General Radio Sound Measurement Instruments

Brooke Clarke, 2012 - 2022

Create Sound
    1307 Oscillator & 1552 Level Calibrator
    1552 Level Calibrator
    1562-A Sound-Level Calibrator
    1559 Microphone Calibrator
Measure Sound
    1565-A Sound Level Meter
    1565-B Sound Level Meter
    1560-P96 Type 1971-9605 Microphone
    1560-P40 X1 or X10 Amplifier
    1560-P52 VIbration Pickup
    1560-4100 Phantom Battery Power Supply
     Cords: AC, 1/4"-Dual Banana, Vibration-XLR, XLR extension
    Adapters: 1560-P98 GR-874 to 2-Pin Audio Adapter, 1560-P82 Earphone Coupler, GR-874 to Dual Banana Jack adapter,
    Stand & Suitcase
Scott Instrument Labs ANSI Type S2A Sound Level Meter


There are a lot of audio accessories, both that convert electricity into sound and convert sound into electricity.  It would be nice if this could be measured and that's what these instruments do.  I think this set was used to calibrate audiometers (hearing testers).


The contents of the suitcase might be divided up into equipment for creating sound, equipment for measuring sound and accessory adapters, cables, power supply, etc.  Most of the equipment has a GR model number of 1560-.

Create Sound

The 1307 Transistor Oscillator was the lead article in the GR Experimenter for August 1954.  They claimed over 100 hours of operation from the three cell Mercury Battery.

1307 Transistor Oscillator &

1552 Sound Level Calibrator

GR 1307
1307 Transistor Oscillator
GR 1307
1307 Transistor Oscillator Battery
GR 1307
                  Transistor Oscillator Battery
The battery is held by a matal strap clamp that's difficult to access.
Why would they solder in a battery and make it so difficult to change.
Ans.  It's a Mercuty battery and was expected to be replaced by
the factory.

The corrosion from the battery locked the Level pot and
the two set screws holding the Level knob on the pot shaft.
Kroil freed up the pot shaft and one of the pot set secrews.
Will need to wait till tomorrow to see of the other set secrw
gets free.

There are patent numbers on the bottom of the battery:
2542575 Alkaline Dry Cell,
2712586 ?
2636062 Electrochemical Cell and Self-Venting Enclosure, Mallory
2462998 Primary Cell with Permanganate Depolarizer
    RE23427 Primary Cell with Permanganate Depolarizer,

Battery Removed
GR 1307
                  Transistor Oscillator Battery Removal

To remove the battery:
Remove the two knobs (requires being able to turn them to access the two set screws on each shaft).
Remove the screw that between the two knobs.
Remove the two nuts on the back of the meter movement.
Fold the board out and remove the hex spacer that holds the battery clamp.

In the photo at left the top electrical contact is (+), you can see "+" scratched on the back of the front panel.

Notice the battery is like a bar bell with ends that are larger in diameter than the center so it can NOT be slipped out.
With the Level pot at max it takes at least 3.4 V (6 ma) of battery to get the meter to read 2V (the calibration value for the 1552-B.
With the battery voltage at 9.0 (5 ma) the level pot has plenty of room on both sides of a meter reading of 2V.
A 9 Volt battery is a little too big to fit where the old mercury battery was.

An A23 battery (12 Volts) is very small.  I soldered a red and black wires and confirmed that the polarity was correct.
Then folded the wires so the point to the opposite end and applied heat shrink tubing.
Soldered into the 1307 without using the bracket since it's way too big and the battery is so light the leads hold it.
All's well.
First application is to see what's going on with the mike on the Harris RF3200ET.

