8. Beat Frequencies

If you play two tones together at very close frequencies you'll hear a kind of pulsing called a "beat frequency." A very easy place to observe this is while tuning a guitar. If the fifth fret of one string is perfectly in tune with the next, everything sounds normal, but when it's a little off you hear pulsating dissonance, the further out of tune you get the faster the pulse gets. This is because of the two waves cyclically reinforcing and canceling each other.

Here's how it works: (Frequency 1 - Frequency 2 = Beat frequency)

So if my E is in tune and I hit 220Hz on the fifth fret, and my A is out of tune and I hit 225Hz on that string, I will hear a 5Hz pulse. This isn't a 5Hz tone, this is a pulse 5 times a second.

This is how binaural beat stuff works, and why it only works in headphones. Two similar frequencies are played in either ear and the brain creates the pulse itself kinda. When these pulses are at frequencies that sync with brain activity it can control your mood.

You can use beat frequencies to your advantage if you make sure the pulses are in tempo. For example @ 120BPM, we might want a 2Hz pulse, which would pulse once per beat. (4Hz would be an 1/8-note, 8Hz = 1/16-note.)

The best way to make them is with multiple test tones. Most synths don't give you the super accurate frequency control you need for this kinda thing.