Sound pressure levels and decibels
For human’s sound is what we are able to hear, but in fact, sound is a form of energy which enables us to hear. The sound energy is transmitted through the air in the way that air particles move back and forth whilst the energy moves forward. When measuring sound, the basic indicator used is the sound pressure. The pressure variations produced when a sound wave propagates through air are very small compared with the static atmospheric pressure. Due to a very large scale of sound pressures changes audible to humans in practice the sound measurements are expressed in decibels. Use of decibel means that sound measurements are relative to a reference pressure.
Leq, Peak, Dose
The sound energy causes changes of the air pressure. Changes of the air pressure are detected by the membranes of measurement microphones, essential parts of sound level meters. The units of an air pressure are Pascals and the smallest change of atmospheric pressure that humans can detect is a change of 20 uPa and the highest change can go up to thousands Pascals. To manage such a big range of variability of air pressure the decibel scale is used (dB). The sound decibel is a logarithm scale that uses a reference of 20 uPa and compresses a large scale of Pascals to manageable scale of decibels. Because of the decibel scale the sound measurement results are referred as sound pressure levels.
Sound that varies with time is usually represented as Leq or Peak results, expressed in decibels. The Equivalent Continuous Sound Level (Leq) is most commonly used becasue it has a direct relationship to the energy content of the signal. The Leq is a form of representation of the energy of sound signal and is used to asses the level of danger to human hearing. The Peak value is the maximum amplitude value that the signal reaches within the period of time.
Time of measurement is a key parameters in sound measurements. Because the nature of sound signals is their variability in time, to asses sound pressure level a measurement period has to be defined in which the variations of the signal are integrated and averaged.
Incase of occupational noise the LEQ is measured in the period of 8 hours of a working day and such result is referred as the Daily Noise Exposure Level (LEX). The daily noise exposure can be also presented as the % of the daily limit, and such representation of the result is called a Noise Dose.
Sound level meter
Sound Level Meter is an electronic device equipped with a microphone that detects air pressure changes and converts them into an electrical signal. The electrical signal is then converted into a digital one so it can be displayed on a display or recorded in the meters memory.
The range of sound level meter is defined by the dynamics of its analogue/digital converter. Typical dynamics of A/D converter is 110 dB, therefore a typical measurement range in decibel scale starts from 20 dBA and goes up to 130 dBA. The 130 dBA is already a threshold of pain for a human ear.
Real sound signals are variable and combine numerous signals combined. Sound level meters use real-time frequency analysis in 1/1 or 1/3 octave bands in order to assess the contents of low and high frequencies in the signal. The output of such an analysis is called frequency spectrum and it represents noise in consecutive, frequency ranges called octaves or third octaves.
Sound level meter is a combination of a microphone, a preamplifier, a signal processor and display device. Performance of the sound level meter is describes as a its class in accordance to IEC 61672-1 standard. As a minimum, the sound level meter shall have frequency weighting A and Fast time weighting. Sound level meters conforming to class 1 shall also provide frequency weighing C. Frequency weighing Z is optional.
The microphone shall be removable to allow insertion of electrical test signals to the input of the preamplifier.
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