1552 Sound Level Calibrator

GR 1552

1552 Sound Level Calibrator Cavity
GR 1552

1562-A Sound-Level Calibrator

 (not part of suitcase set)
Uses a Wien bridge oscillator (Wiki) (a form of RC oscillator) with an amplitude stablising thermistor.  Note:  the HP 201 used a lamp as the stablizing element.
Under some conditions the thermistor is not hot enough to be working correctly, hence the "Start" control setting to warm it up & test the battery.  If the thermistor is cold popping and crackling sounds will be heard instead of clean tones.
Control End
Shown in 125 Hz position (red background).
Circle to the right is the battery test lamp.
Start/Bat Test - OFF - 2000 - 1000 - 500 - 250 -125 Hz
The frequencies are each twice the next lower one.
Probably not designed for communications, but lab
mike calibration.
General Radio
                  1562-A Sound-Level Calibrator Control End
Source End
Open end I.D. = 1.053"
Adapter I.D. = 0.939"
knurled screw on side allows opening case &
acts as the ground side of a 1/4" phone jack.
General Radio
                  1562-A Sound-Level Calibrator Source End
Battery Compartment
Takes 9 V battery with terminal on each end.
NEDA 1600
Eveready 226
CM345 Battery test lamp (6 V @ 60 ma) at lower right.
Voltage at lamp base vs. lamp current:

With 9 V at battery terminals, switch in START,
lamp voltage is 6.3V caused by R110 (62 Ohm) and
Q105 Vces (0.3V).

General Radio
                  1562-A Sound-Level Calibrator Battery Compartment
4 each AAA batteries can be used for the battery.
It's only 6 V instead of 9 V and so takes longer for
the thermistor to come up to temperature, but then works

A problem when testing the Harris RF23200ET mike is
that the lowest two frequencies (125 & 250 Hz) are below
the 300 to 3000 Hz voice band.
Using button cells is not a good idea since the current
this unit draws (a few ma) is way in excess of the max
design current for the button cells so they would have a
very short life.

This gives an advantage to the 1552 in that it can be driven
from an external oscillator, like the HP 33120 Function Gen.

GR 1559 Microphone Calibrator

(not in this set). Manual - I have a book (haven't remembered the title yet, probably related to Sonobuoys or Helmholtz resonators, that talks about the equivalence between a mechanical model and an electrical circuit.  That's to say an electrical impedance measurement can be translated into a mechanical circuit diagram.  The manual is titled: Microphone Reciprocity Calibrator.  The method uses a known volume inside a piezoelectric tube with a microphone at each end.

Conrad Hoffman: The General Radio Corp. 1559-B Microphone Reciprocity Calibrator - (Wiki: Measurement microphone calibration)

3327071 Method of and apparatus for computation particularly suited for microphone absolute calibration, Basil A Bonk, General Radio, 1967-06-20, 381/58; 708/843; 73/1.82; 367/13 - This is the basis of the GR 1559.

Measure Sound

GR 1565-A Sound-Level Meter

GR 1565-A
                  Sound-Level Meter
GR 1565-A Sound-Level Meter
GR 1565-A
                  Sound-Level Meter
GR 1565-A Sound-Level Meter Opened.
Battery clips for a common "C" cell.
The mike can be removed and the -P40 Amplifier installed.
GR 1565-A
                  Sound-Level Meter Opened

GR 1565-B Sound Level Meter

Calibrate with 1562.

GR 1565-B Sound Level Meter

GR 1560-P96 Type 1971-9605 Microphone

Type 1971-9605
General Radio Co
Serial No 7111

1560-P96 adapter to 3-pin male A3

GR 1565-B
                            Sound Level Meter

GR 1565-B
                            Sound Level Meter

Accessories mentioned in manual:
  • 1562 Sound-Level Calibrator (p/n: 1562-9701)
  • 1560-P96 Input socket adapter to Switchcraft Type A3M (p/n: 1560-9696)
  • 1560-P52 Vibration Pickup (p/n: 1560-9652)
  • 1560-P83 Earphone Coupler connects audiometer earphone to SLM.
  • 1560-P73 25' Extension Cable (p/n: 1560-9673).
  • Adapter Cables:
    • 1560-P77 Micro plug to 3/4" spaced banana plug (GR274) (p/n: 1560-9677)
    • 1560-P78 Micro plug to std. 1/4" phone plug (p/n: 1560-9678)
    • 1560-P79 Micro plug to BNC (p/n: 1560-9679)
    • 1560-P80 Micro plug to std. 1/4" phone jack (p/n: 1560-9680).
  • Type 1560-9590 Tripod (with 1/4"-20 thread)
  • 1560-9521 Windscreen

