Sound advice on room treatment: [ Ссылка ]
The basics of room acoustics are simple - use lots of thick absorbers to reduce the reverb time in the room.
Of course you don't have to start off with a lot, but keep adding as you can afford it until you've reached a point where they are working effectively. But don't fall into thinking that you can get away with using more thinner panels - they need to be a minimum of 6" (150mm) thick, otherwise you are wasting space and money. And you are also killing too much of the high frequencies that you need to preserve to make a room that sounds good.

The key with thick absorbers is that they start working right away in the right way. They help to fix the problems that every small room has, and that's room modes and excess energy in the room. They are the easiest, the least complex and generally the most effective way to treat a room.
While there are other effective methods, like Helmholtz and diaphragmatic, they are harder to design, build and put in a place where they'll work best.
There are also electronic bass traps, but they are quite expensive and may not be more effective than standard absorbers.

So on to the idea of cancelling room modes by timing a signal to do that.
I used the ringing bell as an example, but stopping that from continuing to ring would be trivially easy compared to doing that with an enclosed volume of air inside a room. And assuming you could do it and have it work as it needs to, you'd be unable to change anything in the room or it would stop working. That includes adding more people to the room or leaving the door open - it would throw off that delicate balance.

And like I said in the video, to stop the ringing you need to make a sound that's not part of the original signal to do that, and that's distortion. Any change to the original signal is, by definition, distortion.
The best, proven method is with the brute force absorption I talked about. Room acoustics have been studied for hundreds of years and in the last 100 every possible method has been looked at in detail and tried.
That doesn't rule out further discoveries in the future, but they would have to be much more sophisticated than the method described above.

Worth mentioning that I didn't mention diffusers. Those are only used after you've put in as much bass absorption as you possibly can. Diffusers only work on higher frequencies and have no impact of the bass frequencies.
Also my absorber charts show the flow resistivity as 10,000 for both the foam and the fiberglass / rockwool. This isn't accurate, but is "close enough" to show the glaring differences between the thicknesses. That was the point - making the panel thicker so they will be effective down low. The flow resistivity has an effect, but not as much as making the absorber thicker.


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