BRUEL & KJAER 4148, Instructions And Applications

Page 1: ...1 2 Condenser Microphone Type 4148 A precision condenser microphone needing only 28 V polarization Frequency range 4Hz to 16 kHz Sensitivity 12 5 mV per N m2 ...

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Page 3: ...d Applications Half inch Condenser Microphone Type 4148 Precision Condenser Microphone for measurement purposes in conjunction with a Sound Level Meter or Preamplifier 2619 Features low polarization voltage March 1970 ...

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Page 5: ...o o o o o o o o o 0 14 Sensitivity Frequency Response 14 Free field corrections 17 Directional characteristics 19 Dynamic Range Noise and Distortion 20 Equivalent Volume 20 Polarized cartridge capacity 20 Long term stability 0 0 21 4 Environmental influences o o o o o o o o 0 o o o 23 Influence of temperature 23 Influence of ambient pressure 24 Influence of humidity 24 Influence of vibration 25 5 ...

Page 6: ...ntenance 28 7 Microphone Calibration 29 Pistonphone Type 4220 29 Sound Level Calibrator Type 4230 29 Microphone Calibration Apparatus Type 4142 30 Electrostatic Actuator UA 0033 31 8 Accessories 33 9 Spec ifications 39 ...

Page 7: ...varying the polarization voltage between 28 and 120 V DC The microphone has a flat frequency response between 4 Hz and 16kHz and a wide dynamic range Its most outstanding feature is excellent long term stability under a great range of environmental conditions An excellent battery driven precision sound measuring system is obtained in conjunction wit h a precision sound level meter Type 2206 or a s...

Page 8: ...tput voltage to the sound pressure can be obtained within a wide frequency range and an extended dynamic range A condenser microphone posesses asmall capacitance and therefore requires a preamplifier of special design for useful operation as can be seen from the following discussion A simplified circuit diagram of a condenser microphone and preamplifier is shown in Fig 1 1 and the equivalent circu...

Page 9: ...uit should not exceed Ct of the transducer more than absolutely necessary Hence the need for integral construction of the cartridge and preamplifier impedance converter 1 3 Definition of Free field Pressure and Diffuse Field Response The sensitivity of a transducer for different frequencies is normally expressed as the ratio of its electrical output to its mechanical input and is in general a func...

Page 10: ...n the sound field before the microphone is placed in position i e e Mf at agiven single frequency The diffuse field response level relative to reference response Mr can be defined as Mf Rf 2Q Iog1 o Mr dB The definitions of Free field and Pressure Response are illustrated in Fig 1 3 Free Field Response o Pressure Response vo ts_ f J Mi crop hone CZ Wavelength Sound pressure P 0 _ _ __ ________ 0 i...

Page 11: ...0Hz where the wavelength is 10 times the diameter of the microphone The free field response of a microphone is of interest when the transducer is operated in an open space many wavelengths removed from a sound source or any reflecting boundaries In closed cavities couplers and artificial ears the pressure response is important Furthermore in some instances the pressure response is agood approx1mat...

Page 12: ...The diffuse field response level relative to reference response Mr can be defined as Rd 20 log 10 dB The sound field in very reverberant rooms can be considered as a diffuse sourid field 10 ...

Page 13: ...gned primarily for the B K 2206 which requires 28 V polarization but can work with up to the range of 100 V This means that the cartridge is a precision measuring microphone but may of course be used as a studio microphone as well Oiaphra Back plate Hole for static pressure equalization Fig 2 1 Schematic construction of a condenser microphone cartridge Furthermore the microphone is mechanically an...

Page 14: ...g 2 2 Fig 2 2 4148 dimensions The holes in the backplate control the damping of the diaphragm resonance The damping is also controlled by adjusting the spacing between diaphragm and backplate The 4148 is overdamped in order to obtain a free field response which is as flat as possible The holes for pressure equalization are situated in front of the grid and coupler mounting thread and allow air to ...

Page 15: ...by a 20 pm wide slot in the spacer ring between the quartz insulator and housing The slot resistance determines the acoustic cut off at the lower limiting frequency The time constant of the pressure equalization is around 0 1 sec 13 ...

Page 16: ...minal open circuit sensitivity of 12 5 mV per N m 2 or 38 dB re 1V per N m 2 i e 1 25 mV Jbar 58 dB re 1 V Jbar when used with a polarization voltage of 28 V The microphone cartridge may however be operated with any polarization voltage up to 120 volts The resultant change in sensitivity can be seen in Fig 3 2 The output voltage of the transducer is approximately proportional to the polarization v...

