Dual driver T18

[jbell]

The cold weather has really been bugging me, 'cause my shop isn't heated. Well today, it hit almost 40, so after work, I made some dust. I figured my hybrid grand (1916 starck -- gutted and loaded with a gem pro2 realpiano) needed some more bass...

Recognize it? After the PL dries I'll try it out, and post some measurements.
Then a grill and some black band-it, and it'll look like it belongs.

[Jeff Beaird]

OK I'll bite... a dual driver T18?

[jbell]

you got it. It's turned on it's side, and 24" tall, not the standard 18" My hope was the extra 6" would do for the T18's 40hz response, what the extra 6" does for the T36's response.

A real grand can hit 113db, @ 27.5hz. I'm hoping this can get there...

If this works well, I've got a plate amp, that'll just barely fit where the access cover is supposed to be. I'll put the amp on the 'bottom' and some 1.5" feet for clearance.

With the grill in place, it won't even be recognizable as a horn

[jbell]

well I fired it up for 5 minutes before heading to work today. A couple things stand out.

1. I have to get my mouth braces put in place, the unbraced mouth panel flexes over 1/4" Had to hold it still while listening....

2. this thing makes very little sound unless facing a wall.

3. It goes loud, but not low. Hornresp predicted that the extra 6" would help a little, but not a lot.. and I'd say my ears agree. Had to cross at 50hz@12db/oct to get rid of midbass 'ring'

4. I didn't break in the drivers before building, the build was a 'spur of the moment' deal to take advantage of some warm(er) weather, so take anything I say today with a grain of salt.

5. After building a few different tuba's, I've come to the conclusion that I just don't like a 'horn' sub sound for my main music listening. Yea, they make spl, and for PA that's extremely important, but I just can't make them sound the way I want. For jazz, I like the sound of my NHT 10. For rock, and overall best deep bass sound, the HSU 12, for HT effects, the THT. It'd be nice if a single sub could 'do it all' I guess I just haven't found that yet...

6. I've come up with a slightly different assembly method that makes things a TON faster, and guarantee's everything is square. I take the baffle board and the board that makes a 'T' and pl/screw them together first, before attaching them to the side panel. This does 2 things, first it eliminates 'progressive errors' and it guarantees that when you attach the 'T' to the sidewall, that it's square with the world. I use no clamps, only screws for the panels, and brads for the braces. I think my total glue up time for my T18 was under 2 hours.

[SoundInMotionDJ]

6. I've come up with a slightly different assembly method that makes things a TON faster, and guarantee's everything is square. I take the baffle board and the board that makes a 'T' and pl/screw them together first, before attaching them to the side panel.

Pics?

[jbell]

pics of the glue up? It went fast enough, didn't take any. (I'll do that next time) However, any tuba plan, you can find the joint that makes a 'T' That is usually about connection 4 out of 8 or so. If you draw your lines on the 'T' board, and then connect it, just like you'd do with a typical panel to the side, you'll have a perfectly square connection, that when connected to the side, perfectly squares itself that way as well. Only thing to note is when you connect the 'T' and drive the screws down tight, It'll make a perfect 90degree connection. You need to back them off just a little when connecting to the side, as that connection angle is never exactly 90. Once screwed down tight to the side panel, you can tighten back up the 'T' connection screws.

And to tell you how easy this makes things go.... I had both drivers installed on the baffle board before I started glue up... and a pair of 55-2421's are not lite.

When I did my THT, the 'T' was panel 1 to 2. I did that first, then panel 3,4 (which is why you see a glue mess on panel 2/4 connection, it was the last joint for the driver enclosure, and I smeared it with pl, to make sure everything was air tight.

[bgavin]

Interesting idea. I would have thought P1 to P3 would have been the choice. P3 locates midway on P1. Using a square on P3 plus the joint with P1 would make P3 perfectly plumb.

[jbell]

The THT is a bad example, and is a unique tuba , in that it's not a complete spiral. On a spiral, like most tuba's are, it makes more sense. (I also drew the lines for the panel 1/3 connection, to make sure it was square with the world as well.)

The most critical connection (to make sure the throat is perfect) is the connection between the baffle board and the 'T' board. If you start there, the rest works itself out. Also in a spiral, it's usually about halfway through the connection panel process. Breaking that up into 2 parts, cuts your 'progressive' errors that you may end up with in half, so in theory, you are twice as accurate without being any better wood worker. I like that....

[WB]

Just my opinion , but I think those braces are far too short. That's got to be a recipe for vibrating panels.

