Finding The calibration Reference of The Sound Level Meter: |
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The calibration reference sound pressure level is 20µPa, the threshold of hearing. The nominal sensitivity of the provided microphone is 50 mV/Pa, so at the threshold of hearing, the microphone produces:
50x10-3 x 20 x10-6 = 1x10-6 Volts, or 1 microvolt (1µV)
This is used as the reference for the dB scale of the Sound Level
Meter, so 1µV = 0dBµV, & corresponds to 0dB ref.20µPa,
alias “dB SPL”.
Calibration For Microphones
For both methods below, check the mechanical zero on the dial with the instrument turned off before proceeding.
Piston Phone BRÜEL & KJÆR 4220
Ref Precision Sound level meter 2203 “Instruction Manual”,
Sect.3.3.2 Acoustic Cal. (external Source)
An external reference noise source is applied to the microphone.
In our case, the 4220 Pistonphone produces a 250Hz sound pressure
level of 123.8dB ref. 20dBµPa at a nominal ambient
pressure of 760mmHg in a coupler of predetermined volume terminated
by a B&K microphone with protecting grid. The SLM is adjusted
to give the stated reading, and an accuracy of within 0.2dB can
be achieved.
This method calibrates the complete instrument to dBµV
or dB20µPa, whereas the internal reference
oscillator method below does not include the microphone and pre-amplifier.
Internal Reference
The stable internal reference provides a 1 kHz 50mV signal that represents the assumed sensitivity of the microphone (50 mV/Pa). An allowance for the variation of the actual sensitivity of the microphone from this is made as an offset when adjusting the gain of the SLM. To find this value, which is given for every microphone as the Open Circuit Correction Factor “K0” (refer in this example to the “Calibration Chart for Condenser Microphone Cartridge Type 4144” in the Appendix). Our 1” microphone has a “K0” of +0.4dB. The Gain of the Preamplifier for the type of SLM is also subtracted from “K0” to give the true correction factor. For the B&K 2203 SLM this is “Typically”–1.2 dB, giving a total offset of +1.6dB. Microphone extension cables also have an insertion loss, which must be included as well:
K = K0 – Input Gain – Adapter Gain
To Calibrate, the Attenuator dial is switched to “Ref”
to apply a tone at the input of the first attenuator (from the “Reference
Oscillator” in the block diagram)
To adjust the instrument, use a screwdriver on the front panel.
Follow these steps:
Check the mechanical zero, check the batteries (select “Batt”),
then switch to “Fast” & “C” or “lin.”
after a 20 second warm up select “Ref”. Adjust the meter
on the RED “K-factor” scale to equal the total “K”
factor as discussed.
Calibration for Accelerometers
The Artificial Mastoid is a type of accelerometer, and produces an electrical output in proportion to the force or acceleration applied by a bone vibrator. The mastoid produces an absolute output for a known input expressed in millivolts per Newton. The method of calibrating the Sound Level Meter to measure this Voltage is discussed in the section on the “Artificial Mastoid” later.