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post #1 of 32 Old 02-25-2020, 07:44 PM - Thread Starter
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Question Medium Low Tuned Thoughts

So, it is what it is when it comes to what woofers I have. I have FOUR RSS390-HF. I don't want massive box's in our new home so I wanted to try a LLT but with 6.5 CuFt.

HPF- 12HZ BOX tune 10HZ 6.5CUFT each, PORT 2"X18.5"X33" and 500W.

Does this seem visible to you pros or do you have better options?
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post #2 of 32 Old 02-25-2020, 07:47 PM
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Quote:
Originally Posted by Remy.Alexander View Post
So, it is what it is when it comes to what woofers I have. I have FOUR RSS390-HF. I don't want massive box's in our new home so I wanted to try a LLT but with 6.5 CuFt.



HPF- 12HZ BOX tune 10HZ 6.5CUFT each, PORT 2"X18.5"X33" and 500W.



Does this seem visible to you pros or do you have better options?


Where’s your port velocity at?

Chris
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post #3 of 32 Old 02-25-2020, 08:05 PM - Thread Starter
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Where’s your port velocity at?

Chris
Think it was low to mid 200hz range. Don't have PC in front of me lol
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post #4 of 32 Old 02-25-2020, 08:07 PM
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Medium Low Tuned Thoughts

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Originally Posted by Remy.Alexander View Post
Think it was low to mid 200hz range. Don't have PC in front of me lol


Not the first port resonance, the port velocity.
I’m sure resonance is fine with a 33in port.

Chris
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post #5 of 32 Old 02-25-2020, 08:12 PM - Thread Starter
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Ah, forgot to pay attention to that 🤦*♂️
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post #6 of 32 Old 02-25-2020, 08:15 PM
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Medium Low Tuned Thoughts

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Originally Posted by Remy.Alexander View Post
Ah, forgot to pay attention to that *


I’m firing up HR in a bit anyway, I’ll run the model.

6.5cu is the net volume?

Chris
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post #7 of 32 Old 02-25-2020, 08:19 PM - Thread Starter
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Originally Posted by a77cj7 View Post
I’m firing up HR in a bit anyway, I’ll run the model.

6.5cu is the net volume?

Chris
Yes sir, 6.5 is a manageable size that won't eat up family room.
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post #8 of 32 Old 02-25-2020, 08:19 PM
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Generally when you tune that low in a smaller box, your port velocity will remain reasonable as a consequence of the response being well rolled off by that point. Think of the response as similar to sealed but propping up the very bottom end a couple dB's. I think it's a solid avenue, just don't expect the headroom of the big ported boxes or the small size of sealed alignments. It really is in between. One advantage is you can, for the most part, get away with smaller port diameters than you otherwise would have designed for, again the tradeoff being that you ain't getting flat frequency response way down low (and if you try using DSP, your vent velocity will scream upward as expected).

I'm considering such an alignment for my upcoming 21" designs. Attached is a dual 21" sealed vs. a little bigger with an 11hz tune, using a 6" diameter port for two 21" drivers, capping power at about 2kW. Not suggesting this, just showing what I mean. With 2kW, vent velocity is under 8m/s at 20hz, 20m/s at 12hz, and peaks at 33m/sec at 7.5hz. Far from idea, this is a workable design in the real world, particularly with damn near any high pass filter in the system.


Chris
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post #9 of 32 Old 02-25-2020, 08:20 PM
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Originally Posted by Remy.Alexander View Post
Yes sir, 6.5 is a manageable size that won't eat up family room.


Well, yes. I’m just asking if that accounts for driver, port, and bracing displacement.

Chris
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post #10 of 32 Old 02-25-2020, 08:28 PM
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Originally Posted by Chris Popovich View Post
Generally when you tune that low in a smaller box, your port velocity will remain reasonable as a consequence of the response being well rolled off by that point. Think of the response as similar to sealed but propping up the very bottom end a couple dB's. I think it's a solid avenue, just don't expect the headroom of the big ported boxes or the small size of sealed alignments. It really is in between. One advantage is you can, for the most part, get away with smaller port diameters than you otherwise would have designed for, again the tradeoff being that you ain't getting flat frequency response way down low (and if you try using DSP, your vent velocity will scream upward as expected).



