Dual Loaded Table Tuba by a Teenager

[Sydney]

What was quoted from the plans is what I referred to.

[Bill Fitzmaurice]

In the article was stated that the open side was to be placed within a couple of inches of a surface ( like slightly off the floor ) to provide additional acoustic load to the back side of the driver.

The plans say:

To preserve the tuning of the cabinet the side with the access hole should be 1½ to 2
inches away from the boundary, be it a wall or floor. If you lay the cab on the floor
use legs to lift it 1½ to 2 inches.

Could someone clarify this driver access issue? Bill? I won't run mine with an access cover as the plans don't say so. I'm not sure if this excursion issue is a problem, I just don't want to risk ruining my drivers.

If you can see the excursion you don't have the cab properly placed per the plans. Why not?

[bassmonster]

If you can see the excursion you don't have the cab properly placed per the plans. Why not?

It's not finished yet and I didn't corner load it in the living room because I was still fooling around inside the driver compartment with wires and leak checks. I still need to paint mouth/Duratex outside and install the mouth braces, but after the PL had finished curing in my living room (it's too cold outside), I couldn't resist but to fire her up, albeit with only 5v max.

Once it's done it'll be properly boundary loaded in my room, I was just worried about damaging drivers, as while I can hear my old ported 8" complaining, there's no distortion from the TT.

[bassmonster]

What was quoted from the plans is what I referred to.

Ah, I got confused, my bad. When you said:

I have the original construction article from AudioXpress ( NOT the plans ).

I assumed the article might have said things differently from the plans, but I haven't read the article.

Is "acoustic load" similar to air pressure? The access panel on the THT and other BFM horns is supposed to provide "backpressure", if you will, to limit excursion of the driver. Does placing the driver access hole close to a boundary provide a similar pressure (albeit not as much)?

Thank you for the responses Sydney and Bill. I learn something new here every day!

[bassmonster]

You might think of doing music events if full-on home and auto sub nuttiness are out. Least you might get paid to wreak bass havoc.

Of course I know who Elvis Presley is! Anyway, this got me wondering what a pair of these dual TTs would sound like v-plated and corner loaded in a gym. I have a karaoke gig at my school coming up at the fall festival, our setup is in the corner of the gym, so I suppose I'll take the TT out and let her rip. I'll eventually build another one of these anyway (about a year) once the current one goes in my car, once I get a car amp installed I'll have to bump both of them at once.

[Tom Smit]

Quick question. I was playing some tunes on the TT today at about 4-5v, and looking closely at the drivers I can see them "excursioning" (what's the word for it?) at 30-35hz quite a lot for only 5v. I would say it's about 1/4" off a very very rough estimate. It's less than it would be in free air, but from a sealed system I would expect a lot less excursion, especially at such a low voltage. I'm worried about over-excursion at war volume. Is this a non-issue? I wonder what the voltage limit for the two JBLs would be? They would be taking around 150RMS apiece from the Bash at full volume, and if the speakers are "excursioning" that much at a low volume, I'm a bit worried what will happen when running at full bore.

25-28v.

When I was breaking in a LAB12 with 10v it seemed to travel a fair amount, and I too wondered how it would do under full power. It has now been exposed to 50v and is OK.

[Gregory East]

In a TT?

[UROK]

Ah, I understand, now. Obviously not read TT plans. [Actually, I'm thinking a couple of 20" Lab15 THTs for my living room...]

And, gym TT now! AutoTT. Bedroom TT. Gym TT. This thing will need a shrink when it's older! Confused identities. Multiple personalities. May even need to be admitted for driver reassignment surgery. BM, you have a trans-tuba on your hands!

Seriously, go for it. Get into that side of things now. I can hear a chorus of 40+ year old guys agreeing with me and encouraging you to explore recorded or live sound. I was at a psytrance night here on Sat night - great night, friendly positive atmosphere, pretty good music, 4 big Mackie subs, powered monitors, using house mains. But the four subs were split, spread across front of the stage! There was a perfect corner right there too! Just needed to turn one of those boxes round and cluster the others next to it for mega bass.

If, as a teenager, you start avoiding a. playing psytrance if at all possible, b. buying an expensive Mackie or whichever set up instead of building BFD, and c. silly implementation mistakes, this could be, at the very least, the most enjoyable hobby in the world. Who knows you could maybe make some cash to buy all those auto add ons?

[Sydney]

Is "acoustic load" similar to air pressure?!

Air is a compressible gas mix and it's compliance provides resistance; in other words it impedes movement.
Hence the word impedance.
http://www.phys.unsw.edu.au/jw/z.html
http://www.animations.physics.unsw.edu. ... edance.htm

[Tom Smit]

In a TT?

Haha, no. The LAB12 went in a Tuba60 which I put in a gym.

[bassmonster]

So I was able to do a quick war volume test today. I had planned to really flex the TT's muscles above half volume. I played some deadmau5 (Ghosts 'n' Stuff). Maximum I hit was 18v, not even half volume. At this voltage, stuff on the shelves was visibly walking, and the cement floor was shaking beneath my feet. Mind you, it isn't even corner loaded yet. I was scared to go any farther on the volume, for serious fear of damaging objects on the shelves. Also because of my drivers. In the low notes and drum kicks I was hearing a port chuffing/distortion noise, but I don't think my drivers were blown. Examining excursion, it didn't look like overexcursion to me, not even as much as breaking in at 10v. I think the source of the chuffing and rattling noise was actually from the objects on the walls and such shaking. Would this be a fair assumption?

Thanks for the replies and advice everyone. Sydney, those acoustic pressure links were very helpful but the second link was to a newbie like me.

[Sydney]

Use the phrase "acoustic impedance" in Google Books to get to "Loudspeakers: for music recording and reproduction"
By Philip Newell, Philip Richard Newell, Keith Holland
On page 7 Is a simpler description: 1.4.2 Mechanical and Acoustical Impedance

[bassmonster]

Thank you for the research, Sydney.

[bassmonster]

Won't be able to work on the finish for quite a while due to school.

On the upside, I temporarily relocated the TT today. At half volume pictures were falling off shelves, and me standing in front of the TT, the bass flapped my pants and seemed to resonate within my chest. It's not quite the cognizant punch that direct radiators give, but it's definitely immense. It seems to surround you instead of .

[CoronaOperator]

. It seems to surround you instead of .

Now you know what all the fuss is about with horns. Super clean, effortless heaven. Like a big warm fuzzy hug.