When I was at school, which was a long long time ago, I was taught (if I remember correctly) that you could cancel a wave by emitting a wave directly at the source of the same frequency but out of phase. Therefore I’m trying an experiment to see if this works.

Here’s the problem.

The next-door neighbour has installed a hot tub. The hot tub emits a low frequency hum, which I assume is around the 50hz range, but I need to measure it. It’s not loud, but I can hear it all though the house. It’s one of those penetrating sounds.

Here’s the solution.

If I emit the same frequency at the hot tub through a sub woofer, but out of phase, I should cancel the noise.

Cheers
John