BRUEL & KJAER 2631 Guía del usuario

Microphone Carrl•r System

Typa

2631

Microphone Carrier System

Type

2631

A carrier system designed

for

detection of capacitance variations

in the frequency range

150 kHz. Used with a condenser

microphone input it provides

ideal first stage processing of low

frequency pressure variations such

as sonic bangs, thunder storms,

short duration pulses etc.

MICROPHONE CARRIER SYSTEM

TYPE 2631

December 1969

CONTENTS

INTRODUCTION . • . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Microphone

Head

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Control

Unit

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

OPERATION . . . . . . • . . . . . . • . . . . . . . . • . . . . . . . . . . . . . . . . . 9

Input

..........................................

Outputs

.....

................

.................

Lower

Limiting

Frequency

.........................

Balance

.....................................

Gain Adjustment

.................................

Test Oscillator

..........

...............

••

0 • • • • •

Power

...........

...

••••

•••

••

0 • •

••••

• 14

Removal

Module

Panels

..........................

Microphone

.....................................

Calibration

......................................

APPLICATIONS

•.......•............•.•.....•........

Sonic

Bang

Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Measuring Environment

............................

Microphone

and

Installation

........................

Extension

Microphone lower

limiting

Frequency

......

Measurements

and

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 19

Pressure

Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5, SPECIFICATIONS

.••.•...............................

INTRODUCTION

The

B&K

Microphone Carrier System Type 2631

has

been

developed

meet the exacting requirements called

for

in the measurement

low fre-

quency

pressure

variations down

0.01 Hz,

for

which special measurement

techniques

are

required.

The

basic

2631 Carrier System

composed

Head

and a Control

Unit

containing the power supply

and

output

amplifier. A condenser microphone

(not provided)

can

plugged directly into the

Head.

Any

pressure

differen-

ces

present

the microphone's diaphragm

are

converted

capacitance

variations which

are

detected by

detector contained in the

Head.

The

2631

has

adynamic

range

95 dB

("A"

Weighting)

and

unlikemost conven-

tional systems the demodulated

output

from

the

head

allows the possibility

reasonably long connecting

cables

the control unit.

In order

realise the very

low

frequency

response,

special

condenser

microphones

are

employed in which the

pressure

equalisation hole

sealed,

thus bringing the lower frequency

limit

down from 3 Hz

below

0.1

Hz.

However, given

such

a microphone there would

considerable

difficulty

choosing a cathode follower and preamplifier

used

in conventional

sy-

stems.

The

input

impedance required would

in order

101

0 ohms

which would

exceedingly

difficult

realise in conditions

high humidi-

ty.

The 2631

has

the advantage that its

input

impedance.

below 100 ohms

and

therefore

not

affected

moisture.

In the

mode adjustment

provided

for

manual balancing

and

compensation

the system

for

·different transducer capacitances.

Balance

clearly indicated on a monitoring meter. Balancing in the AC mode

auto-

matic.

The special

measi,Jrement

techniques called

for

in the analysis

slowly

varying air

pressures

and

short duration

pressure

pulses

such

found in

sonic

bangs,

fluid

mechanics, wind tunnels,

pressure

chambers

and

thunder

storms make the

B&K

Carrier System eminently suitable

for

such

applica-

tions.

-20

-4-0

-60

Rel.Amplitude

Freq.

.001 .01

100Hz

~,8/~:t

Fig.

1 Idealised N

wave

sonic

bang.

-p~--~----------------------

Tt'me

Fig.1.2 Fourierspectrum

sonicbani.

Taking for example

the

idealised N wave

the

sonic bang from an

aircraft (Fig.1.1) ,

the

peak overpressure, p1 may be 5 lb/ft2 (250 Nfm2)

possibly even higher for low flying aircraft, and

the

pulse duration,

the

order 0.03

0.4

sec. A Fourier integral of such an N wave shows

that

its

spectral

content

(Fig.1.2) peaks

about

and ranges several decades

above and below this frequency. For accurate representation

such spectra

-necessary

have a measurement capability from 0.01

10 kHz as

recommended by

the

I.S.O. Draft Proposal 43/1 N 11. The frequency range

the

2631 Carrier System extends from

150

kHz and

thus ideally

suited

this and

other

applications.

DESCRIPTION

GENERAL.

