Brüel & Kjær 4170 User manual
Probe Microphone
Type 4170
A
horn-coupled
probe
microphone
designed
for
sound pressure
measurements
in
places
where
normal
microphones
cannot
be used.
It features a
wide
frequency
ra~e
,
a very
high
acoustic
orifice
impedance
and a
built-in
preamplifier
allowing
direct
connection
to
the
B & K Measuring
Amplifiers
,
Frequency
Ana-
lyzers and
Spectrometers
. Delivered with an
individual
calibration
chart
of
sensitivity and
frequency
response.
BRUEL&K'-'A:.R
I ,
..-
,
4170
PROBE
MICROPHONE
TYPE
4170
October
1974
CONTENTS
1.
INTRODUCTION
AND
SPECIFICATIONS
(PRODUCT
DATA)
...........
.................
............
.........
1
2.
OPERATION
...........................................
..
..
..
.......................
....................
.......
..............
........ 3
2.1. GENERAL .....
.......
.. .. ....
....
.......
....
......... ...... ...
..
....
...... ......
..
.............
....
.
....
3
2.2. CONNECTION TO THE
MEASURING
INSTRUMENTATION
.....
..
.....
.
......
..
........
3
2.3. CALIBRATION
....
...
....................
...
......................
...
....
........ .. ......
....
.......... 4
2.4.
SOUND
PRESSURE
MEASUREMENT
...
... .... ... ....... ......
....
...............
..
.....
....
. 5
Operating Temperature ...
....
....
...
.......
.
......
...
....
... ...
......
............:
..
...
......
..... 5
Wall
Transmission ....... ........
..
........
....
......
.
......
............. ....
...
..
....
.. .. ..
....
..... 5
Dynamic Range .. .
.....
.............
..
.
....
.. ....... ..... ....
..
... .. .... ..................
....
.
.......
6
2.5. PROBE EXTENSION ..
.....
.. ...
......
.............. ....
....
................ .....
..
.................. 6
3.
DESCRIPTION
.
......
..
...........
......
.
..........
.
..
....
..
...................
........
....
...........................
..
..
......... 7
3.1. PRINCiPLE .. .
.....
.......
.
......
..
.... ...... ........ .. .... ..
..
.. ..
....
..........
..
.........
.....
..
... ... 7
3.2
. CONSTRUCTION
....
....
.....
......
..
.......
.....
.....
.....
.
....
... .... ......
..
.....
....
.
....
'........ 8
3.3.
MICROPHONE
ASSEMBLY
..... .....
.....
.... ..........
..
....
....
.......
.....
..
.......
....
.
....
.. 8
4.
CHARACTERISTICS
............
..
..
..
....
..
..............................
........
........
......
......
.........
..................
10
4 .1 . FREQUENCY RANGE .......
..
.
....
.....
........
.. ..
......
...... ......
..
.......
........
.......
....
..
10
4.2. TEMPERATURE .
..
..... .
.................
.... ....
......
.
...........
............... .......
....
... ......
10
4.3. PROBE ORIFICE
IMPEDANCE
...............
....
.... .. ......
..............
.....
..
.......
.....
.... 11
4.4. NOiSE
.......
..
....
...
..
...................
....... .....
........
...
.....
....
... ... ......... ......... .
.....
.
13
FEATURES:
•
Wide
frequency
range
30
Hz
to
8 kHz
• High
sensitivity
•
High
acoustic impedance
•
All
data
individually
calibrated and supplied
•
Small
size and
low
weight
USES:
•
Measurements
in
the
ear
•
Measurements
inside
intricate
machinery
•
Model
measurements
•
Measurements
in sound
insulating
materials
•
Acoustical
studies
of
musical
instruments
type
4170
Horn-coupled Probe Microphone
The Probe
Microphone
Type
4170
is a
high
precision
measuring
micro-
phone having
the
same
excellent
performance
as
the
other
B & K
measuring
microphones.
The
microphone
is calibrated and
delivered
with
a
calibration
chart
giving
the
frequency
response and
all data necessary
for
accurate
measurements
.
It is designed
to
cover a
wide
range
of
measurements
where
mini-
mum
disturbance
of
the
sound field
is
important
.
This
is achieved by giv-
ing
the
probe a very
small
diameter
and a very
high
orifice
acoustic
im-
pedance. The
4170
uses an
acousti-
cal exponential
horn
to couple a very
thin
probe tube
to
a 1/
2"
condenser
microphone
. The probe is attached
to
the
narrow
end
of
the
horn
and
the
microphone
is connected
to
the
wide
end.
