57 to 113 square meters seems correct. If you then have 9 x 12 square meters in 2 chambers, this will be 108 x 2 = 216 square meters and you should have somewhere between 57 and 113 square meters.
9" width, 12" height, 12" depth. Just enough to fit the 12" driver and have a little room behind the magnet (the 9" side)
Very best,
My mistake here. I thought the prototype was 6 Inch drivers. The calculation will be slightly different.
Pi r2 is the area of driver. So 3.14x6x6 Inch will be 113 for 1 driver
0.5 to 1 gives 56.5 to 113 square inches .
You have 108 square inches and are slightly rusty below the maximum. I would probably have tried a new prototype with 60 to 70 square inches.
Pi r2 is the area of driver. So 3.14x6x6 Inch will be 113 for 1 driver
0.5 to 1 gives 56.5 to 113 square inches .
You have 108 square inches and are slightly rusty below the maximum. I would probably have tried a new prototype with 60 to 70 square inches.
So you think I should reduce the back chambers below 9" length to get closer to 60 square inches? Will this reduce efficiency so that the backwave is less potent and has less cancelation? Or does this produce more energy I think? I can certainly reduce it. The drivers have a 6" depth from flange to magnet, so I did 9" to have 3" of working room behind them for ventilation at the pole and room for some stuffing on the walls for air noise management. I could go down to 7~8 inches I suppose, if losing that 1" matters enough.
For testing, I can fill that cavity with panels using hot glue to test reducing the area in there without re-building.
Very best,
Yea, its 12.25 technically, I rounded, doesn't change much.
Never trust published spec and suggestions on P.E. or from GRS. I measure them physically and measure with DATS. The physical dimensions are almost always different enough to matter and the DATS are always quite different. So I just base things off what I measure.
The piston area and flanges are not even 11" though, so my holes I cut are about that size to reduce volume. I'll see if I can stuff the cavity too, to reduce cavity volume as well.
Very best,
Never trust published spec and suggestions on P.E. or from GRS. I measure them physically and measure with DATS. The physical dimensions are almost always different enough to matter and the DATS are always quite different. So I just base things off what I measure.
The piston area and flanges are not even 11" though, so my holes I cut are about that size to reduce volume. I'll see if I can stuff the cavity too, to reduce cavity volume as well.
Very best,
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So, coming back to slot width:
2.5~3.0 inch vs 6.0 inch slot width measurements. Blue line is 6 inch.
The wider 6 inch slot had a sooner roll off, lower output at 27hz, lower output at 10hz, etc, and the peak resonance of the cavity happens at a lower frequency.
The 2.5~3.0 inch slot cavity had higher output, more extension, more output at 27hz and the cavity peak resonance is higher frequency.
So I'm trying to understand how the larger slot width will add efficiency when here it's showing less. ?
Distortion of the 6 inch slot:
Distortion of the 2.5~3.0 inch slot:
I'm just not seeing anything in these measurements to suggest using the larger slot side.
I'm not refuting it, just giving what info I have. It's entirely possible my measurements are biased or that I didn't do something that I should have. I'm happy to hear any info or learn something here, as to why or how I should approach this slot width, if the larger 6.0 inch slot width is suggested to be more efficient and better. The 33% rule of thumb I read about for the slot area vs total cone area just didn't seem to make sense with two drivers in a cavity vs one, and the output differences here show the more narrow slot was better. But, maybe the slot depth is the reason? It's still a lot of area in that cavity despite the width.
Any ideas?
Very best,
2.5~3.0 inch vs 6.0 inch slot width measurements. Blue line is 6 inch.
The wider 6 inch slot had a sooner roll off, lower output at 27hz, lower output at 10hz, etc, and the peak resonance of the cavity happens at a lower frequency.
The 2.5~3.0 inch slot cavity had higher output, more extension, more output at 27hz and the cavity peak resonance is higher frequency.
So I'm trying to understand how the larger slot width will add efficiency when here it's showing less. ?
Distortion of the 6 inch slot:
Distortion of the 2.5~3.0 inch slot:
I'm just not seeing anything in these measurements to suggest using the larger slot side.
I'm not refuting it, just giving what info I have. It's entirely possible my measurements are biased or that I didn't do something that I should have. I'm happy to hear any info or learn something here, as to why or how I should approach this slot width, if the larger 6.0 inch slot width is suggested to be more efficient and better. The 33% rule of thumb I read about for the slot area vs total cone area just didn't seem to make sense with two drivers in a cavity vs one, and the output differences here show the more narrow slot was better. But, maybe the slot depth is the reason? It's still a lot of area in that cavity despite the width.
Any ideas?
Very best,
The cavity resonance is based on the slot width. The smaller the slot width, the higher the resonance frequency.
In m measurements above you can see the spikes on both widths, in the 225~260hz range in general let's say.
