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Old 8th April 2007, 11:08 PM   #1
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Default Reverse engineering resonant frequency

Okay, so I'm VERY new to DIY Audio (the hobby; even newer to this board). I have a friend who has been designing and building for years. He completed his degree in some sort of electronics engineering and got hired to design high end audio processing devices for someone I've heard of. We promptly lost contact with each other.

Anyway, he taught me a lot about the principles of sound, loudspeaker design (why Bose sucks was the first lesson), enclosure types, crossovers, etc. So I have a good foundation. And I've been studying online for weeks. (It's driving my wife nuts.)

For my first actual project I picked up some JBL G300s (not exactly high end, I know; just decent and perfectly usable for what I have in mind). The enclosure was damaged when the previous owner dropped one. So I pulled it all out and now have two 1" tweets, two 5" mids, and two 8" woofs. (Topic for another day: Why does JBL call these 8" woofers if they only measure 7" to the edge of the surrounds?)

They sound good and almost as importantly for this project, they look good. I'm building the enclosure out of 3/8" clear acrylic. Why? Because it's going to look fantastic, it's really fun to work with, it welds chemically to form perfect seals, and I work in a print shop that uses tons of the stuff and there's plenty of scraps.

I want to do a sealed design. But I need to know how big to make the box. I don't have ANY equipment to analyze the performance so I want to reverse engineer the enclosure that they were sold in to (hopefully?) understand the drivers.

Is this realistic? The one that was still held together well performed well with my Sony receiver. I know that's not impressive to anyone here, but everyone has to start somewhere, right? So it sounded good in my tests and that makes me think it was designed not just thrown together.


Here are the internal dimensions of the enclosure in case someone can help me understand this. I'd rather you tell me how to figure it out or show me all of the the math instead of just telling me the answer, if this is even enough information to work with.

9.5"x20"x8.25"
Port hole in the mdf: 2.875"
Port is 3.875" long, with an external opening of 2.75" tapered down to 2.5" inside the box.
There's a 5" deep dome inside the box insulating the mid from the woofer. Don't know the exact volume there.
There's also some lightweight dampening material against the walls. I'm not sure if that's supposed to be considered when calculating volume.

JBL's website says 50 Hz to 20 kHz, crossed over at 600 Hz and 3.5 kHz. 89dB sensitivity. 8 ohms, 10-50 watts.


So what's the next step? Thank you for your help!


- Jon
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Old 8th April 2007, 11:52 PM   #2
Geoff H is offline Geoff H  Australia
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Hi Jon, welcome to the forum.

First of all, the cabinet resonance could be calculated, but that may not be the resonance of the driver. A bass reflex can be tuned above or below the driver resonance. My guess is that JBL would have gone below.

You really need to determine the resonance of the driver. If going to a sealed cabinet, it would be fairly safe to increase the BR volume by about 20%.

The cabinet should be constructed from sturdy material. I think using clear acrylic will result in a cabinet that will vibrate so much, there will be more sound coming off the panels than the driver. Not good. Hardwood ply is the best choice there, 3/4". MDF would be the second choice. Particle board has it's place elsewhere.

Drivers are usually sized by the OD of the rim.

If you have a PC with a sound card and a small amp, there are programs that allow you to generate tones. With a Digital Volt Meter and a suitable resistor, you can measure the Fr of drivers. TrueRTA is just one of many. Many cabinet designs require more parameters to get the most out of a given driver. Known as T/S specs.

In regard to your good lady, open a charge account at the florist. LOL.

If you post the numbers on the drivers, someone may be able to help.

Geoff.
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Old 9th April 2007, 01:39 AM   #3
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hello jon !

if you want to go closed, the size of the box is the least of your problem really ... build a test box a little bigger than the old one, and fill it until you got bass up to your liking. That wont make the trick for the crossover though , (unless you plan to use the old one) so if you want to design your own, you just gotta invest into some measurement equipment (not necessarily over expensive)

also if jbl says 50 hz in a ported box, you aint gonna get a lot of bass below 70- 80 hz in a closed one, wich ain't really bass for me even if some people can live with that. If i was you i would do a box of the same volume with a port of the same characteristics.

also, geoff is very right about acrylic. If you really wanna do it you need to brace it extensively and then damp it, and even maybe apply some hardening surface treatment. Not easy in anyway, just keep in mind that there are million ways for loudspeakers to suck other than bose's way, and a resonnant enclosure is one. If you feel like you could do it the right way , go ahead, but if you are new to this, mind that you're more safe going the same way than other more experienced people did

have a good day !
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Old 9th April 2007, 06:18 AM   #4
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Thanks for the responses.

Yeah, the general response to the acrylic plan is skepticism, but I assure you it's at least as good as any cellulose-based material at least for a sealed enclosure. If I was using 1/8" or so, or not reinforcing it, I'm sure we'd have a problem. But I've seen some very high end stuff done in acrylic that was really impressive.

