What must headphones be calibrated with?

Headphones must be calibrated with an associated audiometer as a unit because proper calibration ensures that the sound output levels presented through the headphones are accurate and consistent with the audiometric standards. Each audiometer has specific characteristics and response curves, meaning that the headphones used with it must be calibrated to reflect these factors.

Calibration involves measuring the output of the headphones at various frequencies and levels to ensure they match the intended outputs of the audiometer. This process is crucial for obtaining reliable audiometric results, as variations in headphone performance can lead to inaccurate hearing assessments.

The other options do not consider the necessity of pairing the calibration specifically with the corresponding audiometer, which is essential for accurate sound delivery during hearing tests.