Category: (((—Acoustics Engineering—))))

The speed of sound in air at a temperature of 0 degC and 50% relativehumidity is 331.6 m/s. The speed is proportional to the square root ofabsolute temperature and it is therefore about 12 m/s greater at 20degC. The speed is nearly independent of frequency and atmosphericpressure but the resultant sound velocity may be substantially…

Sound power level, Lw, is often quoted on machinery to indicatethe total sound energy radiated per second. The reference power istaken as 1pW. For example, a lawn mower with sound power level 88dB(A) will producea sound level of about 60dB(A) at a distance of 10 metres. If the soundpower level was 78dB(A) then the lawn…

The way sound changes with distance from the source is dependent on thesize and shape of the source and also the surrounding environment andprevailing air currents. It is relatively simple to calculate providedthe source is small and outdoors, but indoor calculations (in areverberant field) are rather more complex. If the noise source is outdoors and…

A sound level meter is the principal instrument for general noisemeasurement. The indication on a sound level meter (aside fromweighting considerations) indicates the sound pressure, p, as a levelreferenced to 0.00002 Pa. Sound Pressure Level = 20 x lg (p/0.00002) dB Peak levels are occasionally quoted. During any given time intervalpeak levels will be numerically…

The decibel is a logarithmic unit which is used in a number ofscientific disciplines. In all cases it is used to compare somequantity with some reference value. Usually the reference value is thesmallest likely value of the quantity. Sometimes it can be anapproximate average value. In acoustics the decibel is most often used to compare…

Sound is produced when the air is disturbed in some way, for exampleby a vibrating object. A speaker cone from a hi-fi system serves as agood illustration. It may be possible to see the movement of a bassspeaker cone, providing it is producing very low frequency sound. Asthe cone moves forward the air immediately in…

Sound is the quickly varying pressure wave within a medium.We usually mean audible sound, which is the sensation (as detected bythe ear) of very small rapid changes in the air pressure above andbelow a static value. This “static” value is atmospheric pressure(about 100,000 Pascals) which does nevertheless vary slowly, as shownon a barometer. Associated with…