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post #1 of 28 Old 02-25-2018, 08:36 AM - Thread Starter
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Help me reverse engineer port tuning of box

A buddy of mine stumbled across 2 DIY subs on CL. He's trying to figure out if they are going to be better than the single SVS PC-Ultra cylinder he has now.

The guy doesnt remember exactly what the port tune is and that will make a large difference in if these are better or not.

The sub being used are 2 RSS390HF-4 (one per box), and I have the external cabinet dimensions from the seller:

External dimensions for each box is 25.5"L x 19.5"W x 26"H, I think I used half inch MDF, doubled up the front baffle and countersunk the driver

I also have the port diameter (3") and length (20.5" deep flared on both ends)

The builder thinks that each is tuned to 30-32hz, but when I put 6.5cf (approximate internal volume), a 30hz tune, and 2 x 3" ports, I DO NOT get 20" of length in WinISD. If I adjust it to around a 20hz tune, then it seems to be right....

Can someone double check me as if the sub is tuned to 30hz that makes a difference as this is a HT only system.

I know 2 15" ported subs should be much better than a single 12" ported, but if the 15's are ported high, they might not reach down low enough for HT use. So I'm trying to figure it out.

The other thing is that the SVS is powered, these are not. I realize a iNuke 1000 or 3000 DSP is an easy fix, but depending on how much power the subs can take (which is a product of port tune and length to keep both excursion and port velocity low) helps decide how much EXTRA he needs to spend besides just purchasing the subs.

So if someone could help me reverse engineer what the port tune is, or verify if it is INDEED ~30hz that would be much appreciated.

PS, my friend is NOT looking to DIY himself, but doesnt have a problem buying DIY subs, assuming they pass WAF

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post #2 of 28 Old 02-25-2018, 09:31 AM
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Making a few assumptions

* rough guess on driver displacement of 0.08ft^3
* No bracing
* Puts net volume before ports at 6.34ft^3

With 2x3" ports at 20.5in, I get a little over 16hz tuning. Swing and a miss on 32hz.

Who builds a sub enclosure out of 1/2"?
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post #3 of 28 Old 02-25-2018, 09:33 AM - Thread Starter
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Quote:
Originally Posted by ZX3ST View Post
Making a few assumptions

* rough guess on driver displacement of 0.08ft^3
* No bracing
* Puts net volume before ports at 6.34ft^3

With 2x3" ports at 20.5in, I get a little over 16hz tuning. Swing and a miss on 32hz.
He mentioned he built them using the recommended specs at the time, however recommeneded for ported is 8cf @ 16hz tune....... So the tune makes sense, just not the size.

I was hoping they were tuned lower than 30 as if they were anywhere in the low 20's the pair should be better than his current SVS sub, depending on how much wattage each one takes

Now to just take a gander at port velocity, max wattage and cone excursion lol......

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Originally Posted by ZX3ST View Post
Who builds a sub enclosure out of 1/2"?
Not sure, I think everyone normally uses 3/4 which would make the enclosure more like 6cf internal

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post #4 of 28 Old 02-25-2018, 11:09 AM
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Can you guys go over and check the subs out in person? You should be able to tell if he used 1/2" or 3/4" MDF, and if you can play test tones and check cone excursion you should be able to find the tune out that way pretty accurately. How much are they going to cost?

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post #5 of 28 Old 02-25-2018, 11:27 AM - Thread Starter
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Quote:
Originally Posted by aron7awol View Post
Can you guys go over and check the subs out in person? You should be able to tell if he used 1/2" or 3/4" MDF, and if you can play test tones and check cone excursion you should be able to find the tune out that way pretty accurately. How much are they going to cost?
He can, I cant..... I'm 7 hours one way from him lol, just the resident sub expert between the two of us.

I've kinda told him he really needs to go over there and see/hear them, or see if the guy will bring em over to his room to play with for a full demo.

I wont tell you the cost, but its LESS than a cost for both drivers so its a steal of a deal since the enclsoures are thrown in for free. He also has a unbalanced MiniDSP and 2 Crown Com Tech 410's he is willing to add in (for a undisclosed price).

Good call on the test tones.

Just play some 20 and 30hz tunes through and see how far the driver moves pretty much right? Only issue is since seller doesnt remember what the MiniDSP is set to he could have set the highpass higher than the port tune and we'd never know..... but thats probably unlikely...

I've told him a iNuke 3k DSP would also solve the problem, with the exception that we would just set the highpas at 20hz and be done (he's not the best with computer stuff so even though I know you can drop it below that with the trick here, its easier to just set it at 20hz).

We would need to set the voltage limiter I think to keep the subs from getting too much wattage, but otherwise the iNuke is the easier option for sure.

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post #6 of 28 Old 02-25-2018, 11:43 AM
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Unhook the Mini.
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post #7 of 28 Old 02-25-2018, 12:57 PM
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I get the about 16Hz tune as well. You did say 2 ports not 1, correct?
Modeled to xmax using 400W average power (to one driver) and a 16Hz high pass:


Excursion:


Velocity:


Box size is a bit small for the tune but I've seen worse... it'll do the job.

