BRUEL & KJAER 4117 User guide

Piezoelectric Microphone
Type 4117
High quality piezoelectric micro-
phone for measurement and
monitoring purposes. Each micro-
phone
is individually calibrated.

Piezoelectric Microphone
Type 4117
Reprint July 1972

Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 5
1.1. Purpose
of the Piezoelectric Microphone . . . . . . . . . . . . .. . .. . . . . . 5
1.2. Definitions
of Free-Field and Pressure Response . . . . . . .. . .. . . . . . 5
1.3. Random Incidence Response ·(Diffuse Field Response) .. . . . . . .. . 6
2. Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . 7
2.1. Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 7
2.2. Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 9
2.3. Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . 9
2.4. Charge Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 10
2.5. Frequency Response ......................................... 10
2.6. Free-field Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. 11
2.7. Directional Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . 11
2.8. Dynamic Range .............................................. 11
2.9. Phase Shift .................................................. 12
2.10. Volume Correction ........................................... 12
2.11. Temperature Characteristics .................................. 12
2.12. Influence of Ambient Pressure ................................ 12
2.13. Influence of Humidity ........................................ 13
2.14. Influence
of Vibrations ........................................ 13
2.15. Influence of Load ............................................ 13
2.16. Cable Capacitance Sensitivity Correction
...................... 16
3. Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Sound Level Calibrator Type 4230 .............................. 17
3.2. Pistonphone Type 4220 ....................................... 19
4. Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1. Microphone Extension Cable AO 0062 .......................... 21
4.2. Microphone Extension Cable AO 0061 . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3. Windscreens UA 0082 and UA 0207 . . . . . . . . . . . . . . . . . . . . . . . . . . .. 22
4.4. Miniature Cable AO 0037 ...•.................................. 23
4.5. Mininoise Cable AC 0010 ...................................... 23
4.6. Adapter JP 0028 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.7. Microphone Stand UA 0049 .................................... 24
4.8. Random Incidence Corrector UA 0055 . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5. Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.2. Condenser Microphone Preamplifiers .......................... 27
5.3. B &K Preamplifiers .......................................... 27
5.3.1. Preamplifier Type 2623 ........................................ 27
5.3.2. Preamplifier Type 2616 ........................................ 29
5.3.3. Vibration Pick-up Preamplifier Type 2625
........................ 30
5.3.4. Charge Amplifier Type 2624 ................................... 32
6. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

1. Introduction
1.1. PURPOSE OF THE PIEZOELECTRIC MICROPHONE
The Bruel & Kjrer one inch Piezoelectric Microphone Type 4117 has been
designed
for the general purpose Sound Level Meter Type 2205, which
fulfils the requirements of the IEC Recommendation Publication 123. Reasons
for choosing a piezoelectric microphone instead of a condenser microphone
are its
low price, its very high capacitance, and the fact that is does not
require polarization voltage. This also means that it is less affected by
water condensation on the back
of the diaphragm than would be the
condenser microphone.
All these advantages justify the use of the piezoelectric microphone as a
general purpose microphone in cases where low price and good quality
are desired. The microphone does not fulfil the requirements for precision
sound pressure measurements, however, so in cases where this is a demand
one
of the B &K condenser microphones should be preferred.
1.2. DEFINITIONS OF FREE-FIELD AND PRESSURE RESPONSE
The Free-Field Response of a microphone is the ratio of the RMS output
voltage to the
RMS sound pressure existing in a free field (plane sound
waves) at
the microphone location with the microphone removed.
The Pressure Response
of a microphone is the ratio or the RMS output
voltage
to the RMS sound pressure, uniformly applied over the diaphragm.
Free-Field Response= ~o
Pressure Response= ;,
vo~ts _rv ------L__Mic_roph-one~IC::::::::::---~------
Wavelength
Sound
pressure p
0
-------;------
00 incidence
(62160
Fig. 1.1. Definitions of Free-field and Pressure Response.
5

