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 -1 xm -4
Bru.1 & It;.:.. s"iiel & Kjc»r Btu.1 & Kj m,
CCDDDCCCCOCccoooooaooooooooo aooo oooooo oooaOOCODoaa
aru.1 & Kjc:er Potenti-..ter Ron~ , ..2L.d8 R.clili...: -B.M.S... l_ lim.. Fr." .; ~l "'t . Sp..d ; _l _mm / WC . Poper $peed; _mm /$.c.
Copenhov-n 50~5 10 75
dB'"
• r2(F '60 dB
860
Frequency
~~ x 15 70dB
6 AS
- --20 10 80 dB
.30
;=;==" 'F' 90 dB
~
215
""'"'--
519".: __ 0 0 100 dB o 0
OP 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
000000 DO 00 0 0 0 0 DOD DOD 00 0 0 00 0 00 0 0 0 DO 00 0 00 0 0 0 a 0 0 00 0 0 0 0
d8 I.cotdI"ONo., 1 SIQn. : V.T. 0..... 5·6·73 ,~ , SOdS z-t.-j . DABClin .
2 • H~
,,143
, 1)
Jlbt.ac....:
Open front e nd
10dB
Closed front end
-I -
.-
--
50000 """"
-, .-Jt«. . 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
OOOOOOODOODOOOOOOOOOODOOOOOOoooooooooooooooOOOOOO[
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
DOOOOOOOOOOOOOOODDODDOODODODOOODOODODDDOOODDOODDDC
dB
5
40
3
2
10
o
Recording No .:
2 Hz 5 10
QP 1143 Pot. Range:
Sign .: Date:
(bl
(cl- (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