GR 1987 Manual - IET Labs, Inc.
Contents
1. PC BOARD FINGER CONNECTION OR MALE CONTACT Ym FEMALE CONTACT lt MATING CONTACTS SIGNAL FLOW MAJOR SIGNAL PATH INTERRUPTED SIGNAL LINES SIGNAL NAME IDESTINATION CODE SOURCE CODE SIGNAL NAME CODE 2 8 7 SHEET NO ws DEVICE NO Lh oewice ein no TYPICAL CIRCUIT TEST VOLTAGES Deme Qnem VOLTAGES EXPLAINED tN INSTRUCTION BOOK SERVICE NOTES ROTARY SWITCH NUMBERING CODE ASA Y32 16 and MIL STD 16C R2 REAR OF WAFER CONTACTS FIRST CONTACT CW FROM STRUT SCREWABOVE KEY VIEWED FROM FRONT WAFER NEAREST PANEL 18 NO1 ROTOR SHOWN CCW wre yen te ees N L SUPPLY SPLITTER LOW BATTE TEST zi Microphone Figure 5 2 Schematic diagram M MK 39 REFDES C 1 n 2 C 3 C 4 5 C 6 TR 1 CK 2 CR 3 ZR 4 Q 1 R 1 R 2 R 3 a 4 R 5 a 6 R 7 R 8 8 9 8 10 R 11 a 12 R 13 R 14 a 15 R 16 M 17 R 1a R 19 5 i Jg 1 ELECTRICAL PARTS LIST CALIBRATOR PC BIARD DESCRIPTION CAP TANT 120 UF 20PZT 10V CAP TANT 129 UF 20PCT 10V CAP CER MOND O LNF 20PCT 50VGP CAP MICA 3591 PF 48 CT 500V ZAP MICA 3591 PF LPCT 500 CA CER MOND Q lU 20PCT 50V3P ZENER COXXXX 50V 1PCT 4W DIJDE 1N4151 75PIV IR SI DIGDE DA1703 40PIV IR 50UA ST DI DOE DAL703 4OPIV IR 500 SI TRANS E 501 CS 33MA T0106 RES COMP 100 5PCT 1 4W RFS COMP 109 5PCT 1 4 RES2. PRECISION INSTRUMENTS FOR TEST AND MEASUREMENT OMOGRAPH FOR APPLYING ALTITUDE CORRECTION TO BAROMETRIC PRESSURE TETER PRESSURE ALTITUDE OF STATION BAROMETRIC PRESSURE IRRECTED IN SEA LEVEL FEET ABOVE SEA LEVEL STATION MILLIMETERS IES nay MILLIBARS MERCURY US 1987 SERIES t 1050 10000 10000 9000 9000 550 es 9 Minical Sound Level Calibrator 8000 8000 750 1000 7000 7000 9 6000 6000 5000 000 600 500 User and Service Manual 8 950 4000 4000 3000 3000 2000 2000 eso 900 1000 1000 650 6 1000 1000 900 is 850 TOO m 950 800 23 75 1000 22 750 2 1060 700 800 NOTE 1 foot 0 3048 meters Copyright 2014 IET Labs Inc 1987 im March 2014 8 I ET LABS NC Long Island New York Email info ietlabs com e ET LABS NC Long Island New York Email info ietlabs com Formerly manufactured by TEL 516 334 5959 800 899 8438 FAX 516 334 5988 IET Formerly manufactured by TEL 516 334 5959 800 899 8438 FAX 516 334 5988 GenRad www ietabs com GenRad www ietlabs com Specifications Output Sound Pressure Levels 114 dB or 94 dB re 20 under reference conditions Output Frequency 1000 Hz 396 Reference Conditions TEMPERATURE 20 C 68 F ATMOSPHERIC PRESSURE 1013 mbart 760 mm of Hg 30 in of Hg RELATIVE HUMIDITY 65 MICROPHONE EFFECTIVE VOLUME 0 5 cm 03 in nominal for GR 1961 Electret Condenser Microphone Accuracy of Sound Press
3. 2 6 1 General The cavity of the 1987 will accommodate the following 1 in microphones the GR 1961 Electret Condenser the GR 1971 Ceramic the Western Electric 640AA and the Tokyo Riko MR 103 Adaptors must be used for other sized microphones The 1 2 in microphone adaptor P N 1987 7061 is an accessory included with the 1987 It must be inserted into the cavity of the transducer assembly when calibration is being performed on an instrument with a GR 1 2 in electret condenser 12 Theory Section 3 3 1 GENERAL The 1987 Minical calibrator isa single frequency secondary standard sound source that can cal ibrate sound level meters in the field The 1987 provides the user a quick means of checking sound level meters at 1 kHz and at two reference sound pressure levels 114 dB and 94 dB The 1987 cal ibrates 1 in nominal microphones directly with out an adaptor and a variety of other standard microphones with adaptors supplied or available as accessories 3 2 CIRCUIT DESCRIPTION 3 2 1 General Figure 3 1 is a block diagram of the 1987 etched board and schematic diagrams are given in Section 5 All of the electrica components except the transducer and battery are contained on the calibrator s only etched board P N 1987 4700 shown in Figure 4 1 The block diagram shows the following main elements of the 1987 circuitry e battery and power supply splitter battery low test circuit e bridge oscill
4. 201 3339P W97 202 RN5593010F RN55 D7 502 F RN5591783F RCROTG154J RCRO07G1 02 J RCROTG 5125 TT00259 CA324G Or MECHANICAL PARTS Fig 5 3 Fed Mfg Qty Ref No Name Description Purpose GR P N Code Mfg P N 2 1 Pad Foam Has one adhesive side 1987 7035 24655 1987 7035 locates and cushions battery 1 2 Desiccant K it Silica gel SO cartridges 5863 3001 24655 5863 3001 two each per kit provides humidity control 1 3 Transducer Electromagnetic provides 1987 1000 24655 1987 1000 Assembly acoustical output 2 4 Spacer 0 112 40 thread both ends 1987 6010 24655 1987 6010 accepts screw securing top housing to bottom housing 1 5 Hex Nut No 10 48 part of 7910 7910 0540 95146 TTO0259 0540 toggle switch 1 6 Label Power Steel indicates sound 1987 0405 14655 1987 0405 Switch pressure level 1 7 Adaptor Plastic adaptor for 1 2 in 1987 7061 24655 1987 7061 Microphone microphone 2 8 Screw 0 112 40 x 0 375 Phillips 7044 1103 24655 7044 1103 head inserts in hole in battery compartment and threads into 1987 6010 spacer 1 9 Bottom Housing Plastic contains transducer 1987 1005 24655 1987 1005 Assembly T in cavity and desiccants 1 10 Gasket Housing Styrene butadiene 1986 7045 24655 1986 7045 elastomer provides moisture and dust control 1 11 Battery 9 V alkaline provides power 8410 3400 90303 MN1604INS to instrument 1 12 Frame Window Styrene butadiene 1987 7085 24655 1987 7085 elastomer provides moistur
5. CATHODE END OF DIODES PIN 1 OF IC S PIN OF RELAYS PiN NO OF CONNECTORS COLLECTOR OF TRANSISTORS Figure 5 1 Etched board assembly viewed from component side 38 PRESSURE TO FREE FIELD CORRECTIONS GR Microphone Adaptor Correction Manufacturer and Type Used dB Flat Response to Random Incidence Microphones GR 1961 9610 or 9601 none 0 0 1 in electret GR 1962 9610 or 9601 1987 7061 0 0 1 2 in electret GR 1971 1 in ceramic none 0 0 GR 1983 Sound Level 1987 7061 0 1 Meter 1 2 in ceramic GR 1954 Noise 1987 7076 0 0 Dosimeter 3 8 in and ceramic 1987 7095 Flat Response to Perpendicular Incidence Microphones GR 1961 9611 or 9602 none 0 3 1 in electret GR 1962 9611 or 9602 1987 7061 0 1 1 2 in electret B amp K 4131 1 in O Ring 0 2 condenser 5855 6920 B amp K 4133 1 2 in 1987 2050 0 0 condenser B amp K 4135 1 4 in 1987 2060 0 0 condenser Measurement Conditions Atmospheric Pressure 760 mm 30 in of Hg see para 2 9 for other pressures t Additional correction required due to microphone effective volume refer to para 2 11 TABLE OF CONTENTS Specifications Condensed Operating Instructions Introduction Section 1 1 1 Purpose m Rs 12 Description vu uuo xx 1 3 Controls Indicators and Connectors 1 4 Operation Section 2 2 1 Unpacking and Inspection 2 2 Battery Install