My cavity is 12" deep with a 2.5~3.0 width, so let's say 13.5 to 14" half of that cavity, as a quarter wavelength, so 4x that is the full wavelength. 13.5 inches is 0.3429 meters. 0.3429 x 4 = 1.3716 meter full wavelength. (343 m/s) / 1.3716 = 250hz. So this crude calc based on my slightly approximate dimensions shows a peak resonance should happen around 250hz with a 2.5~3.0 inch slot width with my 12" depth in the cavity. It shows on the graph, so I would say that checks out.
Now, on the 6" cavity, it would be 12" depth + half the cavity so 3", or 15" total for half that cavity as a quarter wavelength. 15" = 0.381 meters. 4 x 0.381 = 1.524 meters full wavelength. (343 m/s) / 1.524 = 225hz. And that is seen on the graph to, so checks out.
So both are showing that high peak resonance frequency in that 225~250hz range, predicted by calc, but shown by measurement. The slot variation width barely changes it. 3" to 6" didn't change it by much, only a 25hz shift down. This tells me that the slot depth and total width of that cavity actually matters overall I think. Unless I'm missing something (entirely possible!)?
So if my depth wasn't 12", and was instead, something like 8" and had a 3" slot cavity width, it would be 8" into the cavity and 1.5" half width, for a total 9.5" for the slot cavity as a quarter wavelength, so 9.5" = 0.2413 meters. 0.2413m x 4 = 0.9652 meters full wavelength. (343 m/s) / 0.9652m = 355hz. So that smaller cavity with the same width shifts much higher in frequency as the slot depth is reduced. So the actual width isn't the only factor. It's the total cavity baffle dimension as a quarter wavelength.
So to tune higher, the cavity itself needs to be either less deep, or less wide. Obviously you cannot go less deep with larger drivers, so then the width has to come down a little. But as I showed in my measurements, it only changes a tiny bit because of my large 12" depth. People working with smaller drivers can tune much higher in this way, and allow for crossovers much higher into upper bass to lower mids (6.5" drivers for example needing a smaller cavity) which makes sense for a loud speaker design. But this is not necessary for a subwoofer, as I'm not trying to get 300~400hz from a subwoofer. Just 10hz to 100hz.
To tune lower, it would take a much larger cavity. Example, let's work backwards. 200hz peak. Well, 343 m/s / 200hz = 1.715 meter wavelength as a full wavelength, so divided by 4 for the quarter wavelength = 0.428 meters. That's 16". So my slot would be 12" depth and 4" half width to get this tuning. The total slot width would be 8 inches! to get this with a 12" depth.
But then with a huge 8 inch slot width and 12" depth, do the drivers mass load anymore at all? I don't know. Maybe not? It's greater than 33% surface area in that configuration.
In my case, I could reduce the cavity by only cutting a hole the size of the moving piston of my driver, which is just about 10" and a hair of room for the gasket to move without touching anything. And then reduce the cavity around that to its size. Then the cavity could be reduced to about 11" depth at best with this size driver, but it's only 1 inch, so it won't shift much. So, not worth the extra work and effort, I think.
Anyhow, I think that's the logic? Unless I'm off base big time, but I'm totally open to correction if I am!
Very best,
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Yes, so its fixed based on driver size. 12" driver is going to have a minimum of 10~11 inch dimension in the cavity to fit the piston. The width of the slot will be the smallest dimension with the least impact on this total dimension for figuring the peak resonance and yes, as you said, determines the lowest frequency resonance based on quarter wave relative to that dimension. This is why my 3" and 6" slot didn't change much, because my 12" dimensions are so much greater that the slot width didn't impact it much for resonance purposes.
My question then becomes, does loading the drivers change at 6" here, vs 3" here. Per my measurements, the 3" slot width was marginally better, so I can only assume its because they were loading and driving Fs down and Qts up?
Very best,
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Ok thanks, so with 12" drivers, I know I already have to keep it below 200hz, and ideally will low pass at 100hz or just above that. I did that in my EQ version sweep to show all that resonance peak energy gone and it sounds appropriate when I listen without that super loud peak.
I do wonder how things will change when I scale this to 8 drivers and have a large tall open chamber on the back side, 9" wide, 50~55" tall and 12.5" deep. The front slots will still be around 11" deep, 3" wide, 11" tall. I may try to squeeze the chamber smaller if I can, but gaining 1 inch doesn't change the peak frequency much of the cavity. Each cavity will be individual per pair of drivers, instead of one long open cavity for all 8 as this was advised against.
Very best,
You will have a resonance from the rear around 60-70Hz. Not what you want. Divide the rear chambers into 12" heights like the front.
So, I'm coming back to the compression and wondering how the slot behaves here. I took a 3 inch slot to compression. But I don't know if its actual compression or if it's the slot's compression and the drivers couldn't overcome the pressure and so were effectively limited there. Or were they just a excursion limits? I didn't take my 6" slot model to compression limits because it was cardboard and the 3" model looked better at the time.
So, here's the 3" model taken to what looks like compression. But, above 30hz it's not compressing, just under 30hz. So I'm not sure if that's compression, excursion limit (xmech) or maybe the slot pressure getting too high? When I took up the levels, the compression under 30hz is obvious, but above 30hz it kept gaining SPL, so there's something limiting it under 30hz and I'm curious if it's the slot or if it's the drivers. So I need to do a 6 inch model to test this and see how the drivers behave with a wider slot to reduce the cavity pressure and see.