A block diagram

the

B&K

Type

2631 Microphone Carrier System

shown

Fig.2.1. A detailed description

the

systems design follows

below.

Oocillator

Oetoctor

Proamplifior C8ble Test oscillator Miller

integrator

I I Output

amplifier

I I

Low.r

l•m

Freq.

wlector

Microphone

Head

Control

Unit

Fig.2. 1 Block diagram

the Microphone Carrier System.

MICROPHONE HEAD.

The

condenser microphone,

not

provided with

the

2631 can be plugged

straight into

the

Microphone Head and will

act

the

detecting transducer,

the

variable capacitance

which forms an integral

part

the

Head's reso-

nant

circuit.

This resonant circuit

driven by a

M Hz, crystal controlled oscillator,

and as

the

capacitance

the

condenser

microphone

varied by sound

pressure variations

its diaphragm so

the

impedance

the

resonant circuit

coupled inductively

a circuit which

detects

the

impedance changes, and

produces a

proportionate

output

voltage.

The

demodulated

signal

then

amplified by an integrated circuit preamplifier and fed by cable

the

Control Unit containing

the

output

amplifier.

the

demodulated

signal

taken from

the

Head and its

output

impe-

dance

below 1

the

effect

cable capacity

the

response

the

system

considerably reduced. See Fig.3.3. Thus any reasonable length

cable can be used

connect

the

output

from

the

Head

the

Control Unit.

The

polarity

the

system

such

that

an increase

capacitance

the

input, or an increase

pressure

the

microphone diaphragm produces an

increase in

output

voltage.

When

the

mode compensation

obtained for

the

effects

diffe-

rent microphone capacitances and thermal variations

the

capacitance

diodes

the

resonant circuit, by applying voltage feed back from

the

Control Unit's

output

amplifier

the

variable capacitance diodes. This

ensures

that

the

resonant circuit

automatically

tuned

for transducer capa-

citances between

and

pF. This

adequate for any

the

B&K

1 inch

Condenser Microphones, however, for

1/2

inch Microphones a special adap-

tor

Type

0271

r~quired.

CONTROL UNIT.

The

demodulated signal from

the

Microphone Head

fed

one

the

inputs

the

Control Unit's integrated, differential

output

amplifier.

Voltage

feed-back

applied from

the

output

the

other

input of

the

amplifier.

The

amount

feed back applied determines

the

gain which can

be manually adjusted by

the

Gain Adj.

potentiometer

control.

The

Control Unit's low impedance

output

suitable for con-

nection

practically

any

type

recording or· measuring equipment. Out-

put

voltage

mA depending on

the

setting

the Gain Adj.

control.

The

Lower Limiting Frequency

controlled by

the

output

amplifier'sfeed-

back loop

means

a Miller integrator with switchable time constants.

The

feedback signal

then

applied

capacitance diodes

the

resonant

circuit

the

Head.

Automatic

compensation with a preselected time con-

stant

thus

provided

for

different transd·ucers

the

modes.

the

mode, compensation

set manually

the

BALANCE control on

the

front

panel. Balance and overload indication

provided

the

monitoring

meter, also

the

front

the

equipment.

the

mode

the

response

the

Carrier System (without microphone

Fig.3.4) falls. by approximately 1.5 dB

the

LOWER LIMITING FRE-

QUENCY selected. It should be noted

that

the

selected frequency

not

the

cut

off

frequency

(-3

dB point). However,

a microphone

used

that

has

the

same

lower

limiting

frequency response

that

selected on the 2631,

then this would become the

cut

off

frequency. The response

the Carrier

System in the

mode

that

the microphone

used.

Without

micro-

phone the response extends

from

150kHz.

A separate test oscillator

inductively coupled

into

the feed back loop

after the Miller Integrator and injects a 1 kHz signal

into

the Head's

reso-

nant circuit. Here the 1 kHz signal

puUs

the capacitance diodes producing a

capacitance variation. The variation

detected and amplified, and thus

provides a test

for

the whole 2631 Carrier System.

Also contained in the control

unit

a stabilized power supply which

provides +15 and

-15

Volts

DC.

for

circuitry

contained in

both

the

Head

and

the Control Unit. The supply

mains driven and the approximate

current consumption

and independent

voltage.

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