GR 1560-P40 X1 or X10 Amplifier

Hi input Z, low noise for 1" Ceramic mikes.
Fig 6-16b pg 101 Noise Meas Handbook
2 Socket End
GR 1560-P40
GR 1560-P40 X1 or X10 Amplifier
XLR 3-Pin end
GR 1560-P40

GR 1560-P52 Vibration Pickup

Has 1/4-20 tapped hole in the center of the back.
Two clearance holes for mounting.

Missing the short cable with XLR connector that would  mate with the 1560-P96 adapter.
GR 1560-P52


GR 1560-4100 Phantom Battery Power Supply

This box was covered with tape residue and was missing a number of screws.
GR 1560-4100
The Battery test lamp is a GE 344 and it measures 100 Ohms, seems too high.
10 V 14 ma equates to 714 Ohms hot resistance, the cold resistance will be lower.
GR 1560-4100 Phantom Power Supply w/ Ni-Cad batteries
GR 1560-4100
GR 1560-4100 Phantom Power Supply w/ Ni-Cad batteries

Using a bench power supply set to 65 Volts and current limited to 18 ma,
each battery has been soaked for about a day.  They each show a charge of
slightly more than 12 Volts.  When the Battery/Charge switch is on Battery
There is 20+ volts on the XLR socket contacts 1 (ground) & 2 (+20.87 V).


AC Stacking Line Cord for GR 1560-4100 Phantom Power Supply
GR A.C. Line
1560-P95 1/4" Phone Plug to Dual Banana Plug Cable for
GR 1560-4100 Phantom Power Supply Output
GR 1/4"
                  Phone Plug to Dual Banana Plug Cable
Vibration Pickup to XLR 3-Pin Plug to feed signal to 1560-4100 and
receive phantom power from 1560-4100
GR 1560-P52
XLR Extension Cord
                  Extension Cable


1560-P98 GR-874 to 2-Pin Audio Adapter
GR 1560-P98
1560-P98 GR-874 to 2-Pin Audio Adapter
1560-P82 Earphone Coupler, Audiometer Calibration Set
GR 1560-P82

GR-874 to Dual Banana Jack adapter
Patent No. 2548457 Coaxial Connector for High-Frequency Transmission Lines, GR, Apr 10, 1951, 439/583; 174/21.00R; 174/21.0CA; 174/88.00C
GR 874 to Dual
                  Banana Plug
1560-?  Stand
GR 1560 Stand
1560- Suitcase
GR 1560 Suitcase

Scott Instrument Labs Type 453A ANSI Type S2A Sound Level Meter

This sound level meter comes in a carry case with a Type 457 calibrator and an instruction booklet.
The calibrator can be used stand alone to generate a -106 dB A-weighted 1 kHz tone when +9 volts is applied to the tip and ground to the ring terminal.

Note: the CAL 1/8" mono jack is in parallel with the battery supply (after the ON switch) so can be used as an external (always on) DC power input jack.

The meter displays a range of 10 dB, hence there are two index marks for the range switch, one for the left side of the meter and the other for the right side.

This particular meter is not working.  Maybe there are some bad electrolytic caps? No.  The problem was corrosion on the 9V battery connector, it's now working and calibrated.
(only off less than 1 dB).

