What do you think about something like this devices?
They are not forbidden as i know, so it is totally legit to use them. They just fill room with ultrasonic noise overwhelming microphones and preventing your private conversations recording while it is not disturbing for humans (humans cannot hear ultrasonic)
I have such. And i use it when i have private conversations about something i don’t want to be shared with Google, Meta or other evil corporations
The effectiveness will vary based on the microphone capabilities.
Also, quite expensive. 
“Ultra-sonic” means above the range of hearing. Most microphones record within the range of hearing because that’s where we communicate. So unless you’re trying to specifically mask ultrasonic sounds recorded by microphones specifically recording that. This is a bit like being in a parking lot full of cars and trying to drive off in the car next where you parked. It’s not technically impossible your key would randomly just happen to work on that car, just highly unlikely.
Easier solutions would be to play music or white/brown/pink (or a mix) noise at such a volume that any microphone recording the sound would get overwhelmed with that sound and have a hard time recording quieter sounds.
And the easiest solution would be: Do not install any App from Google, Apple, Meta or other evil corporations! 
We need an anti smart glasses recorder as well 
@SwampTrainer you are mostly wrong. Recording breaks fully (see how it will look here
Unfortunately i cannot control what my guests or homemates have
Aw OK for those situations, yes, it may be useful
Recording “fully breaks” - on the apps used here on the devices shown here. Not all hardware is the same, not all codecs or processing are the same, not all jammers are the same.
Even a random site I found that is pro-ultra-sonic jammers agrees that $1,000+ is where quality lies, so you’re gambling on an expensive box that, maybe best case scenario, built to work on low-end phones and that’s it.
You seem sold already and are fighting anyone disagreeing with you. Why not buy the thing and show us all how well it works?
Get a Faraday box , all guest phones go into the box - your house, your rules.
As i said:
I works in most of my cases
Unfortunately, thos won’t prevent recording 
But generally good practice
Cover the outside with thick foam placed in another room far away from the party.
Animals can hear in ranges of sound that humans can’t. I hope people using that device aren’t driving their or their neighbors’ pets deaf or crazy.