Check the Pulse: Figure Out a Track's BPM

Here's how to use software to calculate a track's BPM

by DJ Loop Skywalkerr (a/k/a Craig Anderton)

If you want to match beats, you first need to know the beat...to be speicifc, BPM, or Beats per Minute. When the party is grooving along at 126 BPM and you want to move things up a notch, you usually don't want to reach for something that drags along at 85 BPM.

Most of us have a pretty intuitive sense of which tracks match, and hey, there's always cueing up something beforehand. But I'm impatient, y'know? I don't like sitting there with headphones glued to my ears trying to figure out what the beat is. So I started subjecting any tracks in 4/4 to the "Skywalkerr Beat Determination Method" to figure out the BPM of the various tracks I like to use...and you too can figure out the BPM of your favorite tracks. Here's how.

TOOLS

• You need a computer—Windows and Mac fanbois can fight it out, but anything that runs digital audio editing software will work.

• Then there's the digital audio editing software. For Windows, check out Sony Sound Forge, Steinberg Wavelab, Audacity, and a bunch of others. There are also "lite" versions of some of these, and shareware editors for minimum $$. For the Mac there's Wavelab and the usual shareware programs. For best results, use a program that lets you loop a section of audio—most do.

• You need to import the tune to be analyzed into the 'puter. Fancy software lets you open the track so you can see it on-screen in all its geeky, waveform-happy glory. Or if you’re old school and have CDs, patch your CD player's output in your computer’s audio input and record into your editor. Fidelity doesn't matter that much, you just want to see the waveform.

TECHNIQUES

Here's the deal:

1. Select one measure (four beats) of audio. This is why the software's ability to loop audio is important, because it's easy to hear the measure when it repeats.
2. After you get close to defining a measure, zoom in to time the measure as closely as possible.
3. Read the measure's duration.
4. Apply the SMBF (Skywalkerr Magic Beat Formula) to derive the BPM.

Look at the Sound Forge screen shot (Fig. 1). I put it into a paint program and added some graffiti to show off the important stuff.

Fig. 1: One measure of audio selected in Sound Forge.

You can see the kick drum pretty clearly—it's those four spikes, which I've highlighted with an orange line. Clicking on the Loop button (with a red square around it) plays the selected piece of audio over and over. You can adjust the audio's beginning and end to "tune" the loop as close to a measure as possible.

The calculation accuracy depends on defining the measure as exactly as possible. So, zoom way in to the measure's beginning, and see if you can start the selection exactly where the kick drum hits. Fig. 2 shows a zoomed in section at the beginning of the measure. Note the giant hump that indicates the kick start. Use the same process to set the selection's end point.

Fig. 2: Zooming in closely shows exactly where the kick drum begins.

MATH

Now the important part. Back at Fig. 1, note the readout outlined in orange. This shows the selection length, in this case, 1.800 seconds. The magic formula is:

240/selection length in seconds = BPM

So get out your calculator (in Windows, go Programs > Accessories > Calculator), enter the numbers, and you end up with:

240/1.800 = 133.33 BPM

So that's it : 133.33 is the tempo, in BPM.

EXTRA CREDIT!

Remember where I said that "the accuracy of the calculation depends on defining the measure as exactly as possible"? If you want to get really accurate, measure the length of more than one measure, and take an average. Now we have a brand new and totally exciting formula:

(N * 240) / D = BPM

Where N is the number of measures, D is the duration of the measures, and BPM stands for...well, you know what it stands for.

That wasn't so hard, right? Now you find the BPM of anything with a pulse. Check it out.

DJ Loop Skywalkerr spins in clubs you haven’t heard of, in places where you don’t want to be. He uses a PC, Sonar, and several ancient tone generators to make his own tracks. 

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Craig Anderton is Editorial Director of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.