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A test of this type was carried out on the B & K Precision Sound Level
Meter Type 2203 to determine the effect of vibration on the output. The
test set-up is shown in Fig.5.1 and consists of an Automatic Vibration
Exciter Control Type 1025 driving the Exciter Body/General Purpose Head
Type 4801/4812 combination via the Power Amplifier Type 2707. The
Sound Level Meter signal output with first microphone and then dummy
load connected were recorded using a Level Recorder Type 2305 and are
shown in Fig.5.2 and curves I and II.
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Fig.5.2. Output signal of 2203 (dB re 2 x
1(;-5
N!m
2
J
with a table level of
1 g RMS. Curve I is obtained with the microphone in place, while
the curve II is obtained with the microphone substituted by an
equivalent impedance which is sound and vibration insensitive
5.2.
SWEEP RANDOM TESTING
Wide-band random frequency testing can be very expensive because of
the equalization filters required to shape the test frequency spectrum to
comply with the specifications set. An economic alternative to wide band
testing is sweep random testing. It consists of one or more narrow bands of
noise, sweeping (or dwelling) according to a detailed test programme and
emphasizing various important frequency regions.
The test arrangement shown in Fig.5.3 is set up for two such bands. The
Sweep Random Generators Type 1042 drive the Power Amplifier Type
2707 and the Exciter Head/Body combination Type 4812/4801, via isolat-
ion transformers. Each generator sweeps back and forth in a different
frequency band. The associated Heterodyne Slave Filters Type 2021 ensure
that the compressors are fed with the feedback signals they require for
correct operation independent of spurious accelerometer and table reson-
ances, and of the other generators' signal.
43
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