BRUEL & KJAER 2631 Instructions And Applications Download Page 22 | Manualshive

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Summary of Contents for 2631

Page 1: ...rier system designed for detection of capacitance variations in the frequency range DC to 150 kHz Used with a condenser microphone input it provides an ideal first stage processing of low frequency pressure variations such as sonic bangs thunder storms short duration pulses etc ...

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Page 3: ...MICROPHONE CARRIER SYSTEM TYPE 2631 December 1969 ...

Page 4: ...2 Gain Adjustment 12 Test Oscillator 0 0 13 Power 0 0 0 0 0 0 0 14 Removal of Module Panels 14 Microphone 15 Calibration 15 4 APPLICATIONS 18 Sonic Bang Measurements 18 Measuring Environment 18 Microphone and Installation 18 Extension of Microphone lower limiting Frequency 19 Measurements and Analysis 19 Pressure Measurements 23 5 SPECIFICATIONS 26 ...

Page 5: ...ondenser microphones are employed in which the pressure equalisation hole is sealed thus bringing the lower frequency limit down from 3 Hz to below 0 1 Hz However given such a microphone there would be considerable difficulty in choosing a cathode follower and preamplifier as used in conventional sy stems The input impedance required would be in order of 3 101 0 ohms which would be exceedingly dif...

Page 6: ...dB 0 20 4 0 60 p Rel Amplitude Freq 001 01 I I 10 100Hz Hz 8 t Fig 1 1 Idealised N wave ofsonic bang p t I Tt me Fig 1 2 Fourier spectrum ofsonic bani 4 ...

Page 7: ...ntegral of such an N wave shows that its spectral content Fig 1 2 peaks at about 1 t Hz and ranges several decades above and below this frequency For accurate representation of such spectra it is necessary to have a measurement capability from 0 01 Hz to 10 kHz as recommended by the I S O Draft Proposal 43 1 N 11 The frequency range of the 2631 Carrier System extends from DC to 150 kHz and is thus...

Page 8: ... straight into the Microphone Head and will act as the detecting transducer the variable capacitance of which forms an integral part of the Head s reso nant circuit This resonant circuit is driven by a 10 M Hz crystal controlled oscillator and as the capacitance of the condenser microphone is varied by sound pressure variations at its diaphragm so the impedance of the resonant circuit is coupled i...

Page 9: ... to one of the inputs of the Control Unit s integrated differential output amplifier Voltage feed back is applied from the output to the other input of the amplifier The amount of feed back applied determines the gain which can be manually adjusted by the Gain Adj potentiometer control The Control Unit s low impedance output 10 n is suitable for con nection to practically any type of recording or ...

Page 10: ... 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 is detected and amplified and thus provides a test for the whole 2631 Carrier System Also contained in the control unit is a stabilized power supply which provides 15 and 15 Volts ...

Page 11: ...Carrier System Type 2631 can be attributed to the fact that all controls input and output sockets etc are confined to the FRONT and REAR PANELS Fig 3 1 and 3 2 respec tively of the Control Unit The unit s top bOttom and side panels are Fig 3 1 Front panel 9 ...

Page 12: ...perating and calibrating procedure plus the range of suitable B K condenser microphones for the Carrier System follow in this chapter INPUT INPUT to the Control Unit from the Head is via a standard B K 7 pin microphone socket on the front panel The socket also provides the Head s supply and feed back voltages 10 ...

Page 13: ... system with capacitive loading is shown in Fig 3 3 Note that a DC voltage of up to 12 V can exist at the OUTPUT if the system is not balanced and is in the OC mode 10V 7 0 1 0 01 0 1 10 Fig 3 3 Response of carrier system for capacitive loading of Control Units output LOWER LIMITING FREQUENCY The LOWER LIM FREQ can be set to 1 0 1 0 01 Hz or to DC accor ding to the knob position on the front panel...

Page 14: ...ti clockwise position Once balance is obtained it should be returned to the DC position for measurements The DC BALANCE knob controls a single turn fine adjustment potentiometer and a multi turn coarse potentiometer DC balance of the output in the AC mode is automa tically adjusted to within 20 mV but if it is important that the output signal has no bias then further adjustment of the signal using...

Page 15: ...nction of the measuring system Operation is by depressing the Test button on the front panel The amplitude of the oscilla tor signal is variabl e and is adjusted by a multi turn potentiometer acces sible by removing the left side panel viewed from the rear of the unit see Fig 3 6 Note that since the test oscillator injects a signal into the Head the output level of the signal will also be dependen...

Page 16: ...fuse located at the rear of the equipment is only accessible when the bottom panel is removed REMOVAL OF MODULE PANELS The BOTTOM PANEL of the 2631 Control Unit is removed by releasing the large coin operated finger screw at the bottom of the front panel see Fig 3 1 Quarter to half a turn should be adequate to release the catch The panel is then removed by sliding it towards the rear of the unit T...