Page 17: ...t about 11 kHz which is controlled by the compliance and mass of the diaphragm itself and the inside volume of the microphone This resonance is damped very carefully by air friction in the strictly controlled air gap between the diaphragm and the O OOO OO O OODOODOOO O O DDODODDDOODOOODOOOOOOOODOOOOOOO ___ __ LU F w F Iouolo 0010 OP1123 10 SdB Fig 3 3 Frequency response at different polarization v...

Page 18: ...ole The electrical low frequency cut off 3 dB is determined by the preampli fier input resistance and the total capacitance in the input circuit as can be seen from the equation in Section 1 2 This frequency is where and R c 27T RC input resistance of preamplifier cartridge capacity input capacitance of preamplifier Typical values for Rand C when the preamplifier is Type 2619 are giving 3 co 0 0 c...

Page 19: ...urves are shown in Fig 3 5 a and b By adding the free field correction to the pressure response the frequency response for the various angles of incidence is obtained A definition of angle of incidence is shown in Fig 1 4 The free field corrections are less dependent on the angle of incidence if a Nose Cone is screwed onto the microphone cartridge see Accessories 10 Free field corrections for type...

Page 20: ...150 0 018 S21ao So S3o S1so free field corrections at the respective angles of incidence 18 10 dB 5 0 5 Free field corr ctio s fo type 4148 with protecting grid v o v i Joo fL_60 v V I Q2 Vi o c 180 1 Y _ f I C I I 2 3 4 2d0 9 _ 1f 180 150 I J O incidence I I I I I I 5 6 7 B 9 10 kHz 15 20 Fig 3 5 b Free Field Corrections when Pressure Characteristics are Determined by Electrostatic Actuator Metho...

Page 21: ...racteri stics measured by means of a Polar Diagram Recorder theTurntable Type 3921 in conjunction with the Level Recorder Type 2305 is shown in Fig 3 6 and can be considered as typical Fig 3 6 Typical directional characteristics Better omnidirectional properties can be obtained by screwing a Nose Cone UA 0052 onto the cartridge instead of the protecting grid see Accessories 19 ...

Page 22: ...Volume It may be important to know the microphone equivalent volume when the cartridge is used in acoustic coupler measurements or in artificial ears be cause the equivalent volume has to be added to the volumes enclosed in the coupler Here it is convenient to express the acoustic driving point im pedance of a microphone Za i e the complex ratio of the sound pressure P to the volume velocity of th...

Page 23: ...as an input capaci tance of only 0 5 pF so that the sensitivity is reduced only 0 2 0 3 dB 3 7 Long Term Stability Microphone sensitivity may change during a long period of time even if no accident or mishandling has occured These changes are due to changes in the materials making up the device The major contribution to change in response is metal flow in the diaphragm sag 109 u 0 101 I Ltl 107 I ...

Page 24: ...ow or sag appears fastest at high tern peratures the resultant sensitivity increase at 1500 C is about 0 2 dB per hour Fig 3 7 shows an extrapolated plot of sensitivity stability as a function of temperature thus allowing calculation of the approximate stability at any temperature within the range For example the drift at room temperature is about 0 2 dB per 1000 years For further information on t...

Page 25: ...n sensitivity is 1 5 dB over the range of 500C to 1500C 580F to 302oF dB 4148 Typical Change in Sensitivity 1 0 ___ 1 1 1 1 1 20 20 40 60 80 100 120 140 160 c Fig 4 1 Dependence of the microphone sensitivity on temperature The temperature coefficient is between 0 000 dB oC and 0 015 dB C The influence of temperature on long term stability is described in Section 3 7 Note that long exposure at high...

Page 26: ...erformance under low pressure and varying altitude conditions see B K Technical Review No 1 1960 4 3 Influence of Humidity When a microphone is moved from warm to cold environs there is a risk of water condensation between the diaphragm and the back plate making the cartridge noisy The moisture will escape in 1 or 2 minutes through the pressure equalization hole Using the 2619 preamplifier on 120 ...

Page 27: ... e at right angles to the diaphragm the sensitivity to vibration is mainly determined by the mass of the diaphragm and the air column on both sides of it The microphone is sensitive to vibration acting in this direction but due to the low diaphragm mass the signal produced from vibrations having an acceleration of 1 g is as low as an equivalent sound pressure level of 80 dB 25 ...

Page 28: ... incidence is oo i e the diaphragm surface should be perpendicular to the pressure wave b for free field measurement the microphone should be sufficiently far away from the nearest surface to avoid the effects to reflections Reflect ing surfaces destroy the free field conditions 5 2 Operation with the 2619 Preamplifier The characteristics and power supply requirements of the 2619 are given in its ...

Page 29: ... V or any voltage over 120 V If desired the 2619 4148 combination can be mounted on microphone stand UA 0049 5 3 Operation with Cathode Followers 2614 and 2615 The same warnings about polarization voltage and the teflon casing apply with the 2614 and 2615 as with the 2619 5 4 Operation with Sound Level Meter 2206 2205 UA 0208 The 4148 has been designed especially for use with the 2206 and it is an...

