# Reference tone

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A **reference tone** is a [pure tone](/source/Pure_tone) corresponding to a known frequency, and produced at a stable [sound pressure level](/source/Sound_pressure_level) (volume), usually by specialized equipment.

## In media

The most common reference tone in [audio engineering](/source/Audio_engineering) is a 1000 Hz tone at −20[dB](/source/Decibel). It is meant to be used by audio engineers in order to adjust the playback equipment so that the accompanying [media](/source/Electronic_media) is at a comfortable volume for the audience. In [video production](/source/Video_production), this tone is usually accompanied by a [test card](/source/Test_card) so the video programming may be calibrated as well. It is sometimes played in sequence between a 100 Hz and 10 kHz tone to ensure an accurate response from the equipment at varying audio frequencies. This is also the ["bleep" tone](/source/Bleep_censor) commonly used to [censor](/source/Censorship) obscene or sensitive audio content.

## In music

Many [electronic tuners](/source/Electronic_tuner) used by musicians emit a tone of [440Hz](/source/A440_(pitch_standard)), corresponding to a [pitch](/source/Pitch_(music)) of A above [Middle C](/source/Middle_C) (A4). More sophisticated tuners offer a choice of other reference pitches to account for differences in [tuning](/source/Musical_tuning). Some specialized tuners offer pitches used commonly on a particular instrument (standard guitar tuning, fifth intervals for string instruments, the open tones for various brass instruments).

## In telecommunications

In [telecommunication](/source/Telecommunication), a **standard test tone** is a [pure tone](/source/Pure_tone) with a standardized level generally used for [level alignment](/source/Level_alignment) of single links and of links in [tandem](/source/Tandem).[1]

For standardized test signal levels and frequencies, see [MIL-STD-188](/source/MIL-STD-188)-100 for [United States Department of Defense](/source/United_States_Department_of_Defense) (DOD) use, and the [Code of Federal Regulations](/source/Code_of_Federal_Regulations) Title 47, [part 68](/source/Part_68) for other Government agencies.

## References

## External links

- [Downloadable reference tones](http://www.freesound.org/packsViewSingle.php?id=2103), from [The Freesound Project](/source/The_Freesound_Project)

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Adapted from the Wikipedia article [Reference tone](https://en.wikipedia.org/wiki/Reference_tone) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Reference_tone?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.