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2 x 12" sealed cabs and Qtc
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zigzagwanderer View Drop Down
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    Posted: 04 November 2021 at 11:50pm
Hi , This is my first time putting a system together and building speakers etc so if there are any glaring errors feel free to point them out 

In the next couple of weeks I plan to build a 2 x 12" sealed mid cab loaded with Fane 12mb's and at the moment intending to cross it at around 200hz and 1100 hz 
This is to go above 2 scoops and a 2 x 15" reflex cab.
For practical reasons like stacking I need the cabinet to have a volume of around 60 litres 
According to WinISD this would have a Qtc of 0.506 
But I could put a false back in the cab to reduce the volume and raise the Qtc to maybe around 6 - 6.5
Ive spent hours reading about Qtc and sealed cabs and am not getting anywhere.
With WinISD the spl , cone excursion etc all seem fine either way but with my lack of practical experience I dont know what difference it will make to the actual sound at the above frequencies and if i should be trying to raise the Qtc 
Will it make a noticable difference to the sound and if so in simple terms can you explain what ?
Music wise the final system will be playing old and new roots reggae  




Edited by zigzagwanderer - 04 November 2021 at 11:55pm
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Keen View Drop Down
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Points: 1203 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Keen Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 6:48am
should be easier on the ear if you raise the QTC.

Build both, compare. Let us know
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toastyghost View Drop Down
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Points: 10919 		Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 8:01am
Don Davis & Eugene Patronis Jr's book covers this in really good plain English, but here's the pertinent part:


And some general 'rules of thumb'


If you'd like to know 'why', then borrow the book from your library
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zigzagwanderer View Drop Down
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Points: 48 		Post Options Post Options   Thanks (0) Thanks(0)   Quote zigzagwanderer Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 3:05pm
Thanks 
I picked up a copy of The loudspeaker design cookbook over the weekend and it had the same graph and some simple text describing the effects too.
Looks like raising the Qtc is definatley the way to go.
Does allowing around 3L per driver (fane 12mb) sound about right ?
If the weather holds out will hopefully be getting on with it this weekend Smile
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snowflake View Drop Down
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Points: 3122 		Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 4:07pm
that graph can be slightly misleading as it doesn't show that for a given speaker Fc and f3 are shifted upwards as you reduce Vb to raise Qtc. 

Box Q

Edited by snowflake - 09 November 2021 at 4:08pm
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toastyghost View Drop Down
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Points: 10919 		Post Options Post Options   Thanks (0) Thanks(0)   Quote toastyghost Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 5:54pm
Originally posted by zigzagwanderer zigzagwanderer wrote:

Thanks 
I picked up a copy of The loudspeaker design cookbook over the weekend and it had the same graph and some simple text describing the effects too.
Looks like raising the Qtc is definatley the way to go.


That depends on what you want - accurate response, or maximum ‘boom’ at the low corner. Personally, if the Qtc is going to be 1 or higher I’d start to consider a ported cabinet.

Originally posted by snowflake snowflake wrote:

that graph can be slightly misleading as it doesn't show that for a given speaker Fc and f3 are shifted upwards as you reduce Vb to raise Qtc. 

Box Q


It doesn’t explicitly show both effects on the same plot no, because it’s normalised to the cutoff frequency. Textbooks tend to like to show things in isolation like this. I’m sure you’re aware, but others might not be and it would be remiss to have people start mistrusting excellent resources like that book

Edited by toastyghost - 09 November 2021 at 5:56pm
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bob4 View Drop Down
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Points: 1843 		Post Options Post Options   Thanks (0) Thanks(0)   Quote bob4 Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 7:25pm
Originally posted by zigzagwanderer zigzagwanderer wrote:

But I could put a false back in the cab to reduce the volume and raise the Qtc to maybe around 6 - 6.5


Refer to the graph in snowflakes post.......

Originally posted by snowflake snowflake wrote:

that graph can be slightly misleading as it doesn't show that for a given speaker Fc and f3 are shifted upwards as you reduce Vb to raise Qtc. 

Box Q 

Maybe it is not a good idea to do that.


Edited by bob4 - 09 November 2021 at 7:27pm
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zigzagwanderer View Drop Down
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Points: 48 		Post Options Post Options   Thanks (0) Thanks(0)   Quote zigzagwanderer Quote  Post ReplyReply Direct Link To This Post Posted: 09 November 2021 at 11:06pm
Originally posted by bob4 bob4 wrote:

[QUOTE=zigzagwanderer]
But I could put a false back in the cab to reduce the volume and raise the Qtc to maybe around 6 - 6.5


Apologies that was a typo , should have read .6 - .65
 
I've had a play again this evening with WinISD and should be able to get the volume low enough to give a Qtc of .707 , which i think im going to aim for.
Is there a minimum amount of space that should be left between the back of the driver and back of cab , will 15 - 20mm be ok ?  
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