| Sound Pressure |
The amplitude (“strength”) of a sound wave in an arbitrary point can be described with the sound pressure P. In analogy to the voltage of an electric, alternating current, the amplitude is expressed with the effective value Peff Because of the wide dynamic range of the human ears we have to deal with sound pressure values from about 0.000020 Pa=20 μPa to 200 Pa=200000000 μPa. So independent of the used unit for the sound pressure (pascal Pa or micro Pascal μPa) we will get numbers with a lot of 0’s, which are not easy to read and understand. Therefore the logarithmic unit decibel (dB) is introduced to express the sound pressure values with easy to handle numbers from about 0 to 140. To do that, a reference value Po is needed, and for the sound pressure this value Po=20 μPa represents the average threshold for hearing for young people. Sound pressure values expressed with the decibel unit [dB] are named: Sound Pressure Level Lp (sometimes also SPL) The energy emitted from a sound source is called the sound power and is expressed in Watt (W). Due to the same reasons with a very large dynamic range, also here a logarithmic decibel definition is usually used: Sound Power Level Lw=10*log(Psource/Pref) dB with the reference value Pref=10-12 W A sound source with a mechanical emitted sound effect of 1 W has a sound power level of Lw=10*log(1 W/10-12 W)dB=120 dB The energy flow in a sound wave can be expressed with the Sound intensity in W/m2 or a similar dB definition with Io=10-12 W/m2 Also other terms within vibration and acoustics are expressed with a dB unit using different reference values. Thus e.g. for vibration different dB definitions are used to describe the acceleration, the vibration velocity or the displacement of a vibrating point. Using a dB unit results in easy to handle numbers, but you have to ensure that you know which physical term and which reference value are used. Otherwise there is a risk of mixing values for e.g. sound power level emitted by a source and the sound pressure level, which can be heard (and measured) in a certain point in some distance. Equally when measuring, there are several different standard methods seen. And also the surroundings/ environment for the measurements are very determining for the outcome of the data material. It is therefore impossible to compare measurement figures from different companies, without also looking at all the related parameters. |
