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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: 30/03/21

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Haas Effect

The Haas Effect is a psychoacoustical effect named after Helmut Haas who first described it in 1949 and clarified his findings in 1951, although it was actually discovered by Lothar Cremer the previous year and called 'the law of the first wavefront'. It is also known as the Precedence Effect which is a far more descriptive term. 

If one sound wave arrives at the ear shortly after another, the two are heard as a single sound, with the first arrival being used to determine the perceived sound location for the merged sound, even if the later sound is louder. For simple transient sounds, the time window for the two sounds to merge is below 5ms, but for more complex sounds like speech it can be as much as 40ms. A longer gap between the two sounds is usually perceived as an echo.

It is this precedence effect that allows accurate sound localisation in reverberant locations, since only the direct sound determines the perceived source location, and the later reverberant reflections are merged into the first sound.

However, if the second sound is significantly louder than the first it can become dominant in the perception of source location. It was determined that for time differences of up to 30ms, the first arrival determined the perceived source location even if the second arrival was as much as 10dB louder. Only when the second arrival was around 15dB louder did it become dominant in determining the source location. 

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. (Previously defined as 'cycles per second or CPS).

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. Also known as a 'low-cut' filter.

High-range (highs)

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


Random noise caused by random electrical fluctuations.


A speaker horn is a flared apperture used to convey sound from a transducer into the air. The expanding dimensions of the horn effectively act as an acoustic transformer to couple the high impedance environment of the air in front of the transducer's diaphgram with the low impedance of the atmosphere around the speaker. This arrangement allows for near-ideal energy transfer and so allows the source of sound to operate with a much higher power efficiency than a direct-radiating driver (typically around ten times better).  

In modern applications, horns are normally used with compression drivers, where the diaphgram of the compression driver is larger than the throat apperture of the horn, and generates a very high sound pressure level. A 'phase plug' is often employed at the throat of the horn, to help guide the sound waves from the transducer's diaphgram into the horn, minimising local destructive phase cancellations and extending the operating bandwidth of the complete unit. The dimensions of the flared exit of the horn can be chosen to help control the angle of horizontal and vertical dispersion.

Horns are most commonly used with midrange and high-frequency drivers, but they can also be used in larger folded configurations for low frequency applications, as well as in portable megaphones and pole-mounted PA systems. 

SOS Contributor Paul Ward describes the operation of a horn thus: "But how, I hear you ask, does a horn increase efficiency? It’s all about radiation impedance. When a naked driver diaphragm moves in a fluid (air, for instance), the power it transfers depends on the fluid’s density. A big difference between the density of the diaphragm and the density of the fluid is equivalent to an impedance mismatch in an electrical circuit. Imagine trying to power yourself along by doing a front crawl swimming stroke in air rather than in water. It doesn’t work in air because air is very much less dense than water. The higher density of water stops the molecules from moving sideways to get out of the way of your gracefully sweeping hands. Fish can move rapidly in the water with small fins, while birds need big wings to stay in the air, all for the same reason. Adding a horn to the front of a speaker works because, down at the business end, otherwise known as the throat, the horn essentially stops the air moving sideways and getting out of the way of the diaphragm. The length and flare of the horn then progressively blend the radiation impedance at the throat into that of free air.


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.