Measurement – there are a number of points that should be taken into account when measuring sound pressure level: Jet takeoff from 25m (possible eardrum damage)įood blender – operator, busy urban streetĪccelerating vehicle from 50kmhr -1 at 7.5m The human ear interprets loudness on a scale much closer to a logarithmic scale than a linear scale.ĭB(A) – a sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons. For example, sound pressure radiated by a submarine may vary by eight orders of magnitude depending on direction. Quantities of interest often exhibit such huge ranges of variation that a dB scale is more convenient than a linear scale.In acoustics this is often dropped.Īcousticians use the dB scale for the following reasons: Note: whenever quoting any level as a dB care should be taken to specify the reference level. A change of 3dB is commonly thought to be the smallest change in sound pressure level that can be remembered. This is much more convenient.Ī difference of 20 dB between two sounds means that the more intense one has 10 times the amplitude (100 times the power) of the softer. The same range expressed in dB SPL is 0 -140 dB SPL. the limits of hearing in terms of absolute pressure level cover the range from 20µPa to 200,000,000 µPa. The logarithmic nature of the decibel allows us to compare two values of enormously different magnitudes with conveniently small numbers. In the latter case a sound pressure level is defined in decibels in which case a reference pressure of 2×10 -5Pa is used as a base pressure and the pressure measured by a microphone is related to this standard base pressure. The main application is, however, in the acoustic field to define response limits in audio equipment and as a means of defining noise levels. The decibel scale is often used to express the signal to noise ratio, frequency and amplitude response limits in the majority of instrument specifications. The Bel was the amount a signal dropped in level over a one-mile distance of telephone wire. The bel is the logarithm of the ratio of two powers, and the decibel is one tenth of a bel. The human ear responds logarithmically and it is convenient to deal in logarithmic units in audio systems. Sound Pressure Level or SPL – ten times the logarithm of the ratio of the time-mean-square pressure of a sound, in a stated frequency band or with a stated frequency weighting, to the square of the reference sound pressure of 20×10 -6Pa.ĭecibel – a description of the decibel is a good starting place: A is an abbreviation for A-weighting – a simple filter that is applied to make the measurement more meaningful by by weighting the response to more closely match the human earīelow I’ve broken these expressions out into a bit more detail and have started to add some of the calculations to our workshop calculator that you can download as an excel workbook.dB is an abbreviation of decibel – the sound pressure scale is vast and so we take the logarithm of the measurement to make the numbers easier to handle.When we measure noise level we normally measure the sound pressure level and express the result as something like 85dB(A) Facebook Tweet Pin LinkedIn Email Print Yummly
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