IMF Monitor Transmission Line Subwoofer

[Johnloudb]

I'm starting work on 2 transmission line subwoofers, and just wanted to share. I'm using 2 old IMF monitor loudspeakers for the boxes. The T line is tuned to around 22 - 25 Hz.

The original driver was a KEF B139 and here are the specs:

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The I've pretty much decided on one of the follow drivers for the sub:

Tang Band 10inch Sub

or

Dayton 10inch HO sub


I'll probably try the Tang Band driver first since it's flat like the KEF :T . Can't beat that logic. It has the same Fs of 25 Hz and Sd of 350 sq. cm. like the KEF and about 3 times the Xmax. It has a lower Qts of .31 (vs. 0.37 for KEF) but a smaller Vas of 1.6 cu.ft. (vs. 4 cu.ft. for KEF).

And I welcome help, although apart from driver suggestions there's not much to help with. I really want to try this Tang Band unless there are strong objections. It's less expensive, has a flat frequency response. I'm looking for a music sub primarily. I can always try something else later.

Here is the IMF Speaker. I've filled in half the opening and I'm just about ready to cut a round hole for some driver.

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[ThomasW]

The TangBand has a major cone mode at 250Hz. IMO that could be very problematic unless you're using an extremely low crossover point...

[dlneubec]

John,

You might want to check out Zaph's review of the Tangband, if have not seen it. It is most of the way down the page.
Zaph's tidbits

[dlneubec]

How about the Dayton RSS265 HF, it is on sale at $115 right now, or the RS270?

[Johnloudb]

Thanks guys!! :T So much for flat woofers.

Maybe, I'll go with the RS270. It matches the KEFs T/S parameters pretty well. I may even make this a 3 way design down the road, depending on how the woofer works out.

I'd like to try the RSS256HF, but given it's low Fs of 22 and high Q it might produce to much output from the T line. But then again it might also be great.

One question. Do transmission lines allow for more output from a given driver or just greater low frequency extension?

[joecarrow]

One question. Do transmission lines allow for more output from a given driver or just greater low frequency extension?

In a word, no.

A transmission line's bass performance can be similar to a ported box, or it can fall somewhere closer to an infinite baffle or aperiodic enclosure. There are definitely some real benefits to transmission lines, but lower extension and increased output aren't what you get.

A transmission line can be a very dead enclosure, since the unsupported spans of cabinet panel can be quite small. What a transmission line is really great for is absorbing the sound from the back of the cone (at least the higher frequency ones) and preventing them from bouncing around in the box and reflecting back into the room through the cone. I think that this is most beneficial in the mid-bass, 100-500 hz, where a lot of acoustic fibers and foams are largely ineffective.

If you look at B&W's Nautilus (the snail shell speaker), you'll see what I'm talking about. They use tapered terminated tubes (not so different from a transmission line) to improve absorption of the internal sound waves.

For pure subwoofer use (60 hz and lower), I don't think that transmission lines have any clear benefits over ported or sealed. The nice thing with the RS270 in a transmission line is that it can play cleanly high enough to transition to a dedicated midrange, and can therefore benefit from the aspects of transmission lines that I mentioned.

[cjd]

RS270 also wants a crazy amount of volume in its TL for best results, tho...

C

[Johnloudb]

In a word, no.

A transmission line's bass performance can be similar to a ported box, or it can fall somewhere closer to an infinite baffle or aperiodic enclosure. There are definitely some real benefits to transmission lines, but lower extension and increased output aren't what you get.

A transmission line can be a very dead enclosure, since the unsupported spans of cabinet panel can be quite small. What a transmission line is really great for is absorbing the sound from the back of the cone (at least the higher frequency ones) and preventing them from bouncing around in the box and reflecting back into the room through the cone. I think that this is most beneficial in the mid-bass, 100-500 hz, where a lot of acoustic fibers and foams are largely ineffective.

If you look at B&W's Nautilus (the snail shell speaker), you'll see what I'm talking about. They use tapered terminated tubes (not so different from a transmission line) to improve absorption of the internal sound waves.

For pure subwoofer use (60 hz and lower), I don't think that transmission lines have any clear benefits over ported or sealed. The nice thing with the RS270 in a transmission line is that it can play cleanly high enough to transition to a dedicated midrange, and can therefore benefit from the aspects of transmission lines that I mentioned.

Well, I asked a dumb question. But, transmission lines are usually tuned lower than typical ported boxes. So, they typically have greater low freq extension for a given driver.

[Johnloudb]

RS270 also wants a crazy amount of volume in its TL for best results, tho...

C

Yeah, it's a big box.

