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Unsure about this design (twin vented sub for HT). Please help me! :)


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#1 of 33 Javier_Huerta

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Posted November 20 2002 - 01:31 PM

Hello all!

After giving *a lot* of thought to this, I finally decided to give DIY a double chance this winter season. Hey, a man has to keep himself busy thru the cold days!

Anyway, I'll be building an Adire EBS enclosure for my MartinLogans *and* a second sub for my Home Theater.

I can't be too picky about the drivers, since living in Mexico really limits my choices. I ordered the 250W plate amp and the Tempest from the States, but I'll buy the stuff for my second sub here in Mexico.

I'll be picking up a project Manuel de la Flor started Posted Image. A twin 12" sub with Cerwin-Vega! drivers. You can take a look at the drivers here. The specs for the sub, taken from Manuel's post, are:

Quote:
12" unit:
voice coil diameter : 2½
speaker displacement : 1.98 liters
FS: 22 Hz
Qts : 0.355
Qms : 12.880
qes : 0.365
Vas : 176.22 liters
Xmax 17 mm
Revc 3.6 ohms


I have designed a box using WinISD with the following specs:
[list=a][*]320 liters[*]Tuning frequency: 25 hz[*]Vents: 2, 3" each, tuned to 25 Hz.[*]2 drivers.[/list=a]

I'm getting a very flat frequency response with a *very* slight dip (around -0.2 dB) centered at around 75 Hz and a 0.7 dB peak at 30 Hz, with -7 dB at 20 Hz. Supposedly, I'll have 118 dB SPL maximum power right until around 25 Hz, with 110 dB SPL at 20 dB.

I have a few questions, though.
[list=a][*]Am I missing something? Do I still have to check anything else before comitting?[*]Is this true? Can such a cheap driver provide 118 dB SPL in a properly built and tuned enclosure?[*]Does the driver seem to be any good?[*]Any tips for extending the low end? I think the 20 dB response is way too low.[*]Any other pitfalls you think I might fall into?[/list=a]

I'll thank you a lot for any help you can give me, since it all seems too good to be true. The drivers will cost me around $150 USD. I know it's no Tempest at the low end, but I'd guess that for home theater, the box I designed should be enough.

Thanks!!!!

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#2 of 33 Chris Tsutsui

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Posted November 20 2002 - 03:04 PM

May I ask what amp you will be using that will allow 118db @25hz with those 2 drivers.

With output like that I think they would be great for home theater. I don't know that much about the accuracy of the different simulators such as winisd since when I used the adire simulator I got different specs when designing a tempest box.

As for trying to get a better response, I thought winisd automatically chooses the smoothest specs for a smooth response. If you tuned the box any lower then the response could get worse.


#3 of 33 Greg Monfort

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Posted November 20 2002 - 03:32 PM

I get just shy of 116dB/two drivers/250W total/m in 320L net/25Hz Fb. Will they do it, probably. Will they do it at low distortion, I'm not betting on it.

Regardless, this is too high an Fb for a typical room, much less for a HT sub, and too high a Q for 'tight' music reproduction so lowering Fb to at least 20Hz with two flared 4" vents is in order IMO.

GM
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#4 of 33 Dan Hine

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Posted November 20 2002 - 03:39 PM

I think I recall reading that the stated 17mm is not really the xmax but rather the xmech. This would make a huge difference in actual performance wouldn't it?
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#5 of 33 Greg Monfort

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Posted November 20 2002 - 04:19 PM

WRT distortion, yes. It may actually spit out a higher SPL in its upper BW due to all the added harmonics even though the suspension will be 'tightening up' with increasing excursion if there's enough power available.

Really though, with only 250W on tap it shouldn't be a major issue even if the Xmax is half this.

GM
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#6 of 33 Javier_Huerta

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Posted November 21 2002 - 06:32 AM

Chris wrote:
Quote:
May I ask what amp you will be using that will allow 118db @25hz with those 2 drivers.
A 250W plate amp from Parts Express, although I'll start by using a 100W/ch Harman Kardon receiver. Since the subs have dual voice coils with 4 ohm impedance, I can wire their coils in series and then wire both subs in parallel to produce a 4 ohm load.

Greg wrote:
Quote:
Regardless, this is too high an Fb for a typical room, much less for a HT sub, and too high a Q for 'tight' music reproduction so lowering Fb to at least 20Hz with two flared 4" vents is in order IMO.

