Brainwave Entrainment : Some basic types

I'm going to go over some of the more common methods/sources of brainwave entrainment here. This list isn't exhaustive. I'm really just scratching the surface.

I welcome any feedback you may want to offer - I don't know everything, so if you think I am botching up some detail below, feel free to call me on it in the comments section.

Additionally, if you think there are better ways for me to present this information, let me know. 


I Visual Entrainment :

Lights flickering X times per second can be used to entrain in the brain into various mental states. Point in case - a strobe light flashing 20 times a second can be used to energize a crowd of people.

A variation on this theme - if one watches something moving/oscillating X times a second, this can also potentially be used to entrain the brain. (Example : A hypnotist swinging a timepiece or some other object on a chain to put somebody into a trance.)

II Motion Entrainment : 
 
Any periodic motion a person experiences can also have brainwave entrainment effects.

Examples : A rocking chair, swinging on a swing, a mother gentling rocking a child in her hands, etc.

III  Electromagnetic Entrainment

Electromagnetic fields generated by appliances, power lines and other sources can also potentially entrain the brain. It's one of the reasons why some sources will advise against sleeping with an alarm clock too close to your head. (The EM field of an alarm clock is much higher than the delta and theta ranges associated with sleep - it's thought for that reason that the alarm clock can interfere with sleep.)


There are also 'geomagnetic' fields created naturally which can entrain the brain just as an artificial EM field can.

IV Felt Entrainment 

I was debating whether to include this category, but I think it's worth mentioning (since really bridges the gap to audio below). Any felt stimuli can potentially have brainwave entrainment effects. Think a device that generates vibrations, or somebody doling out a massage. Also, a recurring beat/rhythm  from subwoofers - instances where it's more 'felt' than heard.


V Audio Entrainment

For me, this is the main category that I devote most of my time to playing with (although I do like to do visual entrainment also with strobe lights.)

Any repeating\recurring\periodic sound-based stimuli can potentially entrain the brain. This can be done by changing up any number of qualities about the sound :

* Volume - the sound fades in and out X times a second. (This is the classic 'gapper' effect sometimes heard in electronic music, and you may here it referred to as 'isochronic tones'.)

* Pitch - the pitch of the tone goes up and down X times a second.

* Panning - Using headphones, the tone cycles between the left and right ear X times a second.

This is just scratching the surface - literally any characteristic of the sound, if changed up X times a second, can potentially entrain the brain. Other characteristics that can be modulated -- the EQ, the tone color (timbre), the waveform (sinus, sawtooth, triangle, etc), and the phase between the left and right channels.

(Side Note : Cool Edit's 'Brainwave Synchronizer' feature made use of phase modulation. Cool Edit was ultimately acquired by Adobe, and became Adobe Audition -- the Brainwave Synchronizer effect was named the 'Binaural Auto-Panner'. I believe it still makes use of phase modulation.)

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One other method of audio entrainment involves binaural beats and monaural beats.

Before we go any further, I want to make sure to distinguish between binaural beats and binaural recording, which are two different animals.

Binaural recording is a method of recording sound intended to create a realistic 3D environment. This is often with a microphone shaped like a human head, so it captures sound exactly as a human head would.

Now, with that out of the way, let's get into binaural and monaural beats :

* Binaural beats : This is a method of brainwave entrainment listened to through headphones. To generate an entrainment frequency, two audible tones are used, one in each ear. The difference between the two tones should subtract out to the intended entrainment frequency.

For example, if you want to entrain to 5 HZ (which would be used to promote sleep), you could do this by putting a 200 HZ tone in the left ear, and a 205 HZ tone in the right ear. The brain is going to try and reconcile the subtle difference between the two tones in each ear -- it will end up hearing a single tone, but it will also hear a series of beats - five beats a second. These 'beats' can entrain the brain.

One thing to note - if you play the audio through speakers instead of headphones, you'll end up getting monaural beats instead of binaural beats (see below). [There is an exception to this, but I don't want to over-complicate things - yet.]

* Monaural beats : Monaural are very similar to binaural beats, except instead of having the two tones separated between the left and right ears, you just overlap the two tones period. So, using the example above, you would layer a 200 HZ tone over 205 HZ tone. The end result can be played through standard speakers - headphones are not necessary. Where as binaural beats are formed by the brain trying to reconcile the difference in tones between the left and right ears, monaural beats are the result of the tones mixing in open air after coming out of the speakers.