[Johnloudb]

I finally got this driver in and and took some rough measurements with my 12 year old Radioshack dB meter. I've read that the old one is more accurate. The response is pretty flat 100Hz down to about 3dB at 31Hz and there is some output at 25Hz, at -8dB. The response just about mirrors that of the IMF monitors published specs.

This sub sounds very clean. The best I can tell, with my little T-amp diving them. :B I need finish the other sub and I am building a crossover for them. It will be a variable freq 4th order Bessel crossover. And add phase and a couple equalization controls.

Hole cutout for Dayton 10 inch:

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I filled in half the old hole and cut out the round one. The white stuff is just primer and I plan on painting that black when I get some black paint.

So far so good.

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[Johnloudb]

I just noticed that the Qts parameter posted for the RS270S-8 is different on Dayton's spec sheet (Qts=0.41) than that posted on Parts Express website/catalog (Qts=0.35). It's not really a big concern to me since the driver seems to work well. But, who do you trust? I see this a lot. Anyone know the reason for this? Does PE measure drivers not broken in? Or, does it just vary from driver to driver?

[joecarrow]

It's all quality control- all components (drivers, resistors, capacitors, magnetrons, etc) operate with some degree of error from their specified value. The better ones are more tightly controlled, and manufacturer warranties generally cover replacement of parts that are too far out of spec. Qts is a product of Qes and Qms, so you're right that it is affected by mechanical break-in, but it is also affected by other manufacturing variations.

The very best speaker designs demand a measurement of the actual speaker when it arrives, and accounts for real performance in the filter design. In your case, things seem to be working out alright, but the bottom line is that the only one you can really trust when it really matters and you demand accuracy is yourself. I guess you could also pay someone to do the measurement on your parts and have them shipped back to you, but it's the same thing.

[Kal Rubinson]

BTW, there is a superior crossover configuration for this which, also, greatly enhances the advantageous removal of the MF,HF,VHF drivers to a separate box, as IMF did in later versions. The designer was Crabbe and it was published in HiFiNews. A similar crossover, also in HFN, was used by P.Atkinson in his design.

Kal

[Johnloudb]

It's all quality control- all components (drivers, resistors, capacitors, magnetrons, etc) operate with some degree of error from their specified value. The better ones are more tightly controlled, and manufacturer warranties generally cover replacement of parts that are too far out of spec. Qts is a product of Qes and Qms, so you're right that it is affected by mechanical break-in, but it is also affected by other manufacturing variations.

The very best speaker designs demand a measurement of the actual speaker when it arrives, and accounts for real performance in the filter design. In your case, things seem to be working out alright, but the bottom line is that the only one you can really trust when it really matters and you demand accuracy is yourself. I guess you could also pay someone to do the measurement on your parts and have them shipped back to you, but it's the same thing.

I'll probably get that Dayton WT3 woofer tester and some speaker design software in the near future. It'd be nice to know what I'm working with.

[Johnloudb]

BTW, there is a superior crossover configuration for this which, also, greatly enhances the advantageous removal of the MF,HF,VHF drivers to a separate box, as IMF did in later versions. The designer was Crabbe and it was published in HiFiNews. A similar crossover, also in HFN, was used by P.Atkinson in his design.

Kal

I was thinking of making this a full range speaker, but I've decided to make it just a sub. Although, I may still stick a 5" full range driver in where the mid-range is. Maybe a Fostek driver, with high sensitivity and cross it over to the Dayton.

I've got some RD50 and Usher 8" drivers that I need to put to use in a line source speaker. Thomas gave me some ideas with his RD75/Eton speaker design, so I going to something similar. So I need to build a test baffle and start working on that.

[Kal Rubinson]

I was thinking of making this a full range speaker, but I've decided to make it just a sub. Although, I may still stick a 5" full range driver in where the mid-range is. Maybe a Fostek driver, with high sensitivity and cross it over to the Dayton.

I've got some RD50 and Usher 8" drivers that I need to put to use in a line source speaker. Thomas gave me some ideas with his RD75/Eton speaker design, so I going to something similar. So I need to build a test baffle and start working on that.

I guess we do not share the same predilections. I would keep and tweak the IMF, not transform it into something else.

Kal

[Johnloudb]

I guess we do not share the same predilections. I would keep and tweak the IMF, not transform it into something else.

Kal

It's not that I wouldn't like to restore the IMF. The IMF was an excellent speaker. However, according to my dad one speaker broke and ruined his amp. The KEF drivers would also pop, on some dynamic recordings. The midrange was also somewhat colored, in comparison to our Nelson/Reed 804B speakers.

I could test the tweeter and mid drivers, but a new crossover would be in order due to the new Dayton woofers. And like you say, my sonic aspirations lie somewhere else.

