AutoTuba - First build of many....

[jcmbowman]

I recently undertook what will doubtless be the first of many BFM builds, an AutoTuba.

I stumbled across the BFM site while looking around online for some subs to be used for live and DJ performances. I was actually on one of the forums discussing the LABsub where someone mentioned "those crazy Fitzmaurice Designs" along with a number of other disparaging comments regarding the undoubted superiority of the LABsub, so I fired up google and two months later, here I am.

In a former career path I was a full-time sound guy for one-off rave events around the midwest, and my favorite toys to take out on the road for shows were these massive 30" x 60" x 60" subs known affectionately as "earthquakes." They were apparently modeled after the speakers designed for the movie "earthquake" that came out in the 70s. They were deadly in the 20-80hz range, and I now understand why - they were primitive folded horn subs. (of course they were loaded with horribly inefficient JBL 18" subs that sucked down all the power you could hope to throw at them.)

So on the quest for perfect bass I decided to give the BFM way a test run with an Autotuba build. The build process was not too terribly hard, although I will most definitely be using a table saw for all future BFM builds. I built the standard 14"x15"x32" AT with the MCM driver. The birch ply I got from a local lumberyard was a bit on the warped side, and I only had 4 clamps to work with, so it took me about a week or so from start to finish, but a good part of that was just trying to work around my son's sleep schedule, since the wife wouldn't be too keen on my waking him up at 1:30a to do a trim cut on a brace.

I will eventually be carpeting it, but I'm waiting for better weather so I can do the sticky stuff outside (my workshop is in the basement and I'm not fond of the idea of spraying toxic flammable glue in an enclosed space that I share with a gas furnace.), so I will post pictures once I have it carpeted.

I installed it in my car and fired it up very timidly (not wanting to blow my new creation on the first day) and as I kept gently turning up the gain on the sub amp I was astounded at the amount of volume coming from that tiny 8" driver. It still sounded very muddy, even after playing around with the EQ and crossover points and all the rest of that, so I was starting to get a little disappointed at all the time I had sunk into the project. I asked my wife to join me for a second opinion and she agreed that it just didn't sound right. She suggested that I should try a different position in the trunk (I had it forward facing). Being the stubborn man, I said "no, that can't be it, I just need to adjust some of the settings, tune it in properly and all that." I spent the next 3 days driving around with mud constantly tweaking and adjusting to get more mud. Then I took it out of the trunk, checked it for leaks with a 30hz test tone, found none, and on a whim decided to put it back in the trunk rear-facing.


WOW.


Let me just say that one more time for the record.


WOW!


I would more fully describe the experience, but the terms in my vernacular to describe such things are not the kind that you use in public forums. Holy %$#$ that thing sounds GOOD. I've since spent the last week offering to drive my buddies around just to hear them ask me how many 15"s I managed to fit into my trunk and grin from ear to ear when I see their slack-jawed amazement that ALL THAT SOUND is coming out of a 120-watt 8" driver.


Needless to say, I will be building more. soon. My wife after hearing it positioned the right way for the first time simply responded by grabbing a tape measure and running over to her car while asking me the exact dimensions of the AT.

Know anybody who wants to buy a couple of double 10" sub boxes? I don't think I need them anymore, and I couldn't in good conscience give them to my friends, knowing how easy it would be to build them an AT of their own.

Well, enough waxing poetic. The only question now is, do I build the T24s or the T36s?

[pgolde]

If I were to start fresh, I would build the new dual 12" T30's, everything you need in a fairly compact size for mobile DJ or Raves (just use at least 4)

[AntonZ]

The only question now is, do I build the T24s or the T36s?

What's the trunk size of your wife's car?





Great review. I have been thinking about building an AT for my car. I tried my T18 but was not overly impressed. So I haven't started the AT build yet. After reading about your experience I'll try again with some different positions. Edit for mty: my car is a european size wagon (over here it is called Nissan Primera Estate, may very well go by another name where you live).

[camrynekai]

[mty]

Hy .. Jcmbowman . Just a question about your trunk . the thrunk does have a exit into the cabin gain or the thrunk dont have comunication with the cabin .What car do you have .Thanks for you review .

[MikeHunt77]

Nice! I've built my AT with the intention of it going in the car, but I'm still using it for HT right now...

I may try playing around with the position in the boot (trunk), and see how much difference it makes...

[jcmbowman]

AntonZ - My wife drives a honda Civic sedan, so there's barely enough room for an AT. If I could fit a T24 she'd already have picked up the plywood for me

mty - I drive a 1993 Audi 90 Quattro. The trunk is a sealed trunk, not a hatchback. The rear seat does does fold down, but it's pretty solid when it's up and locked in the normal position. The top deck of the trunk is relatively solid, but not totally airtight. I already put some weatherstripping in key spots to keep the deck from rattling from the previous sub I had in there. Reading your post reminded me that I have a fold-down armrest in the middle of the backseat that has a pass-through to the trunk (for skis and such type things). Before I head off to work tomorrow I'll fold that down and see how that affects the sound/volume.

I found the whole front vs. rear to be very odd, as there's actually more room for air movement when it's facing forward, since the seatback is at an angle. When the AT is facing back there's maybe 3 inches of space between the mouth and the trunk lid. Perhaps firing straight into the trunk just shakes the whole body of the car more, and that's why it -feels- lower... I'm sure one of the acoustical engineers on here would know better than I.

[Mikey]

Facing forward, it was firing into foam, absorbing the sound. Facing to the rear, the sound is being reflected by metal.

[jcmbowman]

Facing forward, it was firing into foam, absorbing the sound. Facing to the rear, the sound is being reflected by metal.

*smacks self in head*

DUH!

[asad137]

Facing forward, it was firing into foam, absorbing the sound. Facing to the rear, the sound is being reflected by metal.

...and then absorbed by the foam in the seatback.

Asad

[Bill Fitzmaurice]

Facing forward, it was firing into foam, absorbing the sound. Facing to the rear, the sound is being reflected by metal.

...and then absorbed by the foam in the seatback.

Asad

+1. The difference would be the loading offered by the solid surface versus porous.

[jcmbowman]

For the record, folding down that armest in the middle of the back seat added almost +2dB to the level in the car.

And in case anyone is out there lurking like I was a week ago, the settings I've found to be best for driving the AT in my car are a HPF set at 35hz with a slope of 18db/octave, and a LPF set at 80hz with a slop of 24dB/octave. With those settings it's pretty much flat from about 35hz all the way up to 125hz, which is about where the other speakers take over.

[Bill Fitzmaurice]

For the record, folding down that armest in the middle of the back seat added almost +2dB to the level in the car.

Mostly in the higher end of the bandwidth I'm sure. The lower you go the more invisible the seat will be to sound.

[jcmbowman]

For the record, folding down that armest in the middle of the back seat added almost +2dB to the level in the car.

Mostly in the higher end of the bandwidth I'm sure. The lower you go the more invisible the seat will be to sound.

By "higher end bandwidth" you're referring to 80hz-125hz, right? that's still pretty low in my book

[Bill Fitzmaurice]

By "higher end bandwidth" you're referring to 80hz-125hz, right?

Yes, 80 Hz and up isn't too hard to damp, but 60 Hz and below will go through anything.