Headroom (audio signal processing)
In digital and analog audio, headroom is the amount by which the signal-handling capabilities of an audio system exceed a designated level known as Permitted Maximum Level (PML). Headroom can be thought of as a safety zone allowing transient audio peaks to exceed the PML without exceeding the signal capabilities of an audio system (digital clipping, for example). Various standards bodies recommend various levels as Permitted Maximum Level.
Headroom in digital audio 
In digital audio, headroom is defined as the amount by which digital full scale (FS) exceeds the permitted maximum level (PML) in dB (decibels). The European Broadcasting Union (EBU) specifies a PML of 9 dB below 0 dBFS (-9 dBFS), thus giving 9 dB of headroom. An alternative EBU recommendation allows 24 dB of headroom, which might be used for 24-bit master recordings where it is useful to allow more room for unexpected peaks during live recording.
Failure to provide adequate headroom can bring about clipping of brief, higher-level transients.
Headroom in analog audio 
In analog audio, headroom can mean low-level signal capabilities as well as the amount of extra power reserve available within the power amplifiers that drive the loudspeakers.
Alignment level 
Alignment level is an 'anchor' point, 9 db below the nominal level, a reference level which exists throughout the system or broadcast chain, though it may have different actual voltage levels at different points in the analog chain. Typically, nominal (not alignment) level is 0 dB, corresponding to an analog sine wave voltage of RMS voltage of 1.23 volts (+4 dBu or 3.47 volts peak to peak). In the digital realm, alignment level is −18 dBFS.
- AL = analog level
- SPL = sound pressure level
See also 
- Audio quality measurement
- Noise measurement
- Programme levels
- Rumble measurement
- ITU-R 468 noise weighting
- Weighting filter
- Equal-loudness contour
- Fletcher-Munson curves
- Loudness war
- EBU Recommendation R68-2000
- AES Preprint 4828 - Levels in Digital Audio Broadcasting by Neil Gilchrist (not free)
- EBU Recommendation R117-2006 (against loudness war)
- AES Convention Paper 5538 On Levelling and Loudness Problems at Broadcast Studios
- EBU Tech 3282-E on EBU RDAT Tape Levels
- AES17-1998 (r2004): AES standard method for digital audio engineering -- Measurement of digital audio equipment