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BRUEL & KJAER 4220 User Manual

BRUEL & KJAER 4220
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The
Noise
Source
Type
4240
mechanically
produces
an
approximate
"white"
random
noise.
The
advantage
of
the
"white"
noise
is
that
it
:
calibrates
over
a
wide
range
of
audio
frequencies,
i.e.
in
conditions
approaching
an
actual
noise
measurement.
The
noise
source
is a
small
independent
device, well-
adapted
for
field
calibration
check
of
sound
measuring
equipment,
and
requires
no
electric
power
for
operation.
The
accuracy
of
the
noise
source
is,
however,
limited
to
± 1.5 dB.
The
principle
of
the
Pistonphone
is
that
a
sound
pressure
is
produced
in
the
coupler
cavity
by
a
piston
moving
in
and
out.
This
sound
pressure
is
proportional
to
the
variation
of
the
coupler
volume, i.e.
tq
the
stroke
and
area
of
the
piston,
and
the
error
in
the
sound
pressure
level
depends
in
particular
on
the
accuracy
in
the
measurement
of
the
stroke.
Fig.
1.
The
"old"
pistonphones;
to
the
left
an old
mechanically
driven
pistonphone,
and
to
the
right an electromagnetically
driven
pistonphone.
The
piston
phone
was
first
developed
as
a
normally
driven
piston
(crank
+
connecting
rod}, see Fig. 1.
With
this
arrangement
is
was
practically
im-
possible
to
carry
out
a
calibration
at
a
sufficiently
high
frequency
.
To
obtain
200 Hz,
the
motor
should
run
at
12ooo r.p.m.,
and
even
at
much
lower
speeds
than
this,
vibrations
occurred
in
the
bearings
and
in
the
connecting
rod,
with
an
eventual
wobbling
of
the
motor
spindle,
which
produced
an
unacceptable
error
in
the
determination
of
the
stroke.
In
addition,
the
distortion
of
the
piston
movement
caused
by
the
finite
length
of
the
connecting
rod
had
to
be
taken
into
account,
and
due
to
the
variations
of
the
torque
on
the
motor
axle
the
frequency
stability
was
also
insufficient.
For
these
reasons,
the
mechanica
1
drive
has
been
superseded
by
an
electro-
magnetic
drive
of
the
type
used
in
loudspeakers,
enabling
audio
frequencies
to
be
easily
obtained.
The
piston
stroke
has
to
be
measured
optically.
This
can
be
made
with
a
certain
accuracy
by
means
of
a
good
microscope.
How-
ever,
the
use
of
a
microscope
and
the
difficulty
in
obtaining
a
high
sound
4

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BRUEL & KJAER 4220 Specifications

General IconGeneral
BrandBRUEL & KJAER
Model4220
CategoryMeasuring Instruments
LanguageEnglish

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