The two definitions coincide for a microphone having negligible dimensions
with respect to the sound wavelength. In the case
of the B & K one-inch
microphones this is practically fulfilled up to about 1400
Hz, where the
wavelength is equal to ten times the diameter
of the microphone (i.e. 240 mm).
The difference in response is then a fraction
of a decibel (approx. 0.6 dB).
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.
1.1 . The difference P1- 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 the
fitted protective grid).
The free-field behaviour
of a microphone is thus described by means of a
set
of free-field correction curves for various incidences, which should be
added to the pressure frequency curve
of the microphone in each particular
case.
For microphones intended
for free-field work it is possible to give the
diaphragm resonance such a damping that the normal incidence free-field
corrections are compensated
for up to frequencies well above the resonance
frequency, in
order to obtain the flattest possible frequency response.
1.3. RANDOM INCIDENCE RESPONSE (DIFFUSE FIELD RESPONSE)
The random incidence response of a microphone for a given frequency is
the RMS value
of the free field sensitivity for all angles of incidence of the
sound wave.
It corresponds to the diffuse field sensitivity of the microphone,
the diffuse field being a sound field in which the sound energy 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) has 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.*)
When
the spectral distribution of the sound varies with the angle of incidence,
correct integration is only possible in the range where the microphone is both
linear and omnidirectional. Omnidirectional microphones are also necessary
in the case
of rapidly moving sources (aeroplanes, motorcars, etc.).
*) 80, 830, 8
60
---- S180 being the sensitivity of the microphone at angles of incidence of
0°, 20°, 60°, ---- 180°, the random incidence (diffuse field) response S is given by the
formula:
S2 = 0.018 (S02 + 8180 2) + 0.129 (8
302 + S 15
l) + 0.224 (8
602 +8120
2) + 0. 258 Swl
6

2. Description
2.1. CONSTRUCTION
The mechanical construction of the Piezoelectric Microphone Type 4117 is
shown schematically in Fig. 2.1. The active element is a ceramic bender which
consists
of two layers of PZT (Lead Zirconium Titanate) electrically connected
in parallel in
order to achieve a high capacitance. The capacitance is In
the order of 4 nF. The bender is supported at both ends by means of bronze
ribbons
to which it is soldered. The diaphragm is a thin metal foil. As the
pressure from the whole diaphragm area must be transferred to a point on
the bender the diaphragm is given a conical shape, which makes it very stiff
and therefore act a piston. Along the edge
it is corrugated, and the apex is
attached to the bender by means
of an epoxy resin glue.
Diaphragm
==~~~~-Capillary tube for
pressure equalisa-
tion
Fig. 2.1. Schematic drawing of the piezoelectric microphone.
When the microphone is exposed to a sound pressure the diaphragm vibra-
tions are imparted to the ceramic bender as variable forces tending to bend
it. Due to the piezoelectric effect a variable potential will be developed
in
the bender, ·uhich is proportional to the force and therefore to the sound
pressure.
To dampen the natural resonances
of the diaphragm and the bender, a conical
back plate has been placed very close behind the diaphragm. In the back
plate there are a number of holes and these together with the
air In the
space between the diaphragm and the back plate give the necessary damping.
The spacing is adjustable and in the production calibration this
Is used to
obtain the desired frequency response
of the microphone.
At the low frequencies the response
of the microphone is affected by the
influence
of a pressure equalization arrangement. This arrangement consists
7

~&109?
Fig. 2. 2. Cut-away drawing of Type 4117.
of a capillary leakage hole through which the equalization of the static
pressure on both sides
of the diaphragm is obtained at a suitable rate. The
hole is situated in front
of the grid mounting thread. The pressure equalization
is then obtained also in the case
of closed cavity or flush mounting measure-
ments. The time constant
of the pressure equalization is 0.05 seconds corres-
23.11mm-60 NS 2
(0. 910 .. -60 NS 2)
22 . 55mm
(0 .888 .. )
---------- t ----------.H-- -----.-
M 22 x0 . 75
Fig. 2.3. Physical dimensions of the microphone.
8