6. an O ring and minor dimensional differ ences inside the B amp K adaptor NOTE The GR 1987 2050 and GR 1987 7061 adaptors are not interchangeable e GR 1987 2060 Microphone Adaptor for B amp K Type 4135 36 1 4 in microphone insert in the same manner as 1987 7061 adaptor e GR 1987 7066 Microphone Adaptor for Shure Bros Model 98108 1 1 8 in microphone insert in the same manner as 1987 7061 adaptor e GR 5855 6920 O ring for 8 amp K Type 4131 32 1 in microphone place it in the cavity and with the index finger work it into the groove all the way around the inside of the cavity e GR 1987 7076 Microphone Adaptor and GR 1987 7095 Adaptor Cap for 3 8 in microphone used on GR 1954 Noise Dos imeter proceed as follows a Set the microphone cap P N 1987 7095 flat side down on a bench or table b Insert the 3 8 in microphone into the cap with the front side up Make sure the micro phone cable slides into the groove on the side of the cap as the microphone is inserted c Place the GR 1987 7076 Microphone Adaptor over the cap such that the groove in the side of the adaptor fits over the cable from the microphone d Insert the microphone cap adaptor assembly into the 1987 transducer cavity in the same manner as the adaptor described in para 2 6 1 was installed CAUTION The transducer should not be in contact with a surface subject to vibration when calibration is performed This can introduce an ambie
7. 1987 Again observe the reading on the 1982 it should be 94 dB 1 25 dB The wider tolerance allows for linearity errors within the sound level meter itself Any 1987 that fails to perform properly should be returned to an authorized GenRad service facility as described in para 4 1 and 4 2 4 4 DISASSEMBLY The circuitry of the 1987 is contained on one small board To gain access to this board which is normally required for troubleshooting procedures only proceed as follows a Disassemble the housing of the 1987 as des cribed in para 2 5 and Figure 2 1 b In order to remove the board first remove the nut and lockwasher that secure the power switch toggle switch to the housing of the 1987 c Remove the one screw on the component side of the board this will allow the board to drop out of the housing d The board can be powered externally by connecting a 9 Vdc supply or battery to WT 3 and WTA see Figure 4 1 4 5 TEST AND CALIBRATION Special facilities are required to calibrate the output level of the 1987 Minical so that it in turn can be used to calibrate sound measuring instru ments in accordance with ANSI and IEC standards Such facilities are available at the GenRad factory and service centers and are used in the initial fac tory calibration or repair of the 1987 units These facilities must include a standard microphone such as the Western Electric Co Type 640AA or its 34 Figure 1 1 Cont
8. SILICCN 500 OHM 10 PCT RFS FLW 15 4K 1 PCT 1 84 RES CGMP 1 0 M 5PCT 1 4W RES FLM 453K l PCT L 8W RES FLM 4 22K L POT 1 8W RES FLM 12 7K 1 PCT 1 8W RES FLM 50 OHM 1 PCT 1 8M RES FLM lo OHM 1 PCT 1 8W RES FLM 1 4K 1 PCT 1 84 PIT CERM TRM 2000HM 20PCT T POT CER4 TRM 22PCT 4T RES FLM 391 OHM 1 PCT 1784 AES FLY 75K i PZT 1 80 RES Fim 178K 1 PCT 1 8W RES COMP 150 K SPCT 1 4W RES COMP 1 0 lt 5PCT 1 4W RES COMP 5 1 K CHM 5PCT 1 4 SAITCH TOG MIN 3POS DPDT xTATITE ISL CA324G 140 QUAD GP AMP P N 1987 4700 PART NO 4450 5616 4450 5616 4400 2050 4851 0272 4851 0272 4490 2050 4083 1102 6082 1001 6082 1049 5082 1049 8215 0100 6099 4105 6099 4105 5740 1110 6250 2154 6999 5105 6250 3453 6250 1422 6250 2127 6250 0150 6250 9100 6250 1140 6049 0424 6049 0425 6250 0301 6250 2750 6250 3178 5099 4155 6099 2105 6099 2515 7910 0549 5432 1056 F4C 56289 56289 72982 72136 72136 72982 24655 14433 14433 14433 17856 81349 81349 25008 81349 81349 81349 81349 81349 81349 81349 81349 80294 80294 81349 81349 81349 81349 81349 81349 95146 86684 MFG PART NUMBER 150012 7x0010R2 150012 7xX0010R2 8131 M059 651 104M OM2Z0FD3591LPFIPCT4CR DM20F2359 1PF IPCT 4C 8131 M050 651 104M 6083 1102 1N369 IN4152 IN4152 501 RCRO7 G104 J RCROTG104J TG 1 8 RN55D1542F RCROTG 105J RN55045233F RN5504221F RN550127T2F RN5501502 F RN5501 OROF RN5501401F 3339
9. cease 3 2 4 Bridge Oscillator The bridge oscillator circuit is comprised of a 1 kHz bandpass filter U1A a soft limiter R8 CR3 and CR4 and a temperature compensat ing attenuator R4 and R3 R14 The bandpass filter U1A is an active filter operational amplifier with a bridge null network in the feedback loop The overall center frequency stage gain and Q are determined by the ratios of the five bridge com ponents The phase shift across the network is zero at center frequency The stage gain is approx imately 34 dB In order to make the circuit oscillate the output of U1A is returned to its non inverting terminal through an appropriate attenuator The soft limiter consisting of R8 CR3 and CRA limits the output of the bandpass filter to ensure long term stability If the U1A output were fed back directly to its input without passing through this limiter oscillation would still occur but would build up in amplitude until limited by the output swing of U1A The soft limiter main tains the U1A output at a constant level of approx imately 1 2 V pk pk The distortion produced by the limiting is later filtered by the 1 kHz bandpass filter The soft limited signal is attenuated by R4 and R3 14 The latter two resistors R3 and R14 are used to temperature compensate the natural temperature shift effects of CR3 and CR4 The 28 THREADED SPACERS ASSEMBLY SCREW OFFSET FROM MIDDLE HOLES IN BATTERY OF HOUSING T
10. 0 1987 2050 1987 7095 1 2 IN 1 4 3 8 ADAPTOR ADAPTOR CAP Figure 1 3 GR 1987 9600 Microphone Adaptor Set available accessory Table 1 3 GR 1987 9600 MICROPHONE ADAPTOR SET Description GR P N Box for adaptors below 1987 1800 1954 Microphone Adaptor 1987 7076 1954 Microphone Adaptor Cap 1987 7095 O Ring forB amp K Type 4131 32 5855 6920 1 in microphone Adaptor for B amp K Type 4133 34 1987 2050 1 2 in microphone Adaptor for B amp K Type 4135 36 1987 2060 1 4 in microphone Adaptor for Shure Bros Model 1987 7066 98108 1 1 8 in microphone 6 Service and Maintenance Section 4 CAUTION The servicing instructions in this section should be performed by qualified personnel only 4 1 CUSTOMER SERVICE Our warranty at the front of this manual attests the quality of materials and workmanship in our products if malfunction does occur our service engineers will assist in any way possible If the difficulty cannot be eliminated by use of the following service instructions please write or phone the nearest GenRad service facility see back page giving full information of the trouble and of steps taken to remedy it Describe the instrument by name catalog number serial number and ID lot number if any Refer to front and rear panels 4 2 INSTRUMENT RETURN 4 2 1 Returned Material Number Before returning an instrument to GenRad for service please ask ou
