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BRUEL & KJAER 4220 User Manual

BRUEL & KJAER 4220
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The
necessary
correction
for
variation
in
atmospheric
pressure
is
read
directly
in
dB
off
a
barometer
supplied
with
the
pistonphone.
In
the
ordinary
range
(0°-60°C),
temperature
has
practically
no
effect
on
the
pistonphone's
cali-
bration.
The
Pistonphone
Type
4220 is delivered
in
a
wooden
case
with
the
barometer
and
different
adaptors.
Principle
of
Operation.
The
principle
of
operation
of
the
Pistonphone
Type
4220 is
shown
in
Figures
4
and
5.
The
two
pistons
are
symmetrically
driven
by
means
of
a
cam
disc,
mounted
on
the
shaft
of
a
miniature
electric
motor.
The
cam,
which
is
made
of
specially selected
tempered
steel, is
machined
to
a
high
degree
of
accuracy,
according
to
the
law
r = a + b
sin
4 a (Fig. 5).
*)
When
rotating,
the
cam
will
give
the
pistons
a
sinusoidal
movement
at
a
frequency
equal
to
four
times
the
speed
of
rotation.
Consequently
the
cavity
volume
is
varied
sinusoidally
and
the
RMS
sound
pressure
produced
will
be:-
where:
2 APS
P = r X
Po
X
,,
- dynes/cm2
Vv2
r =
Cp/C-r
=
ratio
of
specific
heats
for
the
gas
in
the
cavity
(r
=
1.402
for
air,
at
20°C
and
1
atm.).
P.
=atmospheric
pressure,
expressed
in
dyne/cm2.
A.
::a
area
of
one
of
the
pistons.
S
::a
peak
amplitude
of
motion
of
one
piston
from
the
mean
position.
V = volume
of
cavity
with
the
pistons
in
mean
position
+
the
equivalent
volume
of
the
microphone.
p
The
sound
pressure
level
in
dB
is
consequently:
S.P.L. = 20 log
~·
with
Pt
= 2 X
10
-4
flbar
= o.0002 dynes/cm2
=threshold
of
hearing.
It
is
assumed
in
the
above
equation
that
Ap
X S < <
V,
and
that
the
com-
pression
is
adiabatic.
These
conditions
are
satisfied
entirely
in
the
case
of
the
Pistonphone
Type
4220,
where
AP
X S/V < o.0002,
and
where
the
heat
con-
duction
correction
would
be
less
than
o.05"dB
at
20Hz.
**
)
It
is possible
to
define,
with
great
precision,
the
different
quantities
involved
in
the
equation.
The
piston
stroke
S,
which
was
difficult
to
measure
in
the
case
of
an
electromagnetically
driven
pistonphone,
is
simply
here,
one
fourth
of
the
difference
between
the
maximum
and
the
minimum
diameters
of
the
cam. Accurate
measurement
of
this
difference,
though
relatively
small,
offers
no
problem
by
using
a
precision
micrometer.
In
addition,
as
the
pistons
are
*)
A cam disc
corresponding
to
the
law
r = a + b sin 2 a is available on request.
With
this
disc
the
reference
frequency is
125
Hz
and
the
distortion
less
than
1.5
.%
.
*
*)
Experiments
on
the
Piston
phone
Type
4220
have
shown
that
the
compression
can
be
considered as
adiabatic
down
to a frequency of 1
Hz
(the
error
being
approx.
o.3
dB
at
1Hz).
7

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BRUEL & KJAER 4220 Specifications

General IconGeneral
BrandBRUEL & KJAER
Model4220
CategoryMeasuring Instruments
LanguageEnglish

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