What was the primary function of standardized test films or 'tone reels'?

Allowing projectionists to adjust amplifier dials to match known signals

Standardized test films, known as 'tone reels,' were indispensable tools developed during the calibration phase. These reels contained precisely engineered audio signals, such as pure tones at specific, known frequencies and defined modulation levels. A projectionist would play this reel, using external meters or documentation provided with the reel, and then manually adjust the controls on their amplifier setup until the measured output matched the documented reference values. This meticulous process served as the nascent form of calibration, ensuring the playback chain—from amplifier to speaker—was set correctly before the actual feature film, which had its own mix standards, was shown.