11. 1 0 V causing CR1 to conduct If the battery voltage has a 1 5 V drop from 9 0 V to 7 5 V the anode voltage of CR1 becomes 3 5 V and its cathode voltage becomes 1 5 V now the comparator input voltage drops 27 BATTERY low TEST Gu CRI vc BRIDGE LIMITER CRB CRE POSITIVE FEEDBACK KHZ BAND PASS FILTER UIA TRANSDUCER DRIVER CURRENT FEEDBACK 5 wa T CURRENT oorr Apousr MENT SOURCE WB g Ri2 RI3 1 Fis 448 9408 Figure 3 1 Block Diagram 26 when dry These two cylindrical cartridges are about 4 cm 1 5 8 in in length and 8 mm 3 8 in in diameter One of the desiccants is visible through the small circular window on the 1987 housing this desiccant will change color from pale blue to pink when it has absorbed moisture and such an indication means that the two desiccants should be replaced by dry ones Three spare desic cant kits each kit contains two cartridges are included with the accessories supplied they are contained in an air tight package The desiccants are made of a super activated silica gel 502 fill They have a high moisture absorption capability and are fast drying They can be rejuvenated by baking at 80 C 170 F until they turn blue To replace the desiccants proceed as follows refer to Figure 2 1 a Remove the battery compartment cover and battery detailed in para 2 4 b Remove the two Phillips head screw
12. 2 5 dB for a setting of 114 dB Therefore the sensitivity of the instrument under test would be adjusted for a read ing of either 92 5 dB or 112 5 dB in this example 18 ATMOSPHERIC PRESSURE mm of Hg 760 700 600 500 400 10 DEVIATION FROM CALIBRATED SOUND PRESSURE LEVEL dB 12 24 3 7 49 6 1 KM 4000 8000 12000 16000 20000 FT ALTITUDE Figure 2 3 Variation in sound pressure level with a change in altitude and atmospheric pressure 19 wi Transformer Integrated Circui Diode Zener Socket for Plug In Plug Transis Resis Crystal Network P a R 5 x Y z Diode Lamp Fuse Jack Retay Relay Coil Relay Switch Battery Capacitor Inductor Meter Motor REFERENCE DESIGNATOR ABBREVIATONS 8 BT CR DS F J K KL KS t NOTES WAVETORTIS SHOWN NRE AT Qu A LPI CIFIFL LE VEL OF 1 dB AT RHE R 323 k Vg ww 176 LM324 CRA ifo as 155 7 e wT ut m riore BASE DIASRAMS rov Ew 1448 LEVEL CAL Rie R9 00Vaw e 150 yaw RI ust 249 78 una Vers 9448 LEVEL CAL IKHa OSCILLATOR TRANSDUCER DRIVER 4 RESISTOR VALUES IN OHMS K 10 OHMS M310 OHMS CAPACITOR VALUES IN MICROFARADS p 10 FARADS IXXXX FRONT PANEL LEGEND KXXX REAR PANEL LEGEND SCREWDRIVER ADJUSTMENT Q wTz WIRE TIE Q TP TEST POINT
13. 741 Condensed Operating Instructions TO ACTIVATE THE INSTRUMENT a Set power switch to desired calibration level 94 dB or 114 dB b Listen for 1 kHz audible tone If no tone is present replace battery see para 2 4 TO CALIBRATE A SOUND MEASURING INSTRUMENT a Observe 1987 desiccant through window If pale blue satisfactory if pink replace desiccants see para 2 5 b If microphone adaptor is required insert it in 1987 cavity to fit microphone under test see para 2 6 c Set instrument under test to 100 dB if 1987 is set at 94 dB or 120 dB if 1987 is set at 114 dB Instrument may be set to FAST or SLOW response and C or A weighting d Place 1987 on microphone of instrument under test read level and adjust its sensitivity A pressure to free field correction see para 2 10 must be added to this reading for some micro phones refer to following table this correction in dB is to be added to the reading of the instru ment under test before comparing it with the calibrated output of the 1987 either 94 dB or 114 dB Handbook of Noise Measurement This book by Dr A P G Peterson and Ervin E Gross Jr of the GenRad Engineer ing Staff covers thoroughly the subject of noise and vibration measurement Copies are available from GenRad at 9 00 each post paid in the United States and Canada GR P N 5301 8111 MM DENOTES END OF CAPACITORS ARM OF POTENTIOMETERS ROTOR OF TRIMMER CAPACITORS
14. 78 A cross check of the above calibration can be made by comparing the results using the 1987 calibrator with the results using the 1954 Indicator s calibrator Some discrepancy is likely and the tolerance of the 1987 output level and the 1954 Indicator output level must be taken into account when making this comparison 24 microphone Adaptors for microphones other than GR 1 2 in and 1 in microphones are available in an accessory kit P N 1987 9600 which is described below The GR 1987 7061 Microphone Adaptor is inserted by gently pushing it into the cavity until the three detents inside the cavity fall into the groove around the outside of the adaptor Make sure it is firmly seated To remove the adaptor grasp its outer flange that extends out of the cavity and pull it out 2 6 2 GR 1987 9600 Microphone Adaptor Set This optional set is an accessory available by order from GenRad It includes six microphone adaptors that are necessary to accommodate various microphones on instruments under test The base of each adaptor is marked with its appro priate part number Figure 1 3 and Table 1 3 illus trate and describe the adaptors in this set The six adaptors in this set are inserted in the transducer cavity as described below GR 1987 2050 Microphone Adaptor for B amp K Type 4133 34 1 2 in microphone insert in same manner as 1987 7061 adaptor This adaptor appears similar to the GR 1987 7061 1 2 in adaptor except for
