a) The number of dB increases possible above the operation level (0 VU) before unacceptable distortion occurs.

b) In composition, the space between a subject’s head and the upper boundary of the frame.

c) The difference between the nominal level (average) and the maximum operating level (just prior to “unacceptable” distortion) in any system or device. Because it is a pure ratio, there is no unit or reference-level qualifier associated with headroom: simply “dB”; headroom expressed in dB accurately refers to both voltage and power.