Audiometric headsets
In the 60's GS used the telephonics TDH-49 10 ohm earphone as a transducer because it had a better frequency response in the higher frequencies than the TDH-39. In the 70's they switched to the TDH-50 60 ohm earphone because it was less effected by resistance in jack panel phone jacks. Response curves for TDH-39, TDH-49 and TDH-50 earphones can be found under the Nostalgia tab.
Recently I found this vintage headset on Ebay with the original inspection tag still attached, tested by me in 1968.
Note:
Resistance build up in phone jacks was the overwhelming cause of calibration errors .
The second greatest cause was loose earphone screws.