Page 17: ...icrophone Type 4144 and 4145 can also be sealed using the Sealing Kit Type UA 0240 With these microphones however the 0 1 Hz limiting frequency can not be guaranteed The Microphone Head normally accepts a 1 inch condenser microphone but for high frequency high intensity measurements a B K 1 2 inch micro phone Type 4148 with an Adaptor Type UA 0271 should be used This is especially applicable in th...

Page 18: ...r calibration should be set up as shown in Fig 3 8 in an environment free from external pressure distur bances Balance is then obtained for the 2631 with the microphone placed 4146 Microphone 2631 M ic r o phone He a d Control Unit 2631 DC Yo I tme ter Fig 3 8 Coarse calibration instrument set up in a horizontal position The DC mode is then selected and the microphone still in a horizontal positio...

Page 19: ...meter deflection will be 100 mV Once calibration has been obtained for a microphone the inconvenience of recalibrating that particular microphone can be avoided by using the test oscillator The amplitude of the oscillator should cbe adjusted _ to provide some convenient indication on the measuring and recording instrumentation up to 146 dB indication available with the 1 inch condenser microphone ...

Page 20: ...should not subtend a solid angle greater than 0 004 steradians Free air conditions apply when the microphone can be mounted at suffi cient height and when obstructions in the upper half space do not subtend an angle greater than 0 004 steradians This may necessitate an altitude of more than 100 m for the whole sonic bang pressure signature to be deter mined However a lower height may be found suff...

Page 21: ...ing frequency is shown in Fig 4 1 Here the effective volume is increased by enclosing the microphone in a cavity also with an equalisation vent The volume of the cavity and its equalisation vent should be designed to have a time constant K R2 C2 or about 100 sec This should give a microphone lower limiting frequency of 0 01 Hz suitable for sonic bang measurements Measurement and analysis A typical...

Page 22: ...2C2 Fig 4 1 Extending microphone lower limiting frequency showing equivalent circuit Therefore it is advised that both recording channels or possibly a number of tape recorders are used but with different gain settings This ensures adequate signal to noise and that one channel utilizes the full dynamic range Frequency analysis of the sonic bang pressure signature can be given in the form of either...

Page 23: ... Measurement arrangement for sonic bangs function A convenient spectral density function related to the energy spec tral density by a constant factor having the dimensions of acoustic impe dance is given by where and E S cv cn p t X e iwt dt j cn Po 20 uN m 2 fo 1 S dB p t is the overpressure as function pf timet and w 2 1T f ...

Page 24: ... 2020 has the 1 B output attenuator useful in spectral density measurements Tape Recorder 7001 Measuring Amplifier 2606 level Recorder 2305 i I I I Fig 4 3 Analyzing arrangement utilizing B K Filter Set Type 1614 15 11 9 197 For 0 01 to 20Hz frequency components analysis is performed as before but because of the lower limiting frequencies of the level recorder and filters a frequency transformatio...

Page 25: ...ents which can be fitted to the B K 1 2 inch Microphone Type 4134 is shown in Fig 4 5 Except for the two pressure intake nozzles the adaptor should form an airtight seal around the microphone The pressure to be measured should be applied to the microphone diaphragm and the reference pressure to the equalization hole via the appropriate intake nozzle For the 4134 Micro phone the maximum measured pr...

Page 26: ...Intake Equalization Vent Fig 4 5 Schematic of microphone pressure adaptor 5 Volts I 2 Pressure 1P 3 4 Volts m m H2 o 9 j f Fig 4 6 Experimental results from calibration of 1 4 inch Microphone Type 4136 adapted for pressure measurements ...

Page 27: ... 2631 may be used with advantage are in fluid mechanics pressure chambers and wind tunnels In the latter case pressure measurements may be made on moving air foils Fig 4 7 or on the transient response of pressure fronts along wind tunnels In all these applica tions the 2305 Level Recorder will prove useful in providing a graphical display of pressure behaviour with respect to time Direct ion of Mo...

Page 28: ... connectors Built in test oscillator System alone 40 70 pF 0 Hz 150 kHz 6 pF 6 x 1Q 3 pF 0 3 x 1Q 3 pF 0 25 x 10 3 pF 0 15 x 10 3 pF 0 1 X 1Q 3 pF 0 04 pF day 0 0015pF OC 1 2 30 V pF DC 0 01 Hz 0 1 Hz 1 Hz System with 1 Microphone 4146 0 1 Hz 7 kHz 500 Nfm2 0 5 Nfm2 Peak 62 dB RMS 60 dB RMS 55 dB RMS 50 dB RMS 3 Nfm2 day 0 1 m2JOC 20 100 mV N m2 As for microphone Automaticat 0 01 0 1 1 Hz and manu...

Page 29: ... and feet Height Width Depth 132 6 mm 5 22 in 61 mm 2 40 in 200 mm 7 87 in Weight of box 1 65 kg 3 63 lb Dimensions of microphone head Diameter 23 8 mm 15 16 in Length 76 mm 3 in Cable length 2 m 6 5 ft Weight of microphone head 0 31 kg 0 68 lb 27 ...

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