Not another bleeding omni FFS!

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Cressy Snr
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#16 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

Faital Pro - 10FE200

FS/QEs = 74 near as dammit, so the driver falls into the area that means it can either be used in a sealed or a ported box.
That's about all you can get with today's drivers, I've found so far. Most, are not expressly designed with IB loading in mind any more. That's partly why the Doc mod can be so effective with modern drivers, in that it can make a driver, that would not normally be comfortable in a sealed box, usable in one.

Had to push the total bass system Q up to 0.9 when calculating, to get the box size reasonable.
Leaving Qtc at the old sealed box value of 0.707 gave a ridiculously huge box, as it often does these days, but 0.9 isn't a bad compromise.

We have a -3dB cutoff at 58Hz and an enclosure resonant freq at 71Hz. The 12dB crossover filter will push down the efficiency, so we have a pretty good chance of avoiding boom with wall placement, judicious use of damping, and the driver firing along the floor.

I normally start with the midrange and deal with the rest later, but as the mids and treble are now a known quantity, I can build the bass boxes first and get the in-room response even at the bottom end, before tackling the top boxes.
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#17 Re: Not another bleeding omni FFS!

Post by simon »

Cressy Snr wrote: Tue Jul 18, 2017 2:13 pm 8 ohm, 10 inch bass driver under test with 2nd order, low pass filter at about 180Hz -3dB

Image

I also tested a Doc modded 12 inch driver a few days ago.
Nice even responses from the both of them in free air.
The pair of them can shift plenty of air, that's for sure.
Ah, those motor run caps. I wish I'd got a box if them when we clubbed together for ours. Haven't seen them on the web for a long time.

(Good to have you back BTW :-))
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#18 Re: Not another bleeding omni FFS!

Post by simon »

Cressy Snr wrote: Tue Jul 18, 2017 4:10 pmHad to push the total bass system Q up to 0.9 when calculating, to get the box size reasonable.
Leaving Qtc at the old sealed box value of 0.707 gave a ridiculously huge box, as it often does these days, but 0.9 isn't a bad compromise.

We have a -3dB cutoff at 58Hz and an enclosure resonant freq at 71Hz.
Speaker dummy here asking simple questions - sorry if this isn't the right place Steve. I'm trying to learn a bit more about speaker design but could do with a Morgan-type equivalent text; how do you go about raising the Qtc to 0.9? It's the Q of the speaker as a whole as I understand it, so what do you do practically to raise it?

-3dB @ 58Hz - is that the ballpark best a 10" driver can do for IB. It's something I've been thinking about so interested in practical experience.

(BTW there's some fascinating discussions about speakers at the moment, here and Max's thread - thanks, I'm reading and trying to learn a bit.)
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#19 Re: Not another bleeding omni FFS!

Post by Toppsy »

simon wrote: Tue Jul 18, 2017 5:03 pm
I'm trying to learn a bit more about speaker design but could do with a Morgan-type equivalent text; how do you go about raising the Qtc to 0.9? It's the Q of the speaker as a whole as I understand it, so what do you do practically to raise it?

(BTW there's some fascinating discussions about speakers at the moment, here and Max's thread - thanks, I'm reading and trying to learn a bit.)
I sympathise with you Simon. You could have the benefit of the design knowledge and experience of Scott contributing to such threads and perhaps starting a new one, on say on a novices guide to speaker design. However Scott has decided never to post again on AT whilst Princess Sweetcheeks Honeybun Dunn is still actively being able to post and pollute his threads that he has consistently done in the past.

Again apologies to you Steve this is perhaps not the place for such a debate and I shall not post any more off topic to your speakers build thread, but it had to be said.
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#20 Re: Not another bleeding omni FFS!

Post by chris661 »

Without Scott here, I'll give it a shot.
Thanks for the info, Colin.

So the Qts (and Qms, Qes) is inherent to the driver. However, once you put the driver into a cabinet you've got another spring acting.
Since air itself has relatively little damping to contribute, system Q (Qtc, standing for Q-Total-Cabinet) increases. A smaller box gives a higher Qtc, while a very large box will have a Qtc that tends towards the Qts of the driver. You pretty much reach Qtc=Qts when the cabinet is a true infinite baffle (like the IB subs you see). The air spring is basically taken out of the equation since you're trying to compress a near-infinite volume.

It's a little difficult to explain without going through all the derivations.
Qtc = 0.9 is perfectly fine in my book. Rooms have effects of the order of +/-10dB, so the 1dB rise before rolloff from the Q>0.7 is nothing to worry about.

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#21 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

simon wrote: Tue Jul 18, 2017 5:03 pm
Cressy Snr wrote: Tue Jul 18, 2017 4:10 pmHad to push the total bass system Q up to 0.9 when calculating, to get the box size reasonable.
Leaving Qtc at the old sealed box value of 0.707 gave a ridiculously huge box, as it often does these days, but 0.9 isn't a bad compromise.

We have a -3dB cutoff at 58Hz and an enclosure resonant freq at 71Hz.
Speaker dummy here asking simple questions - sorry if this isn't the right place Steve. I'm trying to learn a bit more about speaker design but could do with a Morgan-type equivalent text; how do you go about raising the Qtc to 0.9? It's the Q of the speaker as a whole as I understand it, so what do you do practically to raise it?

-3dB @ 58Hz - is that the ballpark best a 10" driver can do for IB. It's something I've been thinking about so interested in practical experience.