Page 30: ... serial number and of the trouble Water Condensation inside the cartridge is likely to make the transducer noisy but the influence of humidity is virtually nil section 4 3 Handling Precautions and Cleansing of the Microphone Diaphragm The diaphragm which is only 2J Jm 0 00008in thick is carefully stretched during the transducer s assembly by a special procedure in order to have the required charac...

Page 31: ... the ad vantage of being usable for calibrating an entire measuring and or recording system but it must be remembered that the standard deviation of an entire system may be as high as 1 5 to 2 For laboratory situations the reciprocity technique is another accurate me thod giving both sensitivity and frequency response data It is possible to keep the probability of error for this technique below 1 ...

Page 32: ...ly time consuming and is employed mainly for calibration of standard microphones This absolute calibration method uses 3 micro phones coupled in turn to each other acoustically One acts as a transmitter and the other in turn as a receiver When the volume of the coupler and the current through the transmitting microphones are known the absolute sensi tivity of the individual transducers can be calc...

Page 33: ...ay be used seperately when only the frequency response is to be investigated The actuator is placed on the microphone cartridge with protecting grid removed the cartridge being held vertically Three insulating glass studs contact the erlqe of the cartridge so that the Fig 7 4 Electrostatic Actuator UA 0033 31 ...

Page 34: ... calibrated with the actuator UA 0033 certain correc tions have to be applied This is because of the high compliance of the 4148 diaphragm and the actuator influences the response both electrically and acoustically The pressure response curve given in the calibration chart of each 4148 cartridge was obtained with a specially designed actuator which gives accurate results If the normal UA 0033 is u...

Page 35: ...ousing in order to avoid adisturbing ground loop and it is plated for corrosion protection When using the wind screen with the rain cover See below screwed onto the microphone cartridge the signal lead to the electro static actuator inside the rain cover is fed through a slot in the tightening ring Fig 8 1 Windscreens UA 0082 and UA 0237 Wind Screen UA 0237 This is a ball of specially prepared por...

Page 36: ...8 3 Free field corrections are shown in Fig 8 4 The construction of the Nose Cone is clearly seen from the sectional drawing in Fig 8 5 It is shaped to give the least turbulence to air flow The fine wire mesh around the circumference allows sound waves to penetrate to the microphone diaphragm while the truncated cone inside the mesh reduces the air volume in front of the diaphragm The reduction of...

Page 37: ...Fig 8 3 Directional Characteristics with Nose Cone UA 0052 35 ...

Page 38: ...7 8 9 10 kHz 15 20 Fig 8 4 Free field corrections with nose cone The Stand is a Iightweight portable tripod of rugged construction It has rigid telescopic legs and can be used not only for supporting the 8 K microphones but also for the somewhat heavier Sound Level Meters Fig 8 6 Extension Cables Several standard lengths of extension cable are available the cables being specially designed and supp...

Page 39: ...the cables several lengths can be coupled together without influencing the frequency range of the measuring arrangements For very long cables however the effect of cable capacity upon the distortion and frequency response of the overall measuring arrangement is not always negligible Fig 8 6 Microphone Stand UA 0049 37 ...

Page 40: ... pF m 57 pF m 57 pF m Tape Microphone Cable AR 0001 This very flexible flat 7 cored microphone cable is used when sound insulation and reverberation measurements a re made in buildings and where it is necessary to carry a microphone cable through closed windows and doors The flat cable can easily follow sharp bends the thickness of the cable being 0 2 mm Cable length around 30 em 38 ...

Page 41: ...uency 900 phase shift Polarization Voltage Polarized Cartridge Capacity Diameter 13 2 mm 0 52 12 7 mm 0 50 Height 16 3 mm 0 64 15 2 mm 0 60 12 7 mm 60 NS2 1 2 12 5 mV per N m2 at 28 V polarization 38 dB re 1 V per N m2 free field oo incidende o2 4Hz to 250Hz _ 1 0 dB 250 Hz to 5 kHz 0 5 dB H 1 0 5k zto10kHz_ 1 5 dB H 1 5 d 10 k z to 16 kHz _ 2 0 B 3 dB point between 1 and 2 Hz Between 0 08 and 0 1...

Page 42: ...Sensitivity axial Humidity Sensitivity Accessories 40 16 pF at 250 Hz Up to 15ooc Between 0 00 dB OC and 0 015 dBJoc 0 0025 dB mm Hg Approx 80 mm3 Drifts less than 0 1 dB century at 21oc extrapolated less than 0 2 dB hour at 1500C Approx 80 dB SPL at 1 g Less than 0 1 dB in the absence of con densation Standard range for B K 1 2 conden ser microphones ...

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