Need help with choosing port size for Usher 8948A MLTL (folded) - diyAudio
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Old 13th September 2009, 02:41 AM   #1
sardonx is offline sardonx  Canada
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Default Need help with choosing port size for Usher 8948A MLTL (folded)

OK I don't have the software to use MJK's worksheets and I'm not sure If they'd be useful ATM since the box is mainly built already. So perhaps someone can help me.. The woofer is the Usher 8948A, and it's going in an enclosure 35.25"Hx8"Wx11"D and here is a rough sketch of the idea:
http://techtalk.parts-express.com/at...1&d=1242398034

The outer shell of the box is already built minus the rear plate, as internals are yet to go in. My questions is what port size should I use?
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Old 13th September 2009, 02:44 AM   #2
sardonx is offline sardonx  Canada
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8948A specs

*VCdia: 1-5/8" *Le: .39 mH *Impedance: 8 ohms *Re: 5.8 ohms *Frequency range: 45-5,000 Hz *Fs: 45 Hz *SPL: 87 dB 2.83V/1m *Vas: .69 cu. ft. *Qms: 2.43 *Qes: .45 *Qts: ..38 *Xmax: 6.0mm
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Old 13th September 2009, 05:36 PM   #3
pkitt is offline pkitt  United States
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Are the cabinet dimensions you list internal or external, and if they're external, what are the internal dimensions? What's the distance from the Usher's center to the (inside) top and is that distance required or can it be different if necessary? From the sketch you linked, it appears you're assuming or intending the line to be folded. Is that correct? If so, its length may or may not be appropriate for an optimum response from the Usher. Were you hoping for a specific F3 range? I'd be happy to model an ML-TL with Martin's worksheets for you if you want.
Paul

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Originally Posted by sardonx View Post
OK I don't have the software to use MJK's worksheets and I'm not sure If they'd be useful ATM since the box is mainly built already. So perhaps someone can help me.. The woofer is the Usher 8948A, and it's going in an enclosure 35.25"Hx8"Wx11"D and here is a rough sketch of the idea:
http://techtalk.parts-express.com/at...1&d=1242398034

The outer shell of the box is already built minus the rear plate, as internals are yet to go in. My questions is what port size should I use?
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Old 13th September 2009, 07:56 PM   #4
sardonx is offline sardonx  Canada
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Hi Paul! I appreciate the help The dimensions I gave are the internal dimensions. The volume is about 50 litres. The distance from the top inside to the center of the Usher is 10.125 inches. I have already cut out the tweeter hole but not the woofer hole. So I can still move the woofer down some distance if needed but not up. I would prefer to keep it where it is though as I'm following a plan for the crossover and baffle size. If moving the woofer down wont affect mids/highs I'd be happy to do it if it means better bass.
And yes, you're correct the line is going to be folded. I approximated the line at around 1/4 wavelength if my math was correct. So if it needs to be modified hopefully it will need to be shorter not longer, as that's feasable. A room response into the 30's would be nice. I realize the driver's Fs is only 45Hz...

Oh and the brace which will make the box into a TL hasn't been installed yet either. I figured I will taper it but you may have a better/more exact solution.
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Old 13th September 2009, 09:41 PM   #5
pkitt is offline pkitt  United States
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In addition to the T/S values you listed, I added an Sd of 150 square cm and calculated a Bl of 8.68. I modeled a folded ML-TL just as you drew it and came up with the system response I show below. The port I ended up with is just a hole in the back wall, 2.5" in diameter; your folded line pretty much dictated that. As you can see there's a really big dip in the response between 100 and 200 Hz. Most ML-TLs have a dip in this frequency range but it's usually much, much smaller in magnitude, and sometimes almost not there at all. OTOH, F3 is pretty good at ~35 Hz. I think your folded line at ~70" long is too long, and a more appropriate length for either an ML-TL or a 10:1 tapered TL would be more like 40-45". BTW, if I understand you correctly, the long, vertical divider you were planning to insert to create the folded line you were going to angle it, sort of create a tapered ML-TL? If you did, the line's effective length would be even longer than 70" making it even more too long. I did model an ML-TL in your box but without the divider, making the line's length be just the height of the box; with your existing width and depth, the box had too much volume, causing a big peak at the knee of the curve. If the box depth is reduced to 8" or 7", keeping the width at 8", a much better looking response curve is obtainable. Of course you could still make a folded but shorter line in your existing box, ending up with an area at the top, back that would be unused. I'll play around with that and see what I can come up with.
Paul

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Originally Posted by sardonx View Post
Hi Paul! I appreciate the help The dimensions I gave are the internal dimensions. The volume is about 50 litres. The distance from the top inside to the center of the Usher is 10.125 inches. I have already cut out the tweeter hole but not the woofer hole. So I can still move the woofer down some distance if needed but not up. I would prefer to keep it where it is though as I'm following a plan for the crossover and baffle size. If moving the woofer down wont affect mids/highs I'd be happy to do it if it means better bass.
And yes, you're correct the line is going to be folded. I approximated the line at around 1/4 wavelength if my math was correct. So if it needs to be modified hopefully it will need to be shorter not longer, as that's feasable. A room response into the 30's would be nice. I realize the driver's Fs is only 45Hz...