Quote:
WRT distortion, yes. It may actually spit out a higher SPL in its upper BW due to all the added harmonics even though the suspension will be 'tightening up' with increasing excursion if there's enough power available.

I'm a bit confused. Is Fb the tuned frequency of the box? I could try lowering it but I guess I'd be losing some extra SPLs. I thought SVS tuned their subs at 25 Hz in order to get max output for HT. Nevertheless I could try lowering the frequency to 20 Hz. My big question is, how can I calculate the "Q", or how do I set it under WinISD? I know I should be looking for something around 0.7-0.5, but I don't know how to implement this using a box design program.

Also, what is "WRT distortion"? Sorry if it's a well-known acronym, but I didn't really understand. Posted Image

Dan wrote:
Quote:
I think I recall reading that the stated 17mm is not really the xmax but rather the xmech. This would make a huge difference in actual performance wouldn't it?
Yeah, I know about this. I used a 9 mm Xmax value for the simulation.

So...

Do you think I should try and look for some better drivers? Or maybe use 4 subs instead of 2? I know I should definitely get lower distortion figures from this arrangement. (I guess this would give me a tremendous sonic impact).

It's a bit hard, living in Mexico and not being able to choose from the cream of the crop. Posted Image

Thanks *A LOT* for helping me through this! Posted Image

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#7 of 33 Javier_Huerta

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Posted November 21 2002 - 07:20 AM

Maybe this driver will be a better choice... please let me know what you think Posted Image

dbDrive CW12D Dual Coil Subwoofer:

FS: 43
Qts: 0.89
Qms: 7.01
Qes: 1.03
Vas: 1.27 cu ft.
Xmax: 0.30 inch
Re: 6.7 ohms
Pe: 300W RMS
Le: 8.26 mH
Sens: 86dB

I can get this one for around $5 more. Is it worth it? Sorry if it's a dumb question... the Xmax looks quite a lot better than the other one, but the sensitivity (I think) is too low.

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#8 of 33 Brian Bunge

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Posted November 21 2002 - 09:11 AM

The Fs is waaaaay too high and the Xmax is even worse. An xmax of .3" is equivalent to 7.5mm.
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#9 of 33 Javier_Huerta

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Posted November 21 2002 - 09:43 AM

Hmm. So I would be better off with the first subwoofer, or should I keep looking?

Funny thing is, I traced the response curves on WinISD, and on a 1000 and something liter box I got sub-20 Hz response with the second sub.

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#10 of 33 Javier_Huerta

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Posted November 21 2002 - 09:43 AM

Sorry. Double post.
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#11 of 33 Dan Hine

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Posted November 21 2002 - 09:53 AM

Quote:
Funny thing is, I traced the response curves on WinISD, and on a 1000 and something liter box I got sub-20 Hz response with the second sub.



Do you really want a 36cubic foot box in your house? Posted Image

How much less are these drivers going to cost you than a Tempest? I'm just curious.
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#12 of 33 Ryan Schnacke

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Posted November 21 2002 - 10:09 AM

After owning an SVS 25-31 for over a year and building a sub tuned to 21-22Hz I have to say that 25Hz has worked very well for me. It digs plenty deep - things start going subsonic for me at around 28Hz - and shakes the place very well. I built the deeper tuned sub to compare, but honestly I must say that 25Hz is enough for me. And higher frequency rumble filter frees up my sub amp from some of the ridiculously low inaudible stuff that could otherwise bring it to its excursion limits - or beyond.

Since the subs are DVC, 4 ohms each you'll get the ideal 4 ohms on the plate amp. But while you're running off the receiver, waiting for the plate amp I suggest you wire each woofer's coils in series and then connect each woofer to their own channel from the receiver. That way the receiver's output devices are feeding an easier 8 ohm load on both channels rather than a difficult 4 ohm load and forcing one channel to do all the work.

I know you're thinking "I'll get more power with a lower impedence". But think about it ... you can run 4 ohms to one channel and pull maybe 200 watts from that channel or you can run 8 ohms to 2 channels and get 100 watts from each. The sensitivity gain from the 4 ohm load is offset by the fact that you're leaving the 2nd channel unused.

#13 of 33 Javier_Huerta

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Posted November 21 2002 - 10:24 AM

Dan said:

Quote:
Do you really want a 36cubic foot box in your house?

