I encode data in audio by energising
5 frequencies between
I play the WAV on a sports stadium (20k seating, so BIG) PA rig, placing a microphone somewhere in the spectator seating area.
This is the data (I did it at
As can be seen, there is some kind of 'ringing' occurring. Now the frequency channels are using are all integer multiples of a fundamental frequency (
~100Hz), and the ghost frequencies appear at exact harmonics. If I crank up the volume I also get energy midway between harmonics.
There is one further effect: Ghosting at
1/2 f. This is most visible if I analyse a sweep (
As can be seen, much of the energy at frequency
f has been redirected to
Can anyone explain these unwanted artefacts?
PS Often I observe a perfectly valid SE question attracts several instant down/close-votes before making it back to the surface. It always makes me scratch my head a little. It seems rather feeble to downvote without supplying a comment to summarise the decision logic. e.g. Is it OT / malformed / inarticulate / lazy / etc, and how so?
PPS Someone has now suggested _intermodulation_ in a comment. Now this is valuable information. The question together with this keyword, once catalogued by Google, adds to the effectiveness of the Internet as a resource for Information. Yet the question is teetering on 4 (un--publicly-justified) close-votes. I do not think this is right. :/