ponding to a - 3 dB cut-off frequency of 3 Hz approximately. The time
constant may be increased, however, by inserting a thin wire in the capillary
tube. The diameter of the hole is 0.25 mm (0.01 in). The physical dimensions
of the microphone are shown in Fig. 2.3.
Caution
As even a
light touch may damage the diaphragm of the microphone no
cleaning
of the diaphragm should be attempted. The grid is an effective
protection against mechanical damage and should
not be removed unless
special adjustment
of the pressure equalization is required.
2.2. SENSITIVITY
Each microphone is individually calibrated and supplied with its own calibra-
tion chart (see Fig. 2.4). The calibration is carried out at 250 Hz with a load
capacitance of 100 pF. Corrections have been made for the barometric
pressure so that the sensitivity is given at 760 mm Hg. The voltage sensitivity
is in the
order of 0.3 mV/,ubar. .
~cooooooooooooooaoooooaooooooooo
........,.,.Hr.,M-Ht.llleludlftlaloMI
~of100P' :
.. Q.. ~ .t.e. •VI,.tJM- •• -.?.e.t .. . ,.,v, ,._
~Gaf* l !J"'IIHI:
c- .... .1:~ ....... " ·
c.Mit'- .. Tell:
T ~ tu .. .. l..f ....... •c
l ~c P ... eu re .7.1t(L .. ,Mrn tto.
1\e!~W.Hum ld t f)' ,,J.f.. ••••• 'llo
D ate •• (.'?;.f:.~f. .. Sl 11,.. tu ,. •• /?.· . !! .- ....• ..
....,.... ,_. e OOifllolenl : ~ -0 . 11 dB tor
T lO'Mop_... .. ~ .
-- bP""
~~-
--
10000-::.::
Fig. 2.4. Typical calibration chart as supplied with each microphone.
2.3. CAPACITANCE
The capacitance of the microphone cartridge is given on the calibration chart.
Cartridge capacitance comes into
the question when the low frequency cut-off
of the measuring system is computed, as it determines the effect of loading
on the microphone. The nominal capacitance is 4 nF. Because of this high
capacitance
of the microphone the input impedance of the amplifier need
not be very large, e.g. a 2 MQ impedance gives a low frequency cut-off at
about
20Hz.
The cartridge capacitance is measured at 1000 Hz in a capacitance bridge,
comparing with a standard capacitance equal
to the nominal capacitance of
the microphone.
9

2.4. CHARGE SENSITIVITY
This sensitivity can be calculated from the voltage sensitivity and the
equivalent capacitance
of the microphone. Charge sensitivity of a microphone
is expressed in pico-coulomb/
,ubar and is independent of the capacitive
loading on the cartridge. It is determined by multiplying the voltage sensitivity
with the microphone capacitance.
Schorge = Svoltoge X Cmlcrophone
Since the voltage sensitivity is given In mV/,ubar and the capacitance in nF
the charge sensitivity will be in pC/,ubar.
2.5. FREQUENCY RESPONSE
An individual frequency response curve Is attached to the calibration chart
supplied with each microphone. The free-field curve Is derived from the
pressure response curve recorded automatically by means
of the electro-
static actuator method.
13
dB
10
5
0
-5
10
Free fiel~ correctio~s
/
I-- for Type 4117
LV
/v
/
v
l/v
/
oo
v v
vv
/ v
V/3o· v
~
~
V VV 'f\cfl.
60" \'0<-'
6
~/
~
v /
co(1
~O_/•
~
~
/
/..: .
.....
180"
··-1 J
..........
v.:.~
/
90~
·· .
-
~ ...
......
······
~
...,Ji' <
...........
"
r-(· ..
~
~' ......
~
'\. 120"
................ -::
~
150"'
'~
-..c.
f----
r\
~\
~J
1L ~'),.
~
o• Incidence
f----
"'b..
~
I I
1kHz 1.5
3 4 5 6 8 9
10 12
Fig. 2.5. Free-field correction curves to be added to the pressure
characteristic of the microphone .