15. HAT COMPARTMENT ACCEPT ASSEMBLY ELECTROMAGNETIC SCREWS TRANDUCER N LABEL AND NUT REMOVED FROM POWER DESICCANTS SWITCH BATTERY 2v ASSEMBLY SCREWS COMPARTMENT ALKALINE REMOVED FROM COVER BATTERY BATTERY COMPARTMENT Figure 2 1 GR 1987 disassembled b If the battery is up to operating voltage and the 1987 is functional a tone of 1 kHz will be audible c If thisaudible tone is not present the battery should be replaced Refer to para 2 4 and 2 2 for battery removal and installation procedures 2 4 BATTERY REMOVAL CAUTION The battery should be removed whenever the 1987 will not be used for 6 months or more to prevent corrosion on the terminals To remove batteries for replacement or storage proceed as follows refer to Figure 2 1 a Loosen the captive thumbscrew ccw direc tion that secures the battery compartment cover b Grasp the thumbscrew and lift the cover away from the 1987 housing c Place one finger or tool like a small screw driver on the terminal end of the battery and push it to compress the foam on the other end with the same finger or tool pull up slightly until the battery pops out 2 5 DESICCANT REPLACEMENT The 1987 contains two desiccant cartridges inside its transducer section refer to Figure 2 1 in order to keep the interior humidity low relative to any high external humidity The desiccants are used to minimize the effects of humidity upon the transducer which pr
16. TION Ambient sources of noise or vibration can cause a false calibration indication this can be especially significant at the lower 94 dB level a Insert the proper adaptor if one is required into the cavity of the 1987 to fit the particular microphone under calibration refer to Table 1 3 b Set the 1987 power switch to the 94 dB or 114 8B position depending on the desired level of calibration Select the level closest to the upper limit of the level range intended for measurement c An audible tone of 1 kHz should be present from the calibrator if not the battery may need replacement refer to para 2 4 d Place the 1987 Minical over the microphone of the sound measuring instrument being calibrated e On the instrument under test set the LEVEL RANGE control to the range having 100 dB as its upper limit if 94 dB was selected on the 1987 or if 114 dB was selected on the 1987 choose a range with an upper limit of 120 dB The instrument may be set to FAST or SLOW response and C or A weighting f Read the level on the instrument under test and adjust the sensitivity control for the correct indication of the 1987 level selected in step e It is important to note that a pressure to free field correction must be made to this indication for some microphones this subject is discussed in para 2 10 Normal variation of temperature and atmospheric pressure will have negligible effect on the sound pressure level developed fr
17. ation 2 3 Battery Check 2 4 Battery Removal 2 5 Desiccant Replacement 2 6 Microphone Adaptors 2 7 Operational Check 2 8 Calibration of Instruments 2 9 Altitude and Pressure Corrections 2 10 Pressure to Free Field Corrections 2 11 Microphone Volume Correction n 2 12 Use with GR 1954 Noise Dosimeter Theory Section 3 3 1 General Service and Maintenance Section 4 4 1 Customer Service 4 2 Instrument Return 4 3 Performance Verification 44 Disassembly 4 5 Test and Calibration 4 6 Trouble Analysis Parts List and Diagrams Section 5 5 1 Electrical Parts List and Diagrams 5 2 MechanicalParts Parts Lists and Diagrams Section 5 5 1 ELECTRICALPARTSLIST AND DIAGRAMS Figures 5 1 and 5 2 are respectively an etched board diagram and a schematic diagram of the circuits and components in the 1987 An electrical parts list gives descriptions and part num bers for the electrical components shown in these figures 5 2 MECHANICAL PARTS Figure 5 3 shows locations of mechanical parts on the 1987 and the accompanying table gives their descriptions and associated GR and manufac turers part numbers 5 8 FEDERAL MANUFACTURERS CODE The federal manufacturers code numbers for part number identification codes are listed at the end of this secti
18. ator e transducer driver The paragraphs below discuss these circuits As differentiated from a primary standard calibrator that contains a transducer with a known level of response traceable to the National Bureau of Standards see para 4 5 25 2 12 USE WITH GR 1954 NOISE DOSIMETER The calibration of the monitor on the GR 1954 Noise Dosimeter can be checked using the 1987 in place of the 1954 Indicator s calibrator The GR 1987 7076 Adaptor and GR 1987 7095 Adaptor Cap are required these are included in the GR 1987 9600 Microphone Adaptor Set The calibration procedure follows a Insert the 1954 Monitor into the 1954 Indicator as normally done for aroutine calibration check refer to the 1954 Instruction Manual b Set the dB RANGE switch on the 1954 Monitor to 80 130 dB and the ON HOLD switch to ON Insert the 1954 Microphone into the 1987 cavity using the 1987 7076 Adaptor and 1987 7095 Adaptor Cap refer to para 2 6 3 for instruc tions on installing adaptors d Set the power switch on the 1987 to 114 dB e Reset the monitor by depressing DISPLAY and RESET simultaneously on the indicator f Depress the CALIBRATE button on the 1954 and hold it for 10 s until the display reaches a steady value Adjust the CAL control on the monitor for a reading on the indicator of 0 087 This number applies to the 1954 9710 Monitor which has a 5 dB exchange rate The calibration number of the 1954 9730 Monitor is 0
19. e and dust control 1 13 Window Clear plastic provides 1987 7090 24655 1987 7090 Desiccant observation of desiccant 1 14 Cover Battery Aluminum covers battery 1987 2000 24655 1987 2000 Compartment compartment 1 15 Screw Cover 0 112 40 thread secures 1987 6001 14655 1987 6001 battery compartment cover 1 15 Bushing Knurled Fits over head of 1987 6001 1987 6005 24655 1987 6005 screw 1 16 Top Housing Plastic contains battery 1987 2010 24655 1987 2010 Assembly compartment power switch and etched board 1 17 Switch Toggle DPDT Power switch S1 7910 0540 95146 TTO0259 94dB OFF 114dB includes No 10 48 hex nut and lock washer Figure 5 3 Mechanical parts
20. evel recommended by the manufacturer for calibrat ing the absolute sensitivity of a sound level meter Tt Ref Draft Instruments for the Measurement of Sound Level Sound Level Meters to supersede IEC Publications 123 179 and 179A upon final approval International Electrotechnical Commis sion 1 Rue de Varembe Geneva Switzerland Table 1 1 CONTROLS INDICATORS AND CONNECTORS Fig 1 1 Ref Name Description Function 1 Battery Recess in 1987 Contains 9 V alkaline Compartment housing access battery for providing obtained by instrument power removing cover secured by thumbscrew 2 94 dB OFF position OFF disconnects 114 dB toggle switch instrument from its PU pm internal battery B 1 Ce ety ed ibi c source 94 dB selects cal ibrated output level of 94 dB re 20 yPa 114 dB selects calibrated output level of 114 dB re 20 uPa 3 Desiccant Small circular Provides observation Window window on of desiccant to 1987 housing determine its con dition pale blue means satisfactory pink means replacement necessary 4 Transducer Contains Provides acoustical WTA WT3 Assembly electromagnetic output signal to cal CONNECTS BATTERY transducer and ibrate a sound HERE FOR EXTERNAL POWER 1 in cavity for measuring instrument microphone cavity allows tight insertion acoustical coupling to a microphone Figure 4 1 Circuit board P N 1987 4700 35 g Select the 94 dB level on the