An
acoustic
matching
im-
pedance in
the
microphone
end
of
the
horn
equalizes
the
frequency
re-
sponse of
the
assembly. A
frequency
response
of
30
Hz
to
8 kHz
within
4 dB is
thereby
obtained (Fig. 1).
In
order
to
measure
correctly,
a probe
microphone
must
be more
sensitive
to
sound
pressures
at
the
probe orifice
than
it
is
to
sound
transmission
through
the
side
walls
of
probe and
horn
. In
the
4170
the
sound
attenuation
of
the
walls
is
greater
than
20
dB at all
frequencies
between
30
Hz and
10kHz
. Fig.2
shows
the
sensitivities
of
the
4170
with
open and closed probe
orifice,
when
placed in a plane
sinusoidal
sound wave.
An
Adaptor
DP
0181
for
fitting
the
4170
to
the
Pistonphone Type
.4220
or
the
Sound Level Calibrator
Type
4230
is supplied
for
calibra-
tion
.
The probe has an
outer
diameter
of
1,25
mm
and a
length
of
60
mm
.
The
horn
is
120
mm
long. The
horn
and
the
probe are
normally
covered
by a stainless steel tube,
which
pro-
tects
them
and gives
them
a
uniform
outer
diameter
. The protection
tube
may be removed
if
only
the
probe
can be inserted at
the
measuring
spot.
A
preamplifier
is
mounted
inside
the
handle
of
the
4170
and con-
nects, via a cable and a standard
7-
pin plug,
directly
to B & K's extensive
range
of
Measuring
Amplifiers,
Fre-
quency
Analyzers and Spectrome-
ters.
The
built-in
preamplifier
is
electri-
cally and
mechanically
similar
to
the
B & K 1/
2"
Microphone
Preamplifier
Type
2619
and
the
condenser
mi-
crophone used corresponds to
the
11
2"
Condenser
Microphone
Type
4134.
For detailed
information
on
the
2619
and
the
4134
see data sheets
for
Microphone
Preamplifiers
Types
2615
to
2627
and Condenser
Mi
-
crophone Cartridges Types
4133
to
4163
respectively.
Frequency
Response
":
30
Hz
to
8 kHz +
3,
- 1 dB re
250
Hz
(O
OC to
50
°
C)
2
Sensitivity
at
250
Hz"
:
Typical -
60
dB
re 1V I Pa
(1
mVI
Pa)
Distortion
:
< 1%
for
SPL's
below
170
dB
Acoustical
Impedance
of
Probe
Orifice
:
>
4xl0
8
kgxs
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860
Frequency
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6
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-
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10
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dB
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90
dB
~
215
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1124
10
100 200
Multiply
F'eq"'«w:y
Scal.
by 1 500 1000 2000 5000
Zo,.
Lo.o!
, -100 dB
re
1 V(Pa
10000
20000 40000 D A 8 C lin.
1612
12
112
AB
C lin. 173286
Fig
.1.
Frequency
response
of
the
4170
B,u.1
"Kimr
~
Obitoct
: Probe Microphone 4170 B,uet & Kjmr
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DO
00
0 0 0 0
DOD DOD
00
0 0
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173520
Fig.2.
Sensitivies
of
the
4170
with
open
and
closed
probe
tube
Specifications 41
70
Noise:
(equivalent
SPL
from
20
Hz to
20
kHz)
25
x
10
- 3
Pa
(62dB
re 2 x
10
-5 Pal
Maximum
Permitted
Temperature
of
Probe
Tube
:
120
°C
(248
°
F)
(Maximum
allowable
temperature
of
preamplifier
is
60
°
C)
Dimensions:
Probe
Diameter
:
1,25
mm
(0,05
in)
. Probe
Length
:
60
mm
(2,3 in)
Diameter
of Protection
Tub
e:
5,5
mm
(0,22
in)
Length
of Protection Tub
e:
170
mm
(6,7
in)
Maximum
Diam
eter of
4170
:
28
mm
(1
,1 in)
Maximum
Length
of
4170
:
315
mm
(12.4
in)
Weight
:
300
g
(0,66
Ib)
Accessories
Included:
Adaptor
DP
0181
Individually
ca
librated
2.1.
GENERAL
,
2.
