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Narrowband Sound Sources for active Room Mode Cancellation

bleblaah

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Hey ASR community,

I only just started to dive into room acoustics so I'm sorry in advance if this is an obviously stupid idea somehow, but I was wondering:

Given it's possible to create a sound source which emits just a single frequency, couldn't you just put those into the corners of your room and cancle out several room modes?
As I understood, room modes are standing waves with pressure maxima at the walls and corners (regarding only the axial and tangential modes for now) and those maxima are oscillating between the positive and negative extreme values. Couldn't you just cancle out a room mode by putting a sound source in a corner which plays only the frequency of the particular room mode but in the opposite phase? It would be just like noise cancelling but with one isolated frequency or a very narrowband signal at least. So each time the room mode is at the positive maximum, the sound source is at the negative maximum and vice versa.

I could imagine the sound source as being a custom built subwoofer with crossovers and EQ to supress all frequencies except the one of the room mode. Or maybe even putting two woofers opposite to each other that both play the full spectrum signal but the desired room mode frequency is EQ'ed to the opposite phase in one of them. So only that room mode frequency wouldn't be cancled out between the woofers and is able to itself cancle out the room mode in that place.

Did I miss something crucial?

Thanks in advance.
 
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rationaltime

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This seems like a good exercise for thinking about room resonance.

The resonance issue is not with the incident (first) wave, but with the reflections.
We would not want the incident wave cancelled. Maybe we would not be bothered
so much if a narrow band of the source were cancelled. Often though the resonance
occurs at audible frequencies. Would it be all right to cancel certain notes?

Room resonance is probably low Q, meaning frequencies near the peak are
somewhat persistent. To be effective maybe the signal shaping would need to be
not so narrow band. You might be able to measure the Q by seeing how long it
takes for a signal near resonance to decay.

Driving two signals out of phase may work for cancellation at one place. Really,
out of phase is like a delay. Combining with the delayed signal, isn't that called
a comb filter? That might be a way to generate resonance.

That is my initial reaction.
 

NTK

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I could imagine the sound source as being a custom built subwoofer with crossovers and EQ to supress all frequencies except the one of the room mode. Or maybe even putting two woofers opposite to each other that both play the full spectrum signal but the desired room mode frequency is EQ'ed to the opposite phase in one of them. So only that room mode frequency wouldn't be cancled out between the speakers and is able to itself cancle out the room mode in that place.
I think the PSI AVAA (active bass trap) may just be what you are thinking about.
 
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bleblaah

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This seems like a good exercise for thinking about room resonance.

The resonance issue is not with the incident (first) wave, but with the reflections.
We would not want the incident wave cancelled. Maybe we would not be bothered
so much if a narrow band of the source were cancelled. Often though the resonance
occurs at audible frequencies. Would it be all right to cancel certain notes?

Room resonance is probably low Q, meaning frequencies near the peak are
somewhat persistent. To be effective maybe the signal shaping would need to be
not so narrow band. You might be able to measure the Q by seeing how long it
takes for a signal near resonance to decay.

Driving two signals out of phase may work for cancellation at one place. Really,
out of phase is like a delay. Combining with the delayed signal, isn't that called
a comb filter? That might be a way to generate resonance.

That is my initial reaction.
Well I think you wouldn't aim for the wavefront coming from the speaker to be cancelled. In this case it would be just like a bass array. To cancle out only the room modes you could place the hypothetical narrowband sound source in the corner, even facing the walls or something. Again, I'm not an expert. Maybe theres not even a difference between the initial wave coming from the speakers and the standing (reflection) wave both in terms of directivity and phase and its not even possible to only affect one of them.
Anyway, I don't think comb filters would occur if the interfering (delayed) signals of the narrowband sound source and the room mode/speaker are so narrowband.

@NTK
Yes the AVAA seems to work similarly but it's way out of budget for me and it's designed to adapt to whatever room you put it into, using a microphone to measure the room modes I guess. So this is a universally working solution, just like active noise cancelling but for bass only.
I think the big benefit of my idea, if it even works, would be that it's a relatively low budget DIY solution that's customized to fit your individual room modes.
You could just buy a few crappy subwoofers and make them produce the narrowband frequencies you want. Or you buy a crappy woofer membrane, completely put it into a self built ported box so only the port resonance frequency is escaping which is then tuned to your desired room mode whatsoever.
Seems to me that this could be a method to tacle espeacially the lowest room modes, so the porous absorbers in your room can be designed much shallower.
 

Flaesh

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even putting two woofers opposite to each other that both play the full spectrum signal but the desired room mode frequency is EQ'ed to the opposite phase in one of them. So only that room mode frequency wouldn't be cancled out between the woofers and is able to itself cancle out the room mode in that place.

Did I miss something crucial?
DBA (double bass array) aka C.A.B.S. Or SSS.
 
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