2.6. FREE-FIELD CORRECTIONS
The pressure increase, which is caused by the reflections of free-field sound
waves on the microphone diaphragm, becomes appreciable above 1 kHz. The
corresponding correction curves are given in Fig.
2.5. The frequency response
for the various angles of incidence is obtained by adding the free-field
correction to the pressure response supplied with each cartridge. The IEC
Recommendation Publication 123 gives a practical rule for calculation of the
sensitivity
for a diffuse sound field (random incidence) from the free-field
sensitivities at definite angles (see page 6 for definition and formula). This
is used to calculate the random incidence correction curve given in Fig. 2.5 .
The random incidence response
of the microphone is obtained by adding this
curve to the individual pressure response.
2.7. DIRECTIONAL CHARACTERISTICS
In Fig. 2.6 are shown typical directional characteristics at various frequencies.
Fig. 2.6. Typical directional characteristics of the piezoelectric microphone.
2.8. DYNAMIC RANGE
The lower limit of the dynamic range is set by the inherent noise level of
the amplifier used with input loaded by the microphone capacitance. Used
with the B &K Sound Level Meter Type
2205 this limit is 32 dB SL for 5 dB
signal to noise ratio.
The upper limit
of the dynamic range is set by the harmonic distortion in
the complete measuring system. The microphone itself will handle sound
pressure levels of up to
140 dB with less than 4 °/o harmonic distortion.
11

2.9. PHASE SHIFT
90° phase shift occurs at the resonance frequency of the ceramic bender and
diaphragm system. The resonance frequency is approximately
4.8 kHz for
Type
4117.
2.10. VOLUME CORRECTION
The volume correction is defined as the difference in effective volume between
a coupler coupled to the
4117 and the same coupler coupled to a standard
microphone front, see Fig.
2.7. The correction is approximately 1 em' at
1 atm.
IJ
.--- Standard ~
Microphone 4117
Front
r/J 1S . 6mm
Depth 1.95mm
p Coupler
Fig. 2.7. Volume correction for Type 4117.
2.11. TEMPERATURE CHARACTERISTICS
The microphone is calibrated at room temperature but it may be used in the
temperature range
-10 to + 70°C. Storing temperature is -10 to + 90°C.
A typical temperature sensitivity curve is shown in Fig. 2.8.
dB
+1 +------+------~----~------~----~----~----~
~r------r------r------r----~-------r----~----~
Fig. 2.8. Typical temperature sensitivity curve.
2.12. INFLUENCE OF AMBIENT PRESSURE
The microphone sensitivity will vary less than -0.15 dB for + 10 °/o variations
in ambient pressure. For larger changes in ambient pressure the frequency
response
of the microphone will be modified, especially towards the higher
frequencies because
of the change in mechanical damping. The frequency
response at different ambient pressures is given in Fig.
2.9.
12

-""'"'~
Pressure • •
frequency
d,rre;:r at
ambient
pressures
... ..., __ _
- ---
.,., __ _
,_._ ___ ,.,
~*" 1 -zs-aa==
W..S..-- .-
,_,.. __ _
..........,,...,...., :-o o
10
OP1123 10
10000
Fig. 2.9. Influence of the static ambient pressure on the frequency response
of Type 4117.
2.13. INFLUENCE OF HUMIDITY
A sudden drop in ambient temperature may in some cases cause moisture
condensation to take place between the diaphragm and the back plate, and
on the
ceramic bender. The characteristics of the microphone may thereby
be changed
temporarily in respect of sensitivity, frequency response and leak
resistance. The influence
of relative humidity in the absence of condensation
is less than
0.1 dB.
Caution
The
Silica Gel Cap UA 0135 should not be used with the 4117 because of the
violent pressure changes introduced by mounting and removing the cap.
2.14. INFLUENCE OF VIBRATIONS
When the microphone is exposed to vibration the diaphragm and ceramic
bender system will be set into motion. This will introduce an output voltage
as If
the microphone was exposed to a sound field. An acceleration of 1 g
perpendicular to the microphone diaphragm gives approximately the same
output voltage as would a 100 dB sound pressure level.
2.15. INFLUENCE OF LOAD
The signal from the piezoelectric microphone appears as an alternating
voltage
across a capacitive impedance. To detect this voltage the microphone
is connected to an amplifier via a cable. The loading of the microphone Is
hence composed of the leak resistance and capacitance of the cable and
the input impedance of the amplifier. The equivalent circuit of a microphone
with external loading is
drawn in Fig. 2.10.
13

