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Old 8th July 2012, 03:32 PM   #11
Shpilev is offline Shpilev  Estonia
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Quote:
Originally Posted by djk View Post
Alternate 31hz box design.

Driver Properties
Name: 75W104-8 (2003)
Type: Standard one-way driver
Company: Audes
No. of Drivers = 1
Fs = 31 Hz
Qms = 2.83
Vas = 100.5 liters
Cms = 0.689 mm/N
Mms = 37.5 g
Rms = 2.633 kg/s
Xmax = 4.5 mm
Xmech = 6.75 mm
P-Dia = 202 mm
Sd = 320.5 sq.cm
P-Vd = 0.144 liters
Qes = 0.31
Re = 6.4 ohms
Le = 0.933 mH
Z = 8 ohms
BL = 12.37 Tm
Pe = 130 watts
Qts = 0.28
no = 0.931 %
1-W SPL = 91.84 dB
2.83-V SPL = 92.81 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box w/ Active HP Filter
Shape: Prism, square (optimum)
Vb = 1.5 cu.ft
Fb = 31 Hz
QL = 6.983
F3 = 31.2 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 3 in
Lv = 10.74 in
-----------------------------------------
Active 12 dB/oct. HP Filter
Fx = 31 Hz
Qx = 2
I punched in those same numbers and kept getting an F3 of 60 something Hz. Then I noticed the "Active 12 dB/oct. HP Filter". After adding this the F3 dropped down dramatically to 31 Hz.

How does that work? What exactly is this "Active 12 dB/oct. HP Filter" and how is it doing this? As I understand it - it's just a filter that filters out the very low frequencies to prevent the subwoofer from trying to reproduce frequencies that it cannot anyway and thus prevent potential damage. Why would this change the graph in such a way?

Last edited by Shpilev; 8th July 2012 at 03:53 PM.
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Old 8th July 2012, 05:17 PM   #12
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by Shpilev View Post
I punched in those same numbers and kept getting an F3 of 60 something Hz. Then I noticed the "Active 12 dB/oct. HP Filter". After adding this the F3 dropped down dramatically to 31 Hz.

How does that work? What exactly is this "Active 12 dB/oct. HP Filter" and how is it doing this? As I understand it - it's just a filter that filters out the very low frequencies to prevent the subwoofer from trying to reproduce frequencies that it cannot anyway and thus prevent potential damage. Why would this change the graph in such a way?
Hi,

Quick interpretation of the BR + suggested filter...Is it a bad idea?

b
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Old 8th July 2012, 09:57 PM   #13
Shpilev is offline Shpilev  Estonia
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I'm so confused now.
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Old 8th July 2012, 10:18 PM   #14
djk is offline djk
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A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).

The design I gave is a 6th order vented box.

Speakers like the B&W 801, Matrix 800, are 6th order vented, JBL B380 and B460, as are many Electro-Voice designs.
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Last edited by djk; 8th July 2012 at 10:24 PM.
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Old 8th July 2012, 10:34 PM   #15
Shpilev is offline Shpilev  Estonia
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Hmmm... okay. But I'm still confused to what this "Active 12 dB/oct. HP Filter" is supposed to be. Is it something I need to buy and add to my design ? If it's just a filter that blocks out frequencies below its "tuning" then why is this simple thing making my graphs look so much better ?
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Old 9th July 2012, 02:42 AM   #16
bjorno is offline bjorno  Sweden
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Quote:
Originally Posted by Shpilev View Post
Hmmm... okay. But I'm still confused..?
Of course you are..You've been talking of a BR not a BR System to design... I couldn't make the posted numbers into a 6:th order SYSTEM.
Therefore I stayed with a boosted 4:th order and forget about the posted efficiencies as they wouldn't apply to the mentioned numbers as the compliance ratios needed are way off.

I know this driver very well and don't want to push the envelopes in simple BR designs, in whatever order and name-calling designs that have no connection with what this low xmax driver can be driven safely.

b

PS: More to look at...
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Old 9th July 2012, 01:53 PM   #17
djk is offline djk
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While the optimum Qts for a 6th order system is 0.312, about any reasonable Qts is usable.

It is also 'kosher' to pretend the motor is a little weaker than it really is, up to about 20% or so. The interesting thing about this pretense is the harder you drive it, the closer it becomes to optimum (Thiel alignment #15). Most of the EV 6th order designs have too much motor. Technically anything other than 0.312 is not a B6, but becomes a C6 (like the B6 but with small amounts of ripple).

A filter is just that, a filter ahead of the amplifier. It can be active, or passive. Most subwoofer plate amplifiers have one on board, one need only change two resistors (in most cases) to get the Q and frequency right for a given design. I usually modify the crossover or equalizer in a system that has a high-pass filter. Again, it is usually just two resistors.
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Old 9th July 2012, 09:23 PM   #18
bjorno is offline bjorno  Sweden
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FYI:

I think the submitted simulations are more in line with djk:s intentions...
Reminds me on the many designs I've made in the late 70:ties...
BTW. I will not bother with more confusing? BR simulations but maybe a T-TQWT/T-QWP I find should provide with more SQ.

b
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Old 11th July 2012, 01:41 PM   #19
Shpilev is offline Shpilev  Estonia
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Wow you guys are really providing me with a lot of useful information, even tough I can't understand a large portion of it haha. I'd quite thankful.

I think my design might not work that well. The large box might make the driver bottom out too much. I had someone hand build an amp for me and it seems like it doesn't have a subsonic filter. So if I have a large box with a high tuning and a low tone of say 15Hz might pop up (pretty ridiculous I know) - I'd be scared of the driver pop'ing as well.

I think I'll be going with djk's design. The 31 Hz box. Vb = 1.5 cu.ft and the vent 3 x 10.74.

If anyone has any objections or other recommendations I'll gladly listen.
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