I'm considering such an alignment for my upcoming 21" designs.



Chris

Seems to depend on driver, normalized motor force seems to be the best indicator, as well as cone displacement.

For my 18ds115’s, with an absolutely ridiculous motor force of 370, you can’t get port velocity under control at lower tunings.

My HST’s have a much lower motor force, but the massive power handling and displacement bite em hard. I’ve got stupid port velocity and chuffing in my 10cu, 11hz boxes.
Take the boxes up to 25cu, upsize the port to get back to 11hz tune... and the port velocity is still about the same. The output at port tune is just way high.
The only way I’ve found to control the port velocity in models is to use the large box, and eq down the response to flat at port tuning.

Chris
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post #11 of 32 Old 02-25-2020, 08:30 PM - Thread Starter
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Originally Posted by a77cj7 View Post
Well, yes. I’m just asking if that accounts for driver, port, and bracing displacement.

Chris
I mean I can add a little bit of I have to. I was assuming that much wasn't much to worry about
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post #12 of 32 Old 02-25-2020, 08:31 PM
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I’m not close enough to my computer to sim this but that seems like a bad idea. The port is massive for the size of the box and tune. It may act more like a sealed and not a low tuned box.

Also a LLT requires an overly large box, this is more like a SLT


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post #13 of 32 Old 02-25-2020, 08:39 PM
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Again, depends on how you stack the deck. Box volume is always going to work in your favor for including larger diameter ports (we can fit 'em) and the woofers like to dig a little lower in the bigger box. If you're planning on rolling off the extreme low end you can downsize the port a bit and like I wrote above, sort of split the difference between traditional sealed and ported boxes. Dan Wiggins wrote about this a couple decades ago. The idea was to keep group delay super low in the audible range, which is something not commented on here anymore. Anyway, you trade the extreme bottom end for a little boost in the "still low" range (aka 16-25hz) compared to sealed. As such you can run a smaller diameter port and not get chuffing issues for the simple fact that it's not doing as much as an EBS, or LLT.

All this goes out the window if/when you use DSP to flatten the response and think you tuned to 11hz, you gonna get 11hz at the listening position. Nope. Ideally, you'd use one of the "in between" designs in a room with pretty solid room gain that you've already determined is a thing, and high pass pretty close to the port tuning one way or the other.

I don't care much about maximum acoustic output for this stuff, it's nice to see but Ricci already covers that with his (excellent) testing; this is real world implementation. In my case, the largest amp I use is a crown xls2502. Bridged, it'll do in the vicinity of 2kW. In a 14 cuft box with dual 21" drivers, if you tune it to 11hz, you simply aren't going to have issues. If you want a 10hz monster, that's the wrong alignment. I think we're probably saying the same thing coming from different directions.
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Originally Posted by Trimlock View Post
I’m not close enough to my computer to sim this but that seems like a bad idea. The port is massive for the size of the box and tune. It may act more like a sealed and not a low tuned box.

Also a LLT requires an overly large box, this is more like a SLT


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Not ideal situation, I know. But either make 6.5 work or I stick with sealed. Although I love sealed, I want the efficiency of ported cabs- I did a LLT with these and they were insane. I later kinda made them MLTL(ish) but anyway they were nuts. I just don't want to do that in this home. Room needs to stay usable for all.
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Originally Posted by Trimlock View Post
I’m not close enough to my computer to sim this but that seems like a bad idea. The port is massive for the size of the box and tune. It may act more like a sealed and not a low tuned box.

Also a LLT requires an overly large box, this is more like a SLT


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Pretty much exactly. I'm just pointing out some people LIKE that "acting more like a sealed box". I've built them before and liked it, whatever that's worth. IMO if you were looking for LLT, it ain't the way to go.
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post #16 of 32 Old 02-25-2020, 09:01 PM
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Originally Posted by Chris Popovich View Post
Pretty much exactly. I'm just pointing out some people LIKE that "acting more like a sealed box". I've built them before and liked it, whatever that's worth. IMO if you were looking for LLT, it ain't the way to go.


I will add that the similar response shape doesn’t equate to “acting like sealed”. The ported design still has the excursion minima and impedance spike at tune, so instead of being amp limited on the low end, is actually much more efficient.

