Critically Damped (Q=0.5) Subwoofer

[tktran]

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.

 

[Timn8ter]

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.

 

[tktran]

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...

 

[Timn8ter]

Actually, I think you can achieve very similar results with the Adire DPL12, PE plate amp and a ~75 litre sealed box.

 

[kelticwizard]

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.

 

[kelticwizard]

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.

 

[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?

 

[kelticwizard]

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.
http://www.diyaudio.com/forums/showthread.php?threadid=8114&highlight=

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.

 

[Bose(o)]

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.

 

[tktran]

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"

 

[kelticwizard]

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

Tktran:

It is a principle of audiophiles that the less processing you put in the circuit, the truer it is to the original signal, therefore better.

A subsonic filter probably introduces phase distortions, etc,. so an audiophile might leave it out.

The funny thing is, the subsonic filter might be more useful to many audiophiles than to AV fans! That is because many audiophiles still play vinyl records. If there is a warp in the record, the subsonic energy can be huge. DVD's and such have not such outrageous subsonic output.

As in all things, you use your judgement. A subsonic filer would likely be left off by audiophiles unless they play vinyl.

 

[mikee12345]

So u limit the response with Qtc=0.5,then EQ it back....seems self defeating excursion wise.


i dont enjoy EQing sealed boxes. for that reason.

why not DAMP the woofer with a port,then u wont need EQ ,its damped already

Sure the Qtc=0.5 box with EQ may sound better,but has limited SPL

 

[kelticwizard]

why not DAMP the woofer with a port,then u wont need EQ ,its damped already

Mikee:

I personally favor the ported box/passuve radiator. Nevertheless, the closed box aficionados have a satisfactory answer to your question.

At or near resonance, the closed box has much cleaner step response than the bass reflex. If you survey the step response in the thread I gave you before, you should know that that the cleanest ported box's ressponse is worse than a sealed box with a Qtc of 1.3. The "classic" bass reflex, where Vas=Vb, and Fb=Fs=F3, has step response worse than a closed box with a Qtc of 1.6.

The whole idea of the article is to give the builder a subwoofer with bass that is under control without the use of an expensive servo mechanism. Either the parameters of the Peerless changed or something happened, because he ended up giving a box with a Qtc or 0.4 instead of 0.5. But even with that fact, the subwoofer he outlines will give extremely clear, controlled bass. There is not that much difference between a subwoofer with a Qtc=0.4, or Qtc=0.5.

A ported or passive radiator will give more bass output. Just not as clean.

 

[tktran]

I received an email from Joe, stating that it's important that I understand that it's not suitable for home theatre. He even uses two in his music room.

Thanks Kelticwizard, I am rethinking my subwoofer choice, simply because I'd like a bit of flexbility.

A sub to fill out the bottom end with classical music, thump with club/techno music and rumble with the best of 'em when it comes to blockbuster action movies.

I also have two rooms, a HT room 4 x 5m and I'd like to try it in the family room, 6 x 8m and opens out into the kitchen (L shaped)

So I will start with the Shiva... and build the 54L box according to Adire's plans.

Then if I feel I need more grunt I can also try their ported designs.

Will be using the same amp specified by Joe:

http://www1.jaycar.com.au/productVi...xxx&pageNumber=&priceMin=&priceMax=&SUBCATID=

Would the shiva benefit with the peaky 30Hz boost?

http://members.ozemail.com.au/~joeras/critical_q_sub_technical.htm

 

[kelticwizard]

Or else you can use the Peerless, put a 4" diameter port with a screwable end cap-available at any home improvement store-and make yourself a convertible sub. Sealed for music, ported for home theater.

The closable port will be 33" long. box volume will be 50 liters, including the port.

You can do it.

Here is the simulation, woofer driven at 28 volts, (196 watts into 4 ohms)

The red line, lower, is the half space response vented.
The red line, upper, is the vented reponse with room.

The white line, lower, is the sealed response inhalf space.
The whitle line, upper, is the sealed response with room

Quite an improvement, don't you agree?

Nothing wrong with the Shiva, just showing you options.

 

[kelticwizard]

Incidentally, the Shiva in a 50 liter box will have a final Qtc of 0.78. Here is the chart and you can see how that compares to a Qtc of 0.5. It won't be hard to extrapolate between 0.7 and 1.0.

 

[tktran]

Wow a 33" port? How will that fit into a 50L box???

 

[kelticwizard]

Incidentally, over in Australia, I don't know how much the Shiva costs compared to the US.

There is an alternative.

Eminence makes the LAB 12, which has a bigger magnet but otherwise has Thiele-Small specs almost identical to the Shiva. It also has a much larger magnet, and a single 6 ohm voice coil. Oh yes, the Lab 12 has a cast frame where the shiva has a stamped frame. However, I have always felt that a well-designed stamped frame is just a sgood as a cast one. Nevertheless, most believe a cast frame is an upgrade over a stamped frame.

Here in America, the LAB 12costs about $50 more than the Shiva. However, I don't know what the price is over in Australia. Eminence has a destributo in Australia, and Eminence tends to charge the same proce for their speakers worldwide. So the LAB 12, in Australia, might end up being cheaper than the Shiva.

Here is the Eminence page for the LAB 12:
http://www.eminence.com/eminence/pages/products02/lab12/lab12.htm

Here is your Australian distributor:
http://www.eminence.com/eminence/pages/support/inquiry/aus.htm


Again, just an option to consider.

 

[kelticwizard]

Wow a 33" port? How will that fit into a 50L box???

PVC elbows, available at any home improvement store.

It's longer than I'd like, but it's possible.

Actually, a 33 inch port might have a quarter-wave resonance at 102 Hz. Maybe that isn't such a great idea.

 

[tktran]

The Shiva currently costs AU$300, the equivalent of USD$210 from the sole Australian distributor.

The 830500 costs this same price from Australia's official Peerless distributor, or about 190 USD elsewhere.(Peerless / Vifa/ Scanspeak are easier to find)

Thanks for the tip- I'll investigate the Lab12 in Oz.

Bob Stout's Loudspeaker Selection Guide quotes the 830500 as only really being suitable for PR box, so the Critical Q subwoofer had me interested, as does Linkwitz's Thor subwoofer.

Incidentally he also measured the Shiva (Mk1/2?), and if i read between the lines correctly, implies that it's not as good as the 830500. As least for his application (dipole bass)