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BRUEL & KJAER 4131 - Appendix; Microphone Calibration

BRUEL & KJAER 4131
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Appendix
Microphone Calibration.
Error in
an
Absolute
Sound
Level Measurement.
Considering
the
various
standard*)
errors
a, b, c -
--
which
are
expected
to
occur
on
each
of
all
the
different
parts
of
the
measuring
arrangement,
the
total
standard
error
is s =
va2
+ b2 +
c2
+ ----
In
the
case
of
the
measuring
arrangement
of
Fig. 1.2,
for
example,
the
differ-
ent
sources
of
error
to
be
considered
in
the
measurement
of
the
sound
pressure
level
on
the
microphone
diaphragm
are
as
follows
(rough
estima-
tion):
(a)
Microphone
cartridge
calibration:
o.2 dB
after
proper
corrections.
(b)
Transmission
loss
of
the
cathode
follower
and
loading
of
the
cartridge:
o.1 dB.
(c)
Polarization
voltage's
deviation
from
200
V:
o.2 dB.
(d)
Voltage
calibration
of
the
2603: o.1 dB.
(e)
Meter
scale
and
attenuators:
o.2 dB.
(f)
Reading
error:
o.1 dB.
(g)
Uncertainty
on
the
frequency
response
of
the
measuring
equipment,
which
should
be
taken
into
account
when
considering
bands
of
noise
having
frequencies
differing
greatly
from
the
frequency
of
micro-
phone
calibration
{250 Hz).
Still
considering
the
equipment
of
Fig.
1.2,
this
uncertainty
is
about
± o.1 dB
from
100
Hz
to
20
kHz,
and
± o.3
dB
from
50
Hz
to
50
kHz
for
a
free-field
measurement
at
0°
incidence
with
the
4135
or
at
90
,o
incidence
with
the
4136.
When
measuring
broad
band
noise
through
the
A-B-C
weighting
networks
of
the
2604
the
probable
frequency
response
uncertainty
is
of
the
order
of
± o.3
dB
.
Equating
o.1 dB
to
1.1
%,
o.2 dB
to
2.2 %
and
o.3
dB
to 3.3 .% ,
the
standard
error
in
the
measured
result
will
thus
be
S = v 2.22 + 1.12 + 2.22 + 1.12 + 2.22 + 1.12 + 3.32 =
5.4%
= ± o.48 dB
and
the
probability
of
obtaining
larger
errors
than
o.8 dB is less
than
o.l.
Note:
In
some
cases
of
irregular
statistical
distribution
of
signal
amplitudes,
errors
may
be
caused
by
the
limited
averaging
time
(damping)
of
the
meter
or
by
the
limitation
of
the
RMS
rectifier
possibilities.
If
a
better
accuracy
is
desired,
the
most
practical
and
reliable
solution
is
to
make
a
direct
calibration
of
the
whole
measuring
arrangement
before
each
*)
The
standard
error
s
is
such
that
there
is
68
%
chance
of obtaining
errors
smaller
than
s,
90
%
smaller
than
1.65 s
and
99
%
smaller
than
2.58
s.
36

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