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Welcome to Sound On Sound's indispensible, regularly updated, explanations of technical terms from the fields of Recording, Audio Production, Music Technology, MIDI, Music Software, Audio Plug-ins, Mac and PC Computing, Live Sound, Acoustics, Electronics and more...

If we do not explain a particular term below, please email and we will add it to our next update.

Last updated: 04/08/20

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Hard Disk Drive (cf. Solid-state Drive)

The conventional means of computer data storage. One or more metal disks (hard disks) hermetically sealed in an enclosure with integral drive electronics and interfacing. The disks are coated in a magnetic material and spun at high speed (typically 7200rpm for audio applications). A series of movable arms carrying miniature magnetic heads are arranged to move closely over the surface of the discs to record (write) and replay (read) data.


High frequency components of a complex waveform, where the harmonic frequency is an integer multiple of the fundamental.

Harmonic Distortion

The addition of harmonics that were not present in the original signal caused by non-linearities in an electronic circuit or audio transducer.


The part of a tape machine or disk drive that reads and/or writes information magnetically to and from the storage media.


The available ‘safety margin’ in audio equipment required to accommodate unexpected loud audio transient signals. It is defined as the region between the nominal operating level (0VU) and the clipping point. Typically, a high quality analogue audio mixer or processor will have a nominal operating level of +4dBu and a clipping point of +24dBu - providing 20dB of headroom. Analogue meters, by convention, don’t show the headroom margin at all; but in contrast, digital systems normally do - hence the need to try to restrict signal levels to average around -20dBFS when tracking and mixing with digital systems to maintain a sensible headroom margin. Fully post-produced signals no longer require headroom as the peak signal level is known and controlled. For this reason it has become normal to create CDs with zero headroom.

Hertz (Hz)

The standard measurement of frequency. 10Hz means ten complete cycles of a repeating waveform per second.

High Resolution

A misnomer, but used to refer to digital formats with long word-lengths and high sample rates, eg. 24/96 or 24/192. Audio resolution is infinite and identical to analogue systems in properly configured digital systems. Word-length defines only the system’s signal-to-noise ratio (equivalent to tape width in analogue systems) , while sample rate defines only the audio bandwidth (equivalent to tape speed in analogue systems).

High-Pass Filter (HPF)

A filter which passes frequencies above its cut-off frequency, but attenuates lower frequencies.

High-range (highs)

The upper portion of the audible frequency spectrum, typically denoting frequencies above about 1kHz.


Random noise caused by random electrical fluctuations.


Normally used in the context of the USB computer data interface. A hub is a device used to expand a single USB port into several, enabling the connection of multiple devices. Particularly useful where multiple software program authorisation dongles must be connected to the computer.


Audio Signal contamination caused by the addition of low frequencies, usually related to the mains power frequency.


A condition whereby the state of a system is dependent on previous events or, in other words, the system's output can lag behind the input. Most commonly found in audio in the behaviour of ferro-magnetic materials such as in transformers and analogue tape heads, or in electronic circuits such a 'switch de-bouncing'. Another example is the way a drop-down box on a computer menu remains visible for a short while after the mouse is moved.


The SI symbol for Hertz, the unit of frequency.