The world of noise measurement and sound level meters is full of unusual terms and standards. Here is an explanation of the most common and most important acoustic terms. If your question isn't answered then please email or call, we are happy to help.
Acoustic Terms
Here are the most important parameters measured by a sound level meter and the more common features fitted. If you are unsure whether you need to measure the Sound Level or the Leq, or if you are confused about the difference between Max and Peak, read on.
The ear responds better to some tones better than to others, so you can hear somebody talking but cannot hear the very low tones of the wind blowing or the very high tones made by the bats flying around outside. To account for this a sound level meter is fitted with filters, the most common being "A" weighting, which is similar to the response of the human ear.
The maximum and minimum are the highest and lowest sound levels measured, with either a Fast or Slow time weighting (see below). The Peak is not the same as the Max, but is peak of the sound pressure wave with no time weighting.
The calculated Noise Rating and Noise Criterion parameters are used for certain indoor noise assessments, often for air conditioning units and similar equipment. To carry out these calculations you need a sound level meter with octave band filters.
A meter with these filters splits the spectrum into around 10 (for octave) or 30 (for 1/3 octave) bands, giving the ability to determine the frequency content of the noise. This can help when assessing very tonal noise. Octave band measurements give an accurate method for calculating the level at the ear when wearing hearing protectors.
Reverberation time measurements are used when assessing the acoustics of a room or other enclosed space, where echoes can build up as the sound is reflected and then absorbed by the walls and other objects. The reverberation time describes the length of time it takes the sound to decay to a given level.
The basic measurement from a sound level meter is the Sound Level expressed in decibels, or dB. When measuring noise that varies all the time, it is better to have a meter that averages the sound level, providing a parameter called the Leq (equivalent sound level) or Lavg (average sound level with slow time weighting). The OSHA occupational noise regulations recommend the use of Lavg, but other regulations, such as those for Noise at Work in the EU, demand the measurement of Leq rather than simple sound level.
Dating back to the times when sound level meters had analog displays, the motion of the meter was written into the standards. Now replicated in digital meters, most regulations will state that you must measure either Fast or Slow, with Impulse being rarely used.
Most noise measurement regulations will state the standards that the sound level meter should meet. It is essential that you choose a meter that satisfies these requirements to ensure accurate and repeatable measurements.
For accurate and repeatable measurements, and especially when measurements are being used for hearing protection or for legal puposes, it is essential that the sound level meter meets the relevant standard. These standards define many different parameters that the sound level needs to meet.
The sound level meter standards (such as IEC61672, IEC60651 and ANSI S1.4) defined different levels of accuracy. The most important ones are "Class 1" or "Type 1", which is the more accurate version, and "Class 2" or "Type 2", which is ideal for most general applications such as noise at work.
Occupational Noise and Hearing Protection - U.S. OSHA Regulations
There is no better way to protect hearing than reducing the noise at source to a safe level. This should be at the heart of all noise control programs. When further noise reduction is not possible you have to reduce exposure times and provide hearing protection.
Most regulations indicate the type and grade of sound level meter that you should use - usually at least a Type 2 Sound Level Meter with Calibrator - along with calibration schedules and direction on the type of measurements that should be made.
The TWA shows a worker's daily exposure to occupational noise (normalized to an 8 hour day), taking into account the average levels of noise and the time spent in each area. This is the parameter that is used by the OSHA Regulations and is essential in assessing a workers exposure and what action should be taken.
In practice this depends on what regulations you are following. The level is usually based on the amount of noise the worker is exposed to over a period of time - the higher the exposure, the shorter the time allowed. If following the OSHA regulations in the USA this level is 90dB(A) for 8 hours and for the EU regulations it is 85dB(A) for 8 hours. These time weighted averages are calculated differently for the different regulations, so it is important to understand those you are following. Most regulations also demand that hearing protection and training are provided at a lower level. In the USA this is 85dB(A) and in the EU this first action level is 80dB(A).
Before issuing hearing protection, it is important to ensure that it is adequate for the task. They should bring the noise levels well below the action level, but not so low as to isolate the worker, add physical discomfort and risk the worker not hearing warning alarms. Here are some methods for selecting the correct hearing protection in order of accuracy and complexity.
NRR Method (requires "A" or "C" weighted noise measurements)
HML Method (requires "A" and "C" weighted noise measurements)
Sound level meters measure extremely small fluctuations in air pressure using a very sensitive microphone. There are two types of calibration for a sound level meter that are intended to ensure accurate measurement and the early recognition of a fault.
This is a calibration check that is carried out using a hand-held Calibrator just before making important measurements. It is done to check that the sound level meter is functioning correctly and also to make fine adjustments to the measurement chain.
Periodic calibration or verification is carried out by a suitably equipped laboratory. This is a far more detailed check than that carried out in the field using a hand-held Calibrator. It includes checks for frequency response, time weightings, linearity, inter-range errors and many more parameters. This is usually performed once every year or every two years, depending on the regulations being followed. A certificate should be issued by the laboratory to show conformance with the standards.
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