MEASURING HIGH NOISE LEVELS
The Campbell Associates team recently went to a clay pigeon shooting range as part of an experiment to capture accurate high sound pressure levels. They took measurements from various shot guns at a distance of 0.5m from the trigger, using a sound level meter (see picture x). This type of measurement is sometimes required to assess the hearing protection used by individuals participating in gun related leisure activities.
The sound level meter used was a Norsonic 140 sound Analyser and was tested in three configurations.
The standard meter which has a 140dB full scale range.
The standard meter in high level mode which boosts the full scale range to 150dB.
The standard meter with a quarter inch high level microphone capsule. This increases the full scale measurement range to 160dB.
The Results:
In the standard mode the system frequently overloaded when the microphone was positioned closer than 1 meter from the gun being fired.
Figure 1.
How does a high range option work – How do you get an extra 10dB from a standard ½” microphone?
When the extended measurement range is activated the standard 200v microphone has its polarization voltage lowered. This is reduced from 200 V to about 70 V. The microphone sensitivity will then be reduced by 10 dB and the instrument will be able to measure peak signals up to 150 dB. The change in the polarisation voltage will lead to a small change in the frequency response for the microphone. This change is automatically compensated when the extended measurement range is selected. Corrections are specific to each microphone type and should not be mixed.
With this option activated in the sound level meter most measurements were achievable.
Figure 2
However at a distance of half a meter from the gun with heavier shot the sound level meter did see overloads with measured levels over 150dBc peak.
Using a quarter inch microphone
With an adaptor, a ¼” microphone will fit onto a standard half inch preamplifier and you can then measure levels with a standard sound level meter.With this microphone it was possible to measure peak levels without overloads with the highest peak of 157.9dBc
Figure 3.
Why quarter inch microphones?
Quarter inch microphones are inherently lower in sensitivity than ½” microphones. The smaller surface area means the microphone diaphragms are less flexible and the same sound pressure impinging on them leads to less voltage coming from the capsule. This reduced voltage is desirable for very high sound pressure measurements as it is high signal voltages that lead to overloads!
How to measure beyond 160dB?
1/8th inch microphone – This allows measurements to 175dB. With a 120V preamplifier supply it can measure to 178dB.
Preamplifier voltage – For extremely high sound pressure measurements it is important to not only have a microphone with low sensitivity; you must also have a preamplifier which can handle high signal voltages to avoid clipping of the signal. For your sound level meter (or PC analyser) to display accurately these measurements a preamplifier with a 120V supply is preferable.