Sound pressure
Encyclopedia : S : SO : SOU : Sound pressure
| Sound measurements |
|---|
| Sound pressure p |
| Sound pressure level (SPL) |
| Particle velocity v |
| Particle velocity level (SVL) |
| (Sound velocity level) |
| Particle displacement ξ |
| Sound intensity I |
| Sound intensity level (SIL) |
| Sound power Pac |
| Sound power level (SWL) |
| Sound energy density E |
| Sound energy flux q |
| Acoustic impedance Z |
| Speed of sound c |
Sound pressure
Sound pressure is the pressure deviation from the local ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic partical velocity and is, therefore, a vector quantity.
The sound pressure deviation p is:
- [p = \frac]
A = area
The entire pressure '' ptotal is:
- [p_ = p_0 + p \,]
p = sound pressure deviation
Sound pressure level
Sound pressure level (SPL) or sound level Lp 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 dBSPL).- [L_\mathrm=10\, \log_\left(\frac]
where:
| Symbol | Units | Meaning |
|---|---|---|
| p | pascals | sound pressure |
| f | hertz | frequency |
| ρ | kg/m3 | density of air |
| c | m/s | speed of sound |
| v | m/s | particle velocity |
| [\omega] = 2 · [\pi] · f | radians/s | angular frequency |
| ξ | meters | Particle displacement |
| Z = c • ρ | N·s/m³ | acoustic impedance |
| a | m/s² | Particle acceleration |
| J | W/m² | sound intensity |
| E | W·s/mm³ | sound energy density |
| Pac | watts | sound power or acoustic power |
| A | m² | Area |
The distance law for the sound pressure p is inverse-proportional to the distance r of a punctual sound source.
- [p \propto \frac] (proportional)
- [\frac = \frac]
- [p_1 = p_ \cdot r_ \cdot \frac]
Note: The often used term "intensity of sound pressure" is not correct. Use "magnitude", "strength", "amplitude", or "level" instead. "Sound intensity" is sound power per unit area, while "pressure" is a measure of force per unit area. Intensity is not equivalent to pressure.
Examples of sound pressure and sound pressure levels
| Source of sound | sound pressure | sound pressure level |
|---|---|---|
| pascal | dB re 20 µPa | |
| Theoretical limit for a sound wave at 1 atmosphere environmental pressure | 100,000 Pa | 194 dB |
| Krakatoa explosion at 100 miles (160 km) in air | 20,000 Pa | [link] 180 dB |
| M1 Garand being fired at 1 meter | 5,000 Pa | 168 dB |
| Jet engine at 30 m | 630 Pa | 150 dB |
| Rifle being fired at 1 m | 200 Pa | 140 dB |
| threshold of pain | 100 Pa | 134 dB |
| hearing damage during short term effect | 20 Pa | approx. 120 dB |
| jet, 100 m distant | 6 – 200 Pa | 110 – 140 dB |
| jack hammer, 1 m distant / discotheque | 2 Pa | approx. 100 dB |
| hearing damage during long-term effect | 6×10−1 Pa | approx. 90 dB |
| major road, 10 m distant | 2×10−1 – 6×10−1 Pa | 80 – 90 dB |
| passenger car, 10 m distant | 2×10−2 – 2×10−1 Pa | 60 – 80 dB |
| TV set at home level, 1 m distant | 2×10−2 Pa | approx. 60 dB |
| normal talking, 1 m distant | 2×10−3 – 2×10−2 Pa | 40 – 60 dB |
| very calm room | 2×10−4 – 6×10−4 Pa | 20 – 30 dB |
| leaves noise, calm breathing | 6×10−5 Pa | 10 dB |
| auditory threshold at 2 kHz | 2×10−5 Pa | 0 dB |
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.See also
- Decibel, especially the Acoustics section
- Acoustics
- Sone
- Weber-Fechner law
- Sound power level
References
- Beranek, Leo L, "Acoustics" (1993) Acoustical Society of America. ISBN 0-88318-494-X
External links
- [Two Part Article on SPL in Blog Form]
- [Conversion of sound pressure level to sound pressure]
- [The level of sound (dB)]
- [SPL of many different sounds]
- [Definition of sound pressure level]
- [Has a table of SPL values]
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