Destructively Driving Modes
- Thread starter dallasjustice
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That's not what I measure. To drive 1,0,0 destructively, the subs need to be in opposite polarity. When I measure and they are in the same electrical polarity the 1st gets much bigger. If they are in opposite electrical polarity, the 1st goes away. I read that bluray forum post before and I don't think that's accurate info.
I will post a REW plot later showing this.
I will post a REW plot later showing this.
You have bad water in Texas so it screws up everything including your audio equipment....
I'm not sure what is going on in your setup but reference Toole pg. 221 diagram (b). "two subwoofers, one on each side, radiating identical signals, destructively drive the mode reducing the amplitude. This is "mode cancellation".
This is the behavior I get from acoustical modeling simulations.
Are the subs fed exactly same signal (no digital delay of rears)? If you are using the old F112 front and the new F112v2 rear it's possible that the amplifiers are different and one is polarity inverting and the other is not. It's more likely something in your setup than the physics being wrong.
That too. And you kept defending the guy who did your wiring despite my feedback to the contrary:No. I think the polarity is reversed on my front subs because I get cheap electricity.
I am using four subs spaced roughly to provide the best overall frequency response, within the limits of the room and furniture. Two are on the side near the front wall, 3 or 4 feet out (beside the mains), and the other pair is on the back wall with each sub about 1/4 in from the side wall. I used REW and the multisub program by andyc56 on AVS (hope that's not a problem, if so I'll delete this and find my link to his site later) ti dial things in, then did lots of measurements and playing with sub phase and Dirac Live filters. Decently flat and with room gain down 3 dB around 7 Hz or so, though I have more fiddling to do, just no time!
That too. And you kept defending the guy who did your wiring despite my feedback to the contrary:
Yes, I do accept payment-in-kind. I once had my bathroom remodelled for a fee. I'm not above engaging in medieval economic transactions. I believe economists call barter a "coincidence of wants."
"An effective method to equalize low frequencies
in rectangular rooms has been simulated
and implemented in these three rooms. The system
uses two loudspeakers in the front wall of the room
to create a traveling plane wave and an extra two
low frequency loudspeakers in the back wall delayed
and in opposite phase to remove the reflection of
that wall. "
Low frequency sound field enhancement
system for rectangular rooms using multiple
low frequency loudspeakers
Adrian Celestinos and Sofus Birkedal Nielsen
You can see what I'm talking about in section 3.2. "Removing the Reflection from the Back
Wall"
This deals with the method I'm using to destroy modal ringing by running a rear sub in "antiphase."
In the paper's discussion:
"By first creating a plane
wave in only one direction of the room which implies
exiting only the axial modes of that direction
and secondly canceling that plane wave using loudspeakers
delayed at the end wall in opposite phase
with the traveling sound, optimal equalization can
be obtained."
Nyal,
Here's some more from the AES paper authored by Fazenda et al to which Klaus originally made reference with regard to the "sink" method to cancelling room modes:
"The ‘active’ aspect of this control configuration comes
into place as the plane wave reaches the rear wall. At
this location, the ‘sink’ speakers reproduce the same
signal in anti-phase which has the effect of cancelling
its reflection from the rear wall and thus any modes
generated along this dimension."
The rear subs act to absorb or "sink" the so called "plane wave" from the front of the room. My understanding of all this stuff and my own measurements makes me pretty confident that the rear subs need to be in "anti-phase" to the front subs. IOW, the rear subs can either be in opposite phase or opposite polarity and they act to "sink" the modal ringing between those two walls.
To be sure, I will measure the impulse peaks for all four of my subs to make sure the polarity switch on each sub corresponds with the sub's actually polarity.
Here's the "sink" configuration in the paper.
Michael.
Here's some more from the AES paper authored by Fazenda et al to which Klaus originally made reference with regard to the "sink" method to cancelling room modes:
"The ‘active’ aspect of this control configuration comes
into place as the plane wave reaches the rear wall. At
this location, the ‘sink’ speakers reproduce the same
signal in anti-phase which has the effect of cancelling
its reflection from the rear wall and thus any modes
generated along this dimension."
The rear subs act to absorb or "sink" the so called "plane wave" from the front of the room. My understanding of all this stuff and my own measurements makes me pretty confident that the rear subs need to be in "anti-phase" to the front subs. IOW, the rear subs can either be in opposite phase or opposite polarity and they act to "sink" the modal ringing between those two walls.
To be sure, I will measure the impulse peaks for all four of my subs to make sure the polarity switch on each sub corresponds with the sub's actually polarity.
Here's the "sink" configuration in the paper.
Michael.
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Here's a simulation two corner subs in and out of phase at (or near) a problematic frequency:
That's me.
A link to the program is in my profile, or you can just click my user name to visit my homepage. It requires time-synchronized measurements, so at present it can't be used with a USB mic. A loopback timing reference is needed. However, John Mulcahy is apparently working on something in REW to get a timing reference of some kind with a USB mic. The REW update was originally supposed to be ready sometime this month, so I'd expect it to be available soon.
That's a valid cancellation technique, but different to the one you referred to in your first post. Welti is not talking about cancelling plane waves. Also you said you were not delaying the rears, which means you weren't doing the "plane wave cancellation" technique.
Just trying to make sure everyone is referring to the same cancellation technique.
In Table 3 of Low-Frequency Optimization Using Multiple Subwoofers, Welti and Devantier specify that for each subwoofer channel, individual delays of 0, 5 and 10 ms are evaluated in the search grid.
Bear with me, I'm learning out loud.
I shouldn't be on your back about it really, because it's great to have people experimenting with this stuff!
One very cool thing I've been meaning to do a video on for a long while is how when you do the mode cancellation if you play a sine wave at the cancellation frequency you turn the volume UP on the rear sub and the SPL decreases at the modal frequency. Very cool stuff.
But in the final analysis the one they recommend were playing same signal to each subwoofer, right?
It is a mono source signal, but after it is split into multiple paths, there is one adjustable PEQ, one adjustable delay, and one adjustable gain per channel. These are adjusted to minimize the Mean Spatial Variance of the combined sub outputs.
Table 3:
Gain: 0, -6, -12 dB
Delay: 0, 5, 10 ms
Filter Q: 1, 4, 16
Filter attenuation: 0, -12 dB
I'd have to go back and re-read more completely to see how they select the PEQ center frequency.
There are two papers by Welti and I think you are talking about the second one, "In-Room Low Frequency Optimization," Todd Welti, Allan Devantier, 115th Convention, Audio Eng. Soc. Preprint 5942" and Nyal is referring to the first, "“How Many Subwoofers Are Enough,” Todd Welti, 112th Convention, Audio Eng. Soc. Preprint 5602."
I cover the technique for the one you mention with a number of real measurements in my article here: http://www.madronadigital.com/Library/Computer Optimization of Acoustics.html
I was referring to a third one, Low-Frequency Optimization Using Multiple Subwoofers, which describes the SFM algorithm, at least how they conceived of it at the time.