The easiest and most accurate way of finding port tune is by taking a close mic measurement of the port.

Another option is plugging the port. You gain the volume of the port and with a little EQ, you have a full bandwidth sealed sub.
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post #8 of 28 Old 02-25-2018, 01:29 PM - Thread Starter
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Yes two ports, each 20.5” long.

I’ve told him to go check em out and gave him instructions beat I could.

Port velocity isn’t bad but but high down low. That said, if we highpass at 20 it drops quite a bit.


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post #9 of 28 Old 02-25-2018, 05:22 PM
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Sounds like the seller is playing dumb. How do you not remember setting up a minidsp? Or the port tune? We wring our hands so much in these things as diy guys. And then to get it way off and guess 32hz? Smells fishy...
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post #10 of 28 Old 02-25-2018, 06:19 PM - Thread Starter
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Help me reverse engineer port tuning of box

I dunno. They haven’t been in use apparently and he built them 5 years ago.

So I dunno. He seems on the up and up personally. My buddy will feel him out when/if he meets him.

He’s got a lot of stuff around he’s been building (like custom wood diffusers) so maybe he just blanked.

He did tell me he built them with WinISD using the recommended specs...

But again I don’t know. If nothing else my buddy is out a bit of time if they don’t pan out.

I also try to remember that lots of guys aren’t as fanatical as us with specs/build threads/documentation.

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post #11 of 28 Old 02-25-2018, 07:04 PM
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I second the test tone method, sub movement/excursion should be at its minimum at fb. You can do it with a mic or SPL meter at close range as was mentioned, or just watch it closely if you have no measurement gear. Got a link to the CL add? Maybe we can gleam something from the ad/pics. You can also do an impedance sweep, but that's a little more complicated, but the most accurate. Sounds like most people agree, the 32 Hz tune sounds incorrect. 1/2" MDF? Weird. Nobody uses half inch MDF especially not on a set of 15s. I'd pass right there if that's the case, forgot what the box is tuned to, you can change the length of the ports if needed. But if that box is really made a 1/2 MDF, it's a non-starter.
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post #12 of 28 Old 02-26-2018, 05:35 AM - Thread Starter
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Can I ask why 1/2 is a non starter? It’s a ported sub so the extra rigidity shouldn’t create an issue I wouldn’t think...


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post #13 of 28 Old 02-26-2018, 06:53 AM
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As long as it's braced good, I wouldn't kick it to the curb.
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post #14 of 28 Old 02-26-2018, 06:55 AM
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Quote:
Originally Posted by EndersShadow View Post
Can I ask why 1/2 is a non starter? It’s a ported sub so the extra rigidity shouldn’t create an issue I wouldn’t think...
With my SI HT18 in a downfiring stonehenge cabinet sitting on a concrete floor, it'll just about rattle your teeth out if you sit on it during bass heavy songs. That's because the entire front and back (er...bottom and top) panel of the cabinet are flexing significantly. 1/2" would be much worse.
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post #15 of 28 Old 02-26-2018, 07:00 AM
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With a down-firing sub, you're dealing with the motion of the sub going back and forth. It wouldn't be so bad if it was front-firing.

Depending on the weight of your box, it wouldn't surprise me to know that your box comes completely off the ground during full excursion.
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post #16 of 28 Old 02-26-2018, 07:15 AM - Thread Starter
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Help me reverse engineer port tuning of box

Quote:
Originally Posted by lknhomeaudio View Post
As long as it's braced good, I wouldn't kick it to the curb.
That was kinda my thought, but I will see what the concensus is. Its front firing. I will see if he has any pictures of the bracing or any build pics, but I'm not holding my breath lol.

I have included the pictures from the listing.



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post #17 of 28 Old 02-26-2018, 10:39 AM
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Quote:
Originally Posted by lknhomeaudio View Post
With a down-firing sub, you're dealing with the motion of the sub going back and forth. It wouldn't be so bad if it was front-firing.
The driver is exerting force on the baffle (and through the bracing, to the opposite wall) whether the sub is down firing or not. It's just kinda hard to sit on the back wall of your sub to feel the vibrations.
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post #18 of 28 Old 02-26-2018, 10:57 AM
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I'm talking about the force applied to the enclosure from the sub changing directions abruptly, not inner sound waves.

The huge benefit of doing "Dual Opposed" enclosures is that the 2 subs cancel the typical enclosure movement. Makes it virtually motionless!

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post #19 of 28 Old 02-26-2018, 01:19 PM
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Quote:
Originally Posted by lknhomeaudio View Post
I'm talking about the force applied to the enclosure from the sub changing directions abruptly, not inner sound waves.
As am I.