21. he 1982 set its controls as follows Power ON DIGITAL DISPLAY cont OCTAVE FILTER FREQ 1k Hz dBRANGE 70 120 DETECTOR zl ie bI FAST b Set the controls on the calibrator being used to a level of 114 dB and a frequency of 1 kHz Install the appropriate microphone adaptor in the calibrator being used to accommodate the 1 2 in microphone on the 1982 and then carefully place the calibrator over the microphone of the 1982 c Observe the 1982 digital display and adjust its CAL control for a reading of exactly 114 0 dB The 1982 is now accurately calibrated to within 0 5 dB d Insert the 1 2 in microphone adaptor P N 1987 7061 in the 1987 transducer cavity e On the 1987 under test set the 114dB OFF 94dB power switch to 114 dB and then carefully mount the calibrator over the microphone of the 1982 f Observe the reading on the 1982 it should be 114 dB 0 5 dB 33 4 2 2 Packaging To safeguard your instrument during storage and shipment please use packaging that is adequate to protect it from damage i e equivalent to the original packaging Any GenRad field office can advise or provide packing material for this purpose Contract packaging companies in many cities can provide dependable custom packaging on short notice Here are two recommended packaging methods Rubberized Hair Cover the painted surfaces of the instrument with protective wrapping
22. ipment A detailed schematic diagram and parts list are included in Section 5 and circuit descriptions in Section 3 The schematic includes waveforms and voltages to aid in troubleshooting the 1987 undergoes any repair that will alter its acoustical output it is essential that recalibration be per formed This is described in para 4 5 It is important to note that a special calibration facility is required for this procedure 36 introduction Section 1 NOTE Batteries shipped uninstalled See para 2 2 1 1 PURPOSE The GR 1987 Minical Sound Level Calibrator is a compact self contained device for checking the calibration of sound measuring instruments 1 is intended for field use on a wide variety of sound level meters analyzers or sound measuring systems that contain a microphone adaptable to its cavity 1 2 DESCRIPTION The GR 1987 Minical produces a 1000 Hz tone at sound pressure levels SPL of either 94 dB or 114 dB An SPL of 94 dB represents the reference SPL recommended in the forthcoming IEC Con solidated Revision Standard t an SPL of 114 dB is sufficiently above most ambient noise or vibration sources to provide accurate calibration where these sources are present The 1987 features a ruggedized package that is resistant to moisture dust and mechanical shock The cavity of the 1987 will accommodate all GR 1 in electret condenser microphones GR 1 in The reference sound pressure level is that l
23. nsure that the oscillator starts 3 2 5 Transducer Driver This circuit is comprised of two level adjust ments R12 and R13 and a current source amplifier U1D The output of the 1 kHz oscillator is selected by one side of the 114dB OFF 94dB switch S1 and is fed through one of two paths to the current summing node of operational amplifier U1D Each path has a gain contro for adjusting the calibration of the 1987 R12 provides adjust ment of the 114 dB level and R13 is the 94 dB level adjustment refer to para 4 5 and Figure 4 1 The current source amplifier U1D is used to drive the output transducer LS 1 which provides the acoustical output to a sound measuring instru ment under test Since the transducer is an electro magnetic device with an acoustical output propor tional to the current in its magnetic field it is placed in the amplifier feedback loop which pro vides a constant current source If the transducer had been driven from a voltage source the current in the field would have depended on the coil windingresistance which is temperature dependent Capacitor C6 is used to roll off high frequencies thus eliminating some upper harmonics Resistor R10 allows testing of the transducer current at TP2 29 to O V 1 5 1 5 V causing its output voltage to also drop When the output of U1C shifts to a low voltage CR2 conducts this places an added load on the oscillator feedback circuit causing oscillations to
24. nt level significant enough to give a false calibration indication 14 to the SPL when it is calibrated by the 1987 Also no microphone volume correction is necessary for other GR 1 in electret condenser microphones GR 1 in ceramic microphones the Western Elec tric 640AA microphone or the Tokyo Riko MR103 microphone The microphone effective volume of the 1 in B K 4131 4132 Microphone has a significant enough discrepancy from the idealized volume 0 5 cm that a microphone volume correction is necessary This correction is given in Table 2 2 it should be added to the SPL indication on an instru ment that utilizes the 4131 32 microphone when that instrument is calibrated by the 1987 When the 1 2 in microphone adaptor P N 1987 7061 supplied with the 1987 is used to cal ibrate a GR 1 2 in electret condenser microphone no microphone volume correction is necessary Likewise when any of the adaptors except the 5855 6920 O ring adaptor for the B amp K 4131 32 Microphone cited above included in the optional GR 1987 9600 Microphone Adaptor Set are used with their intended microphones refer to Table 1 3 no microphone volume correction is necessary Table 2 2 MICROPHONE VOLUME CORRECTION Microphone Adaptor Correction Type Used dB B amp K 4131 32 O Ring 0 4 1 in Condenser 5855 6920 To be added to the sound pressure level indication of the meter under test 23 Sound level meters that comply
