While most modern house tracks conform to a four-to-the-floor rhythm, the BPM can fluctuate dramatically. Over the past few decades, the tempo of 128 BPM has asserted itself as the dominant speed in contemporary dance music, but it’s not the only rhythm in town.
Because of these fluctuations, you may need to calibrate particular tracks to ensure a seamless continuity between tracks. One crafty way of accomplishing this on tracks with dramatically different BPMs is to use a low-pass filter to blur out the drum beat. A high-pass filter may also work, and you should use discretion to decide which filter to use; however, generally speaking, either method will usually suffice.
If you’re wanting to incorporate, say, a techno track (of 140 BPM) into a dubstep mix (of 70 BPM), you may notice two things—one: the BPMs are wildly different, and two: 70 is exactly half of 140. This means that you can actually beatmatch the two disparate genres without having to adjust any tempos (in this case, at least). Of course, such a mathematically elegant solution is fairly rare, so you are obviously going to need to make sure your B track is correctly synchronised to your A track.
Another interesting idea to consider is to retain the tempos of two tracks with wildly differing BPMs. There are a couple of reasons you might want to do this:
Firstly, if done right, you could introduce, say, the original version of Eminem’s “Lose Yourself” after a regular dance track of 128 BPM, because the start of the aforementioned song has a drumless introduction; thus, the track’s BPM—from a sync point of view, at least—is irrelevant. The reason why you wouldn’t change the BPM is that you can exploit the drumless intro while preserving the original pitch, tempo, and feeling of the song.
Secondly, preserving the track’s original tempo can act as method of calling attention to the fact that the mood has changed. Rather than stealthily incorporate a slower (and perhaps more moody) track into the mix, you can instead make more of a show of the fact that the genre or style is changing, giving the new track more impact than it otherwise might have.
It is also a good idea to remember the technical resources you have on your DJ controller. One interesting way to alter the BPM in a smooth, non-clashing way is to sample part of a track and adjust its BPM gradually. For instance: if track A is considerably slower than track B, you can sample a bar or two from track B at the same speed of track A.
You can then gradually increase the tempo in a fluid, concerted way once track A has been completely faded out; such a technique is a useful for changing tempos, not just mixing two tracks of different tempos into the one tempo. Conversely, you can also do this the opposite way: sampling track A and fading it into track B once the BPMs are successfully matched.
Doing this constantly might be jarring, however, so it is usually best to have a baseline BPM that you can default to. Just remember that certain controllers and software don’t pitch-adjust, which, if this the case, you will need to make sure that either the track won’t sound ridiculous at a vastly different pitch; you can usually fix this manually, but automated pitch adjustment is usually best.