OPERATION
The Probe
Microphone
Type
4170
may
be used
either
with
or
without
the
protection
tube
. Fig .2.1
shows
the
4170
without
protection
tube
.
Fig.2. 1. The Probe
Microphone
Type
4170
(protection
tube
removed)
Should
the
probe
or
horn
be damaged (by
bending,
excessive
temperature
on
the
horn,
etc.
),
it
may
be replaced by a spare probe/
horn
tube
UA
0442
(to be ordered separately).
The
replacement
procedure
is as
follows
(reference
may
be made
to
the
cross-sectional
view
of
4170,
Fig.3.2)
:
1.
Unscrew
the
front
end
of
the
housing
using
a 9
mm
spanner
.
2. Remove
the
probe
tube
from
the
front
end by
pulling
it
outwards.
3. Place
the
spare
tube
UA
0442
through
the
front
end
and
push
it
until
a
firm
contact
is
felt.
4. Replace
the
small
rubber
sealing
ring
on
the
tube,
against
the
front
end.
5.
Screw
the
front
end
fully
in.
2.2.
CONNECTION
TO
THE
MEASURING
INSTRUMENTATION
Pins
5 & 6 shorted
for
operation
from
28 V
only
Polarization
Voltage
Ground
(Heating element)
Signal
Casing
to
outer screen
Looki
ng
at
pins
from
rear
of
socket
(or outside
of
connecting plug)
120 V
DC
Supply
28 V
DC
Supply
Heati
ng
element
17
32
02
Fig
.2.2
Pin
connections
(view
from
soldering
side
of
4170
plug)
3
Type
4170
connects
directly
to
the
PREAMP. INPUT
of
B & K
Measuring
Amplifiers
, Fre-
quency
Analyzers, Spectrometers and Power Supplies, via a standard
7-pin
plug
JP
0701.
The pin
connections
are
shown
in Fig.
2.2.
2.3.
CALIBRATION
4
Calibration
Chart
for
Probe Microphone
Type
4170
Se
,ial
No.
"1.:3~
.
1
.
"
.
Sensitiv
i
ty
at
250
Hz
The Probe
Microphone
is i
ndividually
calibrated at
the
factory and is delivered
with
a cali-
bration
chart
giving the
sensitiv
ity at
250
Hz
and
the
frequency
response curve. See
Fig.2.3.
Bruel &
Kimr
Bruel & Kimr Bruel &
Kimr
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Bruel&Kicer
Potentiometer
Range:
__
dB Rectifier:
__
Lowe
r Lim. Freq.:
__
Hz
Wr
. Speed:
__
mm
/sec.
PaperSpeed
:
_mm
/sec.
IIrileI&KjIor
..
dB
o
- 5
.............
.1.
.4 mV/
Pa
- 10
41
70
is used
in
connection
with
a Bruel
& Kj
ce
r
measuring
amp
lif
ier
for
micro
-
phones
Date
....
19,3,74
...............
. -
15
Si
gnatur
e:
.
~
..
.
~
..
...
Be
0088
QP
1124
-20
10
20
Hz
50
100
200
Multiply
Fr
equency
Scale by
500
1000
2000
Zero Level:
-----
Fig
.2.3. Typical calibration chart
5000
10000
20000
40000D
ABC
lin
.
1612
/
2112
ABC
Lin.
740497
Knowing
the
sensitivity
.
it
is possible
to
calibrate
the
measuring
instrument
using
the
in-
ternal
reference voltage. For
this
type
of
calibration, reference should be made
to
the
in-
struction
manual
for
the
measuring
instrument
in
use.
However,
it
is preferable to calibrate
the
whole
measuring
system
from
probe orifice
to
display device. For
this
purpose, the
Adaptor
DB
0181
is supplied
with
the Probe
Micro-
phone. It
fits
both
the
Pi
stonphone
Type
4220
and the Sound Level Calibrator Type
4230
. The Pistonphone produces a
nominal
sound pressure level
of
124
dB reo
20
pPa
at
250
Hz
wi
th
an accuracy
of
±
0,2
dB. The Sound Level Calibrator produces
94
dB re
20pPa
at 1 kHz
with
an accuracy
of
±0,
25
dB. #
For calibrating a
measur
ing system,
fit
the,
Adaptor
DB
0181
to the
4220
or
4230
and
push
the
probe
fully
into
the
Adaptor
. The calibration procedure depends upon
the
meas-
uring
instrument
in use and reference should be made
to
the
relevant
instruction
man-
ual.
