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I Am The Machine
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Discussion Starter · #1 ·
I'm in a Mechanical Engineering Prototyping class right now, and as such have access to a laser cutting machine and plenty of acrylic. I can design the box in CAD and have it computer cut by laser, so I figure I could end up with something that would fit pretty good. I don't plan on this being a permanent solution, just something quick and easy for now until I can build the sub box I really want.

I'm running an Infinity 120.3 DVC sub, running 300W RMS/1000W max off an Alpine V-Power amp that can push 350W RMS. I don't like King Kong thumpin' in my trunk, just nice clean sound.

* It would be a sealed box, approximately 1 cu ft.
* Would .5" acrylic suffice?
* Would this material distort anything, or screw up acoustics or have other negative effects?
* It would be clear - Infinity recommends lightly filling the box with fiberfill. Obviously this would look dorky in a clear box. Would it be OK to leave it completely empty?
* If it makes a difference, the box would be trapezoidal, the front of it being flush with the rear seats in an E39, about a 75* angle IIRC, the subwoofer firing in the forward direction (unless it would be better to have it rear firing.....opinions on this too please )

Thanks.
 

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not very familiar with acrylic, but i have a fish tank made of it. It's pretty good material and probably do good job for subs. maybe just need the think pieces. btw, your project sounds cool. is it a computerized 3d cut?
 

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It's been done plenty of times :dunno: You might want to use 3/4", make sure you predrill the screw holes into both surfaces to avoid cracking. A blow torch helps to get rid of the haze on the cut sides :thumbup: You will probably be happier with the sub firing toward the rear. Good luck and have fun, did you consider etching the roundel into the acrylic and using a fluorescent tube to make it glow ?
 

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You might want to use 3/4", make sure you predrill the screw holes into both surfaces to avoid cracking.
Concur with above. It's been over 30 years since I built a speaker system for my Mech Eng lab. My bass reflex speaker used (2) 15" JBLs and I used 3/4" plywood for the cabinet. Acrylic should be fine as long as it is sufficiently rigid and you can seal the joints properly. If you are going with acoustic suspension type design (sealed box), there are formulas available to calculate the proper volume. You can increase efficiency by going with a vented cabinet design (bass reflex). Again, there are formulas for the vent size given your speaker & box volume. I would add the fiberfill and use an opaque colored acrylic. Good Luck!
 

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I Am The Machine
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Discussion Starter · #5 ·
Thanks for the replies.

King,
Yes, the initial fabrication is computerized. Basically, I would design the box in Autodesk Inventor to get an idea of how I want it to look (because I find that easier to work with initially), then create each panel in AutoCAD. I can save the files in a format that can be read by a machine called the LaserCAMM. It's fairly simply, just upload the file, place your acrylic, and watch it cut. Many of the holes for bolts/screws would be cut by this machine, I would just tap them afterwards.

JB,
I've searched online and noticed it's been done lots of times, but usually it's by the car customizers that specialize in making your ride be gangsta, yo. I just didn't find any good solid info on the audio quality, which is what I'm concerned about. The acrylic is just because I can quickly, easily, and somewhat cheaply fabricate it and have it fit where I want it. I don't want the box permanently, I want to integrate the sub into the trunk better later on (I like my trunk space). Screw holes will mostly be laser cut, then tapped. The others, will be done on a drill press, and yes, carefully and slowly! I'm hoping the 3/4" isn't terribly expensive. I'm thinking this over, and I do think the rear firing may be better visually. Still wondering about acoustically. But the back of a sub in a clear box really isn't much to look at....lol.
I am actually considering etching a small Roundel into it, perhaps a couple inches diameter. Nothing too "blingin". No neon lights for me.....lol.
BTW, on my first project of the quarter in this class, I did etch a Roundel into it :thumbup: We had to build a basic "clock"/pendulum, do a full timing analysis and all that other fun stuff.
"Clock":
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Closeup of etched Roundel:
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Fud,
I don't really know what "sufficiently rigid" is in reference to subwoofers, which was partially the point of this thread. I do know the .5" acrylic seems to be very sturdy and rigid, I just don't know what exactly is necessary.
Infinity publishes their recommended volumes, and shows the graphs of responses to go along with it. I'll just stick with slightly larger than 1 cu ft, to account for the subwoofer inside the box.
I prefer the sound of sealed boxes. The vented ones seem to be a little more muddy, but I have admittedly only heard the Best Buy Special vented boxes...
I'll probably just try both ways w/ and w/o fiberfill. If I can really notice a difference, I'll go with it. If it's just the placebo effect, w/o looks better.
I'd go with opaque, but the only colors we have available look like they were died with Crayola colors. So ricer-clear it is.

