DIY GFRP Sandwich Panel 1x15PR400

[astack]

Behold, fellow bassist, my first build log of a megalight single 15 full range cab. Nearly every bit of knowledge I’ve used for this has come from TB, so I hope this might help others looking to learn a thing or two.

As this is my first build, I kept it pretty meat-and-potatoes on the audio side, which will be a good match for playing style, too. I’m a structural engineer by day, so the glass fiber reinforced plastic (GFRP) composite sandwich construction was where I had some previous conceptual knowledge and wanted to experiment with. (I was a compete noob to working with GFRP a month ago, btw.)

Yes, it's basically a Faital Bassic (edit: knockoff...), but why should Mike Arnopol get all the fun playing with fiberglass and epoxy? I'm not doing this to sell or save money, just sounded like a fun summer project and there are a few "features" I wanted to try out. If it doesn't sound like fun to you, go talk to Mike and buy a Bassic.

So, going forward in this thread, I plan to explain up my research sources, objectives list, the WinISD and enclosure calcs, materials lists, fiberglass panel testing and design, SketchUp modeling and finally the build. I miss anything?

For now, here’s the beauty that makes this all possible:

Time to get to work!

 

[astack]

Lunch time... and not enough epoxy for the next piece (my supply store isn't open on Sundays), so good time for a break.

Specs:
Vb = 3.1 cu. ft.
fb = 42 Hz
f3 = 52 Hz
28.75" H x 18.5" W x 13.5" D
8 ohms
400W (210W excursion)

The construction is:
2 layers 5.6oz S-2 glass
in marine grade resin and fast hardener @ 150 wt% epoxy/glass
over 1" Foamular XPS foam

That's the basics of it. More info to come...

 

[astack]

Welp, managed to get sick over the weekend and have a big deadline coming up at work, so y'all may have to be patient with me here. I'll post about all my background stuff in the mean time I guess. Oh and pics, I know how you guys like pics.

 

[takeout]

Subbed.

 

[JMac- Wiskerface Designs]

Cool build...
Should be a really nice sounding cab. The 15PR400 is one awesome speaker.
Sub'd.

 

[astack]

Thanks for following along guys. It was getting lonely in here. Jmac, thanks for sharing your build, too. It was a contributing cause of this current madness.

With any luck I can pick up epoxy today and get back on track. Woot.

 

[majortoby]

I'm really glad to see these composites making their way into the DIY realm. 'Tis a great time to have a workshop!

 

[astack]

I'm really glad to see these composites making their way into the DIY realm. 'Tis a great time to have a workshop!

I wish I had a workshop! It's weird growing up in a suburban house then living in cramped apartments for years. Some how, I managed to get a backyard space in a duplex in San Francisco. Best I can do at the moment, so that's where the magic happens. We're moving at the end of the month to a similar place, but still not the holy grail of a GARAGE. Some day...

Anyway, first off, I promised pics, so I'll oblige.

Here are my collected supplies on my first day of experimenting:

Like I said, I was a noob to this stuff a month ago. However, I worked as a painter/wallpaperer/etc. throughout high school with my pops, so maybe some of that translates? You have to be a little brave with this stuff as it's not MDF cheap, but the "experiments" are worth their weight in gold so you're not figuring everything out on your actual build. (That said, you'd be surprised though at how the cost compares to that fancy marine ply that some people use.)

The workstation:

Don't forget, epoxy curing is an EXOTHERMIC reaction. And Solo cups don't like heat:

Lesson learned!

Finally, a teaser - the mocked up cab:

 

[astack]

Like I said, TB has been super helpful for background research. Like with GFRP, I knew essentially nothing about speakers and cab design. I've read and reread following threads a million times for this design:

ZacLite 1 and 2
http://www.talkbass.com/forum/f15/diy-lightweight-composite-cab-build-432560/
http://www.talkbass.com/forum/f15/diy-lightweight-composite-cab-build-ii-zl1263-3-way-549976/

Petebass’s composite
http://www.talkbass.com/forum/f15/another-composite-cab-build-inspired-zac-663169/

Ron’s ultralight build
http://www.talkbass.com/forum/f15/ultra-light-weight-bass-cabinet-829262/

JohnK’s Starliner
http://www.talkbass.com/forum/f15/custom-diy-2x15-cab-build-854517/

And a discussion among the fiberglass elite here on materials
http://www.talkbass.com/forum/f15/fiberglass-composite-builds-798355/

Read the stickies, too - they're great.