(BTW there's some fascinating discussions about speakers at the moment, here and Max's thread - thanks, I'm reading and trying to learn a bit.)
OK Simon, (thanks Chris for opening the answering BTW) :)

OK last things first.
Qtc is a user definable parameter peculiar to sealed box speakers as it is linked to box volume for the sytem. Change the box volume and you change the Qtc parameter and vice versa. Put simply, a Qtc of 0.707 is the old number that used to be regarded as the best compromise for damping of sealed boxes vs the bass extension you could get. Raise the Qtc and you lower the box volume, lower the Qtc and you raise the box volume. In general a Qtc of between O.55 and 1.2 will give good results with sealed boxes.

I'll carry on answering your question when I've had my tea. :)
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#22 Re: Not another bleeding omni FFS!

Post by IslandPink »

I can't imagine anyone building a sealed box , for its advantages, would be too happy with Qtc of 1.2 :D
Rather loose and boomy probably...
Hopefully 0.9 will be OK, fair bit better.
Currently I'm listening to a single FX120 on a 30L box as per James's suggestion - Qtc on this is only about 0.55 , sounds very nice indeed....
Like the high Qms Steve - should play at low levels.
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#23 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

Ah but loose and boomy is what the punters want, innit!
Bass up to 11 on top of that and you can shake the windows for miles around.
I hear the buggers every day driving past our house :lol: :lol: :lol:
At least they've grown out of happy hardcore. (I'm thankful for small mercies)

Aye... the Qtc of 0.55 is going to be used with the top box on my speakers, but I'm getting ahead of myself :wink:
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#24 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

OK Simon, carrying on...
-3dB at 58Hz isn't quite as bad as it sounds, as IB speakers roll off far slower at the bottom end than ported or horn jobbies. You're only going to be 6dB down at 30Hz, so a bit of near wall boost and boom! (not literally)

With a ported speaker, you boost output below resonance by tuning the port appropriately. With an IB speaker this is not possible, so they used high compliance roll surrounds and spiders to lower the driver, free air resonance and the room boundaries were used to provide further boost. So now you know why all those old big banger IB speakers were always against the wall in those old 1960s and 70s room shots, as were the "bookshelf" models from the same era. 8)
Last edited by Cressy Snr on Tue Jul 18, 2017 7:09 pm, edited 5 times in total.
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#25 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

IslandPink wrote: Tue Jul 18, 2017 6:24 pm Like the high Qms Steve - should play at low levels.
My thoughts exactly Mark.
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#26 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

Another reason for the ultra low resonance suspensions on the old sealed box drivers is that the second you put a driver in a sealed box, you raise the resonant frequency of said driver, due to the air mass trapped in the cabinet.
It then follows that if you can get the driver free air resonance as low as possible, then you can use the trapped air spring to your advantage because in order to get the driver free air resonance up from a common in those days, 25 Hz up to say 50Hz, you get to use a stiff air spring.

Stiff air springs equate to small boxes, so now you have a bookshelf speaker with the same bass extension as a big speaker with a higher resonance suspension. Efficiency has now dropped (you don't get something for nothing) but who cares when you can now get 50WPC from those new fangled transistor amps.

Alas these old low resonance drivers no longer exist in the hi fi market (unless you are an expert Doc modder) hence my three cubic foot enclosures for a 10 inch driver. But they'll have the slam that no bookshelf speaker could provide.
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#27 Re: Not another bleeding omni FFS!

Post by chris661 »

Cressy Snr wrote: Tue Jul 18, 2017 7:35 pm Alas these old low resonance drivers no longer exist in the hi fi market (unless you are an expert Doc modder) hence my three cubic foot enclosures for a 10 inch driver. But they'll have the slam that no bookshelf speaker could provide.
The drivers do exist, but they ain't cheap - check out the work done by Acoustic Elegance. Those are really top-notch drivers.
Looks like I need to build a more powerful bookshelf speaker :P

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#28 Re: Not another bleeding omni FFS!

Post by Cressy Snr »

I'll have a look at them :)...

:shock: :shock: Bloody hell. Expensive or what.
Earthquake generators par excellence mind. :twisted:

I'll never afford them, but it's good to know that such things at least exist these days.
Might have known it'd be the yanks that would keep ultra low Fs drivers going.
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#29 Re: Not another bleeding omni FFS!

Post by IslandPink »

This is one of the best sealed-box drivers I've found recently :
http://www.sbacoustics.com/index.php/pr ... 34nrx75-6/
Please, someone have a go at a 150L bass section using these !
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#30 Re: Not another bleeding omni FFS!

Post by Dr Bunsen Honeydew »

Cressy Snr wrote: Tue Jul 18, 2017 7:35 pm Another reason for the ultra low resonance suspensions on the old sealed box drivers is that the second you put a driver in a sealed box, you raise the resonant frequency of said driver, due to the air mass trapped in the cabinet.
It then follows that if you can get the driver free air resonance as low as possible, then you can use the trapped air spring to your advantage because in order to get the driver free air resonance up from a common in those days, 25 Hz up to say 50Hz, you get to use a stiff air spring.

Stiff air springs equate to small boxes, so now you have a bookshelf speaker with the same bass extension as a big speaker with a higher resonance suspension. Efficiency has now dropped (you don't get something for nothing) but who cares when you can now get 50WPC from those new fangled transistor amps.

Alas these old low resonance drivers no longer exist in the hi fi market (unless you are an expert Doc modder) hence my three cubic foot enclosures for a 10 inch driver. But they'll have the slam that no bookshelf speaker could provide.
I can supply you doped drivers at cost or show you how to do it yourself.
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