Oh and the brace which will make the box into a TL hasn't been installed yet either. I figured I will taper it but you may have a better/more exact solution.
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File Type: gif 8948A 1.gif (6.7 KB, 277 views)
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Old 14th September 2009, 12:52 AM   #6
sardonx is offline sardonx  Canada
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Thanks a bunch. So that dip is about 4-4.5 decibels. I suppose that is indeed too much of a dip. Looks awful. I actually didn't plan on making the 'brace' slanted but now that you mention it it makes more sense as far as tapering.
I'm basically open to suggestions at this point. If you do play around some more with the modeling maybe you will come to a compromise. I can do it both ways, i could have an unused volume at the top back which i could use for the crossover network. Or I can still cut 3 inches or so from the back without issues. Let me know what you think is the better solution soundwise...-Mladen
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Old 14th September 2009, 06:44 PM   #7
pkitt is offline pkitt  United States
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Default Option 1

I modeled an ML-TL with your cabinet's depth decreased to 7" and with the Usher located where you plan, 10.125" from the top. Therefore, the internal dimensions are now 8"w x 7"D x 35.25"H. The mass-loading port's center is located 2.25" from the bottom, and the port has a diameter of 2.5" with a length of 4". The port can be mounted on the baffle or the rear panel. I used a stuffing density of 1 lb/cu.ft. and a total of 10 ounces of stuffing is required in the top 19" of the cabinet uniformly distributed by density. I've attached the system response graph which shows an F3 of 36-37 Hz and just a bit of peaking at the knee. All of the wiggles and peaks and dips in this response do not exceed +/-0.5 dB. Now I'll look at some other possibilities.
Paul
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File Type: gif 8948 2.gif (6.6 KB, 257 views)

Last edited by pkitt; 14th September 2009 at 07:00 PM.
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Old 15th September 2009, 01:33 AM   #8
sardonx is offline sardonx  Canada
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Actually that sounds like a good solution. I will have a smaller cabinet than I thought I needed and still reach my target cut off.

Just to clarify, the density of the stuffing in the top 19" of the cabinet is really no different than the bottom portion, correct? From your numbers I calculated about 18.5 ounces total, evenly distributed throughout the whole cabinet. By doing just that i should have about 10 ounces in the top 19"?

I have a couple of side questions also (I searched but there's so much info on MLTLs, mostly plans, that's its' hard to find specific information). Does the type of stuffing matter? And do the walls need to be lined with felt/padding in addition to the stuffing? This is my first MLTL and I might as well do it right The project may be up to a month away from being complete but I will post progress pics asap.
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Old 15th September 2009, 03:32 PM   #9
pkitt is offline pkitt  United States
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No, there is no stuffing in the bottom 16" of the cabinet, only the top 19" and with the amount I specified. Martin's software calculates the total amount of stuffing needed based on the cross-sectional area and length along the line where the stuffing ends up being located (which is based on how the "designer" models the line). Do not line the walls with anything. The stuffing can be either Acousta Stuf or polyester "pillow stuffing" like Polyfil. Acousta Stuf in more expensive but once you fluff and tease it before installation, it will stay in place after installation and not tend to fall due to gravity. If you buy Polyfil, or equivalent, in batting form, you can cut it into appropriate rectangular-sized sheets and when squished together it, too, will stay in place. You'll have to add some bracing, like "window frame" braces, and you can locate one of them to act as a stop for the stuffing 19" from the top.

So far I have not been able to come up with a better design but maybe others, like GM, can do so.

Paul

Quote:
Originally Posted by sardonx View Post
Actually that sounds like a good solution. I will have a smaller cabinet than I thought I needed and still reach my target cut off.

Just to clarify, the density of the stuffing in the top 19" of the cabinet is really no different than the bottom portion, correct? From your numbers I calculated about 18.5 ounces total, evenly distributed throughout the whole cabinet. By doing just that i should have about 10 ounces in the top 19"?

I have a couple of side questions also (I searched but there's so much info on MLTLs, mostly plans, that's its' hard to find specific information). Does the type of stuffing matter? And do the walls need to be lined with felt/padding in addition to the stuffing? This is my first MLTL and I might as well do it right The project may be up to a month away from being complete but I will post progress pics asap.
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Old 18th September 2009, 04:42 AM   #10
sardonx is offline sardonx  Canada
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Ahh okay, I got it. I didn't get an email notification that you had replied... I just saw it now.
I have one last question. The cabinet is made out of half inch baltic birch ply. I would like to add another half inch to the top plate as I understand that the pressure nodes are at the top and bottom of the cabinet. At the bottom I will add thickness from the outside but at the top I have to add a plate inside to not mess with the baffle size. So this will make the center of the Usher 9.625 inches from the inside top and the inside height 34.75 inches.
When you get a chance can you calculate the values for that? If it will make a practical difference that is. I'm some time away from getting the woofers so there is no rush. Here is a progress pic:
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