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drewJnew · ACAS4U Referral Ambassador

ok, I'm going to show you how to do perfect beat-matching using simple algebra and a program with time-stretching (GoldWave in this example)...

First, find out the BPM (beats per minute) of the instrumental that you're using. I'm using the "40 oz" instrumental by D12. To do this in an audio editor like GoldWave, select at least 8 bars (8 counts of 4 to the beat = 32 beats... ya know what, if you don't know what a bar is, then this is too complicated for you) and get as close to the beginning of the first beat and end of the last beat as possible by zooming in... you want a perfect loop. Then, using a calculator, take the time of the loop (using at least 3 decimal spaces) and divide it by 32 (8 bars x 4 beats each) to get the average time of one beat (using 8 or more bars will give a better beat average instead of just trying to select one beat).

ex: The length of 8 bars in "40 oz" is 22.895011 sec. so...
22.895011 sec. / 32 beats = 0.71546909375 SPB (seconds per beat)

Now, since there are 60 sec. in a minute, an instrumental of 60 BPM would have 1 SPB (60 / 60 = 1) and an instrumental of 120 BPM would have 0.5 SPB (60 / 120 = 0.5). Therefore, 60 / BPM = SPB... using basic algebra, we can find out that 60 / SPB = BPM.

ex: 60 / BPM = 0.71546909375 SPB
BPM (60 / BPM) = BPM (0.71546909375)
60 = BPM (0.71546909375)
60 / 0.71546909375 SPB = 83.861 BPM
The "40 oz" instrumental has a BPM of 83.861.

Do the same process to find out the BPM of the acapella. I'm using the "wonk" pella by 50 Cent.

ex: The length of 8 bars in "wonk" is 23.7037037 sec. so...
23.7037037 sec. / 32 beats = 0.7407407 SPB
60 / 0.7407407 SPB = 81 BPM
The "wonk" pella has a BPM of 81.

Next, you want to find out what percent you are going to stretch or compress the pella to match the beat. Take the BPM of the instrumental and divide it by the BPM of the pella then multiply by a hundred to get the percentage.

ex: (83.861 / 81) x 100 = 103.532098%

Now in GoldWave, select the part of the pella you want to stretch, click on Effect --> TimeWarp, and a pop-up window will appear with 3 process choices... 1. Rate (the fastest choice, changes the pitch and length of the sound... probably the best choice for my example because the pitch only needs to be changed by 3.532%) 2. Similarity (keeps pitch and changes length... the best choice for anything you want to slow down slightly or speed up, yet keep the pitch) 3. FFT (the slowest choice, but gives better results; keeps pitch and changes length... the best choice for anything that needs to be slowed down or sped up considerably). Select the option that's best for you and in the 'Change (%)' box enter your percentage (103.5321 in my case), then hit OK. Once it's done processing, copy the pella and mix it into the instrumental!

So remember, there are 5 equations that you need to do...
1) Take the length of 8 bars in the instrumental and divide it by 32 which gives you SPB-I (seconds per beat of the instrumental).
2) Take 60 and divide it by the SPB-I which gives you BPM-I.
3) Take the length of 8 bars in the acapella and divide it by 32 which gives you SPB-P (seconds per beat of the pella)..
4) Take 60 and divide it by the SPB-P which gives you BPM-P.
5) Take BPM-I and divide it by BPM-P, then multiply that by a hundred to get the Change Percentage.

Peace & Luv,
dj Four15
http://www.soundclick.com/bands/8/djfour15music.htm

acapella · Site Admin

I'm exhausted - wow What a write-up !

Cheers four15

aaj2006 · Supremo ACAS4U Member

i do everything by ear. i have never funk with BPM's and i wont start now, too late! besides, i dont feel comfortable trusting a machine for BPM's. it feels weird somehow.

drewJnew · ACAS4U Referral Ambassador

Acapellas4U.co.uk

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