Book - School of Science and Technology

460 Ductwork designxAmplitudeyFigure 16.21 Amplitudes of sound powerAmplitudeThe amount of energy imparted to the air at point A is termed the sound power, and isreferred to in terms of watts. Sound power may be visualised as the amplitude at thesource.The amplitude, see Figure 16.21, of sound X is greater than Y:X is a loud sound andtransmits more energy than soft sound Y. The energy involved is represented by powermultiplied by time (W s). Sound power cannot be measured as such; it can however bedetermined from measurements of the sound pressure level measured in a room withknown acoustic properties.Sound receivedThe power of sound received is termed sound intensity and is measured in W/m 2 . Althoughthe sound produced at point A cannot be measured directly, the sound received by the ear±or by a sound meter ± at point B can be expressed in terms of pressure variation.The amount of variation in pressure of the sound waves reaching the receiver ± termedthe sound pressure ± is measured in Pa.The decibel (dB)The units referred to above are absolute values and are not convenient for general use.Units of reference have therefore been invented and are termed the bel and decibel(10 dB ˆ 1 bel). They are logarithmic, so that if two sounds differ in magnitude by 1bel, the magnitude of one is ten times that of the other. Similarly, if the difference is 1 dB,the magnitude of one will be about 26% greater.The decibel may be defined as 10 log 10 of the ratio between two absolute values suchas levels of sound power, sound intensity and sound pressure.. Sound power level (with reference to a datum of 10 12 watt)ˆ 10 log 10 W/10 2 dBwhere W ˆ actual sound power.. Sound intensity level (with reference to a datum of 10 12 watt/m 2 , taken to be thresholdof hearing)ˆ 10 log 10 I/10 12 dBwhere I ˆ actual sound intensity.