Unconventional Speaker Design - thread carried over from multiway forum - Page 5 - diyAudio
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Old 24th October 2012, 10:21 AM   #41
morgoe is offline morgoe  Australia
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Thanks guys. I bought a PVC pipe today, 37mm on the inside was the best I could get. Unfortunately, and I've just realised this now, my design is a lot shorter than the original. If I want an inch spare at the end of the pipe the most it could be is around 76mm. I'm using this calculator and it recommends a ~100mm pipe, unless I go for a smaller diameter.

I am a little confused as to what I should be setting the 'Tuning Frequency' in that calculator to. The Resonant Frequency of the driver is 97Hz, should I be setting the tuning frequency to the same?
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Old 25th October 2012, 02:48 AM   #42
morgoe is offline morgoe  Australia
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I've been doing a little research on ports. I'm still not sure what frequency I should be aiming for when tuning the box. Any quick help on this would be much appreciated. According to the calculator I'm using the original box is tuned to 90Hz, does that sound right?

I was also reading here about external ports. Would this be a reasonable idea, if I'm not able to get enough length without going to a really small-diameter pipe.

I can only put off my decisions about ports another couple of days. I have less than a week to finish the whole thing.

Final question is about the polyfill - this stuff is the same as what's put inside pillows, if my research is correct, right? I can just take it out of an old pillow? Or is it better to buy somewhere?

Last edited by morgoe; 25th October 2012 at 02:59 AM.
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Old 25th October 2012, 03:35 AM   #43
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Quote:
port size
http://p10hifi.net/FAL/downloads/Changing-Port-Size.pdf

dave
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Old 25th October 2012, 11:24 AM   #44
morgoe is offline morgoe  Australia
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Thanks. I don't think I can just simply convert it because - at least according to the calculator I'm using (posted a couple of posts above), slotted ports are calculated differently to non-slotted. The diagram has a slotted port, mine won't be.

According to the calculator the box is tuned to 86Hz I think, not 90Hz as I said above. Since the resonant frequency of my driver is 97Hz compared to the 95Hz of the original, I'll tune it to 88Hz (sound right?).

This means I'll be using a tube of 1.1in with a vent length of 2.7in. Hopefully I can get this size PVC pipe, otherwise I'll have to change it slightly. I'll buy another pipe tomorrow.
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Old 25th October 2012, 10:34 PM   #45
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What frequency it's supposed to be tuned to gets quite involved. There's such a thing as an "alignment" for a bass reflex cabinet. Thiele identified a couple dozen "maximally flat" alignments, and others continued the work. There are a lot of possibilities. The basic parameters are cabinet volume, taking box losses into account, the "Q" of the speaker, and the ratio of the tuning frequency to the resonant frequency of the speaker for that alignment.

This is all very interesting and worthy of study- I've put quite a few weeks into this subject recently- but my understanding is you are in that tunnel-vision deadline world of school where relevant avenues of study remain untraveled unless they affect your grades. Hence, my recommendation for trial and error based on a reasonable guess of the right starting point.

Possibly, this project may have whetted your interest to where you come back and make a deeper study of some of this fascinating field when you're not under school pressures. Good luck!
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Old 25th October 2012, 10:50 PM   #46
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Hi,

Go sealed and use an AV amplifier or 2.1 amplifier with a sub. 90Hz tuning
will overload all over the place for any low bass without highpass filtering.

rgds, sreten.
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Old 25th October 2012, 10:56 PM   #47
morgoe is offline morgoe  Australia
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Quote:
Originally Posted by tapehead ted View Post
What frequency it's supposed to be tuned to gets quite involved. There's such a thing as an "alignment" for a bass reflex cabinet. Thiele identified a couple dozen "maximally flat" alignments, and others continued the work. There are a lot of possibilities. The basic parameters are cabinet volume, taking box losses into account, the "Q" of the speaker, and the ratio of the tuning frequency to the resonant frequency of the speaker for that alignment.

This is all very interesting and worthy of study- I've put quite a few weeks into this subject recently- but my understanding is you are in that tunnel-vision deadline world of school where relevant avenues of study remain untraveled unless they affect your grades. Hence, my recommendation for trial and error based on a reasonable guess of the right starting point.

Possibly, this project may have whetted your interest to where you come back and make a deeper study of some of this fascinating field when you're not under school pressures. Good luck!
Yeah I would love to be able to put a great amount of research in this and get something that's going to be exactly what I want, but I have only 5 more days to finish this up, so I just don't have the time to spend on it. I'd love to come back to this later on when I don't have the pressure of a deadline.

Quote:
Originally Posted by sreten View Post
Hi,

Go sealed and use an AV amplifier or 2.1 amplifier with a sub. 90Hz tuning
will overload all over the place for any low bass without highpass filtering.

rgds, sreten.
I'd rather not put a sub into this, that seems like much more effort. What tuning would you recommend?
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Old 25th October 2012, 11:52 PM   #48
morgoe is offline morgoe  Australia
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Originally Posted by morgoe View Post
I'd rather not put a sub into this, that seems like much more effort. What tuning would you recommend?
Sorry I've realised this is a crazy question. What are the risks of tuning to around 85Hz. Why would this cause overloading? Is it better to tune a bit higher? The frequency graph Click the image to open in full size. is pretty flat up until about 130Hz, it seems pointless to tune higher than 100Hz.
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Old 26th October 2012, 12:56 AM   #49
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I believe Sreten is referring to the fact that -at- the tuning frequency, the cone's motion will be at its minima, that is, barely moving at all. Virtually all the energy is coming out of the port at that point. So the cone is protected from excessive travel (excursion) and thus distortion is reduced.

But below that frequency, this type of cabinet design offers no protection to the driver. So the driver will (unless high-passed) exceed its mechanical limits as the cone excursion increases drastically, thus increasing distortion. Fortunately, at some point, the driver self-destructs.
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Old 26th October 2012, 01:14 AM   #50
morgoe is offline morgoe  Australia
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Originally Posted by rjbond3rd View Post
Fortunately, at some point, the driver self-destructs.
Well that's good news.

I'm a little confused though. Are you saying that I should tune the box above or below the speaker's resonant frequency? And I've heard that within a box, the speaker's resonant frequency increases - the 97Hz in the specs is in free air, correct?
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