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Old 6th May 2011, 11:48 AM   #1
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Default Another (small) Boombox Build

I built a boombox 2 summers ago by simply cramming a pair of 2-way 6.5" car speakers into a flight case along with a Sure amp, 7Ah SLA & 40W SMPS.

Click the image to open in full size.

Click the image to open in full size.

The result works fine, it plays loud & long, it can take a serious beating because of how strong the flightcase is, however no thought whatsoever went into the size/shape of the enclosure for the speakers, I simply found a case that was deep enough & a convenient size to carry.

So this summer I want to build a new boombox but this time do it properly & actually design it before I start building & actually use the best drivers for the price rather than the cheapest car speakers on ebay.

The Boominator is far too big & heavy - I want something in a similar form factor to my current boombox. Most importantly it must be quite 'slim' so that it can be carried on a shoulder strap like a messenger bag. My current boombox is perfect for this, the Boominator's depth isn't.

I know the very basics of enclosures, I can distinguish ported from folded horn, but I've never actually designed an enclosure before, so I need help.

I'm thinking of going for a mono design using 1 larger mid/bass rather than stereo with 2 smaller mid/bass. I'm currently thinking that 10" is a feasible size. Last night I quickly threw together this design, using a Fane Sovereign 10-125 & Fane FPT-80. I have no idea if those drivers are actually suitable together, but they were reasonably cheap branded drivers I found on CPC.

Click the image to open in full size.

Click the image to open in full size.

The 10-125 says it needs a 25-50 litre enclosure. The one in my design is just over 30. What does the size of the enclosure actually mean to the sound? What difference will there be between a 25 litre & a 50 litre enclosure? How much smaller could I make it if I ported it? And how would I stop sand from the beach getting into the port...
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Old 6th May 2011, 12:12 PM   #2
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forget the FPT-80, look into the CD130, almost the same price and sounds so much better! (FPT/APT series just sound a bit nasty to me)

the volume of the enclosure dictates the low frequency response. optimum size would probably be around 30L, with porting you could probably get the F3 for this driver down to around 80hz, which is as low as you can realistically get .

I have a very similar design with two of the 10-300s, CD130 in a 70L enclosure and have a -3 under 70 which sounds pretty beefy

getting rid of the internal divider would give you some free space easily!

Last edited by ZFRaudio; 6th May 2011 at 12:21 PM.
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Old 6th May 2011, 12:12 PM   #3
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forgot to mention, you'll need a cheap screw-on horn to go with the CD130

I used this horn: http://www.bluearan.co.uk/index.php?id=FANFH195

or look at this beyma: http://www.bluearan.co.uk/index.php?...emode=category

^never heard this or even realised it existed, but it looks like pretty good value for money . and beymas compression drivers are generally excellent!

Last edited by ZFRaudio; 6th May 2011 at 12:19 PM.
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Old 6th May 2011, 12:18 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

You could use stereo 4 ohm amplifiers arranged to drive two tweeters directly
and via the crossover arrangement drive one 8 ohm bass / midrange bridged.
(search for details of this one channel inverted arrangement, car audio ?)

Checking the drivers parameters indicates ~ 30L sealed and optimally stuffed.

rgds, sreten.
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Old 6th May 2011, 12:27 PM   #5
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Forget what the manufacturer specifies as optimum enclosure size. That's for indoors use. Outdoors and for a boombox the most important thing is to combat the bass loss, and not have your driver flapping in the wind playing frequencies that you can't hear anyways. So you need a much higher Q than you'd normally use. My preferred tuning (for this use) is Q=1.2 which gives a nice wide lift between 100 and 200Hz where it's needed, and make sure everything under about 90-100Hz isn't played at all.

To achieve that you just make a box calculation for a standard "optimum" BR tuning of Q=0.7. You then divide the Vb with Pi, and multiply the Fb with the square root of 2.

Simple really. And you get a miniscule box that plays all the bass you can actually realistically achieve outdoors.

Last edited by Saturnus; 6th May 2011 at 12:30 PM.
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Old 6th May 2011, 12:34 PM   #6
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Thanks for the advice

The divider is mainly there to provide something to mount the SMPS/battery/amp to. What are the implications of removing the divider & having these components within the woofer's enclosure? Will it affect the sound that there are odd shapes jutting into the space? Could I go half-way & keep the divider but cut holes into it to allow air to move through? Would the enclosure benefit from any other baffling, maybe vertically?

I was planning on using a single TK2050/TDA8920 amp bridged, maybe something like this;

TDA8920 BTH Class D Digital Amplifier Board 2X100W | eBay UK

That should be able to give 50-60W at 8ohms which will be plenty. I've never used electro acoustic tweeters before, do I still need to have a crossover between the mid/bass & the tweeter? From looking at Boominator builds it looks as if they are just wired in parallel.
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Old 6th May 2011, 12:37 PM   #7
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Quote:
Originally Posted by Saturnus View Post
Forget what the manufacturer specifies as optimum enclosure size. That's for indoors use. Outdoors and for a boombox the most important thing is to combat the bass loss, and not have your driver flapping in the wind playing frequencies that you can't hear anyways. So you need a much higher Q than you'd normally use. My preferred tuning (for this use) is Q=1.2 which gives a nice wide lift between 100 and 200Hz where it's needed, and make sure everything under about 90-100Hz isn't played at all.

To achieve that you just make a box calculation for a standard "optimum" BR tuning of Q=0.7. You then divide the Vb with Pi, and multiply the Fb with the square root of 2.

Simple really. And you get a miniscule box that plays all the bass you can actually realistically achieve outdoors.
Sorry, I have no idea what Q or Vb or Fb are! As I said, I've never designed enclosures or done any enclosure calculations before. The Boominator thread says that anything below 100Hz is useless outdoors, but I will also be using my build indoors fairly often so is it worth sacrificing the lower frequencies when indoors to improve the sound whilst outdoors?
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Old 6th May 2011, 12:41 PM   #8
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The Boominator uses (high quality) piezos, at least in it's intended build. Those are completely different from standard dynamic speaker as they in principle don't need a filter, they do however need a series resistor or they'll sound pretty horrible.

If your woofer(s) are very wideband you might consider this as the piezos will only play above about 4-5KHz if used correctly. And let's face it, in a boombox meant for road use, we *can* relax a little on absolute sound quality in exchange for practicality.
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Old 6th May 2011, 12:43 PM   #9
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Quote:
Originally Posted by lost eden View Post
Sorry, I have no idea what Q or Vb or Fb are! As I said, I've never designed enclosures or done any enclosure calculations before. The Boominator thread says that anything below 100Hz is useless outdoors, but I will also be using my build indoors fairly often so is it worth sacrificing the lower frequencies when indoors to improve the sound whilst outdoors?
You can also go for the half-way point and make the Q=1.0, that's the optimum Q for use in large rooms. It will require a larger box size though.

Oh, and you should probably read up on such things when you want to design a box It's instrumental to understanding what you're actually doing.

Last edited by Saturnus; 6th May 2011 at 12:51 PM.
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Old 6th May 2011, 12:49 PM   #10
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So I take it the Fane CD130 will need a crossover? I see a magnet on the back of the driver at least, so it can't be piezo...

The Sovereign 10-125 quotes a usable frequency range at -6dB of 50Hz - 4kHz & the CD130 2kHz - 18kHz so I was planning on crossing them at 2.5-3kHz?

Is there a good place to read for a basic introduction to enclosure design?
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