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| Sound waves are nothing more than pressure waves that enable the air and our eardrums to get in motion. The sound we hear. It makes our eardrums and microphones vibrate. Do not use the expression "intensity of sound pressure". Intensity is really not sound pressure. Compare: Sound pressure, sound pressure level, SPL, sound intensity, sound intensity level. How much is it twice (double, half) or three times louder sound? |
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| Where: | ||
| p1 | = | sound pressure 1 at reference distance r1 from the sound source |
| p2 | = | sound pressure 2 at the other distance r2 from the sound source |
| I1 | = | sound intensity 1 at reference distance r1 from the sound source |
| I2 | = | sound intensity 2 at the other distance r2 from the sound source |
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| The sound pressure decreases with 1/r at a distance from the sound source. The sound intensity drops with 1/r2 at a distance from the sound source. This is often confused and misunderstood because of the principal difference between the sound pressure as a sound field quantity and the sound intensity as a sound energy quantity is not known. As our ear drums of our hearing and also the diaphragms of the microphones are moved only by the sound pressure as effect, sound engineers should consider this sound pressure as sound field quantity more precisely; see: Sound pressure and Sound power − Effect and Cause |
| Pressure as field quantity is never Intensity as energy quantity. |
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| Often we talk only of sound level. However, sound pressure as a sound field quantity is not the same as sound intensity as a sound energy quantity. Levels of sound pressure and levels of sound intensity decrease equally with the distance from the sound source. The sound power level has nothing to do with the distance of the sound source. |
| To use the calculator, simply enter a value. The calculator works in both directions of the ↔ sign. |
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Reference sound pressure p0 = 20 μPa = 2 · 10−5 Pa ≡ 0 dB Pa = N/m2
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Reference sound intensity I0 = 1 pW/m2 = 10−12 W/m2 ≡ 0 dB
Decrease of the sound field with distance from the source
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| Pressure, velocity, and intensity of the sound field near to and distant from a spherical radiator of the zeroth order |
For a spherical wave: |
| A spherical wavefront is formed under the assumption of idealized conditions, such as a spherical radiator of zero order (ie, a "breathing" sphere) as a source for radiation in a homogeneous isotropic medium, usually air. For the dropping of sound pressure p and of particle velocity v we get in the far field: (r is the distance from the measurement point to the sound source).   All sound field quantities decrease in the far field after the 6-dB-distance law 1/r. Exception: The sound velocity goes with 1/r ² in the near field . That is, the size values are halved by distance doubling. The sound intensity increases as the sound energy quantity is proportional to the square of the distance from the sound source decreases permanent from the sound source. Since the radiated sound power from the sound source as sound intensity is distributed on a growing area with the distance, the sound intensity falls off in the same proportion as the area grows larger.  envelope to the spherical sound source, that is, power is independent of the distance r to the sound source.    Sound power Pak in W, sound intensity I in W/m², distance from measuring point r in m, and area A in m². |
| Ear peoples, as sound engineers and sound designers are mainly interested in sound field quantities, and therefore consider here the sound pressure at distance doubling. Acousticians and noise fighters are mainly interested in sound energy quantities, and therefore consider more the active intensity increase at distance doubling. All consider together the same line!  Is not thisbeautiful? Nevertheless, the drop in sound pressure goes with 1/r and the decrease in sound intensity with 1/r2. This should be understood all right. If you have to work and have to care about sound quality checking the sound by ear then you have to think of the sound waves which move the eardrums only by sound pressure variations as sound field quantities. There is also the advice: Avoid to use the words sound power and sound intensity as sound energy quantities. |
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We do not hear the air pressure change as such, but the sound pressure at each ear, which is superimposed to the air pressure. |
Damping of sound level with distance
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