Decibels
Sound Intensity Level (SIL)
- Represents the flow of energy through a unit area; the rate of energy transfer with respect to area.
- A measure of the power density of a sound wave propagating in a particular direction.
Sound Power Level (SWL)
- Sound power level is a measure of the total power radiated in all directions by a sound source.
- Sound power is measured in watts.
- Most everyday sound sources generate a fraction of a watt of sound power.
- 20 watts of sound power would be close to our pain threshold.
- Not to be confused with the power output of an amplifier; loudspeakers are only about 1% efficient, meaning that a 100 watt amplifier only generates 1 watt of sound power!
Sound Pressure Level (SPL)
- The effect of sound power on its surroundings.
- SPL is measured in newtons per square metre (N m-2).
- 0 dB SPL is approximately equivalent to the threshold of hearing (2×10-5 N m-2 or 20 μPa).
People entering the room would feel the temperature, but the radiator is the source of power.
Decibels
- The decibel is the logarithm of the ratio between two numbers.
- It describes how much larger or smaller one value is than the other.
- It can also be used as an absolute unit of measurement if the reference value is fixed and known.
Sound Intensity Level (SIL)
SIL = 10 log10 (I(actual) ÷ I(ref))
Double the sound intensity = increase of 3 dB
Sound Power Level (SWL)
SWL = 10 log10 (w(actual) ÷ w(ref))
Double the sound power = increase of 3 dB
Sound Pressure Level (SPL)
SPL = 20 log10 (p(actual) ÷ p(ref))
Double the sound pressure = increase of 6 dB
SPL dB ratios
| dB | Multiplication factor |
| 0 | 1 |
| +3 | √2 |
| +6 | 2 |
| +20 | 10 |
| +60 | 1000 |
Useful Reference Levels
| Abbrev. | Reference Level |
| dBV | 1 volt |
| dBu | 0.775 volt (Europe) |
| dBv | 0.775 volt (USA) |
| dBm | 1 milliwatt |
| dBA | dB SPL, A-weighted response |
© Matt Bellingham 2003 – 2006
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.