Note:
When
using the
Sound
Level Calibrator Type
4230,
attention
should be paid
to
the
frequency
response curve given by
the
individual calibration
chart
of
the
4170,
since
the
response at 1 kHz may be
higher
than
at
250
Hz. For example, the calibration
chart
of
Fig .2.3 exhibits a +
1,5
dB
amplification
at
1 kHz (compared to
the
250
Hz
level), and a
correction
factor
of
-1,5
dB
must
therefore
be used
when
calibrating a system using
Type
4~30
.
Scale
no
. Instrument Type Sensitivity
Range
SA
0060
2607,2114,
0,8 - 5 mV/Pa (Lin)
2010,2120
SA 0083
2607,2114
0,28 -1,6 mV/Pa (Lin)
2010,2120
SA
0061
2606, 2113, 0,8 - 5 mV/Pa
2130
SA 0073 2606, 2113, 0,28 -1,6 mV
/Pa
2130
SA 0075
2121
28 -100 mV/Pa
4
0536
Table 2. 1.
Meter
scales
for
the
use
of
Type
4170
with
different
B &K
measuring
instruments
However,
several B & K
instruments
offer
the
facility
of
interchangeable
meter
scales, al-
low
ing
direct
meter
reading
of
sound pressure levels
with
different
transducer
sensitivi-
ties.
With
a sensiti
vity
of
typically
1
mV
I
Pa,
the
4170
may be used
with
the
meter
scales
indicated in Table 2.1 (Note
that
with
the
Frequency Analyzer Type
2121
equipped
with
the
meter
scale
SA
0075
, a
correction
factor
must
be used, see section 3.2.1
of
the
2121
Instruct
ion
Manual)
.
2.
4.
SOUND
PRESSURE
MEASUREMENT
After
calibration
of
the
measur
ing system,
sound
pressure level
measurements
may
be
carried
out
in
the
same
way
a~
w i
th
normal
microphones
.
However
, a
few
points
should
be kept in
mind
.
2.
4.1.
Operating
Temperature
The probe
tube
itself
may be
submitted
to
temperatures
up
to
150
°C,
but
if
the
exponen-
tial
horn
is
submitted
to
such
a
high
temperature
,
the
sealant
used
for
the
horn
assembly
may
be damaged. It is
therefore
recommended
that
the
horn
as
such
should
not
be sub-
mitted
to
temperatures
higher
than
120
°C.
In all cases,
it
should
be
remembered
that
the
maximum
operation
temperature
of
the
preamplifier
is
60
°C.
More
details
on
the
4170
temperature
characteristics
may
be
found
in section 4 .2 .
2.
4.2.
Wall
Transmission
When
the
measuring
point
is inside a restricted
volume
and
the
part
of
the
probe
outside
is exposed
to
a sound
field,
it
should
be checked
that
this
has no
influence
on
the
meas-
ured sound pressure level
due
to
side
wall
transmission
. Fig.2
of
the
Introduction
shows
that
the
wall
attenuation
without
the
protection
tube
is at least
20
dB over
the
range
30
Hz
to
10kHz
. It is advisable
to
keep
the
protection
tube
fitted
whenever
possible
for
its
mechanical
protect
ion
benefits
.
5
2.4.3
.
Dynamic
Range
The
wide
band
(20
Hz
to
20
kHz)
inherent
noise
of
the
4170
corresponds
to
an equiva-
lent
sound pressure level of
62
dB re o 201lPa. The upper
limit
of
the
dynamic
range
for
less
than
1%
distortion
is
170
dB. It should be noted
that
if
selective
measurements
are
carri
ed
out
,
lower
levels can be measured. See
Fig.4
.6.
2.
5.
PROBE
EXTENSION
6
For special applications (e. g.
measurement
inside
the
ear
with
the Probe
Microphone
fixed on
the
subject's neck),
it
may be desirable
to
provide the probe
with
a curved
exten-
sion
tube.
This is possible using a nylon tube, a heat shrinkable
tube
, etc. The extension
tube
(which
should be
of
1,2
mm
internal
diameter) may be bent, holding
the
tube
a
few
centimetres
above a
lit
match
.
There is some
restriction
on the extension
tube
length
since
this
has a noticeable effect on
the
frequency
response
of
the
4170
. In practice,
it
is recommended
that
the
extension
tube is
not
more
than
2,5
cm
(1
inch) long.
