[sdiy] LFO, analog computers

[DCMagnuson at aol.com]

In a message dated 2/14/01 1:41:26 PM Eastern Standard Time, matti at devo.com 
writes:

> 1)In trying to produce theta waves in the brain, I wondered if I couldn't 
> simply 
>  set an oscillator to 4-7 Hz, instead of putting one wave with some 
frequency 
> 
>  into one channel, and another with a difference in frequency of 4-7 Hz 
into 
> the 
>  other. Will normal speakers/headphones output such low frequencies? What 
> about a 
>  soundcard? Most give a range of 20-20kHz, but shouldn't lower frequencies 
> come 
>  out fine, or amybe distorted?

If you look at the response curves for most speakers, you'll notice that they 
begin to roll off at low bass frequencies (anywhere from 20 Hz to 200Hz).  On 
many, I think you'll find that they have very very low output in the 4-7 hz 
range.  I don't know what the distortion would be like, however.  

With sufficient levels, you can probably make it work, but remember that with 
such high volumes anything above the bass roll off point will be extremely 
loud (read: hearing damage).  I'd personally use a LPF to avoid passing any 
"accidental" high frequency information to the headphones / speakers.

Usually when I hear about people using VLF, it's done by beating two 
frequencies together (1000 Hz and 1005 Hz, for example).  I think it's a more 
practical method.

Don't forget, in the 4 Hz range, I think you can cause the listener extreme 
discomfort, disorientation, confusion and possibly make them sh!t themselves. 
 These frequencies are the ones used for riot control, etc.... may want to 
keep some rags handy ;)

This has come up a few times on the Rec.Audio.Pro newsgroup, so maybe try 
searching the archives at Deja.com (although the new Google ownership has 
screwed up the interface from what I hear).  Some of the posters talked about 
piston-like low frequency drivers used to generate 1-20Hz waves.  Pretty neat 
stuff, but it required thousands of watts of power IIRC (and the drivers are 
quite expensive).  They were origianlly designed as an experiment in elephant 
communication... apparently they "speak" to each other with a very low 
frequency sound.  It had something to do with the resonant frequency of their 
skulls or something similar.

Sorry for straying so far OT... I'll stop now ;)

Dave Magnuson
http://www.indiemusicsite.com/synth/dave