I disagree strongly on using an undersize port to shape response. Port compression and noise aren’t what I would consider to bd desirable outcomes under any situation. Now, if you use EQ to tame the low end and size the port appropriately for this, It could work.

Btw, my undersized HST’s do have a positive hump in the response at port tune even with the undersized box/port. Velocity is off the rails, and chuffing is present, of course. The effect of a strong motor.

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post #17 of 32 Old 02-25-2020, 09:03 PM
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Originally Posted by Remy.Alexander View Post
Not ideal situation, I know. But either make 6.5 work or I stick with sealed. Although I love sealed, I want the efficiency of ported cabs- I did a LLT with these and they were insane. I later kinda made them MLTL(ish) but anyway they were nuts. I just don't want to do that in this home. Room needs to stay usable for all.


Sorry, we took this thread far off track.

Anyway, I think what you’re after will work fine, as long as cone displacement and port velocity look good. You won’t get the massive low end output of a LLT, but you won’t get the massive box either.

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post #18 of 32 Old 02-25-2020, 09:09 PM
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Ok, I thought the port size looked off...

I just ran the sim. I dropped the box to 6cf to account for driver and bracing displacement.

Tune comes out at 30hz.

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Sorry, we took this thread far off track.

Anyway, I think what you’re after will work fine, as long as cone displacement and port velocity look good. You won’t get the massive low end output of a LLT, but you won’t get the massive box either.

Chris
Hey no worries, I love the back and forth. Good to get a bunch of minds working together.
We get more accomplished when more people chime in(usually lol)

I modeled it the best I could in WinISD, I think I spent 3 hours trying to perfect it. Figured some fresh eyes would be better.

Wife's mad I have to crash- I'll catch ya guys in the AM.
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post #20 of 32 Old 02-25-2020, 09:13 PM
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Originally Posted by Remy.Alexander View Post
Hey no worries, I love the back and forth. Good to get a bunch of minds working together.

We get more accomplished when more people chime in(usually lol)



I modeled it the best I could in WinISD, I think I spent 3 hours trying to perfect it. Figured some fresh eyes would be better.



Wife's mad I have to crash- I'll catch ya guys in the AM.


See last post, something is very off in your port tune.

Give me the external dimensions you’re after and I’ll see if I can make something work.

Chris
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post #21 of 32 Old 02-25-2020, 09:20 PM - Thread Starter
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See last post, something is very off in your port tune.

Give me the external dimensions you’re after and I’ll see if I can make something work.

Chris

Think it was 20D 20W 35H
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post #22 of 32 Old 02-25-2020, 09:24 PM
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Pretty much exactly. I'm just pointing out some people LIKE that "acting more like a sealed box". I've built them before and liked it, whatever that's worth. IMO if you were looking for LLT, it ain't the way to go.


I agree, if you aren’t sacrificing too much space for the port it would work well.

The major issue I see with this is putting in more work for little effect down low. My 10cu/f, 17hz tuned box still puts out over 100db at 13hz but I have little room gain so the effect is very marginal.

Why put forth the effort for very little? Tune a bit higher and take more advantage of the higher tunes IMO. I think the OP would be happy with higher SPL at the sacrifice of ULF.

I’m not trying to dissuade him though, just a consideration.


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post #23 of 32 Old 02-25-2020, 09:28 PM
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Think it was 20D 20W 35H


Assuming 3/4 material and single baffle, your port will take up closer to 1cu/f, the driver and bracing will probably take up .5. That leaves you with 5.3 internal.


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Open to designs guys I just want the best I can do with size constraints. I'll check back in AM. It's 12:30 and I've been up since 5am lol if backing off tune a bit will yeaild better results I'm all ears. I'm just a movie junkie so that sweet down low is like porn for me.
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Originally Posted by a77cj7 View Post
I will add that the similar response shape doesn’t equate to “acting like sealed”. The ported design still has the excursion minima and impedance spike at tune, so instead of being amp limited on the low end, is actually much more efficient.

I disagree strongly on using an undersize port to shape response. Port compression and noise aren’t what I would consider to bd desirable outcomes under any situation. Now, if you use EQ to tame the low end and size the port appropriately for this, It could work.