25. oduces a more stable output 3 2 2 Battery and Power Supply Splitter The power source for the 1987 is a single 9 V alkaline battery Approximately 20 h of continu ous operation are available from a new battery Since the operational amplifier circuits require both plus and minus supplies the battery output is split into equal positive and negative voltages by resistors R1 and R2 and splitter U1B the output of U1B is the reference for ground Capacitors C1 and C2 provide filtering as the battery s impedance increases with discharge 3 2 3 Battery Low Test Circuit This circuit is comprised of U1C O1 CR1 and CR2 The purpose of the circuit is to detect the power supply output and disable the bridge oscillator circuit for a low or dead battery This results in an automatic shutdown capability to prevent the 1987 from generating a low level out of calibration or drifting signal because of insuf ficient battery power This is especially important if the 1987 is used as a system calibrator where it is left unattended while the system levels are mon itored and adjusted The battery low detector works as follows The battery voltage is impressed across the current source Q1 and the 5 V Zener CR1 which has a low temperature coefficient When the battery voltage is 9 0 V the voltage at the cathode of CH1 is 0 5 V 4 5 V plus 5 0 V then the voltage at the input to the comparator U1C is 1 5 V below the cathode of CR1 or
26. om the 1987 Altitude pressure corrections are given in para 2 9 these must be taken into account for a calibra tion made well above sea level 17 g An example of a pressure to free field cor rection follows Condition of measurement Microphone Type GR 1962 9611 1 2 in electret flat perpendicular Correction from Table 2 1 0 1 dB For this example adjust the sensitivity of the instrument under test for an indication 0 1 dB below the level selected on the 1987 For example if the 1987 is set to 94 dB adjust the instrument s sensitivity for a reading of 93 9 dB 2 9 ALTITUDE AND PRESSURE CORRECTIONS The acoustical output of the 1987 is equal to 94 dB or 114 dB depending on the selection at a normal atmospheric pressure of 760 mm 30 in of Hg at sea level A correction must be made to the output if the instrument is used at a pressure other than this normal pressure Figure 2 3 shows a cor rection curve that indicates the change in sound pressure level SPL for a change in altitude and atmospheric pressure Most barometers are cali brated to read pressures corrected to sea level An example of how to use Figure 2 3 follows Condition of Measurement Altitude of 2 4 km 8000 ft Correction from Figure 2 3 1 5 dB For this example the acoustical output of the 1987 is 1 5 dB lessthan the indication on the power switch That is the 1987 SPL would be 92 5 dB for a power switch setting of 94 dB or 11
27. on NOTE Electrical parts information in this sec tion is presented in such a way that all component and circuit data are visible in a single opening of the manual Thus the part location diagram appears on a left hand page while the parts list and schematic diagram are on a right hand apron and tip out page 37 equivalent that possess a known level of response traceable to the National Bureau of Standards Potentiometers R12 and R13 are gain controls that are used to adjust the 114 dB and 94 dB levels respectively re 20 uPa These two controls should not be tampered with unless a full calibration facility exists The two adjustments are accessible from the top of the instrument when the power switch plate is removed this is accomplished by first taking off the nut and lockwasher that secure the small toggle switch to the housing and then lift ing the switch plate off of the unit The two adjust ments referred to are each covered by an adhesive disk when the disks are removed the adjustments are accessible through the two small holes in the top housing one on each side of the switch see Figure 2 1 4 6 TROUBLE ANALYSIS When a malfunction is suspected in the 1987 it is first recommended that the performance verification procedure of para 4 3 be conducted This will usually establish if there is a problem with the unit The 1987 should only be worked on by personnel skilled in the repair of delicate electronic equ
28. onal Electrotechnical Commission 1 Rue de Varembe Geneva Switzerland 22 2 7 OPERATIONAL CHECK To quickly check the operation of the 1987 before using it proceed as follows a Turn the power switch from OFF to the 94 dB position and listen for a 1000 Hz audible tone this indicates that the instrument is working If no tone is evident the battery may need replace ment refer to para 2 4 for battery replacement b Change the switch from 94 dB to 114 dB the 20 dB increase in level of the 1000 Hz tone should be easy to detect audibly 2 8 CALIBRATION OF SOUND MEASURING INSTRUMENTS 2 8 1 General Before a sound level meter or analyzer is used to make a measurement or analysis its sensitivity should be checked The 1987 Minical Sound Level Calibrator pro vides a calibrated output to ensure that the sensi tivity of a particular piece of sound measuring equipment including microphone is properly adjusted The 1987 generates two levels of output 94 dB and 114 dB re 20 Pa at a frequency of 1000 Hz 2 8 2 Calibration The sensitivity of a sound level meter or analyzer is checked by placing the transducer cav ity of the 1987 over the microphone of the instru ment under test The instrument is then adjusted to indicate the known level either 94 dB or 114 dB Figure 2 2 shows the setup for this sensi tivity check with a typical sound level meter The following procedure is suggested as a means to check and adjus
29. paper Pack the instru ment securely in a strong protective corrugated container which has a 25 kg cm 350 Ib sq in bursting test with 5 cm 2 in rubberized hair pads placed along all surfaces of the instrument Insert fillers between the pads and the container to ensure a snug fit Mark the box Delicate Instrument and seal it with strong tape Excelsior Cover the painted surfaces of the instrument with protective wrapping paper Pack the instru ment in a strong corrugated container which has a 25 kg cm 350 Ib sq in bursting test with a layer of excelsior about 15 cm 6 in thick packed firmly against al surfaces of the instrument Mark and seal the box as described above 4 3 PERFORMANCE VERIFICATION The following procedure is intended as a quick check of the 1987 and is distinct from the test and calibration procedures in para 4 5 that require 32 V24N v POUCI MICROPHONE ALKALINE DESICCANT ADAPTOR BATTERY KITS Figure 1 2 Accessories supplied with GR 1987 9700 instruction manual not shown Table 1 2 ACCESSORIES SUPPLIED WITH GR 1987 9700 Description GR P N Minical Sound Level Calibrator 1987 3000 instrument only Adaptor for 1 2 in microphone 1987 7061 Battery 9 V alkaline 8410 3400 3 Desiccant Kits 2 desiccant 5873 3001 cartridges per kit Pouch 1987 0420 Instruction Manual 1987 0100 5 1987 7066 1987 7076 1 1 8 IN 3 8 IN ADAPTOR DUM ADAPTOR 5855 8920 Q RING 1987 206
