Sound Level Meter Components: |
BRÜEL & KJÆR Type 2203 Ref. B&K Instruction Manual, particularly Specifications p8
Microphone
Capacitive Microphone, Sensitivity: 50mV/Pa nominal,
A thin metal diaphragm suspended above a metallic backing plate,
forming a capacitor. A high charge is placed across the two plates.
As the impinging sound wave moves the plate, the capacitance varies,
since the charge is fixed, the Voltage must follow the changing
capacitance – hence our output signal. These microphones are
delicate, with an excellent frequency response.
Weighting Networks
Weighting approximates the inverted equal loudness contours, “Linear” leaves the instrument at dB ref 20µPa
Filters
External filters can be switched in, and are useful in
reducing environmental noise outside the band of interest.
Our SLM is fitted with 11 octave filters shown here. Note
when calibrating 750Hz 1.5kHz, 3 & 6kHz, use the next
highest filter on the dial, as the roll off is steeper on
the high frequency side. You will notice a reduced level if
you are using the incorrect filter! Each filter can be adjusted
for gain individually if the weighting switch is turned on. |
 Octave Filter |
Amplifiers
B&K 2203: Total amplification 110dB. Ref Specifications for input level range.
Attenuators
The first attenuator follows the input stage and is generally left on its minimum setting (70dB).for quiet signals, the two ganged attenuators are adjusted first (transparent dial on the B&K 2203)
Using the attenuators in this order optimises the SLM characteristics
between two extremes:
-First is the possibility of overdriving the input stage and producing
distortion (inaccurate), particularly when inserting weighting or
a filter, and second, the introduction of noise by inserting too
much attenuation at the input rather than at the later stages. –
The noise is then amplified by all the following stages, reducing
the signal to noise ratio. Digital SLMs manage this aspect automatically.
Note Specifications “inherent noise max 40µV (32dBµV)
ref to input”. Ours measured about 30dBµV open circuited
(no microphone) or 31µV Ref. B&K 2203 SLM 3.4.2 Procedure
for Sound Measurements.
RMS Detector & Display
Detector best accuracy +/-0.5dB. Note RMS - applies only to sine
waves. Meter accuracy: 0.2dB for readings above the zero mark on
the scale. Other sounds can be measured, but with reduced accuracy.