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the signal should be compressed
if
the load
impedance
varies considerably
with frequency.
The upper curve was recorded with the analyzer in the linear mode. It
shows the signal level is of the same magnitude as the background noise and
cannot be recorded without using narrow band analysis. The lower curve
shows the same signal recorded with the analyzer in the selective mode
(3.16 Hz bandwidth). Only the fundamental originating from the B.F.O. is
recorded, thus the influence of noise, spurious vibration, and harmonic
distortion is rejected.
7.3.
SOUND ABSORPTION MEASUREMENTS
Measurement of the acoustic absorption coefficient of a material may be
made by inserting a small sample of the material into an acoustic trans-
mission line and measuring the maxima and minima of the standing waves
obtained.
Fig. 7.6 shows a typical measuring set-up using the 2010 with the B & K
Standing Wave Apparatus Type 4002. The Absorption Coefficient can be
read directly from the 2010 if scale SA 0054, available on request, is used.
Selective reception is necessary if measurements are made on materials with
very low absorption coefficient as the minima are at a very low level. The
specific acoustic impedance of a material may also be determined by this
method. Reference should be made
to
the instruction manual for the
Type 4002 for further details.
78
~f.~~
~~:
:
t
'
· -~-
--
~ .
Heterodyne
Analyzer
2010
-
Standing Wave Apparatus 4002
tube terminated by perfect reflector
ttrv;v
tube terminated by absorption material
171405
Fig.
7.
6. Sound Absorption measurements
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