Ultralight Mini-PA Speaker Project - AVS Forum
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post #1 of 2 Old 01-13-2013, 03:05 AM - Thread Starter
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I've got another Burning Man project underway, and this time it is ultralight mini PA speakers that will be part of a bike trailer / art piece. I basically want something loud enough to play music for a small group of people (<50), but also really light. I'm targeting less than 10 lbs per speaker. This is an outdoor event and the environment is flat, wide open desert.

After spending a bunch of time DIY brainstorming, I spotted these Behringer Eurolive B208D powered PA speakers in the Parts Express catalog, and I must say that I'm impressed with their combination of weight and output, as well as size and bass extension:
Active 2-way
Size: 14.7x8x9.8"
Weight: 14.7 lbs each
Max SPL: 113 dB @ 1m
Freq response: 65-20K Hz
http://www.behringer.com/EN/Products/B208D.aspx

Or I could go mono, and get one B212D for 125 dB at 32 lbs total. I might not even need a sub.

In some ways, those commercial offerings are going to be hard to beat. However, there are a couple of negatives for just selecting one of those setups:
* I'd like to be able to run on 12vDC power. I will have a generator on board, but it would be nice to be able to play music without it. And though I could use an inverter, I'd rather not if I can avoid it, as they add weight and waste energy. If I can make my DIY speakers small enough, I might use them on a bike, where I'd really rather not run an inverter.
* Having lighter speakers will allow me to place them higher on the bike trailer for better throw/dispersion. I want people to be looking up, too -- the trailer will be ~10' tall.
* I have no idea if I can trust the specs above. smile.gif
* I like to build stuff, and home-built stuff is celebrated at Burning Man. biggrin.gif


Here's my current, tentative system plan:

SPEAKERS:
* MTM active, ported 2-way
* Closeout $9 Aura NS6 woofers (1.15lbs, 91dB, 50w) http://www.parts-express.com/pe/pshowdetl.cfm?&Partnumber=299-030
* small horn with mini CD, such as the Denovo Mini Elliptical (super light, 100dB, 30w) http://www.diysoundgroup.com/dna-10.html
* enclosure: two sonotubes, capped by plywood on both ends. Here's a sketch:


ELECTRONICS:
* miniDSP for active 2-way crossover
* 5-channel car audio amp, to power HF and LF for the speakers + sub (these amps have high-pass and low-pass filters, which will give me a way to xover to the sub)
* the source will be a phone, and I'll use the phone to control the volume

SUBWOOFER:
* I'm thinking of building a long Sonotube (or tubes) as a stressed member of the trailer frame. The tube will be long, so I'd probably build two subs -- one in each half of the tube, with a divider in the middle. I might start another thread when I'm ready to work on that.
* targeting max SPL in a light package, perhaps @50+Hz. I am not pursuing deep extension.



It seems suboptimal to use two woofers. It must be possible to get more dBs for the pound using a single woofer. But the NS6 is inexpensive and super light. I modeled the lightest neo-magnet pro woofers I could find, such as a few 8" B&Cs and two NS6s are half the driver weight with about the same max SPL (~112 dB). The Deltalite 2510 can beat the output, but it weighs more (net 4.6lbs), and I think I'd have to use a larger waveguide and lower xo than I do with the NS6. Here's a MaxSPL chart for a 2510 and two NS6 (in a small enclosure, which kills extension) for comparison:

Created with GIMP

The NS6 models best with a 1.58 ft^3 enclosure, which is huge -- 10d x 36l" sonotube. But I played around for while in WinISD, and it seems to be okay in much smaller enclosures. You lose extension, and get a big response hump near the bottom. But I think I don't need the extension, and it seems pretty easy to EQ out the hump (with the MiniDSP). However, I don't have much experience with this stuff -- am I missing any important considerations with this small-enclosure / EQ-the-hump-away approach?

Are there any other midwoofers I should be considering? Weight is critical, so neo magnets are almost a must. I'd like to stay under 5lbs total for the driver(s). Cheap is also a bonus, as the $9 NS6 is hard to resist, but I'm willing to spend if the results will be markedly better.


If my woofers have a max SPL around 112 dB, I figure that I just need to match that with the horn, with response low enough to cross over (which should be easy with the NS6), for as little weight as possible. The Denovo Mini Elliptical handles 30w and has a sensitivity of 100 dB/w. I used this max spl calculator and got 114.8 dB @ 1m, which makes me think it will meet my needs:
http://myhometheater.homestead.com/splcalculator.html
Am I missing anything critical with this approach? Maybe dispersion -- what I should clarify that I don't want ear bleeders, but I'm also not pursuing maximum sound quality, either.

-Max
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post #2 of 2 Old 01-14-2013, 10:11 AM
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Sounds sweet, I think you are on the right track. You might want to take a look at this PE thread about a similar sounding set of bar speakers being built, using Selenium 8" mids and a Selenium 220 compression driver. http://techtalk.parts-express.com/showthread.php?235214-Build-Thread-the-otonimus-quot-Scurvy-Dogs-quot - You might get some good ideas based on a similar set of goals.

Other thoughts...
- Multichannel car audio amps are a cheap way to go, as you discovered.
- I wouldn't worry too much about the low bass extension, and possibly consider dropping the sub - most of the bass "slam" people feel is above 50hz, and not having to deal with that low frequency stuff will save a lot of juice. With no corner loading in such an open space, you really need to pile on the power to get a lot of truly low bass.
- MiniDSP can do some pretty cool shelving filters to tame the bass hump (though honestly a midbass hump may be desirable for presence, remember that you are in pro audio territory here, not hi-fi, and the focus on a ruler-flat response is not relevant). Using Charlie Laub's Active Crossover Designer would be the best way to get those fancier biquad filters programmed if you aren't an expert (I'm certainly not).

Also, where do you camp? I help run Pandora's Fixit Shoppe and Lounge, usually on 6 somewhere behind Center Camp - bike repair, whatever repair, and whatever booze of the day, plus 100% solar power.

-Sam
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