Sound Level Meter:
This is used for the evaluation of sound pressures either on a linear or weighted scale. It normally indicates the RMS value of the sound, but can be calibrated to read the peak level. The dynamic characteristics of the meters generally available are:
1. Slow, with a rise time of500 ms.
2. Fast, with a rise time of700 ms
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3. Impulse, with a rise time of 3 5 ms.
4. Impulse hold with a rise time of 35 ms and a slow fall.
5. Peak, and Peak hold, as impulse and impulse and impulse hold, but measuring peak, not RMS level.
There are two grades of sound level meters:
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1. IEC 123 with limited accuracy, particularly at high and low frequencies, but cheap and robust. Useful for rough survey, to be followed by more accurate measurement, if necessary
2. IEC 179 V with a high degree of accuracy, are expensive and delicate. Used when accuracy is essential.
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Octave Band Analyser:
This is used in conjunction with, or as an integral part of the sound level meter, to determine the average sound pressure of the standard octave bands. It is a sufficiently accurate guide to the frequency content of a noise to determine treatment to reduce it, for the selection of ear protectors (defenders) or for identifying the source of a noise. If more accurate information is required, 1/3 octave band analyzers are available.
Magnetic Tape Recorder:
A high quality record is fed directly from a microphone or through a sound level meter. It is used to record the actual noise for storage or for better evaluation. The recorder needs to be run at fast speed to preserve frequency content and, therefore, is also expensive on tape, which also needs to be of good quality.
Cassette Recorder:
This instrument is cheaper and of a lower quality than the above mentioned one, and is used for storage of noise or for rough evaluation.
It is used for digital recording of sound pressure levels for computer evaluation. This system gives no record of the actual noise but the data on the tape can be stored. The equipment is expensive but it can use cheap tape run at low speed as quality is not important.
Pen Recorder:
This recorder can be fed directly, or from sound level meters, etc. or from magnetic tape recording on one of two systems:
(a) A continuous record of the sound pressure level against time is given which can be used for storage, or to obtain approximate statistical levels by inspection.
The movement of the pen can be sampled during the writing operation to give a statistical breakdown of the sound pressure-level distribution (see mechanical statistical analyzers infra). Either the dynamic range of the instrument or the accuracy will be limited, depending on the price.
(b) A discontinuous record in the form of instantaneous sound pressure levels, sampled from the incoming noise. Statistical levels can be found on inspection, but the dynamic range of the device is limited and normally only one level can be found with any degree of accuracy.
Statistical Analyser:
Mechanical:
It is used in conjunction with the pen recorder to give any statistical level or an
Electronic:
It can be fed directly, through a sound level meter or from a magnetic tape recording, to give any desired statistical analysis, within the limits of the equipment. It can be set up to work unattended but will provide a single analysis. It can also be coupled up to a print out to give a permanent record. Accuracy would normally comply with IEC 179 and dynamic range can be 60 dB or more.
Digital Tape Processor:
This is fed from digital tape recordings to provide any statistical analysis or an L^. This system can give high accuracy and good dynamic range, but it is expensive. The processing time can be much shorter than the recording time.