What are the advantages of an Integrating Noise Meter?
What does Octave Band Analysis mean?
Where should one take an actual noise measurement?
Why is field calibration necessary?
Why is it necessary to send the Noise Meter back for a Laboratory Calibration?
Is the Peak measurement important?
Can one use the Max Hold facility for a peak measurement?
Can I buy a meter that can measure the Daily Exposure (Lepd) directly?
What are the precautions for obtaining accurate readings?
The human ear does not hear different frequencies equally. Sound at 1 kHz sounds louder than the same level at say 500 Hz or 4 kHz. The A-weighting filter has a frequency response that tries to match the response of the ear. The response of this filter is shown below.
Sound levels measured by a Noise Meter normally have a flat frequency response. ie they assign the same level across the frequency range. The A-weighting filter
modifies this to represent that level that is perceived by the human ear. All
Noise at Work exposure measurements are carried out with the A-weighting filter.
It is important that the A-weighting filter in the Sound Level Meter (Noise
Meter) is implemented correctly.
Although The A-weighting filter closely follows the human ear at normal levels of sound, it is not accurate at high levels of sound. The ear behaves differently at different levels. The C-Weighting filter is more accurate at these higher levels. The response of this filter is shown below compared with an A-Weighting filter and the rarely used B-Weighting.
"A", "B", and "C" Weighting curves
The C-Weighting filter is not used for Noise Exposure assessments. However, it is used in the selection of hearing protectors.
What are the advantages of an Integrating Noise Meter?
Noise Meters usually have Fast and Slow response times for measurements. The response determines how quickly the unit responds to fluctuating noise. The Fast measurement has a time constant of 125 milliseconds whilst the Slow is around 1 second. The Slow response is used for Noise at Work exposure measurements.
If the noise changes many times over a longer period, say 10 minutes, then even on the Slow measurements setting, the reading would vary a lot and it would be difficult to take a reading. The Integrating Noise Meter overcomes this problem by keeping and displaying a running average of the Sound Pressure Level.
Therefore, an Integrating Noise Meter is invaluable in measuring levels where the sound varies considerably with time.
What does Octave Band Analysis mean?
Sometimes, it may be necessary to see what the levels of the individual frequencies are, when measuring Sound Levels. There are standardised ways of measuring this, and Octave Band Analysis presents the frequency response in bands centred around the frequencies as shown below.
Octave Band Analysis is sometimes used for more detailed work such as choosing specialised hearing protection, or Noise Control. It is very much for the Noise Professional and not normally required for the day-to-day problems regarding Noise at Work.
Where should one take an actual noise measurement?
When carrying out exposure measurements on an individual, the Noise Meter is held a few inches away from the ear whilst they are in their normal working position. When assessing Noise Levels in a building, one needs to take a number of measurements and work with the maximum one. Certain precautions have to be taken to avoid "standing waves". These are explained in more detail below.
Why is field calibration necessary?
The performance of the microphone used in a Noise meter can vary due to a number of factors, such as Atmospheric pressure, temperature and humidity. Field calibration is recommended before each measurement session. As well as allowing for the above variations it also can alert the user if the microphone is damaged.
Why is it necessary to send the Noise Meter back for a Laboratory Calibration?
Although a field calibration check is essential, it only checks the Noise Meter at one frequency (1 kHz). It does not check for correct operation at other frequencies, or the fact that the A-weighting filters and other features are measuring correctly. A proper Laboratory calibration will make a check on all these and be traceable back to National Standards.
It is also necessary at this stage to ensure that the field calibrator itself is still working correctly.
Is the Peak measurement important?
Just as the Integrating facility is useful for slowly varying Sound Levels, one needs to be able to measure Noise that is very fast, for example from a gun shot or a riveting machine. The Fast setting on Noise Meters cannot react fast enough to these sounds and will give a wrong reading. The Peak measurement has to be made with a time constant of 50 microseconds.
It is important to use a Noise Meter with this facility when trying to assess these types of Sounds. Other Noise meters will read low and give you a false sense of safety.
Can one use the Max Hold facility for a peak measurement?
The Max Hold facility is just that. It uses the Slow or Fast time constant to measure the highest level over a measuring session. It will NOT be capable of measuring Peak Sound levels from impulsive sounds such as gunshots or riveting machines.
Can I buy a meter that can measure the Daily Exposure (Lepd) directly?
These are known as Noise Dose Meters or Dosimeters. If an individuals work pattern is so different that you have to follow them around all day, then these are an ideal alternative. You clip the microphone from this onto the shoulder of the person and start the measurement off. At the end of the measurement session, the Lepd data from the Dosimeter can be analysed.
Since the Noise Dosimeter is out of sight of the person taking the measurement, one needs to take great care in interpreting the results from it. Dosimeters are open to abuse, both by the individual wearing it and others, or of accidentally picking up false measurements, if they are knocked.
What are the precautions for obtaining accurate readings?
The following precautions will lead to accurate measurements.
* Keep the Noise Meter away from your body, to avoid reflections.
* Reflections can cause false peaks. So avoid taking measurements near walls, or large objects.
* Air blowing across the front face of the microphone can cause measurement errors. If there is any risk of this, make sure you use a wind shield.