30. r nearest office fora Returned Material number Use of this number in correspondence and on a tag tied to the instrument will ensure proper handling and identification After the initial warranty period please avoid unnecessary delay by indicating how payment will be made i e send a purchase order number 31 Operation Section 2 2 1 UNPACKING AND INSPECTION f the shipping carton is damaged ask that the carrier s agent be present when the instrument is unpacked Inspect the instrument for damage scratches dents broken parts etc If the instru ment is damaged or fails to meet specifications notify the carrier and the nearest GenRad field office See list at back of this manual Retain the shipping carton and the padding material for carrier s inspection The battery must be installed in the 1987 to make it operable Para 2 2 gives the procedure for battery installation A small circular window in the 1987 housing provides observation of a desiccant cartridge that is used for moisture control If pale blue the desiccant is satisfactory if pink replacement is necessary Refer to para 2 5 for replacement procedures 2 2 BATTERY INSTALLATION The 1987 is shipped with the battery removed A 9 V alkaline battery is included with the acces sories and this is the only type of battery recommended for use that will provide an opti mum operating life A new alkaline battery will provide approximately 20 h of con
31. rols indicators and connectors ceramic microphones the Western Electric 640AA or equivalent microphone and the Tokyo Riko MR 103 microphone The 1987 is also supplied with an adaptor that accommodates GR 1 2 in electret condenser microphones An optional adaptor set P N 1987 9600 is available to accom modate the 3 8 in microphone used on the GR 1954 Noise Dosimeter the B K 1 in 1 2 in and 1 4 in microphones and the Shure Brothers 1 1 8 in microphone 1 3 CONTROLS INDICATORS AND CONNECTORS Figure 1 1 illustrates GR 1987 controls indicators and connectors Table 1 1 describes these items 1 4 ACCESSORIES Model GR 1987 3000 sianifies the Minical Sound Level Calibrator only with no accessories Model GR 1987 9700 signifies the instrument included with the accessories illustrated in Figure 1 2 and described in Table 1 2 Figure 1 3 illus trates the six microphone adaptors included in the optional GR 1987 9600 Microphone Adaptor Set an accessory available by order from GenRad Table 1 3 describes these adaptors accuracy traceability to the National Bureau of Standards A precision sound level meter such as the GR 1982 or its equivalent should be used for this performance test a The calibration of the 1982 should first be checked and adjusted if necessary using another calibrator with a known accuracy correct to within 0 5 dB A GR 1986 GR 1562 or GR 1567 can be used for this purpose To calibrate t
32. s of mercury on the right hand scales b Obtain the corresponding value in millibars from the same scales c Move to the left hand scales and find the milli bar value obtained in step b d Read the corresponding value of barometric pressure in inches of mercury from the left hand scales 41 Battery Test Internal circuitry checks condition of battery continuously Calibrator will not operate at all if battery voltage falls below acceptable range Microphone Coupling Transducer cavity accom modates following 1 in microphones GR 1961 electret condenser GR 1971 ceramic Western Electric 640AA and Tokyo Riko MR103 Accessories Supplied Coupler adaptor to accom modate GR 1962 1 2 in electret condenser micro phone and GR 1983 Sound Level Meter micro phone carrying pouch 3 spare desiccant kits battery instruction manual Accessories Available Adaptor set that includes coupler adaptor for 3 8 in GR 1954 Noise Dosimeter microphone coupler adaptors and O ring for 1 in 1 2 in and 1 4 in B K microphones and coupler adaptor for 1 1 8 in Shure Brothers microphone T In the international system of units SI the unit of pressure is the pascal Pa 1 Pa 1 N m 10 dynes cm 10 mbar Ref The International System of Units 51 U S Dept of Commerce National Bureau of Standards NBS Special Publication 330 S D Cat No C 13 10 330 2 U S GPO Wash D C 20402 U S Patent 4 070
33. s that are visible inside the compartment once the battery has been removed c Grasp the bottom section of the 1987 with one hand the top section with the other and pull off the top section the separation will occur at the rubber gasket between the two sections d Each desiccant cartridge is held in place on the transducer housing assembly by two clips one on each end see Figure 2 1 To replace a desic cant slide it out from under the clips and then slide a replacement cartridge back under the clips 11 e Before reassembling the transducer assembly check to see if the two wires that connect to the transducer became unplugged If so they should be connected to the terminals labelled WT 1 and WT2 on the circuit board These two connections are nonpolarized and interchangeable refer to Figure 4 1 f Replace the top section of the 1987 on the lower section Note that the two spacers see Fig ure 2 1 that accept the Phillips head screws are offset from the center of the housing such that there is only one orientation that mates the top section with the bottom section Both the top and bottom sections should be firmly seated into the rubber gasket before the two Phillips head screws are tightened after securing these two screws make sure the gasket has seated properly It can be properly seated by running a fingernail around its edge g Replace the battery and battery compart ment cover 2 6 MICROPHONE ADAPTORS
