Sound pressure level

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Sound pressure level (SPL) or sound level L_p is a logarithmic measure of the rms pressure (force/area) of a particular noise relative to a reference noise source. It is usually measured in decibels (dB(SPL), dBSPL, or dB_SPL).

<math>

L_\mathrm{p}=20\, \log_{10}\left(\frac{p}{p_0}\right)=10\, \log_{10}\left(\frac{{p}^2}{{p_0}^2}\right) \mbox{ dB SPL} </math>

where p₀ is the reference sound pressure and p is the root-mean-square sound pressure being measured.

The commonly used reference sound pressure in air is p₀ = 20 µPa (root-mean-square).

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. See also Weber-Fechner law.

1 Measuring sound pressure levels
2 Examples of sound pressure levels
3 SPL in audio equipment
4 See also
5 References
6 External links

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Measuring sound pressure levels

When making measurements in air (and other gases), SPL is almost always expressed in decibels compared to a reference sound pressure of 20 µPa (micropascals), which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 metres away). Thus, most measurements of audio equipment will be made relative to this level. However, in other media, such as underwater, a reference level of 1 µPa is more often used. <ref>Underwater Acoustics — Federation of American Scientists</ref> These references are defined in ANSI S1.1-1994. <ref>Glossary of Noise Terms — Sound pressure level definition</ref> In general, it is necessary to know the reference level when comparing measurements of SPL. The unit dB (SPL) is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself.

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. <ref>Glossary of Terms — Cirrus Research plc.</ref> If the actual, as opposed to weighted, SPL is desired, many instruments allow a "flat" or unweighted measurement to be made. See also Weighting filter.

When measuring the sound created by an object, it is important to measure the distance from the object as well, since the SPL decreases in distance from a point source with 1/r (and not with 1/r^2</sub>, like sound intensity). It often varies in direction from the source, as well, so many measurements may be necessary, depending on the situation. (An obvious example of a source that varies in level in different directions is a bullhorn.)

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Examples of sound pressure levels

situation	sound pressure level dBSPL
threshold of pain	130
hearing damage during short term effect	from 120
jet, 100 m distant	110–140
jack hammer, 1 m distant / Diskotheque	approx. 100
hearing damage during long-term effect	approx. 90
major road, 10 m distant	80–90
passenger car, 10 m distant	60–80
TVset at home level, 1 m distant	approx. 60
normal talking, 1 m distant	40–60
very calm room	20–30
leaves noise, calm breathing	10
auditory threshold at 2 kHz	0

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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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