[Johnloudb]

Okay,

I'm still messing around with this sub. I decided to try the Dayton 10" sub driver (RSS265HF-4). Only one problem, this is the that the holes are cut for RS270 which needs a 9-3/8" cutout. The sub needs only a 9-1/4" cutout.

I was able to mount one subwoofer without modification but the second hole is a bit larger and a few of the screw holes hit mostly air. Does anyone have any suggestions for fixing the hole?

I was thinking about just filling the edges with some kind of filler. It need to be strong of course. Then I could use some T - nuts which would grab the edge of the baffle.

Any ideas for a good filler or other suggestions are very welcome.

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[Tsloms]

Any ideas for a good filler or other suggestions are very welcome.

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Duct tape?!?

[Johnloudb]

Hey, Thanks for reply!! Really, I didn't think anyone would. I'll do that, thank you! ;x(

[Xander]

I would cut a ring out of MDF (or plywood if you have no MDF lying around). 10" OD, 9 1/4" ID. Glue it to the box. Mount the driver.

It would be a PITA to use some type of filler to mount the t-nuts to. And I don't think anything will be strong enough to work that way.

[Johnloudb]

Yeah, that filler idea probably wouldn't work.

Putting another board another board on the front would make the driver stick out too much and the speaker grill wouldn't fit. Unless you mean fitting a ring of MDF inside an new hole cut in the baffle. Only problem there is I could never cut a hole clean enough in the baffle to make that work.

But, the driver "almost" fits and half the T nuts would hit solid MDF. I'm thinking I'll just cut some wood shims to put where the problem holes are, and use glue and a couple finishing nails to hold them. Then use the T nuts.
Hope that works. If not, I may use a 1/4 inch plywood ring like you suggested.

Thanks for your input.

[vinceb]

maybe the epoxy putty that you knead and build up the edge a bit so that there is a fillet toward the driver so it has a bigger sealing area, and a lump where the screws need to go, and then drill and install your t-nuts with some dabs of epoxy (dont pound them in of course). I envision using a piece of mdf to hold over the edge as a mold of sorts and push the epoxy up against the edge and the mdf from the inside of the cab (your mdf would just cover the part you were working on so you could reach inside) so it's flush without any further work. The putty is very thick so you would want to press it very firmly on the edge to bond it well, it's cool stuff.

*edit - those are cool speakers *

[Johnloudb]

That's certainly something to think about. I may do like a combination of epoxy along the edges and use wood shims where I need the holes. I'll try some epoxy first and maybe just use that.

Thanks

[Johnloudb]

Well the epoxy putty worked great. I hammered in two finishing nails, one on each side of the hole, and let them to stick out slightly to give the epoxy something solid to grip. It's solid stuff and held the screws well, so there was to need for T-nuts.

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With the Dayton RSS 10" sub.

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Still waiting on my crossover to be completed. But a while back I tried some test tones ... very solid bass.

[Johnloudb]

First Off. When I logged into Mission Possible late last night I noticed my IMF Sub thread and three new posts in that were posted "Today."

Then this morning when I logged in to MP my thread was not there and the posts weren't there. :scratchhead:

Anyway, PhilDSP thanks for the input on how transmission lines have a cardioid like response that gives the perceived increase in bass output, or something like that. And, that the woofer excursion is less in low frequencies of a transmission line than for a sealed or ported design.

I can only assume that my browser brought up an old cached version of Mission Possible and that you and the other poster deleted your posts later?

Or, Maybe I was visualizing future posts that you both were about to make, but hadn't yet. Yeah, I got a little ESP going on and occasionally have dreams about future events. Now, I've really screwed things up and created an alternate time line and altered the future forever! Oh well, I want to get these subs and crossover done.

-----------------------------------------------------------------------------------

Well, I've got these subs working, sort of. Trying to sort out the crossover details. I first tried a first order high pass on the mains, and 4th order low pass on the woofers. I didn't get that to mesh well, but the upper frequencies took a hit. Mostly I think it was due to some mid-bass attenuation.

So, I'm thinking of stuffing the port on the mains with fill and adjusting the low pass crossover to 2nd order, possibly. I'll try an LR4 first. I could also build a passive 2 order filter to work with the natural roll off of the sealed cabinets, and have a LR4 high pass slope.

But these to pump out the bass nicely. Flat down to 25Hz in our room. Just have to do some experiments.

I'm using a Radio Shack SPL meter for measurements right now. I can't drag my computer into the listening room, but I may string a 40 Ft. cable in there to hook up a mic. We'll see.

[Johnloudb]

The missing posts I mentioned can be found in this thread:

https://www.htguide.com/forum/showthread.php?p=494638#post494638

Moved by one of the moderators, I guess.