It doesn't hurt to do the advanced search function on all those speaker gurus through the "posts by" and "threads started by," including Duke, BFM, Green Boy, Alex Claber, fdeck, Will33, JohnK_10, 5StringPocket etc. Many others and I'll probably amend this list as names come to mind, but these guys were especially awesome for my research. (Now that I write this, it feels creepy, but hopefully these guys will be flattered and not creeped out? )

This has nothing to do with cabs, but it’s really good at spelling out important structural concepts if you want to dig into it. It's about bikes, but all the basics carry over.
http://www.princeton.edu/~humcomp/bikes/design/desi_6.htm

Bookwise, I’ve kept these books close at hand while learning this stuff:
Principles of Vibration and Sound - Rossing and Fletcher
Way too technical, really, but I like reading math and there’s some useful equations/figures on panel vibrations in here.

Introduction to Loudspeaker Design - John L. Murphy
Pretty basic stuff, but it explains the basics more clearly than anything else I’ve found.

Great Sound Stereo Speaker Manual - Weems and Koonce
Terrible name, very handy reference, though.

Fiberglass & Composite Materials - Forbes Aird
Great overview of the materials and process. Rereading it after my first trial layups it made way more sense than the first time, though.

I love the library.

Youtube was great for fiberglass layup technique, especially these (a tad dry, but gets the ideas across):


Oh and I googled a bajillion things as they came to my mind. Google's awesome...

I think that's the major ones. Total overkill here, in know, but this stuff is fun and there needs to be more of this action going on Hoping to bring some others to the (Ultra) Light Side


PS: Got the epoxy just as they were closing.

 

[Bredian]

Subbed

 

[astack]

A bit of background's in order, I think. (I've written it before, but I swear I'll shut up about it after this.)

I'm coming from the 2x12 Ampeg Portabass cab and PB250 head from the same series. When I bought this, I downsized from an SWR SM-500 and Senior Goliath 6x10. Big drop in performance, but this was the first wave of decent sized portable rigs. And it was awesome. That set up lasted me a good 6 years. I loved having the PB250 in the backpack and well thought out cab (in terms of portability at least) in tow. The removable, telescoping handle and wheels; the slightly angled baffle and the spring handle that can be used to tilt the cab back farther; neo speakers and italian poplar ply. Terrible sensitivity and low end (sealed design), though. But, as I said, it's worked well enough and never given me troubles.

So why change? We're in the next wave of ultra portables and I couldn't resist going the next step.

OK, so as you start to research this stuff, you'll hear it over and over again that cab design is about trade offs. (Uhh or any design process...) So, for me, the first place to start was to set what my goals were and after more research some actual target numbers and finally how to get there.

Objective - Target - Method/Solution
Low weight - Sub #40 - Neos, GFRP, simple or no crossover
Coolness factor - Is there a coolness scale? - One word: Tolex
Not too expensive - Sub $750 - Yeah, not too restrictive, but means no carbon fiber...
Loud enough - 100dB and 500cc - PA grade speakers or lots o' speakers
Easy to move - Ergonomics, handles, etc. - Testing...
Good dispersion - No squares - Vertical alignment, midrange speaker?
Simple design - 1 or 2-way max
Full range cab - 60 Hz f3 max, 7kHz up top - Speaker choice and box alignment

Yeah, that's basically everything anybody ever wants but putting some numbers on it helps balance things out. I think I'll get fairly close on everything when this thing is done, and the weight I should beat out by a good margin, depending on what spinners and spoiler I go with

Note I haven't pointed out an overall "solution" yet (i.e. 2x12, 2x15, etc.) with these objectives. I didn't start with something in mind. Actually that's a lie, I starting by thinking about how cool a vertical 4x10 would be. But by taking a step back and thinking about what I wanted this thing to do and not be, I discovered the 1x15 is a better option. This is just one of those little engineer nick picky things, but I'm glad at this point I don't have 4 10" speakers here in my living room ready to disappoint.

