3
I was looking at how beats work. You use a amplitude wave $A = \cos\left(2\pi f_1x\right) + \cos\left(2\pi f_2x\right) = 2\cos\left(\frac{2\pi x\left(f_1-f_2\right)}{2}\right)\cos\left(\frac{2\pi x\left(f_1+f_2\right)}{2}\right)$ (frequencies $f_1$ and $f_2$). According to Wikipedia, "Because t...