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BRUEL & KJAER 4131 - Random Incidence Response

BRUEL & KJAER 4131
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4135
· 4136- INTRODUCTION
At
higher
frequencies
the
diffractions
of
the
sound
waves
on
the
microphone
produce
an
appreciable
change
in
the
resulting
sound
pressure
acting
on
the
microphone
diaphragm
as
illustrated
in
Fig.
0.3.
The
difference
Pl
- po,
called
free-field
correction,
depends
on
the
orientation
of
the
microphone
with
respect
to
the
direction
of
propagation
of
the
sound
and
on
the
external
dimensions
of
the
microphone
(
in
particular
those
of
the
front
and
of
fitted
protective
grids
or
"
nose
cones").
The
free-field
behaviour
of
a
microphone
is
thus
described
by
means
of
a
s
et
of
free
-
field
correction
curves
for
various
incidences,
which
should
be
added
to
the
pressure
frequency
curve
of
the
microphone
in
each
particular
case.
See
Figs.
1.14-1.16.
For
microphones
intended
for
free-field
work
it
is
possible
to
give
the
diaphragm
resonance
such
a
damping
that
the
normal
incidence
free-field
c
orrections
are
compensated
for
up
to
frequencies
well
above
the
resonance
frequency,
in
order
to
ob
t
ain
the
flatt
e
st
possible
frequency
response.
Random
Incidence
Response
(Diffuse
Field
Response).
The
random
incidence
response
of
a
microphone
for
a
given
frequency
is
the
root-mean-
s
quare
value
of
the
free
field
sensitivity
for
all
angles
of
inciclence
of
the
s
ound
wave.
It
corresponds
to
the
diffuse
field
sensitivity
of
I
he
microphone,
the
diffuse
field
being
a
sound
field
in
which
the
souncl
e
nergy
density
is
uniform
and
the
mean
acoustic
power
per
unit
area
is
the
same
in
all
directions.
The
International
Electrotechnical
Commission
(
publication
no.
123, § 8.2)
ha
s
given
a
practical
rule
for
the
calculation
of
the
random
incidence
sensitivity
from
the
free
-
field
sensitivities
at
definite
angles,
with
coefficients
proportional
to
the
relative
solid
angles
.*)
Fi
gs. 1.3
and
2.2
show
the
random
incidence
frequen
cy
r
es
pon
se
of
the
different
B & K
quarter-inch
microphone
combinations.
The
se
curves
should
be
taken
into
consideration
in
the
case
of
measur
e
ments
in
highly
reverberant
rooms
giving
rather
diffuse
field
condition
s.
Wh
en
the
spectral
distribution
of
the
sound
varies
with
the
angle
of
incidence,
correct
integration
is
only
possible
in
the
range
where
the
micro-
phone
is
both
lin
e
ar
and
omnidirectional
(practically
up
to
2(}
kHz
with
the
quarter-inch
microphones).
Omnidirectional
microphones
are
also
necessary
in
the
case
of
rapidly
mo
v
ing
sound
sources
(
aeroplanes,
motorcars,
etc.).
*) So, Sso, S6o, - - - - S1so
being
the
sensitivity
of
the
microphone
at
angles
of
incidence
of
0°,
30°, £0° , - - - - 180°,
the
random
incidence
(diffuse field)
response
S is given by
the
formula:
S2
= o.018 (So2 + S1so2) + o.129 (Sso2 +
S1
5
o2
) + o.
224
(S6o
2
+ S12o
2
) + o.258
Sgo2
6

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