Btw, my undersized HST’s do have a positive hump in the response at port tune even with the undersized box/port. Velocity is off the rails, and chuffing is present, of course. The effect of a strong motor.

Chris
One of the major attributes to a sealed box is the low group delay in the audible pass band. A very low tuned sub tends to have very low group delay in the audible ranges -hence "similar to". Obviously there are significant differences, the gain in efficiency (and reduced distortion due to less excursion) among them.

Port compression is a function of how much you're pushing through the port. For example if you lived in a fictional world where 10 watts was the maximum output of any amplifier, even your strong motor design would be fine with a simple 4" port no matter what you tuned it to.

It is an oversimplification to size a port diameter based on Vd capability of the driver(s), although that'll definitely get you in the safe zone.

I, too, am very much against port compression. I also am against using an undersize port to shape response -that would be a terrible idea. The box does most of the shaping of the curve, the port either flows enough to be compression/chuffing free enough or it doesn't at that particular tune/power level. It stands to reason, however, that if the response is rolled off, the port isn't going to see the same requirements for air flow capability before compression/chuffing. It's all about what you want to do. If you look closely there are many commercial designs that use tenets of this fact; PSA and JTR among them.

Below is four graphs showing vent velocity freq response and group delay for a dual 21" subwoofer. The blue line is a 15" diameter port tuned to 30hz. The pink line is a 6" port tuned to 8hz. The grey line is sealed. The red line is the same 6" port tuned to 8hz with a 10hz high pass. All are being fed 2000 watts. The smaller diameter port with high pass actually has less velocity, and less compression in the audible pass band at maximum power (2000 watts) although both are certainly good. It'll exhibit LESS compression, and LESS chuffing. This is expected; the 8hz tune is going to give you a frequency response shape that looks very very similar to a sealed box; i.e. the port ain't doing much in this alignment. As it isn't doing much, you don't need it to be as large. This is what happens with a medium box and a very low tune. Note: you can manipulate this, and make it all untrue (or control), with DSP.

This is just basic acoustics/physics. The real question is if there are any good real world applications for this, and depending on the application, yes, the caveat being that it is, absolutely a design compromise. They all are though, which is why we have so many different alignments to choose from.
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post #26 of 32 Old 02-25-2020, 09:49 PM
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Medium Low Tuned Thoughts

Quote:
Originally Posted by Remy.Alexander View Post
Open to designs guys I just want the best I can do with size constraints. I'll check back in AM. It's 12:30 and I've been up since 5am lol if backing off tune a bit will yeaild better results I'm all ears. I'm just a movie junkie so that sweet down low is like porn for me.


Ok, I just ran one real quick using the “modify my marty” spreadsheet. This assumes two port rails.
Box 20x20x36.
Port 1in high, 33in long.
Net volume 5.6cf
500w input power.
Port tune 14.8hz.
17hz, 2nd order HPF
Cone excursion: 17mm
Port velocity: 22m/s





Bump the port to 1.5” high.
Net volume 5.4cf
Port tune: 18hz
18hz, 2nd order HPF
Cone excursion: 16mm
Port velocity: 18.5m/s


I like the 18hz tune better.
Let me know if you want it further modified, or the cutlist posted.

Chris
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Originally Posted by Trimlock View Post
I agree, if you aren’t sacrificing too much space for the port it would work well.

The major issue I see with this is putting in more work for little effect down low. My 10cu/f, 17hz tuned box still puts out over 100db at 13hz but I have little room gain so the effect is very marginal.

Why put forth the effort for very little? Tune a bit higher and take more advantage of the higher tunes IMO. I think the OP would be happy with higher SPL at the sacrifice of ULF.

I’m not trying to dissuade him though, just a consideration.


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Yup, might be a bunch of extra box volume and hassle for not much real world result.
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post #28 of 32 Old 02-25-2020, 10:04 PM
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Quote:
Originally Posted by Chris Popovich View Post
One of the major attributes to a sealed box is the low group delay in the audible pass band. A very low tuned sub tends to have very low group delay in the audible ranges -hence "similar to". Obviously there are significant differences, the gain in efficiency (and reduced distortion due to less excursion) among them.



