Hi. That's another great idea.
Speaking for myself, as a musician with perfect pitch and someone who mixes/masters audio for folks, I would actually like hearing anywhere from one up to five tones, selectable. The volume of each tone could rise/fall as signal level fluctuates. (as long as said tones spell out a pleasant-sounding chord, not an atonal mess) so pitch stays constant, but volume indicates dB, which makes sense.
Here's a handy frequencies of musical notes chart:
Heh, add in some stereo panning and you can indicate positioning, strongest signal currently coming from this spot etc.
There are so many ways someone could manipulate audio in an app like this, many of which I sent to InteraXon waaaay back...
You could have a sound moving left to right, the movement slowing down as the intended brain state is reached. i.e. goal is to keep the sound in the middle between the ears/speakers.
or, any modulation type effect that pulses (phasing, flanging, chorusing etc.) and said oscilation slows down as the ideal state is reached.
or, the sound has less echo on it as the state is reached, i.e. it sounds closer to the listener.
But, that's the audio geek in me speaking.
Most people would probably want to just focus on one or two tones and how they change over time, beta versus alpha for example. (I'm only guessing that's what the Calm app is focusing on to determine active versus calm)
Or, here's a better idea ... rather than separate tones, why not use one fundamental and it's first four or five harmonics?
how about playing one fundamental (pure sinewave) tone for a brainwave type, and the other brainwave types are upper partials (harmonic series) of that tone?
That way, as the levels of the various waves fluctuate, the sound of that one tone gets richer or more plain. (i.e. the timbre changes)
Does that make sense?
Its the relative intensity of those harmonics of the fundamental that help us distinguish the same pitch, middle C for example, on a guitar, piano, saxophone, etc. Same note, different blends of upper harmonics.
ok, that's enough geeking out over here