Microphone I Cable I
Am~
ifier
~
=r=cm
Rm
=~Cc
Rc
==c;
R;
Fig. 2.10. Equivalent circuit of the microphone, cable and amplifier.
The internal resistance Rm of the microphone is essentially infinite at room
temperature. Also the cable leak resistance R. is usually extremely high, and
therefore both Rm and R. can be neglected, giving a simplified diagram as
seen in Fig. 2.11.
c
Fig. 2.11. Simplified equivalent circuit for normal operating frequency range.
The following terms will be used during the considerations:
Q = cl:large induced across the microphone capacitive element (Coulomb).
Sq = charge sensitivity of the microphone (Coulomb/bar).
P = sound pressure to which the microphone is subjected(bar).
s. = voltage sensitivity of the microphone (V/bar).
C = total capacitance in the circuit, including microphone (Cm), cable (C.)
and amplifier (C1) (Farad).
R = 1/G, where G is the total conductance
In the circuit, Including micro-
phone, cable and amplifier (R in Q).
The piezoelectric microphone is a charge generator, and the charge generated
is proportional to the sound pressure to which the microphone Is subjected.
Q (co) = Sq (co) P = S. (co) CmP (1)
Assuming a sinusoidal sound pressure of angular frequency co the current
flowing in the circuit will be
I= jcoQ
(2)
and the output voltage will be
E = 1/(G + jcoC) = jwQI(G + jcoC)
(3)
This shows that when G < /coC, i.e. when the shunt resistance In the circuit
14

Is very high or at high frequencies the voltage depends only upon the
capacitive loading and the microphone sensitivity:
E = jwQ/jwC = Q/C (4)
It Is also seen that the output is directly proportional to 1/C. This must be
taken Into account when long microphone cables are employed.
From equation
(3) it can also be seen that when G >jwC, i.e. for low
frequencies
or low shunt resistance the output is frequency dependent:
E=~WG=~OO 00
This means that the output falls off at the same rate as the frequency at the
low frequency end.
The corner frequency where the output is 3 dB down Is where /G/ =
/jwC/
i.e. f. = 1/2:nRC (6)
where f. Is called the "cut-off frequency".
The Internal resistance
of the microphones is extremely high, always exceeding
10,000 MQ at room temperature (!::S120°C}. The resistance of the piezoelectric
material is lower at high temperatures but usually still higher than
10,000 MQ
1.0
!.---"
-t-
/
~
~elative input voltage
~
--
-f--·- ·-
to the amplifier as
/
a function of frttquency
and loading v
09
0.8
I
--- -- --- ------
1--
---
-- - -
-f-
------
1----
---
-- --- -
-----
0.7
0.6
I
v
0.5
03
v
I R=amplifier resistance
I C=total capacitance
in
circuit (amplifier +
/ cable +microphone)
/
./
v
~
0.4
0.2
0.1
0.01 0.02
0.05
0.1
02 0.5
1 fRC 2
'611S'f
Fig. 2.12. Chart for finding required input impedance when microphone
capacitance and low frequency cut-off is given.
0
dB
-1
-2
-3
-4
-5
-6
15

