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). Many sound intensity measurements are made relative to a standard threshold of hearing intensity I 0 : 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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