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Old 20th November 2017, 04:09 PM   #11
kaputt is offline kaputt  Germany
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It's a very smart concept to get the benefits of velocity transformation without the size issues of a bass horn. It's also way easier to build.
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Old 20th November 2017, 04:49 PM   #12
CharlieLaub is offline CharlieLaub  United States
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I would like to bet that this was impressive in terms of bass. Multiple 15" drivers and a HUGE baffle help to make the pathlength very long, and reduce/minimize the OB short circuit losses that happen at low frequency.

The only issue I see here is a duct resonance from the slot loading:
Click the image to open in full size.

The way the drivers are mounted, opposed, to a slot or chamber that then opens to the "free air" causes a short transmission line to form. This line has a resonance that happens at the frequency corresponding to a length that is four times the "depth" of the slot. Let's say the slot is 20 inches deep (so that I can do the calculation in my head). 4 times 20 = 80 inches, which is about 2 meters. f=c/L and c=350 (rounding up here folks) meters/second, so 2/350=175 per second, or 175 Hz.

Fine, we have some resonance. So what? Well because the aspect ratio of the "slot" is high (it's much deeper than it is wide) the Q of this resonance will be high. You can either (A) avoid this region by crossing over an octave lower, e.g. around 100Hz, or (B) you can try to EQ the resonance to flat. With approach (B) it can be tricky to get the EQ just right. Approach (A) limits the bandwidth, which otherwise with a planar OB could reach as high as the driver can operate. (A) also leave the resonance untreated, and unless the LP filter is steep it will not be sufficiently suppressed. Even acoustic energy coming from a completely different source (the midbass horn) can stimulate the slot's resonance if the Q is high enough, since damping is zero.

Is there another approach? YES: use a wider slot. That will reduce the Q of the resonance and make it easier to work with. While the narrow slot does provide some loading of the driver (reducing Fs and increasing Qts) the resonance is not desirable if its Q is high.

I can suggest making the slot at least twice as wide as it is deep, the wider the better. You will still get the force cancellation, which is one of the benefits of this type of driver arrangement. With a Q more like 1.3-1.4, you can easily incorporate that into a crossover having a corner frequency equal to the resonance frequency by adding a low Q (e.g. Q=0.5) 2nd order low pass filter to get an overall response that is approximately a 4th order Butterworth. This could even be done using passive components only, since a second order Q=0.5 filter is the same as two first order filters of the same corner frequency. Just use two RC filters in series on the amp input, with values spread by 10x to avoid interaction.
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Last edited by CharlieLaub; 20th November 2017 at 04:54 PM.
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Old 21st November 2017, 10:57 PM   #13
Scott L is offline Scott L  United States
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Default multiple 15's

Hi Charlie,
Would this be any better ?
Attached Images
File Type: jpg Infinite baffle.jpg (353.3 KB, 103 views)
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Old 21st November 2017, 11:11 PM   #14
planet10 is offline planet10  Canada
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Quote:
Originally Posted by CharlieLaub View Post
Fine, we have some resonance. So what? Well because the aspect ratio of the "slot" is high (it's much deeper than it is wide) the Q of this resonance will be high. You can either (A) avoid this region by crossing over an octave lower, e.g. around 100Hz
Charlie,

I followed the discussion of this speaker syatem on the JoeList and IIRC, this woofer array was crossed over at 40 Hz. I canít recall how fast, but i think it was steeper than i would have guessed.

dave
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Old 21st November 2017, 11:21 PM   #15
planet10 is offline planet10  Canada
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Quote:
Originally Posted by Scott L View Post
Hi Charlie,
Would this be any better ?

ETF Subwoofer Demo-infinite-baffle-jpg
Quite similar to this Aug, 2000 thot experiment:

Click the image to open in full size.

Push-Push Dipole Concept

dave
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Old 22nd November 2017, 11:52 PM   #16
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by Scott L View Post
Hi Charlie,
Would this be any better ?
It's a little better. The "slot" is wider, so the Q will be lower. I was thinking more along the lines of making the width at least 2x the depth. This looks like width=depth... it's probably OK. You should be able to see the resonance (around 150Hz I would guess) in a nearfield measurement.

These long slot loaded "bass line array" kind of constructions will also have a weak resonance in the up/down direction within the "slot" cavity. All of these resonances are OK if the Q of the resonance is kept low. The most effective way to do that at low frequencies is to make them much less deep than any other (lateral) dimension, or in other words the "slot" should be wide and relatively shallow. Of course the slot must be at least as deep as the driver frame is wide, so the only dimension you can vary is the slot width.

Also, regarding your image and the last post - if you are going to use a bunch of opposed drivers why not arrange them so that there is some degree of even order distortion cancellation? This does NOT do that:
Click the image to open in full size.
Above: illustration of the wrong way to mount the drivers!

Instead, of mounting the two drivers pointing in opposite directions and magnets together, one of the drivers should be mounted backwards so that both drivers "point in the same direction". A pair of drivers operating in this type of system should push air into the slot at the same time. When mounting is done with one driver reversed, one of the driver cones is moving towards the magnet while the other is moving away from it. When the compliance vs excursion varies non-symmetrically (and other parameters, too) then the distortion generated by this will be mostly cancelled out when one driver is reversed. It's worth it when only a simple change to the driver mounting is all it takes. It would look like the attached image.
Attached Images
File Type: png distortion-cancelling driver mounting.png (18.3 KB, 2 views)
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Last edited by CharlieLaub; 23rd November 2017 at 12:03 AM.
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Old 23rd November 2017, 08:58 AM   #17
jazbo8 is offline jazbo8
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Isn't that like the ETF 2017 version in post #2 - 8 of them stacked in a column?
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Old 23rd November 2017, 09:10 AM   #18
kaputt is offline kaputt  Germany
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Quote:
Originally Posted by planet10 View Post
Charlie,

I followed the discussion of this speaker syatem on the JoeList and IIRC, this woofer array was crossed over at 40 Hz. I canít recall how fast, but i think it was steeper than i would have guessed.

dave
It was a 48dB filter I believe. DSP.
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