Thanks guys :) Keep it comin'!
 

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The thing about acrylic is that it is more rigid than wood. However, that's also its downfall. Wood absorbs some of the wave energy and attenuates it, while acrylic will tend to resonate and transfer the energy to the outside of the box, since it doesn't absorb as much. This is why you need to use thicker walls; the extra rigidity will counter the resonance. Using thicker acrylic also allows for a larger surface area of contact where the joints meet- much better, particularly for a sealed box scenario.

One or two other notes . . the general consensus is that stuffing the box effectively increases internal volume to a maximum of approximately 15%. The more stuffing you use, the greater the increase. In other words, if you completely fill the box with stuffing, you can make the box 15% smaller. Your idea of the trapezoidal box is well documented. Making the speaker-mounted wall not parallel with the opposite (back) wall reduces standing waves, thereby increasing the driver's efficiency.
 

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Building a box of each design is a good idea. Just be careful that they should be compared at the same volume level and not at the same power level (volume dial position). The improved efficiency of the reflex design will make it inherently louder making it appear to sound "better". Many confuse louder volume with "better sound". A well-tuned ear can differentiate the differences. Mostly a difference in taste, like tubes vs. ICs.
 

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I have a couple of local buddies that are still fierce competitors in the circuit. Eddie DeJesus is a Team DSL competitor and national champion in sound quality.His truck sounds amazing, when I worked with him he was competing in a Mazda 626 with all Focal

2001 Chevy S10 - True Truckin'
DLS Delivers With This Champion SQ Truck


writer: Tim Sprinkle
photographer: Henry Z. DeKuyper

Florida customizer Eddie DeJesus knows all about understatements. How else could he describe a 1,600-watt, fully re-imagined '01 Chevy S-10 pickup as "nothing really elaborate?"

"It's a basic system," DeJesus says of the truck's A/V setup. "There are cars out there on the competition circuit that have a lot more equipment, a lot more elaborate systems, but our main concern was keeping everything well-balanced."

http://www.caraudiomag.com/features/caep_0803_2001_chevy_s10/index.html

The other is Tommy McKinnie, dominator of all that is SPL

http://www.termpro.com/asp/competitorstats.asp?Competitor_ID=9880






Each has its place in the field even while they are not remotely the same. Some people argue SQ is superior to SPL and vice versa. They are completely differend competitions and each requires a tremendous amount of time and work to pull off at this level. Some are going to say that McKinnie's truck is that obnoxious kid who pulls up at the light :tsk: if that truck pulled up on you at a light and burped the system you would likely crap your pants :rofl: There is an amazing amount of engineering that goes into both of these vehicles.
 

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I Am The Machine
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Discussion Starter · #9 ·
Dudesky,
Thanks! The trapezoidal box was just to fit better with the seats (trying not to waste space) - great to see it has acoustical benefits as well! :rofl:
What I think I'll do is make the box to be about 1.1 cuft inside volume, layer the bottom of the box with about 2.5" of fiberfill, and leave the rest open. This way the fiberfill will perhaps make the box a small amount "larger" (perhaps by 3-5% since it's not fully stuffed) so to speak, and will compensate for the room the sub takes up, effectively putting me at 1 cuft or close.

Fud,
Sorry, I was just talking a about a sealed box with and without fiberfill. The acrylic isn't free, I can't afford to make 2 boxes. :(

Bryce,
Those are pretty crazy. I've seen the black truck before - I think there's a video where it is taken from the side and you can see the windshield flexing with each bass note. He has an insane setup. Personally, I don't care about loudness, I just really want quality, and a small amount of bump, and no distortion. As long as it goes as loud as I like to listen to it, I'm happy.....and that's nowhere near 130dB! Lol.

Quick summary:
  • 3/4" acrylic is best?
  • 1.1 cu ft internal volume OK?
  • Rear firing sub will sound best?
  • All walls perpendicular to each other except rear wall, to be parallel with rear seats?
  • 2.5" thick layer of fiberfill along the bottom - will that be effective enough without looking terrible?
Look good guys?
 

