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