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Old 27th April 2011, 03:25 AM   #1
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Default How to improve these old concrete speakers?

Hi!

I have these concrete speakers at home, they're much older than me. My father helped some physics students who were trying to design concrete speakers, and they gave him these prototypes. He uses this system since then.

I enjoy them, but some time ago I started thinking they could be better. I'm not used to listen to Hi-End systems, but I know these are not flat. I have much to study and learn until I can design good speakers and crossovers, so I'm asking you to give me some opinions on what to do with these speakers in order to get them sounding better!

The cabinets are concrete 30mm thick, with a 15mm plywood baffle (that I think resonates a lot). They are stuffed with a fiberglass foam (which irritates the skin), and the plywood baffle is covered inside with a blanket, maybe cotton. The plywood was placed 20mm from the front end (this is where the screen frame fits), and I believe there is much diffraction at these edges. The internal measures are: 24cm width x 34.5cm depth x 58.5cm height, which gives 48,4L. The drivers are 10" woofers, 3.5" mids and 2.5" tweeters, all paper cone.

Playing each frequency from this site Equal loudness contours and audiometry - Test your own hearing from treble to bass, I notice that they start to fade at 60Hz, play 45Hz very low, and 30Hz almost nothing. I don't know if it's the natural for the human hearing, I'll only be able to say something about frequency response when I get a test microphone (I'll soon buy those Panasonic electret at Digi-Key). Anyway, I don't like how they sound, they're not natural. I know these are cheap drivers, with cheap crossovers, they have bad stereo image (maybe because of the position of the drivers), bad response at medium, and an anoying boost at 12kHz.

Soon I'll try to measure the T/S parameters with Speaker Workshop or something, so I can try to crossover better. It seems the crossover frequency between woofer and mid is 1kHz, don't remember what I thought to be the frequency between mid a tweeter.

With the driver's T/S parameters in hands I could make a good crossover for these speakers, with baffle compensation and everything. To eliminate resonances and diffraction, I thought about pulling off this plywood baffle and replace it for a 3/4" MDF, putting it right at the front, eliminating that 2cm depth (where the screen frame fits).

For a future total rebuild of the speakers, I would like some suggestions on what kind of driver to use. I could get tweeters and mids at Madisound, Parts-Express, etc. Woofers are heavy and it would cost too much to send it here to Brazil, so I would like to know what Fs, Qts, Vas, are good for a woofer to match these cabinets and reach deep and flat bass.

I know the thread seems a little pointless, but I would like to hear from experienced people how can I get these speakers sounding good.

Thanks!
Attached Images
File Type: jpg Concrete speaker with screen.jpg (139.7 KB, 297 views)
File Type: jpg Right.jpg (131.7 KB, 284 views)
File Type: jpg Left.jpg (123.3 KB, 277 views)
File Type: jpg Inside.jpg (108.0 KB, 275 views)
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Old 27th April 2011, 03:46 AM   #2
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re: "I would like to know what Fs, Qts, Vas, are good for a woofer to match these cabinets and reach deep and flat bass", research the drivers that are available to you locally, and sim them in Unibox to see which are suitable for that cabinet. I'd be inclined to do a 2 way using a couple of 6" drivers like these 6.25" - 4 Ohm - Ferrite - WF160WA02 and a dome tweeter.
Get rid of the fibreglass and replace it with polyfill, make the front baffle thicker & flush with the concrete
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 27th April 2011, 10:25 PM   #3
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Actually the internal sizes are: 590mm height x 350mm width x 24mm depth (it's wider than longer - could someone give an opinion about internal reflections and stationary waves for these sizes?). I plan to put an MDF baffle in front of the end of the concrete walls (so the front would be all MDF), allowing me to round cut the edges for less diffraction, and thus gives 270mm depth, and a new internal volume of 56.7L.

At the lower back there's a 80mm x 100mm hole, where fits a metal panel with the connectors and two knobs for adjusting tweeter and mid, and it says "12db/Oct". But I don't believe it'll be usefull, I just thought about using this hole for a port exit.

PeteMcK, the first thing I want to get rid is the fiberglass. The second is the crossover, as soon as I measure the drivers T/S parameters and response, so I can make a proper one.
I found that this 6.25" driver would be better for 15L sealed enclosures for a Qtc=0.7 and an F3=78Hz, what would be a waste of volume and bass potential for my boxes. I could still make them vented, putting the driver in a reduced chamber of 34.4L, reaching an F3 of 41Hz.
I searched for Selenium drivers, a brazilian manufacturer, but their 10" woofers needs much bigger enclosures, and their 8" mid-bass needs much smaller enclosures and don't go much deep.

