I'll explain why this happens to the bass. I hope that everyone at least knows what a sine wave looks like. If not then you can see one here . So I'm going to just talk about two single bass beats and assume that there is no other interference from any other part of the track. Given that:

Bass beat 1 (the live beat) beats at a certain bpm. Your objective as a dj, is to adjust bass beat 2 (the cue beat) to the same bpm and the same phase as bass beat 1.

So you have the sine wave of the live bass and you hear the sound when the wave hits a maximun or a minimum. Look at picture "a" at the above link if you need a picture. The beat starts at 0 on the timeline and every time you hear a beat the wave is at +1 or -1 on the y-axis. This holds true for the cue beat also.

So now comes the time when you have both beats matched and you are ready to play them live at the same time. Well, since you don't know which beat is at +1 and which is at -1 (because they are indistinguishable by ear--if you want to know how you can go about detecting it, I'll elaborate to replies, though it's not an experiment that you yourself can perform without expensive equipment) you know if you have the beats synched, but you don't know if you have them in phase yet. Now, don't confuse being in-phase or out-of-phase with phrase matching. There are two possible senerios:

In-phase means that the waves of both bass beats are at +1 at the same time, and hence at -1 at the same time too. When this occurs, the two waves will add together, and you will get an over-powering/double (1 + 1 = 2) bass signal. This is why most djs will turn the bass eq down on the live track as they bring in the cue track--if they don't, the bass will kick twice as hard and this can cause distortion, and even speaker damage depending on how loud it is.

The second case occurs when the two beats are out-of-phase. This occurs when beat 1 is at +1 when beat 2 is at -1, and vise-versa. When this happens, then the waves still add, though 1 + (-1) = 0, and you have cancelation--you will hear no bass beat. This is the problem that you are inquiring about.

So why you ask, do you hear a beat that is just reduced in volume and not none at all, and also how do you go about fixing this?

First, you hear a bass beat reduced in volume for a couple of different reasons. 1. it is very hard to get the beats "matched" as far as truly and completely matched goes. Your ears, however well trained, can never (unless you're lucky once or twice) ever get two beats perfectly matched. The speed of sound is about 760 miles/hour and varies with temperature etc. You CAN however, get them matched enough so as not even the best trained ear can distinguish them (if you're good. The second reason you hear only a reduction in volume is because more than likely, the bass beat of each track was not made by the same instrument and hence doesn't have the same frequency range as the other beat. Take forinstance, beat 1, with a frequency range from 40Hz to 45Hz and beat two from 43.5Hz to 54Hz. While each beat has its own frequencies, and each one overlaps a portion, there is still part of the beat that is not effected. Beat 1 will have no (very little at intersecting or overlap nodes to get technical) cancelation effect between 40Hz and 43.4Hz while beat two will have no cancelation effect between 45.1Hz and 54Hz. This mean that you get a parital bass signal, but reduced in overall volume due to elimination of certain frequencies.


One thing to note, not to confuse those who use programs like winamp and soundforge. In Soundforge, you can zoom up on the wave of an entire track and it appears to be one single, continuous line. This is not really an accurate picture. If you have winamp, and you have the spectrum analyzer on peaks instead of wave/oscilliscope, then you see all the frequencies of the track being displayed (supposedly each frequency per bouncing line) at any INSTANT in time. Think of this as a frontal view. Soundforge, however displays the side view, so that you can travel along the wave in time, though you cannot see any individual frequency, you can only see the sum of all of the frequencies. You can only perceive the entire waveform in 3 dimensions.