When you download a loop, usually you know the BPM. If not, it's usually an exact number of measures, and is easy to fit to your project's BPM. Most of the time, it's no trouble to fit a loop into a project with a different tempo.
Sometimes, however, you might run across a situation where you don't have a neat, 1-bar sample, and you're not sure what the BPM is, either. In this case, you need to match the BPM by hand. Here's one method to get that done:
1. Isolate one measure from the source sample. You will have to do this by ear. In your sample editor, set it to play the loop on infinite loop and change the length of the loop until it sounds just right.
If there are drums in the source material, a good place to start is on a bass drum hit, which often marks the beginning of a measure. Drums are frequently the easiest way to find one full measure, especially because you can often see them in the waveform display.
Save the loop as a .wav and put it aside.
2. Isolate or render out one measure from your project (at the desired BPM) as a .wav file.
3. Load up the project .wav file and find the length of the file in either number of samples or milliseconds. Write down this number.
4. Load up the loop you isolated as a sample, and do the same thing - write down the length in either samples or milliseconds (use the same measurement for both.)
5. Now, divide the length of the destination loop by the length of the source loop. An example might be like this:
Source: 1828.5 ms
Destination: 1742.3 ms
1742.3 / 1828.5 = 0.952857…
This means that the source is almost 5% longer than the destination. So, if you take this number, 0.952, and multiply the length of the source material by that, you’ll have your BPMs matched up. In this case, we are speeding up the source material to match the destination. If that number comes out longer than 1, you’re slowing it down. Simple!
The best way to do this is to use a time-stretching utility. So, for this example, you’d want to time-stretch the source audio by 95.28% . The great thing is that this value will work for any part of the source material you want to match to the destination BPM. Now, get out there and do some sampling!
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