In a digital and analog audio systems, headroom is the amount by which linear signal capabilities exceed actual signal level. In digital audio, headroom is defined as the amount that 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. In analog audio, headroom can stand for low-level signal capabilities as well as for the amount of extra power reserve available within the power amplifiers that drive the loudspeakers.
Alignment level is an 'anchor' point, 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 (alignment) level is 0 dB, corresponding to an analog sine wave voltage of +4 dBu, corresponding to an RMS voltage of 1.23 V, or an amplitude of 1.74 V. In the digital realm, alignment level is −18 dBFS. 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-voltage transients. This was a common problem with CDs published in the late 1990s and early 2000s which often used heavy level compression, producing a constant level that lacked the sparkling dynamics provided by brief peaks from percussion sounds.
AL = analog level SPL = sound pressure level