Scott Instrument Labs Type 453A ANSI Type S2A
                  Sound Level Meter & 457 calibrator


GR Strobotac
Edgerton Notebooks

Military Audio
Military Audio Accessories
U-229 Audio Accessories
U-229 Pinout by Function
TS-585 Audio Level Meter
HP 4395A Network, Spectrum & Impedance Analyzer

Beltone 12D Audiometer -
Sound Powered Telephone - and the development of early speakers
Crystal Radios and early loudspeaker
Teledyne Avionics TA-3D Acoustic Impedance Meter -
Tuning Forks & Helmholtz Resonators -


GR Handbook of Noise Measurement, Arnold P.G. Peterson, 1963 -1980
Acoustics, Leo L. Beranek 1986 ISBN: 0-88318-494-X
Acoustic Measurements, Leo L. Beranek (MIT), 1949 -1962
Of Acoustics and Instruments* Memoirs of a Danish Pioneer -



2173426 Electric System, Scott Hermon Hosmer, (Wiki HH Scott: Company, man) General Radio Co,1939-09-19 - Model 760 Sound Analyzer - this is the patent that's part of the HP 200 series of audio oscillators as well as the HP 415 SWR meter.
2268872 Variable Frequency Oscillation Generator, William R. Hewlett (HP), Jan 6, 1942, 331/141; 331/183 -  HP 200A Audio Oscillator - lamp stabilization
3012197 Calibration Apparatus, GenRad, Dec 5, 1961,
324/130; 324/123.00R; 330/2; 330/110; 330/112; 330/284; 331/44; 331/107.00R; 331/109; 331/136; 331/183 -
for use in sound level meters.
2322708 Vibration Measuring Apparatus, WE,
2564010 Unattended Repeater Testing by Causing the Repeaters to Oscillate, Bell Labs
2587697 Apparatus for Testing Amplifiers, Stromberg-Carlson,
2647958 Voltage and Current Bias of Transistors, Bell Labs, Aug 4, 1953
2764643 Oscillators, F.H. MacIntosh

Microphone Testing (related to Harris RF3200ET mike/audio input circuit problem)

H413, Microphone Output-level Tester, Navy, Jan 5 1988, 381/58 - defensive patent
4296483 Method and means for measuring geophone parameters, Litton Resources, Oct 20, 1981 367/13; 73/1.85 - related to geophones


Also see References for more on Edgerton and GR strobe lights.

2408764 High-speed flash-photography, Edgerton Harold E, Oct 8, 1946, 396/171, 362/4, 455/66.1, 250/214.00P, 315/230, 455/500, 250/215
2478907 Flash-producing device, Edgerton Harold E, Aug 16, 1949, 315/188, 315/354, 315/353, 315/208, 315/230, 315/241.00R, 315/241.00P, 315/238
2877341 Liquid cooled flash-producing apparatus, Edgerton Harold E, Mar 10, 1959, 362/264, 315/241.00R, 313/36, 315/241.00P
2939984 Flash device and method, Edgerton Harold E, Jun 7, 1960, 313/634, 315/241.00R, 315/237, 313/594, 315/241.00P
2975397 Surface-indicating apparatus, Edgerton Harold E, Mar 14, 1961, 367/134, 200/61.42, 340/686.6, 367/137, 340/692, 340/984
2983850 Condenser-charging circuit for flashproducing apparatus, Edgerton Harold E, May 9, 1961, 15/241.00P, 315/241.00R
2996966 Underwater-flash-producing-and-photographing system, Edgerton Harold E, Aug 22, 1961, 396/28, 362/8, 396/387, 396/180, 340/850, 362/10
3033988 Method of and apparatus for the control of electric impulses, Edgerton Harold E, May 8, 1962, 250/205, 250/214.00P, 315/340, 356/215, 330/59, 315/241.00P, 315/151
3065667 Transparency photographic apparatus, Edgerton Harold E, Nov 27, 1962, 355/27, 315/234, 315/227.00R, 315/241.00P, 355/68, 355/36, 355/64, 396/33


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page created 11 Jan 2012.