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# Sound pressure level

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. It is almost always expressed in decibels compared to a reference source of 20 µPa (micropascals) sound pressure.

$L_p=20\, \log_{10}\left(\frac{p_1}{p_0}\right)=10\, \log_{10}\left(\frac{p_1^2}{p_0^2}\right) \mbox{ dB SPL}$

The reference sound pressure is

$p_0 = 2 \cdot 10^{-5} \mathrm{ Pa}$
= 20 µPa (micropascals), (Pa = pascal = N/m²; N = newton)

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.

## 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.