Beats per minute
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Beats per minute (BPM) is a unit typically used as either a measure of tempo in music, or a measure of one's heart rate. A rate of 60 bpm means that one beat will occur every second. One bpm is equal to 1/60 Hz.
The BPM tempo of a piece of music is conventionally shown in its score as a metronome mark:
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This indicates that there should be 120 crochet beats per minute. In simple time signatures it is conventional to show the tempo in terms of the note duration on the bottom. So a 4/4 would show a crochet, as above, while a 2/2 would show a minim.
In compound time signatures the beat consists of three note durations (so there are 3 quavers per beat in a 6/8 time signature), so a dotted form of the next note duration up is used. The most common compound signatures: 6/8, 9/8, and 12/8, therefore use a dotted crochet to indicate their BPM.
Exotic time and particularly slow time signatures may indicate their BPM tempo using other note durations.
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[edit] Typical Beat per Minutes by Genre
[edit] Dance Music
Beats per minute became common terminology in popular music during the disco era because of its usefulness to DJs, and remain important in dance music.
In this context the beats measured are either crochets in the time signature (sometimes called down-beats, although the term is ambiguous), or drum beats (typically bass-drum or another functionally similar synthesized sound), whichever is more frequent. Higher BPM values are therefore achievable by increasing the number of drum beats, without increasing the tempo of the music.
Hip hop typically uses a BPM tempo of 70-110, while house music is faster: 110-140BPM. Jungle music goes even faster: 140-190. gabber and Speedcore music exceeds 200 BPM with an underlying crochet tempo of around 100-120.
[edit] Extreme BPM
More extreme BPMs are achievable at the same underlying tempo with a very fast drum patterns, or a drum roll. Again, these have a much slower underlying tempo, but increase the BPM by adding additional percussive beats. The Speedcore subgenre claims to top 1000 BPM[citation needed] using this method.
The human ear can distinguish separate pulses up to a frequency of around 15Hz (900 BPM)[citation needed], thereafter the pulses are perceived as a single sound, and beyond around 1200 BPM (20Hz) as a bass frequency and harmonics. This places a natural perceptual limit on the actual BPM of a track, although the PR value of claiming a high BPM is unaffected by whether it can actually be discerned by a human listener.
[edit] Beatmatching
Beatmatching, an art amongst DJs, concerns the speeding up or down of a record in order to match the BPM of a previous track so both can be flawlessly mixed.
DJs often beatmatch the underlying tempos of recordings, rather than their strict BPM, particularly when dealing with high BPM tracks. A 240 BPM track, for example, will normally match the beat of a 120 BPM track without slowing down or speeding up, because both are likely to have an underlying tempo of 120 crochets per minute.
Normally, the pitch and BPM of a track are linked: spin a disc 10% faster and both pitch and tempo will be 10% higher. Software processing to change the pitch without changing the tempo, or vice-versa, is called time-stretching or pitch-shifting. While it works fairly well for small adjustments (± 20%), the result can be noisy and unmusical for larger changes.
BPM can be calculated by hand (count the number of bass drums per 60 seconds, or - to be fast - per 15 seconds and multiply by 4), but some software programs such as MixMeister, Traktor-DJ and PC-DJ can do it automatically by listening for regular volume peaks at low frequencies.
