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Old 22nd March 2012, 10:46 AM   #1
RoboJ1M is offline RoboJ1M  United Kingdom
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Default Learning the basics of speaker cabinet design

Hi,

My names Jim and I'm just starting to learn the basics of speaker cabinet design.

Our first project is a man portable PA built into a British army bergen (rucksack)

I'm trying to get my head around calculating the dimensions of an ordinary sealed cabinet.

The drivers we will be using are here (PDF), because they're cheap, cheerful but come with a set of thiele small numbers.

I've used a calculator I found here (web page)
That worries me though because it's completely black box and is named as a designer for subwoofer enclosures. I don't know whether the maths is different for full range speakers.

Anyway, the bag minus batteries and amp leaves us with about 50liters of space for the cabinet.

Now, putting the numbers into that calculator get me a Qtc of 1.

So far I've read that you're supposed to aim for 0.707 but 0.5 to 1.2 is a usable range.

Unfortunately I don't have all the measurements and dimensions to hand, but I keep not getting round to making this post so I'm just taking the plunge.

Sound quality is not top priority in this project, it's just slightly behind:

1) Cost, apart from the fact nobody has much cash to throw around, the Mark 1 Noisy Bag is intended to be followed by more refined designs so we don't want to spend too much on something that will probably be just the first attempt.

2) Just Getting It Done. Seriously, we've been procrastinating for about 2 years now. It's time to get the hammers out.

3) And then sound quality, because there's no point in putting in all this effort/money if it's going to sound bad.

In fact it's really about striking a balance between all of the above 3 factors.
Speed, Quality and Cost.
Hmm, wait a minute, that reminds me of an old adage... ;-)

Anyway, I'll try to draw up and post some more details this evening (can I upload napkins and fag packets, back of? Does that work )

Anyway, thanks for reading the ramble.

J.

Last edited by RoboJ1M; 22nd March 2012 at 11:45 AM.
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Old 24th March 2012, 07:57 PM   #2
Simon B is offline Simon B  United Kingdom
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Hi Jim,

I'm not going to ask quite why this is to go into a bergen, I'll just presume you're a noisier than average member of the rambler's association.
I should tell you that it's so long since I designed a speaker box that my information came off printed pages and I used a handheld calculator.

Sealed box and vented box are ok for your purposes, bandpass isn't. Are you going for sealed box because it's the simplest? A vented box will get lower cutoff frequency for a given volume, or smaller volume for given cutoff.

Vented box design is more involved and there's less protection for the speaker from damage by being fed high levels of signal below cutoff.

All this is jumping the gun a bit though, you've decided on a driver unit, but the link you give is actually to a specification for two different speakers,
Slick 69.2, a 140W and Slick 69.3, a 160W version.

Maybe if you say which you're talking about, and quote the figures from those specs that you've put into the calculator you've used you'll get more replies on here?

The other thing that I notice is that the specs don't give a figure for where the top end, frequency-wise, of these speakers is - you really need to know that. Googling on "slick 69.2 speaker specification" brings up a few different units, you're appparently talking about a round one, 220mm dia.

The oval 6" x 9" units have a concentric built in tweeter, and are supposedly ok up to 25kHz, high enough - maybe the round 220mm ones do to?

Given the lack of information, the quip about fag packets probably sounds:

1. A bit lazy to British readers
2. Something very different to Americans!

Hope this helps!





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Old 24th March 2012, 08:36 PM   #3
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The drivers you have choosen will not make a box with Q < the Qts of the driver, both of which are greater than max-flat 0.707.

As car speaker they ar edesigned to be mounted somewhere where they are in essentially a large leaky box.

The Vas parameter is in an unknown unit -- for now i'll assume it is meant to be litres.

The lower Vas unit alsohas the higher Q, but with a limited size box you'll end up in about the sameplace... you are best to make the box aperiodic.

dave
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Old 25th March 2012, 06:15 AM   #4
dangus is offline dangus  Canada
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I'm just going to suggest buying a plastic foam-insulated picnic cooler from a charity store. That would get you a light-weight but reasonably stiff enclosure. Pack it with pillow stuffing to reduce/eliminate the honking from internal standing waves. As for lightweight speakers, some OEM car systems come with incredibly lightweight speakers using neodymium magnets. Those may get chucked out by car audio installers who install aftermarket speakers.
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Old 25th March 2012, 10:08 AM   #5
Simon B is offline Simon B  United Kingdom
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Default Weight

Dangus makes a very good point about weight there, the difference with neodymium magnets is dramatic.

