BPM to Milliseconds: Delay, Reverb and LFO
This lesson teaches you to translate your track's tempo into milliseconds, so your delays, reverbs and LFOs breathe with the rhythm instead of blurring it.
So far, you've measured time in beats, bars and grid steps. Effects, on the other hand, often measure it in milliseconds: a delay repeats the sound after so many milliseconds, a reverb waits so many milliseconds before starting. When these durations don't match anything in your track, the echoes fall between the notes and blur the groove. When they match the tempo, the effect becomes part of the rhythm.
One formula: 60,000 divided by the tempo
BPM counts beats per minute. A minute lasts 60 seconds, that is, 60,000 milliseconds. So to find the length of a beat, you just divide 60,000 by the tempo. At 120 BPM, a beat lasts 500 milliseconds, half a second. At 100 BPM, it lasts 600 milliseconds; at 140 BPM, about 428.6 milliseconds.
In a track in 4/4, that is, with four beats per bar, that beat is a quarter note. All the other durations follow from it, using the ratios you saw in the lessons on note lengths and on dots and triplets. The eighth note is half the quarter note, the sixteenth note a quarter of it. The dotted eighth is one and a half eighth notes, so three quarters of a quarter note. The eighth-note triplet puts three notes in the time of one quarter note: so each one is worth a third of a beat.
| Tempo | Quarter note | Eighth note | Dotted eighth | Eighth-note triplet | Sixteenth note |
|---|---|---|---|---|---|
| 90 BPM | 666.7 ms | 333.3 ms | 500 ms | 222.2 ms | 166.7 ms |
| 100 BPM | 600 ms | 300 ms | 450 ms | 200 ms | 150 ms |
| 120 BPM | 500 ms | 250 ms | 375 ms | 166.7 ms | 125 ms |
| 128 BPM | 468.8 ms | 234.4 ms | 351.6 ms | 156.3 ms | 117.2 ms |
| 140 BPM | 428.6 ms | 214.3 ms | 321.4 ms | 142.9 ms | 107.1 ms |
| 174 BPM | 344.8 ms | 172.4 ms | 258.6 ms | 114.9 ms | 86.2 ms |
You don't need to memorize this table. Remember the formula and one reference point: at 120 BPM, a quarter note is 500 milliseconds, an eighth note 250, a dotted eighth 375 and an eighth-note triplet about 166.7. For other tempos, the workshop below does the math for you.
At 120 BPM, a quarter note lasts 60,000 divided by 120, which is 500 milliseconds. An eighth note lasts half as long, a half note twice as long.
| Value | Straight | Dotted | Triplet |
|---|---|---|---|
| Whole note1/1 | |||
| Half note1/2 | |||
| Quarter note1/4 | |||
| Eighth note1/8 | |||
| Sixteenth note1/16 | |||
| 32nd note1/32 |
Click a number to copy it, ready to paste into a plugin.
The math is always the same: a quarter note lasts 60,000 divided by the tempo, in milliseconds. A dotted note lasts one and a half times its value, and a triplet note two thirds of it, since you play three in the time of two. For an LFO, the frequency in hertz is 1,000 divided by the length in milliseconds.
Where do these numbers go? Into the time of a delay that does not sync on its own (the dotted eighth is a classic), into the pre-delay of a reverb (a sixteenth or a 32nd note, so the reverb starts in time), into the release of a sidechain compressor (so the volume comes back up just before the next kick), and into the rate of an LFO, in hertz.
Delay: echoes that land on the grid
A delay repeats the sound after a fixed time, then repeats the repetition, more and more quietly; the feedback control decides how many repetitions there are. If the delay time is an eighth note, each echo lands on the next eighth note: the delay doubles the rhythm without changing it. It's discreet and clean, but it can fill up the silences.
The most famous setting is the dotted eighth. Its echoes fall every three sixteenth-note steps, so they straddle the beats, and they create a new rhythm that gallops against the melody. Look at where the echoes of a single hit on the first beat land.
The echoes land on steps 4, 7, 10, 13 and 16 of the grid: the first four each land on a different spot in the beat, and only the fifth comes back to the first one's spot. Here's what that gives on a short melody with one note per beat. Only the first echo of each note is written out, so you can see the figure that appears.
The eighth-note triplet gives a different color: its echoes roll with a triplet feel, three per beat. On a triplet grid, they land on every cell.
Compare the two grids: the dotted eighth pulls the rhythm forward, while the triplet makes it roll. Both stay locked to the tempo, and that's what makes them musical.
Reverb in time
A reverb has two settings that involve time. The pre-delay is the wait between the sound and the start of the reverb. A few tens of milliseconds are enough to separate the attack of a vocal or a snare from its space: the sound keeps its clarity, and the reverb arrives right behind it. Many producers choose a small note value: at 120 BPM, a thirty-second note is 62.5 milliseconds, and a sixty-fourth note about 31 milliseconds.
The decay, or decay time, is how long the reverb takes to die away. A good starting point is to fit the pre-delay and the decay together into one note value, a quarter note, a half note or a bar, depending on the space you want. At 128 BPM, a half note is 937.5 milliseconds: a snare reverb that dies away in a little under a second will leave room for the next hit instead of piling up on it. It isn't an absolute rule, and a long reverb on a breakdown can be used precisely to wash out the beat.
The synced LFO
You came across the LFO in the lesson on detuning, with the chorus: it's an oscillator too slow to be heard as a note, used to move a setting, like a filter's cutoff, the volume, the pitch or the stereo position. Its speed is set either in hertz, or as a note value when it's synced to the tempo. One cycle per quarter note at 120 BPM corresponds to one cycle every 500 milliseconds, that is, 2 hertz.
Once it's synced, the LFO becomes a rhythmic instrument. On a filter's cutoff, one cycle per eighth note gives a bass that wobbles every half beat, and one cycle per bar gives a slow sweep that breathes with the phrase. On the volume, an LFO synced to the quarter note imitates sidechain pumping, where the other sounds duck on every kick.
Your trap project is at 140 BPM. Work out the length of a quarter note, then the delay time for an eighth note, a dotted eighth and an eighth-note triplet. Also work out the speed in hertz of an LFO that does one cycle per quarter note. Check your results with the workshop, then set up a synced delay in your DAW and compare the eighth note and the dotted eighth by ear on a pluck.
This lesson wraps up the part on sound. In the next part, you'll put all of this into practice, starting with analyzing a track by ear.