Critically Damped (Q=0.5) Subwoofer - diyAudio
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Old 17th November 2003, 05:20 PM   #1
tktran is offline tktran  Australia
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Default Critically Damped (Q=0.5) Subwoofer

Hi,

Does anyone have experience with the Critical Q subwoofer?
http://members.ozemail.com.au/~joeras/sub_index.htm

I got lost one-third down that page, so I tried modelling the Peerless 12" XLS 830500 driver in WinISD 0.44 using both the manufacturer's T/S parameters and the measured T/S from Vacuum State, but did not get anything as expected. So I plugged the values into

http://www.diysubwoofers.org/sld/sealed.zip

and when shooting a for Qtc = 0.5, resulted in a Vb of 33 and 26L, with -3dB points of 60 and 70Hz respectively.

Is this correct? How can this be considered a "sub"woofer?

Does the subwoofer amplifier use some sort of Linkwitz transform circuit or LF equalization? I must have missed it, because could not find anything on the website.
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Old 17th November 2003, 05:34 PM   #2
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He's EQ'd the sub by modifying the amp. There's also a disclaimer stating that if you've never modded an amp to chose another amp the meets the criteria. He freely admits making changes to the power amp section can be tricky. There's a section titled Sub-Amplifier Components Changes where he outlines which components to change to particular values.
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Old 17th November 2003, 05:41 PM   #3
tktran is offline tktran  Australia
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Oh I see. So the EQ can be achieved without doing the "Optional & May Be Very Difficult" Power amp module mod?

Perhaps it will be just easier for me to try shove a 12" Shiva in sealed 50L box...
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Old 17th November 2003, 07:15 PM   #4
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Actually, I think you can achieve very similar results with the Adire DPL12, PE plate amp and a ~75 litre sealed box.
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Old 17th November 2003, 07:32 PM   #5
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Quote:
Originally posted by tktran

Perhaps it will be just easier for me to try shove a 12" Shiva in sealed 50L box...
Below is the simulation by Subwoofer Simulator, written by our own F4ier, for a Shiva in a 50 liter sealed box.

The lower white line is the response in "half space" the way most simulation programs do it. The upper white line is the response with the average amount of room gain.

The room gain is from an frd file written by our member Serow.

The purple line is cone excursion, which actually seems to match quite well with the Shiva's linear excursion.

Even with room gain, the response is only 105 dB at 20 Hz-not great. At 30 Hz, though, you get 110 dB with room gain-pretty good for a box not even 2 Ft³.
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File Type: gif shiva 50 liters sealed.gif (20.0 KB, 951 views)
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Old 17th November 2003, 07:59 PM   #6
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PS: Just want to add that the above simulation was for a Shiva with both coils driven in parallel for a 4 ohm total load. The voltage level was 28 volts, which into a 4 ohm load yields 196 watts.

The reason I selected approx. 200 watts is that reasonably priced sub amps are available in that power range. The Shiva can take more power than 200 watts, if you want to provide it.

PS: The original link that was given for this Critical Q subwoofer was here:
http://members.ozemail.com.au/~joeras/sub_index.htm

You will find that there is a room gain graph given on that page. I would just point out that is the same graph that Serow used when he wrote file for room gain that appears in the graph above.
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Old 17th November 2003, 09:28 PM   #7
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There's nothing novel about this design is there? Isn't it just another low Q sealed box system with bass boost? I'm not impressed by the unsubstantiated claim that it's better than some unnamed servo systems. What's the big deal?
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Old 17th November 2003, 10:06 PM   #8
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Quote:
Originally posted by Dave Jones
There's nothing novel about this design is there? Isn't it just another low Q sealed box system with bass boost? I'm not impressed by the unsubstantiated claim that it's better than some unnamed servo systems. What's the big deal?

Actually, you are right to a certain extent, and the web page author is right to a certain extent.

A sealed sub with a Qtc, (that is the Q when the speaker is put into a closed box), of 0.5 will have less overhang than a servo sub if the woofer has a higher Qtc, like 1.5. The 0.5 Qtc means that "overshoot" is naturally controlled, and that this natural control is better than a higher Qtc rating with sevo control.

The disadvantage of Critically Damped subs is that the output is 6 dB down from the midpint at resonance. This requires a big boost by the amp to bring the output up.

The big advantage is of course, the natural control of the sub.

Here is a thread which I think explains a lot.
Subwoofer Qtc and 'tightness'

I should point out that a sealed sub, even with a Qtc of 1.3 or higher, has less distortion than a ported sub. It is usually easier to get higher output at lower frequencies with a ported or Drone Cone, (Passive Radiator) sub. To which can be added the fact that distortion is less hearable the deeper the bass frequencies.

Still, some people feel the sealed sub's lack of distortion compared to the ported box's is quite hearable.

I believe the author's aim was to give the beginner a fast, relatively inexpensive way to make a reasonably sized sub that will deliver clean, quick bass.
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Old 18th November 2003, 12:14 AM   #9
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Currently I am in the proccess of building a 53L box for the Shiva. The three litres are there for the woofer and bracing, making the system effectively 50L. I believe that I calculated a required 165W of power to achieve Xmax in that enclosure. I love the Shiva driver already and have it sitting on a proto-H-Baffle woofer enclosure that I have.
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Old 18th November 2003, 12:17 AM   #10
tktran is offline tktran  Australia
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Joe also gives options for 3 versions- Audiophile LF (restricted LF), Audiophile and A-V.

But the only real difference between the Audiophile (intended for music) and AV (better for movies) is that the AV version has a subsonic filter.

How can this be detrimental to sound quality? I had always thought that a subsonic filter is a "good thing"
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