I found what I believe would be the optimal woofer for my speakers: https://www.madisound.com/store/prod...roducts_id=222
Fs: 18.36Hz
Qts: 0.318
Vas: 214.9L
It needs a 55L sealed box (it's my number!) for a Q=0.7 with an F3=40Hz. Plus it handles 100W, and has a Xmax=8mm peak, so I could use a Linkwitz transform circuit in the pre so it could reach even deeper bass response! Only con is that it doesn't seem to play well above 500Hz, according to the frequency response graph. And I'd like to use mids crossed over at least at 1kHz, so I could use sealed mid drivers that don't need a separate chamber (that would take air volume from the woofer).

Would really this 10" Eclipse woofer be the best choice? If so, then what would be good mids and tweeters to match? (Nothing too expensive, as the woofers will already cost around US$160 shipped.)
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Old 28th April 2011, 12:42 AM   #4
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How is the baffle sealed to the concrete? It is important that the speakers should not leak.

You could take one out in the open air and test the performance. You might want to build or buy a measurement microphone, here's a cheap one:- Behringer ECM8000 - microphone

You can try testing with a laptop, amplifier and software such as HOLMimpulse, which is supported on this site, just search.

w
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Old 28th April 2011, 12:55 AM   #5
BudP is offline BudP  United States
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The drivers are perfect candidates for EnABL. Their improvement will be remarkable. All of the other things being suggested will just be even better since EnABL only removes resonances, spatial steering of frequencies, like beaming of highs, and increases the dynamic range that is actually intelligible. All of the rest of the speaker performance specs will remain unchanged. If you like you could do a double thrust and bring the same question to this thread, where we will teach you how to do this for yourself. Be sure to read the recent postings by francolargo about his treatment of an olde JBL system.

EnABL - Listening impressions & techniques - diyAudio
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Old 28th April 2011, 01:02 AM   #6
adason is offline adason  United States
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I would suggest to use concrete enclosure for woofer only and put the mid and tweeter on top as open baffle...update the mid and tweeter with nice new drivers and you have a perfect speaker
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Old 28th April 2011, 10:24 AM   #7
Dr_EM is offline Dr_EM  United Kingdom
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If you use that woofer your best bet would be to use a small fullrange driver from 500Hz (or less) upwards. Something like the Monacor SPH-30X is commonly available, reasonably priced and has a lift around 10kHz which will stop the treble being lost so much if you continue to listen with the speakers on the floor. It will definately need a seperate volume inside the box though, only 1-2 litres sealed, won't be hard to make.

The suggestion above is a good idea too, would get some more height to the high frequency drivers as well as probably sounding nicer generally.
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Old 28th April 2011, 02:30 PM   #8
adason is offline adason  United States
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maybe this is an oportunity to enter the world of fullrange speakers as well
use the concrete box for woofers crossed at, lets say, 180-200 Hz, maybe even higher is possible, since they will not act as typical subwoofers, since they will be placed closely to the mids
then place fullrange on top, preferably on open baffle, from 200 Hz up...this will free the fullrange from low frequencies, so it can be loaded a lot, without messing up mids...
this could yield a nice sounding system, much different from what you have now, but likely much better
I swared long time ago to never put midrange in small enclosure, I always hear the back wave bouncing back muddying the sound
once you put midrange on open baffle, you never go back

Last edited by adason; 28th April 2011 at 02:32 PM.
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Old 28th April 2011, 04:03 PM   #9
JZatopa is offline JZatopa  United States
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I would suggest you scrap the front baffle, remove the fiberglass and start from scratch. Use the volume of the concrete enclosure to pick your drivers and you should be ok. Driver technology has come a long way since those were built.

The suggestion to use those just for a woofer and to build a smaller enclosure to put on top of them, which would hold a full range or mid + tweet, is also a good idea. Do you have the tools to build a speaker enclosure?
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Old 29th April 2011, 07:52 AM   #10
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adason, is a closed box woofer with an open baffle mid and tweeter going to sound good? I had a read on Linkwitz notes, but I don't understand much about dipole speakers. My speakers are placed in a small apartment living room, there's furniture at both sides of each one, and they can't be placed with the front much more than 0.5m away from the wall. Would it cause too much reflections at the rear pole of an open baffle, destroying its response?

Dr.EM, I liked this Monacor driver, but it's 4ohm, wouldn't match my system. I thought about the Fostex FE83En ( https://www.madisound.com/store/prod...oducts_id=8820 ), which has the same sensitivity as the Eclipse woofer (low, 88db/W/m), but it seems to me that it has bad response in the upper end, and its rated input is only 5W. I don't know if at 500Hz it would be ok.

Anyway, I would have to crossover this woofer at 500Hz maximum, so doing this at 500Hz or even 200Hz isn't good because of phase shift being evident, right? So what to do?
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