Any thoughts appreciated?!
Very best,
So, here's the 3" model taken to what looks like compression. But, above 30hz it's not compressing, just under 30hz. So I'm not sure if that's compression, excursion limit (xmech) or maybe the slot pressure getting too high? When I took up the levels, the compression under 30hz is obvious, but above 30hz it kept gaining SPL, so there's something limiting it under 30hz and I'm curious if it's the slot or if it's the drivers. So I need to do a 6 inch model to test this and see how the drivers behave with a wider slot to reduce the cavity pressure and see.
Any thoughts appreciated?!
Very best,
I like it, but I'm still curious if a wider slot will have a different response below 30hz. Above you can see compression or xmech limit or maybe the pressure of the smaller cavity width is limiting the drivers. I don't know. So I will try to do one more physical model with a wider slot, like I did before, but take it to compression--something I didn't do last time when I first made the initial physical model. If it looks the same, then I'll call it compression or xmech limit and call it a day. But if it relieves it, then it was likely pressure from the cavity causing a limit and reducing output. If I can get +1 or +2 db back down there, I would absolutely increase the slot width. But if it remains, I'll keep the smaller 3~4" slot width instead.
Very best,
Ok, new iterration completed. 6 inch slot cavity finished and measured to compression and taken to the same levels as the 3 inch slot cavity width and compared.
12" side wings, 9" front width on each side of slot, 6 inch slot width, 12" cavity depth. Total baffle effective width is about 72 inches from the beginning of the side baffles, or 60 inches from the middle of the side baffles (if this is more appropriate?).
Cavity is 12" depth, 12" height and 6" wide. So the quarter wave is 12+6" = 18". Full wavelength is 72" = 1.82 meters. (343m/s) / 1.82 meters = 188hz expected cavity resonance. Still above 100hz, but starting to creep much closer to that area.
Taken to complete compression of the infrasonic frequencies, or at least xmech, not sure which. Either way, they're maxed out here.
Ground plane at 1 meter taken to full compression. Compression started to be noticed around 16 volts and lost almost 1db there. Then at each -1dbFS level increase, more noticeable compression began until I was getting no increase in SPL per level increase as seen below at full compression below 30hz. I was still scaling up above 30hz per change, so the drivers were likely simply hitting their excursion limits and could keep going if they were limited from 30hz and up. But under 30hz, their 8.5mm xmax shows I think here and this is beyond that, this is at whatever their true xmech limits are without clanking. So maybe a hair less than real xmech limit.
Here's the overlay comparison of the 6 inch cavity vs the 3 inch cavity taken to compression at the same levels.
They both compress at the same place, around 27~28hz it's noticed easily and stays neck and neck down to 10hz at full compression. The 3 inch slot actually has higher output above 30hz by about 1 db at the same levels as the 6 inch slot. So to me, this shows that this configuration the 6 inch slot is less efficiency than the 3 inch slot for output through the entire bandwidth. Unless something else is the culprit of this? Still, this shows that there's no advantage to 6 inch slot width on these dimensions with these drivers, taken to excursion limits. So my final slot width in the next build will be 3 inches likely.
Very best,
12" side wings, 9" front width on each side of slot, 6 inch slot width, 12" cavity depth. Total baffle effective width is about 72 inches from the beginning of the side baffles, or 60 inches from the middle of the side baffles (if this is more appropriate?).
Cavity is 12" depth, 12" height and 6" wide. So the quarter wave is 12+6" = 18". Full wavelength is 72" = 1.82 meters. (343m/s) / 1.82 meters = 188hz expected cavity resonance. Still above 100hz, but starting to creep much closer to that area.
Taken to complete compression of the infrasonic frequencies, or at least xmech, not sure which. Either way, they're maxed out here.
Ground plane at 1 meter taken to full compression. Compression started to be noticed around 16 volts and lost almost 1db there. Then at each -1dbFS level increase, more noticeable compression began until I was getting no increase in SPL per level increase as seen below at full compression below 30hz. I was still scaling up above 30hz per change, so the drivers were likely simply hitting their excursion limits and could keep going if they were limited from 30hz and up. But under 30hz, their 8.5mm xmax shows I think here and this is beyond that, this is at whatever their true xmech limits are without clanking. So maybe a hair less than real xmech limit.
Here's the overlay comparison of the 6 inch cavity vs the 3 inch cavity taken to compression at the same levels.
They both compress at the same place, around 27~28hz it's noticed easily and stays neck and neck down to 10hz at full compression. The 3 inch slot actually has higher output above 30hz by about 1 db at the same levels as the 6 inch slot. So to me, this shows that this configuration the 6 inch slot is less efficiency than the 3 inch slot for output through the entire bandwidth. Unless something else is the culprit of this? Still, this shows that there's no advantage to 6 inch slot width on these dimensions with these drivers, taken to excursion limits. So my final slot width in the next build will be 3 inches likely.
Very best,
