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Old 14th December 2009, 06:21 PM   #1
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Default Box design

Hello!

I've used WINISD to design a box for a pair of 2-way speakers (11cm woofer), and well ,I finaly decided that the box should be about 30L in bass reflex configuration to get some "good" bass.

I want to make a box like the one in the picture, but I`d like to know your opinion. Will it have resonance problems with such a "tall" box? Will it affect much the position of the port? (for example in the botom or in the middle)

Click the image to open in full size.

I've also thought about making a TL, but I canīt afford any of the software that is avariable for this single desing...(Is there any free version of the Martinīs mathcad models or TLwrx???)

thanks a lot!!!!
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Old 18th December 2009, 06:55 AM   #2
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Nobody? Should I move this thread to another sub-forum? Thanks
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Old 18th December 2009, 07:29 AM   #3
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The image you posted is really small... but it is large enuff to tell that it is tall enuff that it is going to be a TL, but with the port up by the driver it is going to be strange. The port would need to be near the bottom to take full advantage of the 1/4 wave resonance.

It also looks like a 2-way, which may be why you got little response.

What driver(s)?

You can download Martin's tables and the accompaning spreadsheet to model simple TLs.

dave
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Old 18th December 2009, 08:03 AM   #4
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...alternatively, you can buy a year's personal DIY license of his worksheets for the rather minor sum of $25. If you can afford to buy drivers, material etc., then it's not a vast expenditure. If you need to save up for a month, do so. You'll be better off in the long run.

Re the lack of response, as Dave points out, your query was regarding a 2 way, and this is the section of the forum about wide-band drivers. FWIW, I would expect you to run into more or less severe resonance problems with a box of that aspect ratio if the design does not take these into account.
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Old 18th December 2009, 06:06 PM   #5
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Thanks for your replies!!

Well... I think I better start in the right way...telling you the whole history.

My drivers are Hertz ESK130, which is a mid/low cuality car-audio two-way system:

http://www.hertzaudiovideo.com/Doc/pdf_esk130.pdf

The woofer is EV 130,and his electro-acustic parameters are:

Click the image to open in full size.

As you see, is a 11mm woofer, and low-bass response is very limited (its supposed you willl use a sub-woofer for that in a car...).

The thing is that this system is the one I have,and I can not change it... I just want to adapt it for use at home the better way it could be done (I mean...I do know these drivers were designed for car-audio, installed in a door... not for home use)

Anyway...I used WinISD, and made some tests. First, although I get an EBP near 90, i tested the closed box. This was one of the results, with a 10L box:

Click the image to open in full size.

Then, I did some tries with vented box (simple bass reflex). I think the best configuration is around 20L and tuned near 50Hz:

Click the image to open in full size.

(i got a group delay peak as high as 28ms around 45Hz....but i got similar results with any box volume or tuning frecuency)

The thing is... if i use optimum dimensions for 20L, the box looks very big and "ugly" for such an small drive, and because of that I wanted to build a tall and slim box, which looks better for home. Here is the picture again (good sized xD)

Click the image to open in full size.

So, planet10, it is just a "tall" bass reflex box, not a TL. But I don't know if it will work fine, because, as Scottmoose sais, that aspect ratio could generate problematic resonances... and that is what I was asking xD

But, I also thought about building a TL, because the size of the box will be similar.

I tried using Martinīs tables and the Excel worksheet, (I would do a classic TL), but those tables just can be used until Fs of 70Hz, and the EV130 have an Fs of 89Hz.

And well, I have very limited resources (the drives were free, a gift from a friend who changed his car woofers)...I know 25$ is not much for a year license...but for a single design...is too much. Well... after the license expires (no more updates), I can use the worksheets I downloaded until I want, isn't it? That's why I asked if there were no free old versions or something..

Thanks!
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Old 18th December 2009, 06:57 PM   #6
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Quote:
Originally Posted by TonySnes View Post
Anyway...I used WinISD, and made some tests. First, although I get an EBP near 90, i tested the closed box. This was one of the results, with a 10L box:

...

Then, I did some tries with vented box (simple bass reflex). I think the best configuration is around 20L and tuned near 50Hz:

So, planet10, it is just a "tall" bass reflex box, not a TL.
The Q of your driver is quite high, any sealed box, no matter how large will have a Q higher than i really like to see, your sim has what i would consider a big bump at the bottom. The BR sim you show is worse, you want a smooth roll-off not a rippled one.

And because of the high aspect ratio of the box it will be a (non-optimal) quarter-wave resonantor (a TL) and not a BR, so the sim does not accurately represent what is happening.

What i would do with this driver, is to build as large a sealed box as you can live with (ie the one you have), then place a series of aperiodic vents at the very bottom on the back, and then damp appropriately to make it aperiodic.

You can see an examnple of what i mean by aperiodic vents in this picture:

Click the image to open in full size.

and a basic configuration of how to arrange the damping in the attached cross-section of the PEARL PR-2.

dave
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File Type: gif perkinsPR-2-sm.gif (38.0 KB, 57 views)
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Old 18th December 2009, 07:12 PM   #7
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*Edit -Dave beat me to it*

You're saying you don't think your music is worth $25 / the price of a couple of CDs? Oh well, to each their own.

What Dave is pointing out is essentially the same thing as I was: a tall slender box will end up with a powerful vertical standing wave and end up as an MLTL (Mass Loaded Transmission Line), or MLQWR as I prefer to call them (Mass Loaded Quarter Wave Resonator). You can try to break it up a bit internally, but that doesn't work all that well. Bass reflex is based purely on helmholtz resonance theory, which assumes uniform air particle density in the enclosure and no standing waves. Soon as you move away from that, things start to change; the transition from one load to the other occurs when you need to change the vent dimensions compared to what BR principles would suggest in order to maintain the same Fb.

Anyway, looking at the driver specs. you've detailed, it certainly wouldn't be my first choice for a BR; sealed or aperiodic would be far better bets. Alternatively, you could go with a simple, untapered Alpha TL (straight pipe, sealed at the top, open at the bottom). See attached. This one is set for flattest impedance; 29in long, pipe CSA 48.5in^2 (make it any shape you like providing the area remains the same), driver mounted as close to the top as possible & stuffed 2.2lbs ft^3 with dacron hollow-fibre material or similar. Nice damped response, F10 about 50Hz.
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File Type: gif Alpha TL.GIF (5.6 KB, 66 views)
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Last edited by Scottmoose; 18th December 2009 at 07:16 PM.
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Old 18th December 2009, 07:55 PM   #8
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I have taken the liberty of moving this to Multi-way.
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Old 18th December 2009, 09:26 PM   #9
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Quote:
Originally Posted by Scottmoose View Post
...stuffed 2.2lbs ft^3 with dacron hollow-fibre material or similar
Note that with that damping density you have another (also simplier & modeled) approach to an aperiodic box. Essentially another way of saying the "set for flattest impedance" bit.

dave
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Old 18th December 2009, 09:50 PM   #10
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True. TBH, I was thinking of it purely from the POV of the driver spec[s] as related to Fp rather than a damping perspective per se. Probably as close as you're ever going to get to a 'perfect' TL (or aperiodic load if you prefer... )
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