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Are you actually serious?
This is a tough one, maybe it is, or maybe it is't the hot tub. The only way to evaluate this problem is to make an effrot to measure, document, or identify any frequency hum. The problem is, the potential of finding the source, or the direction from which it comes will be a daunting task. UNLESS of course, you can monitor the noise and then power down each individual hot tub, but I don't think this is practical in your situation, as you may have a "suspect", but you would need their cooperation during the evaluation process.We have had customers who have dealt with situations like this from opposing neighbors that claimed it was their spa, but we went out and coordinated and monitored what was referred to as the noise which continued unabated after we shut the spa down, proving it wasn't the spa. That does not mean the neighbor was convinced with such a non-technical evaluation, causing them to continue with thier opinion, in spite of this proof.There is a low frequency humm phenomenon being talked about throughout the country that has yet to be identified, so far no one is talking about what it is, but theories are abundant.
Why don't you ask the neighbor to shut it off for the night to see if it stops?
IMO, any tub you can hear from 300 meters should be deafening to people actually using it. Some mounting screws on my old tub worked loose and I could hear the hum from my neighbors door 30 meters away, but tighening it up fixed it. Most A/c comressors are far nosier than any hot tub I've been around.
If hot tub noise from 300 meters away bothers you, you're living in the wrong neighborhood. Either that, or train you mind to enjoy it, instead of complaining about it.