at 70°C. This large resistance gives theoretically a low frequency cut-off value
at 0.004 Hz
for the microphone alone but as the time constant of the pressure
equalization is 0.05 sec. the lower limiting frequency
of the microphone is
3 Hz as delivered. The
time constant may be increased, however, by inserting
a thin wire in the capillary tube. The diameter
of the hole is 0.25 mm (0.01 In).
Fig. 2.12 gives a chart for finding the required input resistance for a given
lower limiting frequency and a given microphone capacitance.
Example:
Find the required input impedance for a 1 dB cut-off at 10 Hz using a
microphone with capacitance 4 nF.
Solution:
A 1 dB cut-off is seen to give a value of about 0.3 for. fRC
fRC =
0.3
R = 0.3/fC = 0.3 / (10 X 4 X 10 -9)
R = 7.5 MQ
The minimum acceptable input resistance of the amplifle( is 7.5 MQ.
2.16. CABLE CAPACITANCE SENSITIVITY CORRECTION
The capacitance of a long cable connecting the microphone to the amplifier
will reduce the voltage sensitivity of the microphone. For example a 5 m long
cable with a capacitance
of 90 pF/m will give a reduction in sensitivity of
approximately 10 %. The reduced sensitivity can easily be found if the total
shunt capacitance in the input
circuit is known, i.e. microphone capacitance,
cable capacitance and amplifier input capacitance. The new sensitivity Is
found from the formula:
where
Sv<cl
s.
Cm + 0.1
Cc
c,
Example:
S _ Sv (Cm + 0.1)
v(c) - Cm + Cc + C,
= reduced voltage sensitivity.
= voltage sensitivity given on the calibration chart.
= microphone capacitance given on the calibration
+ 0.1 nF used during the sensitivity calibration (nF).
= cable capacitance (nF).
= amplifier input capacitance (nF).
chart
A microphone has a sensitivity
of 0.310 mV/p,bar and a capacitance of
3.4 nF. What is its sensitivity with a connection cable of 40 m length,
90 pF/m. Amplifier input capacitance negligible.
Solution:
Cable capacitance Cc = 40 X 90 = 3600 pF = 3.6 nF
S _ 0.310 (3 .4 + 0.1) = 0.310
7.x
0
3.5 = 0.
155
mV/ubar
v( c) - 3.4 + 3.6 r
16

3. Calibration
The sensitivity of the microphone is given on its calibration chart, but it may
in many cases be very convenient to calibrate the whole measuring set-up
right from the microphone to the indicating instrument. To do this Bruel & Kjer
has developed two calibrators, the Sound Level Calibrator Type 4230 and the
Pistonphone Type 4220.
3.1. SOUND LEVEL CALIBRATOR TYPE 4230
Fig. 3.1. The Sound Level Calibrator Type 4230.
The B & K Sound Level Calibrator Type 4230 creates a sound pressure level
of 94 dB ± 0.3 dB at 1 kHz in the coupler volume and is a pocketsize, battery
powered
unit for field calibration of microphons. (See Fig. 3.1). The calibra-
tion value obtained for all weighting networks (A; B, C, D and linear) is the
same, as these weighting scales have
the same SPL values at 1 kHz. The
influence of static pressure is very small, thus the calibration signal is
independent of barometric pressure and altitude for ordinary use.
A special
construction of the vibrating system makes the equivalent coupler
volume more than 200 cm3 which is greater than the total mechanical volume
of the unit (125 cm3 ). In practice this means the signal level is independent
of the microphone type and the accuracy of the connection to the calibrator.
Caution
Apply and remove the calibrator slowly in order to avoid damage to the
microphone diaphragm.
17

Specifications 4230
Calibration Frequency: 1000 Hz ± 2 °/o.
Calibration
Pressure Level: 94 ± 0.3 dB re 2 X 1 o-s N/m2 (A, B, C, D or Linear).
Equivalent Coupler
Volume: larger than
200 cm3•
Total Harmonic
Distortion: less than 1
°/o.
Temperature Range: - 10 to + 50°C.
Power Supply: single battery.
1 X 9
V. I EC Recommendation 6 F 22, size
25.5 X 17.5 X 48.5. NEDA 1604.
Examples: Manufacturer IType
Union Carbide Ever Ready No.
216 or 222
Dimensions:
Microphone Types:
Hellesen Type H 10
Varta Pertrix No.
438
Tudor No. 9T4
National U-006 P
40.4 mm (1.58 in) diameter,
96 mm (3.78 In) length. .
1" and 1/2".
3.2. PISTONPHONE TYPE 4220
Fig. 3.2. Construction of the Pistonphone.
18
f66122