Yeah, this was in one of my '50s era DIY books, though for the times just recommended for ~cubical cabs, though in TLs it will just reduce their amplitude (unless staggered/overlapping?) and create a bunch of new, higher frequency ones to more easily damp.
Kind of moot though since I imagine installing them will require sawing out the back plate.
Someone on AVS forum suggested I get some stiff 2" thick board and cut it to size to put in the box as a barrier or angled piece. Probably worth a shot if I can get it into the box somehow. I'm going to play with it all more tonight. I have a good bit of safe n sound I can cut up to line the walls.
well, it is a little "pipe-y" shaped ...
To be fair, my double 15 box that was golden ratios had a resonance of just under 200hz.
It was 39" tall.
so 1129 / (39" x 2 / 12") = wow, 175hz, the math works.
For those of you in band, tuning at 440hz, drop an octave and thats close to where is would "ring" a bit longer.
Or that bit of a drone you can hear in a double 15 box in a movie theater.
Non-pipe nor 2 drivers made it go away, probably lessened it though.
Certainly not a deal breaker for the setup...............
Just a flaw.
Shelf brace should help
To be fair, my double 15 box that was golden ratios had a resonance of just under 200hz.
It was 39" tall.
so 1129 / (39" x 2 / 12") = wow, 175hz, the math works.
For those of you in band, tuning at 440hz, drop an octave and thats close to where is would "ring" a bit longer.
Or that bit of a drone you can hear in a double 15 box in a movie theater.
Non-pipe nor 2 drivers made it go away, probably lessened it though.
Certainly not a deal breaker for the setup...............
Just a flaw.
Shelf brace should help
I tried the shelf as best I can. Seems to be also just lowering that resonant peak in frequency but not actually eliminating anything. 🙁
I think I found a good combo of things to make me happy. I have the bottom of the enclosure filled with polyfill, then a polyfill pillow roughly in the center, on top of the other polyfill to "trap" the fibers. My tuning dropped a bit, from 35 to 32 Hz. But check out how smooth the woofer response is now. Ignore the top end, that's about where the crossover to the CD starts and I didn't hold the mic near the dead center of the woofer.
And the port.
And the port.
Cool 🙂
The shelf is not a good idea, 140Hz is almost 3 meters long (double the height), 30cm sized piece as shelf is about tenth of that. Outside a box an object whose size is tenth to wavelength is invisible, you wouldn't measure any reflection from it.
I haven't measured it but reasoning like this and the logic says the most problematic long modes inside any box are there no matter what, you cannot fit big enough object inside that would have an effect. All they would do is make a maze, even longer path, and even lower and more problematic resonance. All one can do is prevent it happening by positioning the driver and/or dampening.
The shelf is not a good idea, 140Hz is almost 3 meters long (double the height), 30cm sized piece as shelf is about tenth of that. Outside a box an object whose size is tenth to wavelength is invisible, you wouldn't measure any reflection from it.
I haven't measured it but reasoning like this and the logic says the most problematic long modes inside any box are there no matter what, you cannot fit big enough object inside that would have an effect. All they would do is make a maze, even longer path, and even lower and more problematic resonance. All one can do is prevent it happening by positioning the driver and/or dampening.
depends what you goal is 🙂 If the lowest mode is problem, driver in the mid way could fix it. If its such box with fullrange coaxial driver it would possibly end up below ear height giving other problems.
Like you say in earlier post one could also add another driver to the other end of the baffle so that their acoustic center was midway of the box at the mode frequency eliminating the lowest mode. But now you'd be handling a 2.5way xo where phase difference at the mode would need to be taken account, lobing issues, possibly even bigger dimensions to the box and so on. Its always a set of compromises 😉 Making a three way system would solve the conundrum, enough separation of concerns, but leading to some other compromises again.
Like you say in earlier post one could also add another driver to the other end of the baffle so that their acoustic center was midway of the box at the mode frequency eliminating the lowest mode. But now you'd be handling a 2.5way xo where phase difference at the mode would need to be taken account, lobing issues, possibly even bigger dimensions to the box and so on. Its always a set of compromises 😉 Making a three way system would solve the conundrum, enough separation of concerns, but leading to some other compromises again.