Quote:
Originally Posted by lknhomeaudio View Post
The huge benefit of doing "Dual Opposed" enclosures is that the 2 subs cancel the typical enclosure movement. Makes it virtually motionless!
This is true, but not what we were talking about.
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post #20 of 28 Old 02-26-2018, 01:31 PM
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No you're not lol... You're talking about the box pulsating from the lack of bracing. Two totally separate things. You even state it in your post. :/
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post #21 of 28 Old 02-26-2018, 02:59 PM
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No you're not lol... You're talking about the box pulsating from the lack of bracing. Two totally separate things. You even state it in your post. :/
Have you seen the stonehenge? It's braced to high heaven. I was talking about the movement of the driver flexing the baffle and, through the bracing, the back wall. This would happen whether it's downfiring or front firing.

If it was built from 1/2 inch MDF it would only be worse.
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post #22 of 28 Old 02-26-2018, 03:33 PM - Thread Starter
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Quote:
Originally Posted by dorri732 View Post
Have you seen the stonehenge? It's braced to high heaven. I was talking about the movement of the driver flexing the baffle and, through the bracing, the back wall. This would happen whether it's downfiring or front firing.



If it was built from 1/2 inch MDF it would only be worse.


However I’m guessing your sending more than 400 Watts to your sub, which is also 3” larger than these.....


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post #23 of 28 Old 02-26-2018, 07:41 PM
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If the sub is braced well enough, then sure 1/2" could be fine. But we're not talking about throwing a 2x4 across the middle of the side panels and calling it good. The bracing needs to be an integral part of the design to get away with 1/2" and not just as an afterthought. But after seeing the pics they look better than I would have thought. I bet the enclosure is actually made from 3/4" MDF. Take a look at the difference in height between the surface of the front baffle and the driver frame. It's at least maybe 1/4 inch? PE specs the thickness of the basket frame at 10.5 mm (or 0.413") which means if it were 1/2" MDF you'd really only have less than 3/32" height difference. Looks like more than that to me. Could be an illusion of the lighting though. So that's promising. Still have to deal with the tuning though. Still have no idea what's inside the box either. But if your friend is willing to do some basic measurements and possible modifications, might be worth it. You won't tell us what the guy is selling them for, so it's hard to give much more directed advice. If it's a good deal just to get the drivers then maybe take a shot. I'm sure everyone on the forum will be happy to help your friend "fix" them up as needed.
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post #24 of 28 Old 02-26-2018, 07:47 PM - Thread Starter
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Quote:
Originally Posted by aktiondan View Post
If the sub is braced well enough, then sure 1/2" could be fine. But we're not talking about throwing a 2x4 across the middle of the side panels and calling it good. The bracing needs to be an integral part of the design to get away with 1/2" and not just as an afterthought. But after seeing the pics they look better than I would have thought. I bet the enclosure is actually made from 3/4" MDF. Take a look at the difference in height between the surface of the front baffle and the driver frame. It's at least maybe 1/4 inch? PE specs the thickness of the basket frame at 10.5 mm (or 0.413") which means if it were 1/2" MDF you'd really only have less than 3/32" height difference. Looks like more than that to me. Could be an illusion of the lighting though. So that's promising. Still have to deal with the tuning though. Still have no idea what's inside the box either. But if your friend is willing to do some basic measurements and possible modifications, might be worth it. You won't tell us what the guy is selling them for, so it's hard to give much more directed advice. If it's a good deal just to get the drivers then maybe take a shot. I'm sure everyone on the forum will be happy to help your friend "fix" them up as needed.
Ok, arm twist not needed lol.

Guy's selling the pair including enclosures for 250, which is a screaming deal IMHO. 25% less than buying the 2 drivers alone, and they come with enclosures that look decent.

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post #25 of 28 Old 02-26-2018, 08:27 PM
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Quote:
Originally Posted by EndersShadow View Post
Guy's selling the pair including enclosures for 250, which is a screaming deal IMHO.
I agree. These aren't high excursion drivers and the resonant frequency of the box will be higher than the sub band being used. As long as sufficient damping was used there won't be a problem. One could argue that you'll lose .5dB from cab vibrations being out of phase with the driver but it won't be perfectly 180 out of phase on all panels so either way there will be no audible loss of output. And if he breaks the box up from playing them loud that is awesome and will make for a great thread!
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post #26 of 28 Old 02-26-2018, 08:31 PM
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I didn't mean to twist your arm! I respected that didn't want to say the price, but now that we know, yeah seems like a very good deal. Maybe haggle another $20 and go for it. I ran some sims assuming the dimensions you gave and the 3/4" MDF and the tune is pretty low for that size enclosure like Shreds mentioned, but by no means is it out of the ballpark. I came up with a rough estimate of 155 liters, that can vary 1-2 liters based on port length. If he wants to chop off about 6" dropping the length to 14.5" you can raise the tune to an even 20 Hz. It models a little better, lesser chance of exceeding xmax and increases the mid-bass area a dB or so. You can see that a 30 Hz tune would be garbage, but it takes a 5" port to get there. The sealed option is just shown for reference. They do look a lot better than most of the DIY subs I see on CL.
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post #27 of 28 Old 02-26-2018, 08:54 PM - Thread Starter
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I meant it in jest. No hard feelings at all lol.


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post #28 of 28 Old 02-27-2018, 05:30 AM
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Definitely pick it up, for that price! As long as they are in good working order...
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