F'ig
.
2.4
shows
the
Probe
Microphone
fre-
quency
response
for
various extension
tube
lengths. (Note
that
a
10
dB range poten-
tiometer
was
used
for
the recording). Curvature
of
the
extension
tube
has
virtually
no
in-
fluence on
the
frequency
response.
Bruel &
Kjan
-
Me.suring
Object
: Bruel & Kjmr
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5
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3
2
10
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Recording
No
.:
2
Hz
5
10
QP
1143
Pot. Range:
Sign
.: Date:
(bl
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(al '
o
50
100
200
dB
Rectifier
.: Lower
lim
. Freq
.:
Potentiometer:
500
1000
2000
Hz Writing
Speed:
Zero Level:
5000
1
~--20000
mm
/sec. Paper
Speed
:
Fig
.
2.4
. Typical
frequency
response
of
Type
4170
a)
without
extension
tube
b)
with
a
2,5
cm
extension
tube
c)
with
a 5
cm
extension
tube
DAB
C lin.
50000
1
00000
mm
/
sec.
740490
3.
1.
PRINCIPLE
3.
DESCRIPTION
The
principle
of
a
horn-coupled
probe
microphone
is
illustrated
in
Fig.3.1
(a
detailed
cross-section
diagram
of Type
4170
will
be
found
in Fig.
3.2).
The probe
tube
is coupled to a cavity via an acoustical
horn.
which
is exponential
for
Type
4170
. The
condenser
microphone
and its coupling
volume
is
equivalent
to
a
small
acousticaI capacitance.
5,
Probe
Microphone
74049
1
Fig.3.
1.
Principle
of
a
horn
-
coupled
probe
microphone
Matching
at
the
horn
termination
is
ensured
by resistance R
which
is vented
into
cavity
V,.
coupled
in
turn
to
a larger cavity V2
in
order
to
equalize
the
frequency
response
around
the
horn
cut-off
frequency
.
The
horn
acts
mainly
as an
to
impedance
transformer.
the
transformation
ratio being equal
n
Z2
~
Z1
52
where
Z,
and
Z2
are
th~
acoustical impedances at
the
horn
ends and
5,
and
52
the
areas
of
the
horn
section at both ends. The acoustical impedance at
the
microphone
end
is
thus
reduced compared
to
the
probe end impedance.
The
horn
impedance Z2
at
the
microphone
end is given by
where
p =
air
density
c =
sound
velocity in air
52
= section area
of
the
horn
at
the
microphone
end
The
microphone
and coupler impedance is
p c2
---
j w V
where
V is
the
microphone
and coupler
equivalent
volume
.
7
The
higher
frequency
cut-off
(-3
dB point) occurs
when
i. e.
The
cut-off
frequency
is
therefore
higher
for
a
horn
-coupled probe microphone
than
if
the
probe is
directly
coupled to
the
microphone
.
Resistance R is equal to
Z2
so
that
impedance
matching
is ensured over a
wide
fre-
quency
range. Cavities V1 and
V2
form
a closed system in order to
minimize
external
sound pressure
influence
.
3.2.
CONSTRUCTION
Probe
and orn
Fig.3.2 gives a cross-sectional
view
of
the
4170
.
The probe
tube
is a
60
mm
long
tube
with
an
outer
diameter
of
1,25
mm.
In order
to
avoid standing
wave
formation
in
the high
frequency
range,
the
probe orifice is damped
by a
fine
wire
mesh.
Rubber
S
ea
ling
R S
ea
ling Coupling
Thread
(glued)
R
es
i
sta
nce
Microphone
v
Pr
ea
mplifi
er
Fig.3.2. Cross-section
of
Type
4170
740494
The probe is attached to the
narrow
end
of
the
acoustical
horn
which
approximates a
continuous
exponential
horn
by a series
of
eight
concentric steps. The
horn
is
120
mm
long, starting
with
an
internal
diameter
of
0,8
mm
and fi'nishing wi
th
3
mm
. The larger
end
of
the
horn
is coupled to a cavity in
front
of
the
microphone diaphragm
while
the
matching
resistance R
(thin
wire
mesh) is vented
into
cavity V,
which
is in
turn
coupled
to cavity V2 in order to equalize
the
frequency
response at
the
horn
cut-off
frequency.
The probe and
horn
tube UA
0442
can be replaced
in
the
event
of
damage (see section
2.1).
3.3
.