I once thought that a package containing a pair of
bass guitar speakers must have been shipped empty until I opened it, it's that much of a difference.

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Old 25th March 2012, 10:14 AM   #6
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by RoboJ1M View Post
Hi,

My names Jim and I'm...
Hello Jim, and welcome to AudioAddicts anonymous.

Seriously though, is this going to be used mostly for speech or music? I ask because the requirements for good speech intelligibility are rather different than for decent quality music reproduction.
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Old 30th March 2012, 02:07 PM   #7
RoboJ1M is offline RoboJ1M  United Kingdom
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Oh crap, sorry guys, I didn't realise there were so many replies!
That's embarrassing.
Right, I really will sit down, reply to all your replies and write up our ideas/designs properly this evening (it's 3pm right now here)
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Old 2nd April 2012, 06:26 PM   #8
RoboJ1M is offline RoboJ1M  United Kingdom
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OK, I've written up the sketches into something resembling a design.
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Old 2nd April 2012, 06:34 PM   #9
RoboJ1M is offline RoboJ1M  United Kingdom
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Quote:
Originally Posted by Simon B View Post
Hi Jim,

I'm not going to ask quite why this is to go into a bergen, I'll just presume you're a noisier than average member of the rambler's association. I should tell you that it's so long since I designed a speaker box that my information came off printed pages and I used a handheld calculator.


Quote:
Originally Posted by Simon B View Post

Sealed box and vented box are ok for your purposes, bandpass isn't. Are you going for sealed box because it's the simplest? A vented box will get lower cutoff frequency for a given volume, or smaller volume for given cutoff.

Vented box design is more involved and there's less protection for the speaker from damage by being fed high levels of signal below cutoff.
Indeed, I'm probably going to start with sealed as it's the simplest.
Also it's a reference point, we're actually going to try to build it so the speaker it just a box bolted to other boxes and you can change it for a different design.

Quote:
Originally Posted by Simon B View Post

All this is jumping the gun a bit though, you've decided on a driver unit, but the link you give is actually to a specification for two different speakers, Slick 69.2, a 140W and Slick 69.3, a 160W version.

Maybe if you say which you're talking about, and quote the figures from those specs that you've put into the calculator you've used you'll get more replies on here?
Whoops, we're going to use the Slick 69.3.

Quote:
Originally Posted by Simon B View Post

The other thing that I notice is that the specs don't give a figure for where the top end, frequency-wise, of these speakers is - you really need to know that. Googling on "slick 69.2 speaker specification" brings up a few different units, you're appparently talking about a round one, 220mm dia.

The oval 6" x 9" units have a concentric built in tweeter, and are supposedly ok up to 25kHz, high enough - maybe the round 220mm ones do to?
We're going to be using the 6x9, 160W version.

Quote:
Originally Posted by Simon B View Post

Given the lack of information, the quip about fag packets probably sounds:

1. A bit lazy to British readers
2. Something very different to Americans!
Sorry, I just have limited time to get this stuff done, but I've now written up the napkins into something like a design (see above post)

Quote:
Originally Posted by Simon B View Post

Hope this helps!
Indeed it does!

Thanks,

J.
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Old 2nd April 2012, 06:38 PM   #10
RoboJ1M is offline RoboJ1M  United Kingdom
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Quote:
Originally Posted by planet10 View Post
The drivers you have choosen will not make a box with Q < the Qts of the driver, both of which are greater than max-flat 0.707.

As car speaker they ar edesigned to be mounted somewhere where they are in essentially a large leaky box.

The Vas parameter is in an unknown unit -- for now i'll assume it is meant to be litres.

The lower Vas unit alsohas the higher Q, but with a limited size box you'll end up in about the sameplace... you are best to make the box aperiodic.

dave
Thanks Dave, the software kept complaining that the Q had to be more than the Qts of the driver, now it makes more sense why.

I'll go Google aperiodic!

J.
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