34. se Pressure response is character ized by a sound field that produces a uniform sound pressure over the diaphragm surface of a microphone This condition is realized by the 1987 when the microphone being calibrated is tightly coupled within its specially shaped cavity of small volume Most microphones used in sound measuring instrumentation are designed for one of two kinds of response either a flat frequency response to sounds of random incidence in a free field or a flat frequency response to sounds of perpendicular with respect to the plane of the microphone s diaphragm incidence in a free field Since such free field incidence is not simulated by the 1987 with its tight pressure coupling within a cavity of small dimensions correction factors are sometimes necessary for either of these two types of micro phone when calibrated with the 1987 The amount of correction factor that is required depends on the type of microphone being cal ibrated Table 2 1 gives the pressure to free field corrections that are necessary for various micro phones These corrections factors are computed from the difference between the perpendicular or random response that a microphone is designed for and the actual pressure response within the cavity of the 1987 Thus a correction will always result in adjusting the sensitivity of the instrument under test for a reading lower than the level indicated on the 1987 label by the power switch 20 CAU
35. t the sensitivity of an instrument 15 Figure 2 2 Typical calibration setup 16 Table 2 1 PRESSURE TO FREE FIELD CORRECTIONS Microphone GR Adaptor Correction Manufacturer and Type Used dB Flat Response to Random Incidence Microphones GR 1961 9610 or 9601 none 0 0 1 in Electret GR 1962 9610 or 9601 1987 7061 0 0 1 2 in Electret GR 1971 1 in Ceramic none 0 0 GR 1983 Sound Level Meter 1987 7061 0 1 1 2 in Ceramic GR 1954 Noise Dosimeter 1987 7076 0 0 3 8 in Ceramic and 1987 7095 Fiat Response to Perpendicular Incidence Microphones GR 1961 9611 or 9602 none 0 3 1 in Electret GR 1962 9611 or 9602 1987 7061 0 1 1 2 in Electret B amp K 4131 1 in Condenser t O Ring 0 2 5855 6920 B amp K 4133 1 2 in 1987 2050 0 0 Condenser B amp K 4135 1 4 in 1987 2060 0 0 Condenser Measurement Conditions Atmospheric Pressure 760 mm 30 in of Hg Temperature 20 C 68 F ATo be added to sound pressure level indication of meter under test before it is compared with level indicated on 1987 label by power switch t Additional correction required due to microphone effective volume refer to para 2 11 21 2 10 PRESSURE TO FREE FIELD CORRECTIONS When the 1987 transducer is tightly coupled to a microphone within its cavity it creates a micro phone frequency response condition referred to as pressure respon
36. tinuous operation GenRad recommends Mallory MN 1604 or Eveready 522 To install the battery proceed as follows refer to Figure 2 1 a Loosen the thumbscrew ccw direction that secures the battery compartment cover This screw is captivated so that it will remain with the cover once loosened b Grasp the thumbscrew and lift the cover away from the 1987 housing c Before inserting the battery observe its polarity with respect to the battery outline shown inside the compartment d Push the bottom of the battery against the foam to compress it Simultaneously push the other end of the battery terminal end into the compart ment such that it is firmly seated and makes con tact with the spring metal contacts e Replace the battery compartment cover by sliding its rear edge under the rear edge of the bat tery compartment until the thumbscrew lines up with its threaded hole in the 1987 housing Then seat the cover flush on the lip that is built into the compartment and tighten the thumbscrew cw direction to secure the cover to the unit 2 3 BATTERY CHECK If the battery voltage is not adequate there will either be no acoustical output or the output will cut out after being on for a short time To check for sufficient battery voltage proceed as follows a Set the power switch to the 94 dB or 114 dB position net attenuation in this positive feedback load is slightly less than the gain of the bridge filter to e
37. ure Level Under stated reference environment conditions 0 25 dB for cavity alone or cavity when used with any adaptor except 1 1 8 in adaptor 0 5 dB Temperature Coefficient of Sound Pressure Level Less than 0 02 dB C Power Powered by 9 V alkaline battery Mallory MN 1604 or Eveready 522 recommended Battery provides at least 20 h continuous operation Environment TEMPERATURE 10 to 50 C 14 to 122 F operating 40 to 70 C 40 to 140 F storage with battery removed HUMIDITY 0 to 90 RH operating Mechanical DIMENSIONS wxhxd Approx imately 63x63x89 mm 2 1 2x2 1 2x3 1 2 in WEIGHT Approximately 270 gm 9 5 oz Appendix USE OF THE NOMOGRAPH CORRECTION OF BAROMETRIC PRESSURE TO STATION ALTITUDE To obtain a corrected barometric pressure for a station a Determine the station s altitude above sea level b Obtain a barometric pressure reading corrected to sea level from a barometer If the barometer reads only values in millimeters find the corresponding value of millibars from the right hand scales c Place a straight edge across the proper points on the center and left hand scales of the nomograph and read the actual pressure at the station on the right hand scales CONVERSION FROM MILLIMETERS OF MER CURY TO INCHES OF MERCURY To convert from millimeters of mercury to inches of mercury proceed as follows a Find the barometric pressures value in milli meter
38. with the ANSI S 1 4 1971 standard use a microphone idealized for a random incidence response whereas meters that comply with the JECT standards use a micro phone idealized for a perpendicular incidence response 2 11 MICROPHONE VOLUME CORRECTION The sound pressure level SPL developed in the 1987 transducer cavity is dependent upon the vol ume of air between the diaphragm of the micro phone being calibrated and the diaphragm of the 1987 transducer A portion of that volume between the two diaphragms namely the particular volume comprised by the space between the front face of the microphone and its diaphragm is referred to as the microphone effective volume The 1987 transducer cavity is designed to pro duce a 94 dB or 114 dB SPL upon the diaphragm of a 1 in microphone that has an effective vol ume of 0 5 cm 03 in The 1 in GR 1961 Electret Condenser Microphone has this idealized effective volume and thus requires no correction Ref American National Standard Specification for Sound Level Meters ANSI 1 4 1971 pub lished 1971 American National Standard institute Inc 1430 Broadway New York N Y 10018 tRef Recommendations for Sound Level Meters Publication 123 published 1961 Precision Sound Level Meters Publication 179 published 1973 Precision Sound Level Meters Additional Char acteristics for the Measurement of Impulsive Sounds Publication 179A published 1973 Internati
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