MICROPHONE
ASSEMBLY
8
The
microphone
assembly includes a condenser
microphone
and a
preamplifier.
The
microphone
used in
the
4170
is
mechanically
and acoustically
similar
to
the
B & K
Type
4134
. This is a
half-inch
condenser
microphone
designed
for
a
linear
pressure re-
sponse over a
wide
frequency
range
(4
Hz
to
20
kHz, ±2 dB). The open
circuit
sensitivity
of
the
microphone is
typically
12,5
mV
IPa,
but
the probe
attenuation
(approx.
20
dB)
brings
the
overall
sensitivity
of
Type
4170
down
to
typically 1
mV
/Pa.
The
preamplifier
is electrically and
mechanically
similar
to
the
B & K
Microphone
Pream-
plifier
Type
2619
. It features high
input
impedance (>
4000
MO
II
O,8
pF)
and
low
output
impedance «
250)
as
well
as a
wide
frequency range (2
Hz
to
200
kHz). The
power
sup-
ply voltage is
normally
120
V DC,
but
28
V
DC
can also be used
if
degradation in specifi-
cation is accepted.
The
power
supply voltage
(120
V
DC)
and the
microphone
polarization voltage
(200
V
DC)
are provided by
the
PREAMP-. INPUT
of
the
B &K
Measuring
Amplifiers,
Frequency
Ana-
lyzers and Spectrometers via a
7-p
in plug
JP
0701
. Use can also be made
of
a B & K
Mi-
crophone Power Supply Type
2801,
2804
or
2807.
The battery operated Power Supply
Type
2804
can deliver
either
a
28
V
or
200
V polarization voltage;
the
latter should be
selected. Type.
2804
delivers to
the
preamplifier
a
28
V
DC
supply.
Short-circuiting
pins
5 and 6
of
the
preamplifier
is
internally
made
in
Type
2804
.
9
4.
CHARACTERISTICS
4.1.
FREQUENCY
RANGE
. The specified
frequency
range
of
the
Probe
Microphone
Type
4170
is
30
Hz
to
8 kHz (+ 3,
-1
dB
reo
250Hz,
O°C to
50°C)
. A typical
frequency
response curve is given in Fig.2.3.
The dip at approximately 9 kHz is due to the fact
that
the
horn
has not a
continuous
ex-
ponential design,
as
the exponential shape is approximated by steps.
When
recording the frequency response
at
the
factory, a specially designed coupler is
used
as
shown
in
Fig.4
.1. The probe is placed very close
to
the control microphone. It is
also possible to use
the
calibration coupler
DB
0260
which
is included
in
the
B &K Probe
Microphone
Kit
UA
0040
: since
the
coupler is designed
for
probe tubes
with
0,5
- 1 -
2 and 4
mm
outside
diameter,
an extra hole
(1,3
mm
diameter) should be
drilled
in
the
coupler. A
one-inch
condenser
microphone
Type
4144
should be used as
the
transmitter
,
whereas
the
control
microphone
assembly should include a
half-inch
microphone Type
4134
and a Pream
plifier
Type
2619
.
I
I
I
I
I
:
Beat Frequency
Oscillator
1022
. .
!~
'.~.
~
.
.
~.
~
..
~
Assembly
4134/2619
:
....
-----.1
.
L
_________________________________________
~~~i!'!:_
~~v.!'~_h!'!t
_~~i>~~
________
..J
740495
Fig.4
. 1.
Set
-
up
for
frequency
response
measurements
4.2.
TEMPERATURE
10
As
indicated in section 2.4.1,
the
probe
tube
as such can be exposed to
temperatures
up
to
150°C.
Fig.4
.2
shows
a typical
frequency
response
of
the
4170
when
the
60
mm
length
of
probe
tube
alone experiences
temperatures
of
+
140°C
and
-25°C.
Since a
10
dB range
potentiometer
was
used
for
the
measurements,
it
can be seen
that
the
influ
-
ence
of temperature
on the probe tube alone is very
small.
The
influence
of temperature
over the
4170
as a
whole
is larger, especially
at
higher
fre-
quencies. In
this
case, all parts are involved: probe,
horn,
microphone and
preamplifier
.
This is
illustrated
in
Fig.4
.3
which
shows
the
frequency response
with
the complete
in-
strument
exposed to
temperatures
of
+
55°C
and
-25°C.