Next time on our program, I'll run through the alternative configurations I considered and modeling stuff, but I don't want to bore you to much, so there'll be some progress pics, too... Stay tuned

 

[JMac- Wiskerface Designs]

Thanks for following along guys. It was getting lonely in here. Jmac, thanks for sharing your build, too. It was a contributing cause of this current madness.

With any luck I can pick up epoxy today and get back on track. Woot.

Not a problem Man! I like to help cause trouble whenever I can.

I'm really liking that You are going with an epoxy resin over a polyester. It is more expensive, but well worth the extra $ and effort.
I'm also really curious about Your laminating schedule for using the glass with the foam board, which I am sure You'll document with some nice photos.
Best of luck, and I cant wait to see it finished!

 

[takeout]

Yikes. Everything in one place = let's sticky this sucker.

 

[astack]

Not a problem Man! I like to help cause trouble whenever I can.

I'm really liking that You are going with an epoxy resin over a polyester. It is more expensive, but well worth the extra $ and effort.
I'm also really curious about Your laminating schedule for using the glass with the foam board, which I am sure You'll document with some nice photos.
Best of luck, and I cant wait to see it finished!

Thanks! Yeah I'll get to it, but the short of it is polyester doesn't play nice with the polystyrene foam. I think it would dissolve it. And epoxy happens to be stiffer, so it's not the worst.

 

[JMac- Wiskerface Designs]

True that. IME- Polyester resin will turn the polystyrene to "jell-o" without a doubt, whereas polyurethane foam will play nice with it for the most part. FWIW- The epoxy is king of hill without a doubt, hense it's use in aerospace applications.

Again, I cant wait to see it done and I doubt You'll have any regrets on going with the Faital Pro driver.

 

[will33]

Only thing slightly "creepy" might be being listed in your credits there.

I've learned a ton from those guys, but couldn't hope to be some sort of equal.

Cab plans look good. Your f3 and excursion limit look real similair to my 212 Deltalite rig.

I don't know anything about the fiberglass composite stuff. Same stuff applies as far as needing stiff panels. Don't know how inherently stiff they are though/how much brace is needed. Guys who work with it say it's pretty stiff already.

-Will

 

[astack]

Only thing slightly "creepy" might be being listed in your credits there.

I've learned a ton from those guys, but couldn't hope to be some sort of equal.

Cab plans look good. Your f3 and excursion limit look real similair to my 212 Deltalite rig.

I don't know anything about the fiberglass composite stuff. Same stuff applies as far as needing stiff panels. Don't know how inherently stiff they are though/how much brace is needed. Guys who work with it say it's pretty stiff already.

-Will

Ha, great. Well they had to learn it all from someone else, too, right?

Yeah, part of the magic of the composite is the low mass in combination with stiffness. In theory, the resonant freq is proportional to the square root of stiffness divided my mass. So, even if the stiffness is only comparable to ply, the low mass means it'll still start vibrating at a higher frequency, meaning less energy (analogous to why bass amps are higher powered). It probably won't be damped as much, which could be bad, however. Thems the trade offs.

I'm not too worried as I have some extra internal space earmarked for bracing if I need it. I'm planning to do some forced vibration testing on the panels eventually to see what the response really is.

 

[will33]

Cool. That's real designing.

I usually just overbuild to some degree I come up with from who knows where, and deal with a couple extra pounds. Then again, I'm 6'2" / 250lbs. carrying a couple 112's, so, not that big a deal.

 

[astack]

Well, the weight thing is mostly because I can. It's been joked about before on here how the lightweight bass rig thing really only benefits the guitarist and his all-tube head and ceramic speakers... So, I've offered to make matching cabs and head cases for my guitarists we'll see if that happens...

