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properties of the projector, and when picked up by a hydrophone it will
be transformed into an electrical signal. This signal is then passed via the
Amplifier to the Level Recorder and automatically recorded.
In Fig. 5.15 a typical directional characteristic of a piezo-electric projector
is shown.
Recording
of
Frequency Response
of
Hydrophones.
A projector, a standard hydrophone and the hydrophone to be tested are
placed in an appropriate free field under water.
To obtain an automatic recording, the Level Recorder Type 2305 should be
used. The mechanical connection between the motor of the Level Recorder
and the tuning capacitor of the
BFO
being effected by means of the Flexible
Shaft UB 0041.
In Fig. 5.16 the measuring arrangement is shown. The voltage from a
standard hydrophone is fed to the COMPRESSOR INPUT of the BFO thereby
making it possible to keep a constant reference sound pressure level at the
symmetrically placed hydrophones. If the standard hydrophone has a linear
free field response in the measured frequency interval, the recording will
immediately show the free field response of the unknown hydrophone.
Due to the use of the compressor arrangement, the delay time (i.e. the time
needed for the sound to travel the distance from the projector to the hydro-
•
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Bruel & KJatr
+
BrU.I
&
KJ-
C..........
K
5025
•
I•
•
I•
I•
•
I
•
•
Measuring
Object:
dB d
Piezoelectric
4020
,.
Hydro§hone
-
11
........
,
a
""'
..,
Type
X
5492
30
15
I '
\
~·
'
20 10
Zero
Lev~
_ _
Paper
Sp.:_3_
L Lim.
Fr~
f.OQ
Rec. No.:
_ _
b
Pot~
~Dot.:
_ _ _
10
5
' W
r.
Sp.: 200
Sign~---
Rect"---F'*I. Scale • -
o
0
1000-
2000
5000
10000
20000
QP
0123
1000
10000Hz
f6f78f
Fig. 5.17. Curve a: Free field response of a hydrophone obtained with the
set-up of Fig. 5.16.
Curve b: Recording of the reference sound pressure level on the
standard
hydrophone.
39
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