I'm glad to know I'm not insane. :dancenana:

[ThomasW]

Nothing is missing...

8 months after the last post in your 1.5 yr old thread, a new member brought it up dissing your brand of speakers and posting some trivia about old woofers. Then a discussion about how TLs work began.

Basically off topic stuff for your thread, that's why things were moved to the theory section.

Had you simply sent me a PM, I would have explained the actions to you.

[Johnloudb]

Thanks for the explanation Thomas. Perhaps being naive, I wasn't aware you did anything. My computer's Airport sometimes turns off and my browser brings up an older cashed web page instead, so I thought maybe that's what happened. And maybe they had deleted their posts shortly after posting, though that seemed kind of weird to me also.

I was about to PM PhilDSP, but looked up his posts and found the new TL Discussion thread. So, I just had to tell everybody what happened.

But, I'll PM you next time.

Well, nothing happens fast for me, and I was about to bring up this thread again anyway, since my project is about done.

My home built active sub crossover had an issue, when I tried it today and couldn't get any output from the sub. But, should get that fixed shortly.

[Johnloudb]

Well, I've got these subs pretty well integrated to our main speakers now. The Dayton RSS 10" sub is a good one :T and as I suspected needed some break-in. It sounded kind of slow and lethargic at first but that has changed dramatically with some playing time and they are sounding much, much better. In my limited experience with new subs it takes quite awhile for them to really loosen up.

[Johnloudb]

Since Dan asked, his Blades' TL is a tapered line, where the CSA of the line decreases as you move from the closed end near the driver to the open end for the terminus. You do have to pay attention to the air velocity though the terminus to ensure it doesn't get so high as to cause audible noise. If you can its best to keep this air velocity less than 3% of the speed of sound when the system is playing at high loudness, but when the terminus is on the rear, you're generally okay to have the air velocity as much as 5% of the speed of sound. One of the many graphs in Martin's worksheets shows this air velocity, and you can manipulate the dimensions of the line to achieve an acceptably low terminus air velocity. Considering nothing else, making the terminus CSA smaller would lower the system's tuning frequency allowing even less of the higher frequencies to be emitted through it, not the lower frequencies; however, doing so would increase the air velocity even though the system could play to somewhat lower frequencies. "Classic" TL theory is often just that, theory but not necessarily correct or true in every case. There's no reason that a line has to have some multiple of a driver's Sd for its CSA, but you do need enough "room" for the driver to not have a restricted flow path. In the Blades, the overall taper of the line, coupled with its length and volume, determined its bass response and other performance attributes. Regarding that peak in the response of your IMF sub, I can't make any suggestions unless I know more about its design. Is it also a TL?
Paul

I think I was wrong about the frequency response. I just found this test on the IMF Professional Monitor, which we have.

http://www.marklev.com/IMF/Promon/ontest/index.html

I think it was the Studio monitor response that I remember seeing that had a peak below 50 Hz I think. So, I think our room probably has a dip in the 45-60 Hz range since we had the a similar problem with our main speakers. I can add an EQ to my crossover and cut out the slight rise in the 25 - 35 Hz.

I really appreciate your comments on TLs, it's helpful. So, reducing the port size I could extend the response somewhat lower? Would that make the dip at 50-60Hz worse?

I wouldn't mind tuning these to a slightly lower frequency so it would play a bit lower. So, I may play with the port size a bit.

Any suggestions for these would be very welcome. But maybe there's not much else to do?

[speakerman19422]

Is that fiberglass stuffing? I know from being a long time customer of Bud since 1980 building many versions of his aperiodic and t-line cabinets he used polyurethane open cell foam for stuffing after IMF. It is the low density type white turning yellow over time. . The colored foam is a higher density. On my t-line subwoofer the area behind the driver is 11" by 12". A sd of 2.5 Three pieces of 2' vertical evenly spaced glued to the cabinet. The foam is never against the sides of the cabinet in t-lines like some line tunnel and aperiodic damped. Tapeing to one 2" piece to line terminus. The opening is 3'" by 12" at line terminus on bottom front of enclosure.. Every part of line is angled. What is the area behind driver and area of line terminus at bottom front of enclosure. The amount of bass these enclosure produce sound very natural regardless of any impedance peaks that compare them to bass reflex. I only build t-lines now. T nuts can be fun on 1' high density particle board. I found using a clamp to pull them in slowly works the best for me. Then I use the hammer very carefully.

[Johnloudb]

Hi Speakerman,

Sorry, I missed your post. Yes it is fiberglass stuffing. I've always really liked the bass of this speaker.

Paul K. is kindly doing some sims for me using MJKs worksheets. Be interesting to see the end results.