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While the port output seems quite broadband to me, I think you are on the right track. Do you have the means to compare the port output level with the driver output level? I expect that the port output is a bit lower and you'll do good by tuning the port further (the rise at 20Hz seems to indicate your MLTL is tuned at subsonic frequencies).
The port tuning now is about 31Hz. But this is a pro drivers, and Fs naturally is in the 50s or so. But with sensitivity and good power handling you can boost the low end to get it flat. So a rising response in the port makes sense since the modeled response shows the tuning frequency is roughly F10.
It could be based on how I'm measuring. For the port SPL I held the mic right at the mouth of the port and measured. Could be that it was picking up some woofer response as well in the measurement since they're so close. I do know the woofer response is more accurate since I stuffed the port for that one.
What matters to me though is the combined response no longer has a big peak/dip and is smooth throughout the midbass. I'm still waiting for my subwoofers to come in, but I'll most likely cross the surrounds over at 40Hz or 60Hz depending on how well I can integrate them to the subwoofers. Maybe even go up to 80Hz. We'll see. 🙂
I wanted big surrounds that can be crossed lower to help improve the directionality of the midbass, and wood is cheap, and I figured it would be easy enough to just make these floor standers rather than smaller boxes on stands. So even if I end up crossing up in the 80Hz range, I won't be upset. At that point I might just seal them to make the phase integration easier.
What matters to me though is the combined response no longer has a big peak/dip and is smooth throughout the midbass. I'm still waiting for my subwoofers to come in, but I'll most likely cross the surrounds over at 40Hz or 60Hz depending on how well I can integrate them to the subwoofers. Maybe even go up to 80Hz. We'll see. 🙂
I wanted big surrounds that can be crossed lower to help improve the directionality of the midbass, and wood is cheap, and I figured it would be easy enough to just make these floor standers rather than smaller boxes on stands. So even if I end up crossing up in the 80Hz range, I won't be upset. At that point I might just seal them to make the phase integration easier.
Yeah something changed. Your first couple impedance sweeps showed mid 30's tuning. Then when you went back to "clean slate" they are showing low 40's tuning. Then your port measurement looked like 65?
Some of the sweeps I posted were with the port (it's 3D printed) not actually in the box. I was adjusting stuffing through the port hole and then running impedance sweeps. So that's why the tuning shifted into the 40s.
The SPL measurement I believe is right since there's such a rising response on the woofer itself. I said before the F10 is in the 30s naturally, so I'd expect the port SPL to be lower around tuning than some spots higher up naturally. Plus I was holding the mic at the outlet of the port, about 5" away from the bottom of the woofer cone, and I'm sure that affected the graph.
The SPL measurement I believe is right since there's such a rising response on the woofer itself. I said before the F10 is in the 30s naturally, so I'd expect the port SPL to be lower around tuning than some spots higher up naturally. Plus I was holding the mic at the outlet of the port, about 5" away from the bottom of the woofer cone, and I'm sure that affected the graph.
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Did you consider cardioid enclosures? And is there any level below 100Hz in the surround signal?
I didn't consider cardioid enclosures. I've used these speakers in a similar configuration for two channel and love them. Plenty of bass with some DSP. Plus, they have the same compression driver as my L/C/R, and the crossovers were designed by the same person, who voiced them similarly, so there's good coherence in the sound stage.
There's plenty of content below 100Hz in the surrounds, and atmos for that matter. Edge of Tomorrow is an extreme example where all channels have 10Hz tones at -4dBFS. Of course it's pretty unreasonable to expect that all channels play that low, so that's where the subwoofers come in. For me, bass localization starts to be pretty noticeable above 50Hz, so that's why I'm hoping to be able to cross the surrounds in the 40-60Hz range.
There's plenty of content below 100Hz in the surrounds, and atmos for that matter. Edge of Tomorrow is an extreme example where all channels have 10Hz tones at -4dBFS. Of course it's pretty unreasonable to expect that all channels play that low, so that's where the subwoofers come in. For me, bass localization starts to be pretty noticeable above 50Hz, so that's why I'm hoping to be able to cross the surrounds in the 40-60Hz range.