H

Head Unit
Source component, also called receiver or radio. It provides source selection, volume control, and often the first gain stage feeding the processor or amplifier chain. See also: preamp output, line driver, DSP.
Chapter 5.1

Headroom
Margin between the operating level and a named ceiling such as clipping, sample full scale, or true peak limit. Good specs say what the headroom is relative to, for example 1 dB headroom relative to -1 dBTP. See also: dBFS, true peak, limiter.
Chapter 12.8

Helmholtz Resonance
Resonance created by the interaction of the air mass in the port and the air spring in the enclosure. It defines the tuning frequency of a vented box and is one of the core relationships behind port design. See also: Fb, ported enclosure, box volume.
Chapter 10.3

HRTF (Head-Related Transfer Function)
Frequency-dependent filter representing how the listener's head, torso, and ears shape sound from different directions. It matters most in spatial playback and binaural rendering, not just generic stereo tuning. See also: binaural audio, spatial rendering.
Chapter 14.1

Related reference: Audio Levels, Metering, and Headroom.