BANDS OF
RANDOM
NOISE
Bands
of
random noise are often used to produce more realistic conditions
for loudspeaker tests, sound insulation tests, reverberation, etc.
Bands
of
Random Noise may be generated with
the
combination of
1614
or
1615
and
the
B & K Random Noise Generator Type 1402. Octave or
1/3
octave bands can be generated for excitation of, for instance, a loudspeaker
with a
constant
output
power
at
all
frequencies. The 1402 has a 3 dB/octave
weighting
network
which compensates for
the
increase of power with fre-
quency which occurs when constant percentage bandwidths
of
noise are
used.
ANALYSIS
FROM TAPE RECORDER
The
1614/15
Filter Sets can be used
to
directly analyse
the
output
from a
tape
recorder.
The
B & K
Type
7001, with maximum signal
of
1 V
RMS
is
suitable for direct connection
to
the
input of
the
Filter Set. For recording
of
spectrograms,
the
Level Recorder Type 2305 can be synchronized with
the
Filter Set.
LOW FREQUENCY
ANALYSIS
The
Type
7001
Tape
Recorder has a frequency response down
to
DC,
and it
may sometimes
be
required
to
frequency analyse down
to
very low fre-
quencies from
the
tape. This can be done by means
of
frequency transfor-
mation with
the
tape
recorder. The tape
is
simply played back
at
a faster
speed, so
that
all frequencies are multiplied by
the
ratio of
the
playback
to
recording speed. Frequency ratios up
to
40:1 are available from
the
7001, so
that
for
the
2Hz
filter
of
the
1614, 0.05
Hz
components can be analyzed
with frequency transformation.
ANALYSIS
OF TRANSIENTS
Signals
of
short
duration,
pulses, transients, etc. can be recorded on
the
7001 and frequency analyzed with
the
Filter Set by making use
of
the
tape
loop adaptor.
The
signal
is
either recorded
onto
the tape loop directly,
or
else a piece from a
continuous
tape
is
selected and made into a loop. By
combining this
technique
with
that
of frequency transformation, analysis
in
the
range 0.05
Hz
-
20
kHz
is
possible, of pulses, transients etc.
The
technique
is
to
repeat
the
signal by means
of
the loop adaptor, as
often
as
is
required for analysis. Normally this means repeating it once for each
filter
of
the
filter set. Provision
is
made
in
the
7001 to do this automatically
in
conjunction with
the
Filter Set and a
Level
Recorder Type 2305.
38
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