Unconventional Speaker Design - thread carried over from multiway forum - Page 6 - diyAudio
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Old 26th October 2012, 01:27 AM   #51
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The resonant frequency increases relative to Fs in a sealed box. But this is vented of course. The usual starting point is the driver's Fs. Then you can adjust it up or down, but in this case, tuning to Fs is what I'd do.

And then I'd only play it softly at first, with some smoky old jazz records or chamber music (since it's not high-passed). Dub-step would not be advisable.
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Old 26th October 2012, 03:50 AM   #52
benb is offline benb  United States
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Quote:
Originally Posted by morgoe View Post
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?
Yes, the spec is measured in free air.

Tuning the cabinet to (pulling a "reasonable" number out of the air) 80 Hz may give you a reasonably flat response down to 75 Hz or so, BUT ...

The problem is the frequencies BELOW this 75 Hz. Most recorded (pop) music material has a lot of energy well below 75 Hz - the standard electric bass guitar goes down to 40Hz, and the most energetic part of a kick drum is perhaps in the 30 Hz to 70 Hz range. If you put such music into the speaker, there's no "back pressure" at these frequencies, and the cone will have rather large excursion, but not much sound will be emitted at these frequencies (unless you put your ear right up to the cone, but that's cheating). If you play it too loud, even though it may not SOUND "too loud," these lower frequencies could damage the driver. SLOWLY turn up the volume and watch how much the cone moves with the bass.

A sealed box won't extend the low frequency response as much as a (bass reflex) vented port, but it will have the "back pressure" at these lower frequencies, and help protect the driver.

It's possible to make a passive "crossover" or high-pass filter that will take out the frequencies below 70-80 Hz and protect the driver in a bass-reflex cabinet from these frequencies, but for such a low frequency the components are both expensive and physically large, and so it's not practical.

I think it's good advice to have this as a sealed cabinet, at least for when you're showing it off and someone else might turn up the volume. You can make the port later. This will also give you a good idea of the difference between the response of a sealed cabinet versus a bass reflex.
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Old 26th October 2012, 07:38 AM   #53
morgoe is offline morgoe  Australia
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Ok, thanks for that. I'll take that into account when I decide what to do. I'm never going to have these very loud - they're in my living room with loads of other people. I'd like to have something I can reference for my audio engineering work, but the accuracy of the lowend is not a huge issue as my room isn't treated. Something pleasant to listen to for music, video games, movies, etc. at medium volumes is my aim.

Just out of curiosity - will boosting the lowend with a software EQ (or a baffle step compensation circuit) have the same risks?

I'm going to glue the front + rings together tonight and tomorrow, then I can glue the front on, attach the speakers, pack it with polyfill (again - what's the best source for this?), then I can decide about the port and glue the bottom on. Oiling and sanding wherever fits best.
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Old 26th October 2012, 04:55 PM   #54
benb is offline benb  United States
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Googling shows polyfill at Parts Express:
Acousta-Stuf Polyfill 1 lb. Bag 260-317

but the stuff at Walmart should do fine:
Mountain Mist Fiberloft Polyfill Pillow Stuffing - Walmart.com

I recall that baffle step compensation is only 6 dB (or is it 3, I forget), and shouldn't be a problem. Using a bass tone control or software EQ can boost things by 12dB or more, and at medium to high volume can be a worry for any system. The problem isn't always overexcursion of the cone, it is often the power handling (heat generated) of the voice coil.
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Old 26th October 2012, 06:41 PM   #55
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Quote:
Originally Posted by morgoe View Post
pack it with polyfill (again - what's the best source for this?),
Sewing stores/craft stores. Big chains that cater to these markets (Wally World, Target (was Zellers), The Bay, Sears in this neck of the woods)

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Old 26th October 2012, 07:33 PM   #56
sreten is offline sreten  United Kingdom
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Hi,

FWIW there is nothing remotely unconventional about this speaker design
and this thread is full of the conventional mistakes and misunderstandings
of those that know very little to nothing about speaker design.

For the project build them sealed and use EQ (MP3 or PC) to balance them.
They wont go very loud so tout them as very good nearfield PC speakers.
(The good drivers will cream most PC speakers for volume and bass.)

In the longer run for a usuable pair of speakers your going to need a stand
to bring them to ear level and this may as well contain some half decent
bass units, as they simply cannot do bass farfield well, at all, too small.

The stand being an entirely conventional braced box with a base plinth.

Two of these per side sealed and stuffed in parallel is a good starting point :

The Madisound Speaker Store (nothing close available anywhere near the price)

One each side face and glued/linked together for force cancelling
with resilient driver mountings is heading towards unconventional.

An unconventional first order series x/o may or may not work well, if it
does the result will be a fairly unconventional FAST, very good vs. cost.

rgds, sreten.

Sealed stuffing is not critical for a 1.1L driver in 2.5L.
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Last edited by sreten; 26th October 2012 at 08:01 PM.
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Old 26th October 2012, 08:24 PM   #57
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Quote:
Originally Posted by sreten View Post
this thread is full of the conventional mistakes and misunderstandings
of those that know very little to nothing about speaker design.
Hi Sreten,

I really want to learn all I can. If you are willing to point out the flaws, I for one would be really grateful. (I am reading and re-reading all the books I can get my hands on, but admit to being weak on the physics).
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Old 28th October 2012, 10:05 PM   #58
morgoe is offline morgoe  Australia
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Thanks all. Ill leave the port til later and just do a sealed box for now.

I glued my front, top and back panels together. Unfortunately my front panels were pretty warped. Goddamnit. I sanded as best I could but there were still some significant gaps that had to be bridged by the glue. I just hope it'll hold.

I hooked up the speakers and put them in the box, they sounded pretty good. No vibrations in the box, which is good news. There was no bottom panel on , nor any polyfill so it's not what it'll finally sound like. I sine sweeped and it was pretty flat apart from a noticeable spike at 2-4khz. Not outrageous, but noticeable. Just using my ears, so I can't give a dB rating. Any way to alleviate this?
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Old 29th October 2012, 12:52 PM   #59
morgoe is offline morgoe  Australia
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Ok. Holy **** I'm annoyed. Went in to uni today and someone had misplaced the router bit I needed to do the lip of the speaker. Jesus ******* Christ. Because I'm an idiot I accidentally glued one of the baffles on the wrong way (DON'T say anything!), so now I have one amazingly good speaker and one speaker that rattles and vibrates and sounds like ****.

I'm gonna take them both in tomorrow and rout them with a 10mm bit. It means I'll have a huge gap around the speakers, but it's the only way I can fix this problem without spending $60 on a 24 pack of router bits when I only need the one.

Anyway, apart from that, I packed the good speaker with a bit of polyfill and the bottom is being glued on as I type. It sounds incredible. Sealed, no port, and it goes down to 70hz fairly flat. Still a spike at 2-4khz, but other than that its flat.
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Old 30th October 2012, 11:17 AM   #60
morgoe is offline morgoe  Australia
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Finished it up today. Sounds pretty damn good. Thanks for everyone's help, MUCH appreciated. If anyone has any questions I'll try my best to answer.

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