Online Metronome

How the Metronome works

A metronome is only useful if it is more steady than the player, and most browser metronomes are not. They use a JavaScript timer to fire a sound every beat, and those timers slip. Rendering, garbage collection and background tabs all delay them, and browsers throttle timers hard when a tab loses focus. A few milliseconds per beat is inaudible at first and obvious after thirty seconds against a recording.

How this online metronome keeps time

The Web Audio clock is a separate, sample-accurate timeline that runs in the audio thread. This metronome does not play a click when a timer fires. Instead it runs a short lookahead loop: every few milliseconds it asks whether any beats fall due in the next 150 ms, schedules each one at an exact time on the audio clock, and goes back to sleep. The timer only needs to be roughly on time because the clicks are already placed. Long-term accuracy is that of your sound card's crystal, a few parts per million, rather than that of the browser's event loop.

Tempo, in practice

BPM counts quarter-note beats per minute in most notation, so 120 BPM is two beats a second. Tempo markings map roughly like this, though composers treat them as moods rather than numbers:

Popular styles cluster too. Ballads sit around 60–80 BPM, most pop and rock between 100 and 130, house near 120–128, drum and bass around 170. If you have a recording and want its tempo, the BPM detector will estimate it.

Reading the display

• Tempo is the current setting in beats per minute.
• Beat shows which beat of the bar is sounding, such as 3 of 4. It helps when you are counting in or practising an entry after rests.
• Beat length is the gap between clicks in milliseconds: 60,000 ÷ BPM. At 120 BPM that is 500 ms. This is the number to use when setting a delay pedal or a plugin to the song, and the BPM to ms calculator gives the dotted and triplet values as well.
• Bar length is beat length multiplied by beats per bar, in seconds. A 4/4 bar at 120 BPM lasts 2 seconds, which makes it easy to work out how long a section runs.
• Voice shows the waveform behind the current click sound.

Beats per bar and the accent

The first beat of each bar is pitched higher and louder. That accent is what lets you feel the bar rather than just a stream of clicks. Use 4 for common time, 3 for a waltz, 2 for a march, and 6 or 12 for compound time such as a jig or a slow blues shuffle, where the beat divides into threes. Odd meters like 5 or 7 work the same way. Set it to 1 if you want every click identical.

Choosing a click sound

The three voices differ in tone and decay, not just pitch. Click is a short square-wave tick with a very fast decay. It cuts through drums and amplified guitar best. Woodblock is a triangle wave with a slightly longer tail, easier on the ears over an hour of practice. Beep is a soft sine wave. Use it under headphones while recording, because a sine that leaks into the microphone is far less noticeable in the take than a tick full of high harmonics.

Exporting a click track

Bars to export renders a WAV file with a fixed number of bars. It is rendered offline at 44.1 kHz, 16-bit, with the first click 50 ms in, so you can line it up on a bar line and trust that every later click lands where the arithmetic says. Use it to put a click into a phone recorder, a DAW session with no tempo map, a stage in-ear mix or a rehearsal backing track. Sixty-four bars at 100 BPM runs a little over two and a half minutes.

Practising with a metronome

• Start slower than feels necessary. Find the tempo at which a passage is completely clean, stay there until it is boring, then move up four or five BPM at a time. Speed built on errors just rehearses the errors.
• Put the click on 2 and 4. Halve the tempo and treat each click as the backbeat. It is how jazz and pop players practise feel, and it makes rushing obvious.
• Drop out bars. Play along for four bars, turn the volume down for four, then bring it back. If you land on the downbeat, your internal time is solid.
• Record yourself against it. Small drifts are much easier to hear on playback than in the moment.
• Tune first. Timing practice on an out-of-tune instrument is wasted ear training; the tuner takes a few seconds.

Want a groove rather than a click? The drum machine plays full patterns at the same tempo. The metronome runs entirely in your browser; nothing is recorded or uploaded.

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