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| The particle velocity v is not
the speed of sound c = velocity of sound. Unless otherwise stated, the sound pressure is always meant as RMS value.  eff = RMSThe missing values will be calculated. One of these values could be the specific acoustic impedance (characteristic impedance) of air: Z0 = 413 N·s/m3 at 20 °C = 68 °F or Z0 = 410 N·s/m3 at 25 °C = 77 °F. |
| In acoustics sound pressure, particle velocity, and acoustic impedance are linear sound field strength quantities. The sound intensity is a quadratic sound energy strength quantity. |
| Sound pressure p in pascals is not the same physical quantity as intensity J or I in watts per square meter. ... and the sound power does not decrease with distance r from the sound source - neither with 1 / r nor as 1 / r2. |
| Often the sound pressure as a sound field quantity is mixed incorrect with the sound intensity as a sound energy quantity. But I ≈ p2. |
The well known law V = I × R means accordingly (equivalent) in the acoustics p = v × Z.
| Hearing is directly sensitive to sound pressure (ear drums). In stereo the level differences have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be of value in music recordings if it could be. "Intensity" stereophony is better termed as level difference stereophony, because our eardrums and microphone diaphragms are moved by the differences of the sound pressure level. |
| Since our ears do not respond to the particle velocity (sound velocity), but only on the sound pressure changes, the particle velocity is irrelevant on the loudness perception. |
Acoustic impedance of a medium = Sound pressure / Particle velocity
Characteristic acoustic impedance Z0 = p / v
Please enter two values, the third will be calculated.
Acoustic impedance of air is Z0 = 413 N·s/m³ at 20 °C.
Acoustic impedance = Density of medium × Speed of sound
Z0 = ρ × c
Please enter two values, the third will be calculated.
Density of air is ρ = 1.204 kg/m³ at 20 °C.
| Medium |
Density ρ in kg/m³ |
Speed of sound c in m/s at 20 °C |
Acoustic impedance Z0 in N·s/m³ |
| Air | 1.204 | 343 | 413.5 |
| Water | 1 000 | 1 440 | 1 440 000 |
| Brick | 1 700 | 4 300 | 7 310 000 |
| Glass quartz | 2 200 | 5 500 | 12 100 000 |
| Aluminium | 2 700 | 6 100 | 16 500 000 |
| Steel | 7 500 | 6 000 | 45 000 000 |
The decrease of sound with distance
| How does the volume (loudness) decrease with distance from a sound source? How does the sound pressure (voltage) decrease with distance from a sound source? How does the sound intensity (not the sound power) decrease with distance from a sound source? The beginners question is quite simple: How does the sound decrease with distance? |
| For a spherical wave we get: The sound pressure level (SPL) decreases with doubling of distance by (−)6 dB. It falls to the 1/2 fold (50%) of the initial value of the sound pressure. The sound pressure decreases with the ratio 1/r to the distance. The sound intensity level decreases with doubling of distance also by (−)6 dB. It falls to the 1/4 fold (25%) of the initial value of the sound intensity. The sound intensity decreases with the ratio 1/r2 to the distance. The loudness level decreases with doubling of distance also by (−)6 dB. It falls to the 0.66 fold (66%) of the initial value of the sensed loudness. The loudness decreases with the ratio 1/(20.6r) = 1/1.516 r to the distance. |
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