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Active center speaker on a budget

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voodooless

voodooless

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From a distortion standpoint, or a power handling standpoint?
Mostly power handling. It maxes out at about 94 dB already.
If the former, smart selection of the HP frequency helps get away from rising bass distortion, assuming your other drivers & physical considerations allow it.

If the latter, if you restrict the bandwidth, you can increase the overall effective SPL capability (same power handling, less spectrum coverage).
No, it's thermal handling that is the issue here. It has plenty of Xmax to handle > 250 Hz, just not the power handling to match. Now music really has a fairly decent crest factor, so that will give some extra headroom, but it's still a bit tight.

This is excursion with 50W applied:
1643384660172.png
 

howard416

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Mostly power handling. It maxes out at about 94 dB already.

No, it's thermal handling that is the issue here. It has plenty of Xmax to handle > 250 Hz, just not the power handling to match. Now music really has a fairly decent crest factor, so that will give some extra headroom, but it's still a bit tight.

This is excursion with 50W applied:
View attachment 182560
Yes, but...

1) Voices don't hit the same peaks that explosions do. So you're already... what, 10-15 dB down? Remember, 10 dB is roughly what it takes for an apparent doubling (or halving) of loudness. So that's 10% of your power relative to peak requirements, right there.
2) The bandwidth is limited. With a driver like that, maybe you're running 600 Hz - 2500 Hz or so. If I plug that into a formula I have on one of my design sheets (based on an old chart from Fane showing distribution of power vs frequency), it tells me I need 23% of the power relative to the whole speaker, when playing typical music (vs taking an overall speaker SPL reading with a meter). With voices, way more of the power will be concentrated in the midrange - but it's still not doing fullrange duty. Even conservatively, I'd say the mid would be taking 50% of the overall sound output at most. Of course, this will go up as the HP frequency goes down. At 300 Hz I'd use a factor of 75% instead.
3) I'm not including this factor, but short-term peak power handling will be more than the rated 20 W. IIRC, IEC 268-5 uses a signal with 6 dB crest factor, you're not going to be listening to a continuous 97 dB @ 1m, are you?

Basic, stupid math - 0.1 * 0.5 = 0.05 => driver power is 5% of the power you'd need for a full sine wave at your maximum SPL.

Uh, I probably could explain that better. But I hope that makes sense in some way. Long story short - I think you're fine for total output, but again, DON'T expect to do full-range sine sweeps and get clean output @ 105 dB all the way. It's not your use case.
 
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voodooless

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Uh, I probably could explain that better. But I hope that makes sense in some way.
Yes, it makes a lot of sense and you are absolutely correct. I've done such calculations many times :) Still, I would not trust that small driver to handle it all. But I can't substantiate it at all, so sue me ;) > 600 Hz would definitely be fine, but not > 200 as in your initial proposal.

Di you have a link to that calculator? Would be interesting to see. I also always wondered about how frequency-dependent the crest factor in music is. I suspect that there is quite some difference in bass vs mid frequencies.
 

howard416

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Yes, it makes a lot of sense and you are absolutely correct. I've done such calculations many times :) Still, I would not trust that small driver to handle it all. But I can't substantiate it at all, so sue me ;) > 600 Hz would definitely be fine, but not > 200 as in your initial proposal.
Oh, right. I didn't mention it but I figured you would be OK with using a midbass underneath. Bad assumption then. If you are limited to 4 channels, which of course is a reasonable limit, any chance you'd consider a passive crossover between the tweeter and the mid? It'd be pretty small and cheap, the rest could be active still.

More perspective: shouting is only 76 dB 1 meter away. I didn't know that either...
 
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voodooless

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Oh, right. I didn't mention it but I figured you would be OK with using a midbass underneath. Bad assumption then. If you are limited to 4 channels, which of course is a reasonable limit, any chance you'd consider a passive crossover between the tweeter and the mid? It'd be pretty small and cheap, the rest could be active still.
With 4 channels, I would not need that (tweeter, mid, bass (can share 2 channels)), would I? For a 2 channel setup, definitely.
More perspective: shouting is only 76 dB 1 meter away. I didn't know that either...
Yes, 76 dB is actually quite loud already. But If you look at the specific use case: home cinema, you'll need speakers to be referenced at about 75 to 85 dB (depending on preference). Add some 15 dB crest factor (movies have a higher dynamic range than music), and your at 100 dB peaks. And these speakers are not full range, anything with real power is already cut off (let's say < 80 Hz).

But at this point, it gets more and more academic since the Genelecs are very tempting... Not less fun though. I like the "gedankenexperiments" very much. It can be quite satisfying even when not building anything.
 

howard416

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With 4 channels, I would not need that (tweeter, mid, bass (can share 2 channels)), would I? For a 2 channel setup, definitely.
A slot-loaded woofer, like I described earlier, will really not work above ~250 Hz, according to the back-of-the-envelope math I did. With a slot depth of 150 mm (a typical 5" driver diameter), the notch will be at ~570 Hz. You want a 4th-order LP to avoid that as much as possible, and have it no higher than an octave below the notch IMO.

So, you'd need a driver in between the SB65WBAC25 and the woofer, in that case.
 
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retro

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The 11cm chassis diameter is the most important limitation here.
Just found and read this thread..like ur project..!

But...why oh why such a stubborn limitation..?!? 10cm max driver..?!? Man..that's dinky toys for children..;)
Instead, think..how and how much can I raise my tv..? Or, how much can I lower my cabinet..? Both, even better.
And no, I won't accept a simple "can't be done". Instead, show some pics to back it up. I'm sure we can then give you options..;)
You would gain soo much with proper bigger better drivers..

Never let video or furniture rule over ur audio. Or partners, if that's the case here..:cool:
 
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voodooless

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Well, in the end I went the lazy way with a triplet of Genelec 8040. Obviously it’s larger than 10 cm :facepalm: For now the center is on a contraption in front of the cabinet. I hope to move it over the TV in the near future though.
 

retro

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Well, in the end I went the lazy way with a triplet of Genelec 8040. Obviously it’s larger than 10 cm :facepalm: For now the center is on a contraption in front of the cabinet. I hope to move it over the TV in the near future though.
Haha, ok. Well, at least u gave up on ur dinky toys project..:)

Still, why so set on 10cm..?!? Pics please..;)
 

600_OHM

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Oh wow, this reminds me of my first center speaker project way back when.

Meant to be funny, but a kid has to do what he can afford. :)

Sitting around tripping out to some awesome jazz-fusion, I spied an article on how to create an "effects channel" by running some leads from the + positive of the L & R to a single speaker in the center.

KIDS - DONT DO THIS UNLESS YOU ARE ABSOLUTELY SURE YOUR SYSTEM CAN HANDLE IT.

Placed my 6x9 auto speaker with no surround on the shelf. YES, I can hear some "difference signals" through it. (balance must be centered and so forth)

But it wasn't loud enough. No money for an amp. Solution: an 8-inch PA horn speaker. :)

Kinda harsh, but angled towards the ceiling. Ah, that worked! I realized though that for the "effects channel", I didn't really need hi-fidelity just to pull out some trippy notes, because they weren't there all the time.

Ah, the good old naive days....
 
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