Sound Pressure Level
June 03, 2005
Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the energy of a particular noise relative to a reference noise source.
where p0 is the reference sound pressure and p1 is the sound pressure being measured.
It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure.
Measuring sound pressure levels
When making measurements in air, SPL is almost always expressed in decibels compared to a reference sound pressure of 20 ÁPa (micropascals), which is usually considered the threshold of human hearing. Thus, most measurements of audio equipment will be made relative to this level. However, in underwater acoustics, a reference level of 1 ÁPa is more often used. In general, it is necessary to know the reference level when comparing measurements of SPL.
Since the human ear does not have a flat spectral response, sound pressure levels are often frequency weighted so that the measured level will match perceived sound level. The International Electrotechnical Commission (IEC) has defined several weighting schemes. "A weighting" attempts to match the response of the human ear to noise, while "C weighting" is used to measure peak sound levels. If the actual, as opposed to weighted, SPL is desired, many instruments allow a "flat" or unweighted measurement to be made. See also dB(A).
SPL in audio equipment
Most audio manufacturers use SPL to describe the efficiency of their speakers. The most common means is measuring the sound pressure level from the speaker with the measuring device placed directly in front of and one meter away from the source. Then a particular sound (usually white noise or pink noise) is played through the source at a particular intensity so that the source is consuming one watt of power. The SPL is then measured and the product labeled something like "SPL: 93 dB 1 W / 1 m". This measurement can also be represented as a strict efficiency ratio of audio output (sound power) to electrical input(electrical power), but this is far less common. This method of rating speakers using SPL is often deceiving because most speakers produce very different SPLs at different frequencies of sound, often varying as much as ▒10 dB throughout the speaker's usable frequency range (it generally varies less in higher quality speakers). The SPL quoted by the manufacturer is often an average over a particular range.
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