Sound Pressure
Sound
Sound is vibration transmitted through a solid, liquid, or gas; particularly, sound means those vibrations composed of frequencies capable of being detected by ears.
For humans, hearing is limited to frequencies between about 20 Hz and 20,000 Hz (20 kHz), with the upper limit generally decreasing with age.
The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through vacuum.
Sound waves are characterised by the generic properties of waves, which are frequency, wavelength, period, amplitude, intensity, speed, and direction.
Sound and your Ears
Humans hear because our ears convert the vibrations of a sound wave into signals that your brain interprets as sound. When these vibrations enter your ear, your eardrum and a set of tiny bones in your ear (the well-known hammer, anvil, and stirrup) amplify the vibrations. In your inner ear, these amplified vibrations move tiny hair cells that then convert the vibrations into nerve impulses sent to your brain. Your brain then interprets these nerve impulses as sound.
Sound Pressure
Sound pressure level (SPL) or sound level is a logarithmic measure of the rms sound pressure of a sound relative to a reference (ambient) value. It is measured in decibel (dB).
Loudness
Loudness, a subjective measure, is often confused with objective measures of sound pressure such as decibels or intensity. Filters such as A-weighting attempt to adjust sound measurements to correspond to loudness as perceived by the average human. The perception of loudness is related to frequency, intensity and duration of a sound.
Perception of Loudness
The perception of loudness is not the same as sound pressure level. Although the actual formulae is somewhat complex, as a rough rule of thumb, an increase of 10db SPL is perceived to be approximately twice as loud;
- A 20 dB gain would seem to be about 4 times as loud
- A 40 dB gain would seem to be about 16 times as loud
Sound Pressure & Intensity
| Examples | Sound Pressure Level Lp dB SPL | Sound Pressure p N/m2 = Pa | Sound Intensity I W/m2 |
|---|---|---|---|
| Jet aircraft, 50 m away | 140 | 200 | 100 |
| Threshold of pain | 130 | 63.2 | 10 |
| Threshold of discomfort | 120 | 20 | 1 |
| Chainsaw, 1m distance | 110 | 6.3 | 0.1 |
| Disco, 1 m from speaker | 100 | 2 | 0.01 |
| Diesel truck, 10 m away | 90 | 0.63 | 0.001 |
| Kerbside of busy road, 5 m | 80 | 0.2 | 0.0001 |
| Vacuum cleaner, distance 1 m | 70 | 0.063 | 0.00001 |
| Conversational speech, 1m | 60 | 0.02 | 0.000001 |
| Average home | 50 | 0.0063 | 0.0000001 |
| Quiet library | 40 | 0.002 | 0.00000001 |
| Quiet bedroom at night | 30 | 0.00063 | 0.000000001 |
| Background in TV studio | 20 | 0.0002 | 0.0000000001 |
| Rustling leaf | 10 | 0.000063 | 0.00000000001 |
| Threshold of hearing | 0 | 0.00002 | 0.000000000001 |
| 0.25m | 0.5m | 1m | 1.5m | 2m | 3m |
| 70-76dB | 65-71dB | 58-64dB | 55-61dB | 52-58dB | 50-56dB |
References
Wikipedia - Sound
Wikipedia - Sound Pressure
Wikipedia - Loudness
Decibel Table - Comparison Chart
