audioguru said:
A fan is not a speaker. Sound is frequencies above 20Hz. Frequencies below 20Hz are vibrations.
You need something to make vibrations or a good sub-woofer.
An electronic circuit can be made to divide the frequencies of sound into lower frequencies.
Well, of course all sound is vibrations, so it's sort of semantics, but I agree, since you won't really 'hear' so much as 'feel' these frequencies.
For those not familiar with the technology the OP is referring to - it's a fan whose blades have their pitch controlled, allowing it to produce very low frequencies, down to the 1Hz range, with a lot of power; see:
https://gizmodo.com/gadgets/gadgets/worlds-most-powerful-subwoofer-134202.php
However, I think the entire challenge with that is in actually constructing the device, I don't think providing it a signal is much of a challenge. The estimated commercial price of the unit I linked is over $12000, that should be a pretty good indication that it is NOT going to be something easy to make.
I think your idea of "compressing" all sound below 250Hz down to 50Hz sounds pretty strange... surely it could be done, but it doesn't seem like it would produce much worthwhile - you'd just be artificially adding it to existing music - it seems pointless to build a device like this unless you actually had a useful signal to play through it... If existing recorded music doesn't have any signal below about 20Hz, then you'd have to re-record it in a manner that preserved the lower frequencies - if it's been cut out by the recording process then you can't "get it back" from the higher frequency information, you'd just be faking it. I'd also question whether most music even HAS any significant signal below 20Hz, regardless of what the recording process preserved... If it was never there in the first place, then you'd DEFINITELY be completely faking it.
It sounds more like you are trying to build it just to make music more loud and obnoxious, not to improve it...