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The Infinity 120.3 DVC should do well in a rear firing position. You're an engineering student, so I'll entertain you with some technical mumbo-jumbo :D Resonant frequency is the natural frequency of vibration determined by the physical parameters of the vibrating object. The larger the driver size, the lower the resonant frequency. And more important to this situation- the lower the frequency, the longer its wavelength. Which is to say that if you fire a 12" to the rear, it still has enough wavelength to return forward and permeate the cabin area, whereas a 10" or 8" driver wouldn't do as well. Smaller drivers will typically work better when fired forward through the ski pass hole. The other part of this equation is, of course, amplitude. Whatever size speaker you use, you need enough amplification to overcome the dampening characteristics of your environment.

All other things you mentioned look pretty good. If anything, you can always compensate with more or less fiber fill. Hope this helps.
 

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Dudesky, you forgot about the part where you need to have at least a quarter wavelength to hear the sound anyways and for 20hz that happens to be around 24' - so bouncing back off of what will let your 12" woofer be heard better how?

If anything, you'd need the speaker to be in the car behind you to actually hear 20hz in the damn car to begin with.

I'm going off very tired braincells here as I haven't done any serious sound engineering in close to 20 years, but I can tell you you're way wrong on a couple points you just made.

Boston Accoustics (back when they used to be a high end brand) made some killer 8" subs that needed .5cf to play down to 20hz (albeit they were down about 6db at that point - but cabin gain helped). I used to run 4 of them and would put those up against a muddy 12" any day, anywhere. They're fast, and tight which is difficult to achieve with the larger speakers unless you're running push-pull, bandpass, or servo controlled.

As for the OP. You can make your acrylic as rigid as you need simply by adding some ribs to the cabinet. You have the advantage of having a cnc router, so you can make the panels interlock, and cut lots of material away as to not consume valuable box volume.

I have a birch sub box in my car where the majority of the mass is in the sub itself, but it sounds as good as any 3/4" mdf box I ever built (and I built lots of them back in the day). What I did was make cross braces that followed the contour of the basket, so that there wasn't any more than 8" of wall span between bracing. Now I connected all of these to the back 5 sides, with specific intent not to attach to the baffle board. The baffle board helps give your speaker its tone. It's also already super supported by the basket frame itself.

SPL guys are stupid IMO. If you want to win one of those contests, just seal up the car and ignite some high explosives in it. After all, the sound quality doesn't matter as long as you peg the RTI higher than the next guy.
 

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I Am The Machine
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Discussion Starter · #13 ·
Dudesky,
Thanks. I'm high-school physics level familiar with wave travel, so that makes sense. :D
I hear arguments over front and rear firing all the time, and must assume that more factors into the equation that driver size and resulting wavelength, though, otherwise it would be a simple "this is best" approach. Assuming it differs based on application, like the design of the trunk, vehicle type, etc. Regardless of all that, I'm glad to hear rear firing works because it gives me another several cubic inches of trunk space... :D

Bryce,
:slap:

FJ,
That is a good point re: ribbing.
Since I'm a college freshman, cost *is* an issue. 1/2" acrylic would save a lot over 3/4". In your opinion, would some ribbing give me the support I need to use 1/2" effectively, and not interfere with sound wave reflections or other fun stuff?
 

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Dudesky, you forgot about the part where you need to have at least a quarter wavelength to hear the sound anyways and for 20hz that happens to be around 24' - so bouncing back off of what will let your 12" woofer be heard better how?
Correction: 1/4 wavelength of 20Hz is about 14ft at standard atm pressure. But while your main point is true and noted, some of it still reflects back forward, whereas much less would be discernible from the front seats if that large woofer was facing forward. A better setup would probably be to install the box all the way back and firing forward, or to corner load the box near the battery compartment. Neither of which I think the OP is willing to do.

Boston Accoustics (back when they used to be a high end brand) made some killer 8" subs that needed .5cf to play down to 20hz (albeit they were down about 6db at that point - but cabin gain helped).
But 20Hz was not its resonant frequency, was it? 6dB is a lot of loss, considering that to make sound twice as loud generally requires a 3dB increase.

As for the rest of your post, excellent info:thumbup:
 

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SPL guys are stupid IMO. If you want to win one of those contests, just seal up the car and ignite some high explosives in it. After all, the sound quality doesn't matter as long as you peg the RTI higher than the next guy.
If that were true, how would one hold a title for so long ?