Alright, well back to the numbers. Aside from the stiffness as mentioned above (we'll get there), the first thing I was curious about with the fiberglass panels was how much weight I could actually save and how expensive it would be vs. the reigning cab champs of Okume and BB. (Disclaimer: The ply data came from http://www.boulterplywood.com/MultiLayerPlywood_4.htm. I'm sure others can quote other prices and weights, and I'd be interested just for comparison, but this is more about ballparking than precise figures.)

Check it:

For the GFRP, "(2) E e.s. o/ 3/4"" translates to "2 layers of e-glass on 3/4" XPS", fyi.

These are rough numbers again, but looks like our box weight can be halved by ditching ply. The ply weight will jump because of bracing and the GFRP weight will jump because this is at 100% not 150% epoxy. You get the idea...

Great, but this stuff is expensive, right?

Huh. Actually fairly comparable to the higher end ply. First off, note if I wasn't buying the materials locally, I could buy them online for a good bit cheaper. Other point is, for this build I'll be using about 15 sq. ft. of material so 1) the cost of the box is still 1/2 to 1/3 the cost of the speaker and 2) I don't have to buy a full sheet of the pricey stuff like with the ply. So, again, at least for my operating parameters, high quality materials cost a bit, but they're closer than you might have thought. (For fun, 2 layers of carbon fiber each side on XPS would be closer to $24/sq ft. Yikes!)

Ultimately, these graphs where enough to convince me I wasn't totally mad for going this route. They may or may not be for you, and that's cool, too.

 

[astack]

A lot to say about picking a speaker, and I'm not going to rehash all that can be found in other threads. Bottom line is, it's the most important decision in DIY'ing a cab.

There were three things I looked for in a speaker when shortlisting (or figuring out why others had shortlisted it):

1) How's the low end?
WinISD modeling - can I get reasonable bass drop offs without humps at a reasonable box size?

2) How's the mid and high end?
Does the manufacture's published frequency plot look "OK" or not and what do others say about how it sounds? Check some sound samples if possible.

3) How loud can it go?
This is the only semi-original contribution I can make to this topic.

In my objectives, I said I'm looking for 100dB 1W/1m SPL and 500cc Vd. Since Vd is loosely a measure of how much power the speaker can take before farting out, I assumed it could be used to estimate max prefart SPL.

If a doubling of power means a 3dB increase, the equation relating dB's to power would be:

dB = S + 3*log_2(n*P),

where S is the 1W/1m sensitivity spec in dB, n is the number of speakers and P the input power in watts. log_2 is log base 2.

So, I tried the same, but with Vd and compared it to WinISD's max SPL. Aside from a fudge factor of two, it was within +/-1dB, which was close enough for me for a first pass. (I'm sure fdeck has the real equation for this somewhere in those pdf's in the stickies.) So here's my rule of thumb equation:

*** dB_excursion limited = S + 3*log_2(n*Vd*ff) ***

where Vd is displacement (Sd*Xmax in cc) and ff is the fudge factor taken as 1/2 here.

In other words, if you have plenty of power relative to the speaker(s), sensitivity and displacement can be traded off and compared as above, loosely.

So I'm looking for 100dB + 3*log_2(1*500cc/2) = 124dB
And I got 100dB + 3*log_2(1*460cc/2) = 124dB with the 15PR400

Jackpot.

Another interesting thing is, just like when people say more power doesn't make as big a difference and you'd think, more Vd acts the same. The 3015LF has S = 98.4dB and Vd = 850cc SO MUCH MORE DISPLACEMENT. But that's just used to overcome the lower sensitivity: 98.5dB + 3*log_2(1*850cc/2) = 125dB. I think it actual modeled as 124dB, so not as big a boost as you would expect.

(Note: If this is at all interesting to you, check out this thread with Duke and others trying to work out the same problem: http://www.talkbass.com/forum/f15/seach-meaningful-mechanical-power-handling-spec-600790/ They're trying to work out the fudge factor that I just called 1/2 But in the same way that that thread died because there were too many simplifications made, I know there are holes all through my equation above. Eh. Close enough.)