A lot of songs have that empty space (silence) at the beginning. So usually when people have the right bpm it still seems off or wrong because of that silence (long or or very short at the beginning.)
#1. The absolute easiest and quickest way is to load up the original
song and line up the acapella to match it. Make sure the beginning of both of the waves are right at the start of the sequencer. Chop off that silence in the wave at the beginning of the song
and also the pella at the same time.......or chop it til you see the first hit of the beat. Once you chop off both evenly, move them both to the beginning of of the sequencer. Once you chop that silence, you can now find the BPM of the acapella by syncing it to the metronome. I usually save the acapella after that so that I have a "grid version" instead of an original.
#2 O.K...you don't have the original song. You have to have a good ear and ride the metronome. If you have tap tempo use it so you can get a rough estimate of the number. Your metronome is your beat. I usually bring the pella far away from the start of the sequencer before I do this. Two things is going to happen......as you sync the metronome you are also moving the wave til you can bob ya head to the pella and the metronome.
Tick..tock...tock...tock.....[that's your metronome sounding off]. Once you "think" you got it perfect, chop the wave on the tick of your metronome..........(if yours doesn't go tick......use common sense..
). By doing this, you have your pella snapped to the grid so you can later move it to where ever you want to as long as it is on the tick.......and it makes things easier to slide over in case it is off or the bpm is not correct yet. You'll know it's correct because you can play any part of the pella and it's still in the same rhythm with the metronome.
With live music your ass is out of luck. Music that is not on the tick will never be perfectly synced. You are going to have to chop it unless you are playing your music live with no quantize. But most drum machine type music is synced. Some sample beats usually have some numbers behind the decimal. 98.578.