-
Briiel
&
Kimr
Briiel
&
Kimr
Briiel
&
Kimr
000000000000000000000000000000000000000000
0000000
Brliel
& Kjrer
Potentiometer
Range:
__
dB
Rectifier:
__
Lower
Lim
. Freq
.:
__
Hz
Wr.
5peed:
__
mm
/sec. Paper
5peed:_mm
/sec.
Copenhagen
50
25
10
75
•
dBdB
dBdB
,
4020
B
60
Measuring
Obj
.:
(a)
.
3015
(b)
645
2010
430
10-5
215
Rec.
No
_.:
_
Date
_ _
Sign
.:
__
04>
10
20
Hz
50
100
200 500
1000
2000
5000
10000
20000
400000
A B C lin. 0
OP
1124
Multiply
Frequency
Scale
by
Zero Level:
161212112
740492 A B C Lin.
Fig
.4. 2. Typical
frequency
response
of
Type 41
70
when
the
60
mm
probe
tube
only
is
submitted
to
tem-
peratures
of
a) +
140
°C
and
b) -
25
°C
-
Briiel
&
Kimr
Briiel
&
Kimr
Briiel
&
Kimr
0000000000000000000000000000000000000000000000000
IBrliel&Kjrer
Potentiometer
Range:_ _
dB
Rectifier
:
__
Lower
Lim. Freq.:
__
Hz
Wr.
5peed
:
__
mm
/sec. Paper
5peed
:
_mm
/sec
Copenhagen
50
25
10
75
• dB dB
dBdB
,
4020
860
Measuring
Obj
.:
(a)
3015
6
4~
(b)
20
10
430
10~
215
Rec.
No-,,--
Date
--
5ign
.:
__
00
500
2000
c
I;~o-
0
10
20
Hz
50
100
200
1000
5000
10000
20000
400000
A B
OP
1124
Multiply
Frequency
Scale
by
Zero Level:
1612
/
2112
740493
~
B C Lin.
Fig. 4.3. Typical
frequency
response
of
Type
4170
when
the
whole
instrument
is
submitted
to
tempera-
tures
of
a) +
55
°C
and
b) -
25°C
4.3.
PROBE
ORIFICE
IMPEDANCE
The probe
orifice
impedance
must
be as
high
as possible since a
low
impedance
will
noti-
ceably
modify
the
measured sound field. (The acoustical impedance
of
the
probe
orifice
of
Type
4170
is
greater
than
4 .
10
8
kg
.
s-1
. m- 4
).
A set up
for
measuring
the
probe impedance is
shown
in
Fig.4
.4. The beat Frequency
Oscillator
Type
1022
delivers
a sine signal
to
drive a
quarter-inch
microphone
Type
4135
as
transmitter
. The
output
from
the
receiving
microphone
assembly (condenser
microphone
Type
4135
and
Preamplifier
Type
2618)
is fed
to
a Level Recorder Type
2305
or
2307
via a
Measuring
Amplifier
Type
2606.
The coupler used at B& K had a
volume
of
12
mm
3.
11
12
r--
Beat Frequency
Oscillator
1021 200 V
DC
I
Emining
Microphone
4135
Measuring Amplifier
f§Q§.
li
l
~
l
:
i
"
r
__
___
.
i
.
~
I:
~-
'
.
"
6
I
~
-,
I
, ,
Level Recorder I
2305 I
___________________
!.!!~~~'?r.!."!
2~!t_~B_~1
__________________________
J
7_
Fig.4.4.
Set-up
for
measuring
the
probe
orifice
impedance
The acoustical velocity
flux
t/J
of
the
transmitting
microphone
is given by
where
Ke
=
microphone
sensitivity
=
current
through
the
microphone
Current
i
may
be
written
as
where
Ce =
cartridge
capacitance
Ee
=
driving
voltage across
the
microphone
If
t/J'
is
the
acoustical
flux
entering
the
probe,
the
sound
pressure
in
the
coupler
is
where
p
p =
density
of
air
P
c2
-.-(1/;-1/;')
JWV
c =
sound
velocity
in
air
V =
coupler
volume
(the
equivalent
volume
of
both
microphones
may
be
neglected since V is
much
larger).
Similarly
p =
Zp
t/J'
where
Zp
= probe impedance.