I respect that you either don't feel compeled to compete in that arena or just don't understand what it's all about. It's like the auto-X guy saying that IMSA is just ridiculous, they are different competitions. Having worked on winners in both classes (SPL & SQ) I have an appreciation for both. While I personaly would choose the sound quality over SPL I respect that it's not a "stupid" competition and an insane amount of knowledge, engineering, love, and money go into those vehicles.
 

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It's been over 15 years since I built that system and honestly, it was the last car "system" I spent any mental capacity on. I'm pretty sure they were Pro 8.4LF's so you could probably find archives about their specs if you were so inclined. What I liked was that I got 4 woofers in 2cf that would still hit 124db with the top down and were so fast and tight they'd drive nails. I ran them off a Phoenix Gold M100 pushing two 2ohm loads.

14' is probably correct. It's been a good long while since my fingers graced the pages of the master handbook of acoustics. If you can get past the theory that you don't need to know while absorbing the theory that relates, that book has a LOT of good info for budding and seasoned designers in it.

Bryce, the fact that speakers have been designed with quick change cones specifically for SPL contests makes me despise everything the competition stands for. I got into audio for the love of music. Listening to that music in its most pure form (which is subsequently why I've abandoned car audio because it's much easier to compensate for a house than a car's acoustic signature).

Filling a van with concrete and strapping the glass to keep it from popping has nothing to do with music. What you do is make a controlled level of noise for whatever given duration of time is needed to satisfy the rules for a "qualifying run." It's like saying Doodle Bops are the same as Elvis Costello because both of them are musicians with tv shows. What does SPL have to do with enjoying music? My RTA was within 4db of flat at 124db btw. I could push it to 136, but the distortion kicked in.

6db is a lot of loss on paper. The truth of the matter is that human hearing actually needs closer to 9db for an audible doubling or reduction by half. Ears aren't as linear as our recording technologies. ;)

1/2" acrylic with proper supports would likely result in a more rigid structure than 3/4" with wide spans of unsupported area. Just remember to calculate the area consumed by the supports into your total volume calculation or you'll end up with a massive need for a polly pillow to get the booming to go away and that will negate the coolness of your enclosures transparent walls. :)
 

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I Am The Machine
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Discussion Starter · #18 ·
1/2" acrylic with proper supports would likely result in a more rigid structure than 3/4" with wide spans of unsupported area. Just remember to calculate the area consumed by the supports into your total volume calculation or you'll end up with a massive need for a polly pillow to get the booming to go away and that will negate the coolness of your enclosures transparent walls. :)
Perfect, will be cheaper too. I think I'll go that route. 1/2" reinforced...and yes, I'll subtract the volume of the reinforcements. :thumbup:

I remember this class! I built my clock timer in the shape of an Absolut bottle and etched most of the bottle verbiage on it. Then I accidentally fell on it (after it was graded!) and smashed it to pieces.

Back on to this reg. scheduled thread. Good luck!
MAE3 FTW! We'll have to get together at one of the NorCal meets this summer and you can give me insider info on the upcoming classes....lol.

One of the guys made it in the shape of a particularly voluptuous woman - etched everything from cleavage to nipples....:rofl: I couldn't stop laughing when I first saw it. Best design of the class, IMO.

Side note, I know the purpose of the clock was to get us familiar with CAD, analysis, the LaserCAMM, and the machine shop, but really, ours ran for 4 weeks (supposed to be 2.5). If they had just cut the clock project, the extra few weeks would have been great for the robot competition. Oh well.
 

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Perfect, will be cheaper too. I think I'll go that route. 1/2" reinforced...and yes, I'll subtract the volume of the reinforcements. :thumbup:

MAE3 FTW! We'll have to get together at one of the NorCal meets this summer and you can give me insider info on the upcoming classes....lol.

One of the guys made it in the shape of a particularly voluptuous woman - etched everything from cleavage to nipples....:rofl: I couldn't stop laughing when I first saw it. Best design of the class, IMO.

Side note, I know the purpose of the clock was to get us familiar with CAD, analysis, the LaserCAMM, and the machine shop, but really, ours ran for 4 weeks (supposed to be 2.5). If they had just cut the clock project, the extra few weeks would have been great for the robot competition. Oh well.
Sorry- last OT post but I have almost all my old MAE books and tests stored if you need them for super duper cheap. Some were never even used. :confused::angel: I switched out of the ME dept. after 2.5 years and got a degree in something else. I have no use for any of them whatsoever but was always too lazy to sell them.

Lemme know.
 
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