Port compression is a function of how much you're pushing through the port. For example if you lived in a fictional world where 10 watts was the maximum output of any amplifier, even your strong motor design would be fine with a simple 4" port no matter what you tuned it to.



It is an oversimplification to size a port diameter based on Vd capability of the driver(s), although that'll definitely get you in the safe zone.



I, too, am very much against port compression. I also am against using an undersize port to shape response -that would be a terrible idea. The box does most of the shaping of the curve, the port either flows enough to be compression/chuffing free enough or it doesn't at that particular tune/power level. It stands to reason, however, that if the response is rolled off, the port isn't going to see the same requirements for air flow capability before compression/chuffing. It's all about what you want to do. If you look closely there are many commercial designs that use tenets of this fact; PSA and JTR among them.



Below is four graphs showing vent velocity freq response and group delay for a dual 21" subwoofer. The blue line is a 15" diameter port tuned to 30hz. The pink line is a 6" port tuned to 8hz. The grey line is sealed. The red line is the same 6" port tuned to 8hz with a 10hz high pass. All are being fed 2000 watts. The smaller diameter port with high pass actually has less velocity, and less compression in the audible pass band at maximum power (2000 watts) although both are certainly good. It'll exhibit LESS compression, and LESS chuffing. This is expected; the 8hz tune is going to give you a frequency response shape that looks very very similar to a sealed box; i.e. the port ain't doing much in this alignment. As it isn't doing much, you don't need it to be as large. This is what happens with a medium box and a very low tune. Note: you can manipulate this, and make it all untrue (or control), with DSP.



This is just basic acoustics/physics. The real question is if there are any good real world applications for this, and depending on the application, yes, the caveat being that it is, absolutely a design compromise. They all are though, which is why we have so many different alignments to choose from.


Yes, I was pointing out that simply using the port to control response without dsp is a horrible idea.

You can make anything work on low enough power. If you’re going to limit the power and excursion far below driver limits, there’s no point to the larger driver though. Just use one of your extraneous JBL’s instead, much cheaper for similar results.

Pretty sure we’re both arguing for the same points here.

Using the box volume to control response works decent if you can still fit an appropriately-sized port.

Real-world application: hell yes. Understanding how these relationships control performance is completely necessary for box design. While these extreme cases are generally useless, they serve well to demonstrate the effects.

Then there’s my HST’s on 2500w input. After I was annoyed by chuffing with my 10.5cu, 11hz enclosure, I decided to see just how large the box would need to be to get port velocity under control. I gave up at 30cf, with velocity almost as high as at 10.5cf. Response shape was also completely useless, with a huge hump at port tune.
I believe that dsp is REQUIRED to make a low tune work with this driver on 2500w.
In my case, I can’t fit boxes that large anyways, so I’ll live with the chuffing from mine.

Chris
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post #29 of 32 Old 02-25-2020, 11:01 PM
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Somebody: Should I add more subwoofers to my system?
BTH: Yes!

Horned is too big.
High-order is just silly.
Sealed is too weak.
IB is a no-go.
So that really only leaves you with ported or PR.

Ported is like the Goldilocks of subwoofers...
it's not too hot and not too cold. But who ate my Spaghet?
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post #30 of 32 Old 02-26-2020, 04:27 AM - Thread Starter
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Quote:
Originally Posted by a77cj7 View Post
Ok, I just ran one real quick using the “modify my marty” spreadsheet. This assumes two port rails.
Box 20x20x36.
Port 1in high, 33in long.
Net volume 5.6cf
500w input power.
Port tune 14.8hz.
17hz, 2nd order HPF
Cone excursion: 17mm
Port velocity: 22m/s





Bump the port to 1.5” high.
Net volume 5.4cf
Port tune: 18hz
18hz, 2nd order HPF
Cone excursion: 16mm
Port velocity: 18.5m/s


I like the 18hz tune better.
Let me know if you want it further modified, or the cutlist posted.

Chris

Morning-

Awesome stuff there. I like the 18hz tune also. And seeing as I'll be building (4) of them it should be beastly in room. I'd love a cutlist/build list if it's not to much trouble.


This looks very promising, and I appreciate everyone's input. It's a hard task, I know. I promised my wife I wouldn't go ape sh** with cabinets again. Next house no exceptions, must have a dedicated room lol.
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