I'd show you the design I have in mind but it's hard to when the actual proportions aren't finalized. But seeing it would give you an idea about the reinforcement I'm doing. If I could afford it I'd use 1" acrylic, but I really can't. 1" is rigid enough to do it without reinforcement (or at least without much). But the reinforcement is an opportunity for more artistic design. I will be cutting the parts on a CNC router. It cuts anything I draw in Illustrator.


You answered one of the other questions I have: how accurate do I need to be for the volume. I've seen designs where the designer used water displacement to calculate the volume of the crossover components and wiring in order to tune the port. I just don't know how much of a problem I'm going to see from being off by a few cubic inches.

For that matter how bad is it going to be if I was off by a full cubic foot? Would it absolutely suck or just not sound as good as it could? How much does the cabinet need to vary from mathematically perfect to notice a change? This is the sort of thing I want to understand, which is why I'm starting with enclosure design for my first project.

Again, I know acrylic can work because I've seen it done and I work with acrylic regularly. I may do an mdf setup first to test the design. We have a lot of scrap maple veneer mdf as well. But the final box will be acrylic.


- Jon


By the way, in case anyone is unfamiliar, acrylic is also known as plex, plexiglass, polymethyl methacrylate (pmma), Acrylite, Polycast, Lucite, and other names.
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Old 9th April 2007, 08:12 AM   #5
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Fun project, I can't wait to see pics when it's finished.

OK, you say you can't measure, do you have a PC? If so, you could use something like Speaker Workshop to test, and will get much better results in the end. As for the box volume, 10% either way won't make much difference, so just be as good as you can with your calculations, and you should be fine. Building on the bigger side is always best, you can always reduce the volume with blocks, but you can't add volume to a completed enclosure as easily.
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Old 9th April 2007, 08:45 AM   #6
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Windows is not allowed in my home. I have two Macs (a PowerMac and a PowerBook) and a PC loaded with Linux.

Anybody recommend anything there? I really don't know ANYTHING about measuring these things.

Incidentally I finally remembered where my friend went to work. Harman. Heard of 'em?

I remembered when I posted an ad on craigslist and got a reply from my old scout master (wow, that was like 13 years ago) and his email signature identified him as working at Harman. (www.harman.com)

How odd is that? Hopefully he'll stop by to pick up the thing he wants to buy and I'll invite him in for a bit to "catch up" and he can see my project sprawled out on the floor and he'll have nothing better to do than to do some measurements for me. After all, these are JBL units and JBL is part of the Harman Audio Group. (HAG? Not an attractive acronym...)

I suppose I'm just daydreaming now. I remember when my daydreams involved women and cars...


- Jon
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Old 9th April 2007, 11:53 AM   #7
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Default Re: Reverse engineering resonant frequency

Quote:
Originally posted by jonkun227
it welds chemically to form perfect seals
Do you know the secret formula, Jon?
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Old 9th April 2007, 12:02 PM   #8
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Quote:
Originally posted by jonkun227
Windows is not allowed in my home. I have two Macs (a PowerMac and a PowerBook) and a PC loaded with Linux.
Good man, I have a PC just for measuring, and that's all!

Can't find the link, but google fuzzmeasure, I think that has a demo version.
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Old 9th April 2007, 07:47 PM   #9
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Default Re: Re: Reverse engineering resonant frequency

Quote:
Originally posted by Inductor


Do you know the secret formula, Jon?
Hmm. Can't tell what you're actually asking. Not sure if you're asking what to use to weld acrylic or if you're saying something else. My hunch is something else. But I use a commercial product called Weld-On that contains Methylene Chloride. I have to methods:

A) If the pieces fit together perfectly I clamp them together and apply a few drops of this very watery solvent which is drawn by capillary action into the joint. It dissolves both surfaces which are then chemically welded when the joint cures. (Sticks in about 2 minutes, and it's unbelievably strong in 24 hours.)

B) I have a separate jar of acrylic chips dissolved in solvent. It's a thick, syrupy form that fills gaps when the joint isn't perfect. Sometimes the saw blade chips the material and this fills it in flawlessly. I can also pour it into the holes where you would normally use a screw. It fills in the threads and I cap it off similarly to a screw head and I end up with a really good looking connection without hardware.

Of course that makes it hard to make any changes later on...

I polish the edges with a very fine point blowtorch, which melts the rough saw-cut edges and restores the glassy finish.

Quote:
Originally posted by pinkmouse


Good man, I have a PC just for measuring, and that's all!

Can't find the link, but google fuzzmeasure, I think that has a demo version.
Yeah, I'm thinking of moving all the files on the PC over to the PowerMac (Linux is only being used for a file server, which the PowerMac is perfectly capable of), so I can install Windows. But I really don't want to have to. Neither of my Macs are fast enough to handle Virtual Windows, so that's out.

Thanks for the lead!


- Jon
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Old 10th April 2007, 12:34 AM   #10
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