Combining
the
above
equations
Ke
j W Ce
1
jwV
-+---
Z P c2
p
p
Now,
the
measured
voltage
Er
is equal
to
(1
)
(2)
(3)
(4)
(5)
where
Kr
= receiving
microphone
sensitivity
Hence Kr
Ke
j w Ce
--'-..::..-----:-"-
Ee
1
jwV
-+---
Zp
P c2
(6)
(7)
It is
therefore
possible
to
der
i
ve
the
impedance
Zp
from
the
recorded
output
level.
Measurements
on
the
4170
have
shown
that
the
probe impedance
was
equal
to
2 .
10
9
kg
. s- 1 . m- 4
from
20
Hz
to
1000
Hz
. Above
1000
Hz
,
the
term
jwV
/
pc
2 be-
'comes
higher
than
1/ Zp
and
Er
is
constant.
However
,
from
theoret
ical considerations,
it
may
be assumed
that
Zp is
constant
and equal
to
2 .
10
9
kg
. s- 1 . m- 4
until
standing
waves
appear, i. e. up
to
approx. 8 kHz.
4.
4.
NOISE
The
inherent
noise
spectrum
of
the
4170
measured in
third-octave
bands is
shown
in
Fig.4.5.
The
wide
band noise
(20
Hz
to
20
kHz) corresponds
to
an
equivalent
sound
pres-
sure
level
of
62dB
reo
20/lPa
.
-
00000
0 0 0 0
DOC
00
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
-""",,,-
-db
...
4170
-Inherent
-Noise
-----
30
,,~o
IJ.
V
1
------:£010
Jt
ec.No.:
___
_
0..
,----
"'"'-
---
.
,,"
----
~
:'
':
:
,.
, 1
JlV
.
~
_
-"
5
",,,
0
""
dB
"---_
r
Opll<
Sp.:
___
_
M
..wp,.
F
r.q.
XoIe
b,
~_
••
,.
OP
1123
10
100
"'"
1000 2000
.5000
2OJOO
cll
"
oooo
100
1000
'100001 (I6n/2IT1) A
Fig.4
.5. Typical
third-octave
noise
spectrum
of
Type
4170
,
'
db
, ,
, ,
, ,
i-++
I I I •
,
, ,
c ...
u.
.
"
740498
13
BROEL & KJjER
instruments
cover
the
whole
field
of
sound
and
vibration
measurements
.
The
main
groups
are:
ACOUSTICAL
MEASUREMENTS
Condenser
Microphones
Piezoelectric
Microphones
Microphone
Preamplifiers
Sound
Level
Meters
Precision
Sound
Level
Meters
Impulse Sound Level
Meter
s
Standing
Wave
Apparatus
Noise
Limit
Indicators
Microphone
Calibrators
ACOUSTICAL
RESPONSE
TESTING
Beat Frequency Os
cillator
s
Random Noise
Generator
s
Sine
-Random Generators
Artificial
Voi
ces
Artifi
cial Ears
Artificial
Mastoids
Hearing Aid Test Boxes
Audiom
e
ter
Calibrators
Telephone
Measuring
Equipme
nt
Audio
Reproduction Test Equipment
Tapping
Machines
Turntables
VIBRATION
MEASUREMENTS
Accelerometers
Force Transducers
Impedance Heads
Accelerometer Pre
amplifiers
Vibration
Meters
Ac
celerometer Calibrators
Magn
e
tic
Transduc
er
s
Capacitive Transducers
Complex
Modulus
Apparatus
VIBRATION
TESTING
Ex
citer
Controls -
Sine
Exciter Controls -Sine -Random
Ex
citer Equalizers, Random
or
Shock
Ex
citers
Power
Amplifiers
Programmer
Units
Stroboscopes
STRAIN
MEASUREMENTS
Str
a
in
Gauge
Apparatus
Multi
-
point
Panels
Automati
c Selec
tors
MEASUREMENT
AND
ANALYSIS
Voltmeters
and
Ohmmeters
Deviation Bridges
Measuring
Amplifiers
Band-Pass
Filter
Sets
Frequency Analyzers
Real
Time
Analyzers
Heterodyne Filters and Analyzers
Psophometer
Fi
Iters
Stati
stical
Distribution
Analyzers
RECORDING
Level Recordes
Frequency Response Tracers
Tape Recorders
DIGITAL
EQUIPMENT
Digital Encoder
Digital Clock
Computers
Tape Punchers
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BRilEL&K\.IAC.R
DK-2850 Nrerum, Denmark. Teleph.:
(0
1)
800500
. Cable: BRUKJA,Copenhagen. Telex: 1
53
16
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