[jbell]

yep, probably are too short. They are offset, so that helps, (most of the length is braced) but they are too short.

Just used the pic, as I knew I had already posted it.

Did a little more listening tonight. I think what I'm hearing that I don't like is time delay due to the horn path.

It sounds MUCH better behind the couch in the corner, about 5 feet away, and my main L/R speakers about 12-14 feet away. Corner loading does help, but it seems like the bass fades with distance quickly, and tonality bass to main speakers changes dramatically as I move around the room. (which obviously doesn't happen when sub/mains are in basically the same location)

I'll give them about a week for break-in before I do any measurements.

[Sydney]

I have to get my mouth braces put in place, the unbraced mouth panel flexes over 1/4" Had to hold it still while listening....

I wondered about that, when I 1st saw the pic.
( With my T18, and my T24 there was a marked improvement in the "tightness" of the bass after the mouth braces was installed. )
What is the weight of your cab?

[jbell]

cab is pushing close to 50lbs. Dual 55-2421's are heavy.

The mouth braces helped. But I've found something (in my case) that helped more than anything else. That is placing the horn mouth facing straight UP in the corner. (as well as dropping crossover point to about 38-40hz.)

With the horn facing up, is that 'corner loading' ?? Sure sounds best that way.

[Sydney]

cab is pushing close to 50lbs.

That's a testimonial to the lighter weight of ply ( and conversely the extra weight of MDF ).
My T18 with 1 driver made out of MDF weighs 52lbs.
The ONLY other difference is - I added a couple of dowel braces on the 1st panel.

With the horn facing up, is that 'corner loading'

I think it qualifies ... I was hard pressed to hear a lot of difference between orientations in the same position.

[jbell]

OK.. I've finally had time to dial in my dual T-18, and I think I can wholeheartedly recommend it.

It's finally had some time to break in, and sounds much better, I've found out that I need it closer to my listening position than my mains to eliminate the 'in a can' sound, that you get from the horn time lag.

So, I did a 1hz in room test at my listening position, scosche spl meter, C weighted, 2 meters from sub, 3.5 meters from mains.

Here it is:
20-29hz.
86.4, 85.5, 86, 87, 87.5, 89.2, 91, 92.5, 92.9, 93
30-39hz
93,94,95,97,98,98.5,100,102,103,98
40-49hz
89,93,98,100,100,100,101,101.5,102,101.5
50-59hz
101,101.5,102,103,104,103,100,98.5,97.7,96
60-69hz
94,92,90,87,85.5,89.5,92.4,93.7,93.7,93
70-79hz
92.5,92.2,92.2,91.7,90.8,89.7,86.2,80.1,62.2,78.7
80-89hz
80,82,86,87.4,86.5,85.2,84.6,84.9,84.1,84
90-99hz
79,79.5,84,87.7,90,91.5,92.5,93.5,95,96
100-110hz
96,96.1,95.9,95.2,94,92.5,90,89,88,89
110-120hz
89,88.7,88.3,86.6,84,85.2,86,86,85,84.3



Don't you just LOVE acoustics.. I've seen rooms that have 36db swings... yea, I know I have a dip at 40&80 in this particular spot, 1' away, it's a different story, so I'm not too concerned with it. My living room is 17x29x9... just for that reason. NO ACOUSTIC material is in the room.

So, in the end of the day, can a dual driver T18 get loud and sound ok in room -- yes is the answer.

It has real output from 30hz on up. Just cross it low, about 40hz is just about exactly right on the money. (funny... I remember bill said 40-50hz was about right on the money in the plans....)



EDIT:
After thinking about it for awhile, I came up with a way to tame the 55hz spike, without killing 80hz. I added a simple 16mh (or may be 18, couldn't read the writing real well) choke I had in my stash of stuff, and then moved the cross back up to a typical 80hz.

The green line tells the story -- MUCH better. and the 1meter response doesn't show the 40hz dip, that my coffee table location showed. In fact it was the opposite... 114db.. most likely a floor to ceiling resonance due to the cabinet facing straight up, which is why I'm +6 and 1meter and -6 at 2meters. It's less than a db variation from 50-130. (I only did every 10hz, not every 1hz on this test)

The night and day difference the choke made makes me think that any one with a T18 would benefit, not just me with a 24" cabinet and dual drivers.

This is no joke, I turned it up a little and clocked it at 126 @40hz... My meter only goes to 130, so I stopped there. BTW -- it's being powered by ONLY a nht 120watt amp... My feelings about this sub have turned 180degrees after adding the 6db/oct choke....

[crockettnj]

If I may, whats a choke?

Thanks for the review