How much less are these drivers going to cost you than a Tempest? I'm just curious.

Let's see... hmm... s/h should cost around $100, and taxes would come at around 50% of the total cost ($250).

So I guess I'd end up paying around $375 for the Tempest. Definitely *not* good.

Also, believe it or not, size is not an issue! The sub will be used on a dedicated HT room. I have permission to use it any way I want it to.

(I can imagine more than a couple of you fainted from envy Posted Image )

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#14 of 33 Greg Monfort

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Posted November 21 2002 - 11:52 AM

>I'm a bit confused. Is Fb the tuned frequency of the box?
====
Correct.
====
> I could try lowering it but I guess I'd be losing some extra SPLs.
====
Depends on how much room gain you have. You may have to tune it even lower to get a ~flat response if you have a small room.
====
>I thought SVS tuned their subs at 25 Hz in order to get max output for HT.
====
Don't know, but if they are then a rumble filter would be required based on the number of movies I've rented that had <25Hz signals.
====
>My big question is, how can I calculate the "Q", or how do I set it under WinISD? I know I should be looking for something around 0.7-0.5, but I don't know how to implement this using a box design program.
====
You can't do either one per se. All you can do is look at the slope of the roll off between F3/Fb and compare it to sealed box slopes. For reference purposes, a max flat (WINISD, BoxPlot, etc., default) alignment is Q = 0.707, i.e., 12dB/octave roll off.
====
>Also, what is "WRT distortion"? Sorry if it's a well-known acronym, but I didn't really understand.
====
'With regard to', or 'with reference to'.
====
>Do you think I should try and look for some better drivers?
====
Only if these don't make enough SPL with ~8-9mm of Xmax or you can do better for less IMO.
====
> Or maybe use 4 subs instead of 2? I know I should definitely get lower distortion figures from this arrangement. (I guess this would give me a tremendous sonic impact).
====
Well, there's no replacement for displacement when it comes to making high SPL at low frequencies, so I guess it depends on your pocketbook/space available.
====
>Also, believe it or not, size is not an issue! The sub will be used on a dedicated HT room. I have permission to use it any way I want it to.
====
Then build a false wall to make an IB enclosure, mount eight C-Vs and EQ to suit. This will solve any potential limited Xmax or distortion issues. Posted Image

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#15 of 33 Brian Bunge

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Posted November 21 2002 - 11:57 AM

Considering a 36ft^3 box would be roughly a 40" internal cube (this is without any bracing) my next question is how much will it cost you to rent a fork lift to move it from the shop into the room. And will it even fit through the door. With something that size you might as well cut a hole in the ceiling and build an IB subwoofer.

EDIT: OMG, I'm starting to think like Greg! Posted Image
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#16 of 33 Javier_Huerta

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Posted November 21 2002 - 12:08 PM

Greg, I can't thank you enough for your explanation. I'll look further into the "Q" issue, which is, I think, the only thing that separates me from building a good enough box.

I've researched into other alternatives I could buy around here. All are JL Audios, Punch, Kicker and automotive stuff. Their specs seem worse than the CV (especially in Xmax and free air frequency) and they are more expensive.

I think I'll *try* to buy four drivers and mount them in the corners of my room (I'm building it, so I'll make it perfectly symmetrical and built to avoid resonant room modes). If I can't, I'll go with two of these subs and try to design the best box I can. I'll try your suggestions and report back about my findings. All things considered, I'd rather go with Cerwin-Vega instead of the other brands, which seem to be focused on midbass punch.

Brian, you are right! I forgot someone had to build the thing, even if I had the space! Thanks for reminding me about that Posted Image

So, I guess the Cerwin Vega drivers are kinda OK from all your opinions... am I on the right track, then?

Sorry for so many questions. I am researching every post you make, and learning lots in the process. I can't thank you enough for this. Posted Image

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#17 of 33 Greg Monfort

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Posted November 21 2002 - 01:07 PM

>Greg, I can't thank you enough for your explanation. I'll look further into the "Q" issue, which is, I think, the only thing that separates me from building a good enough box.
====
You're welcome! Just remember that the lower it's tuned the lower it's Q is between F3/Fb and the steeper it will be below Fb with the attendant higher group delay. If Fb is >20Hz, then the extra GD isn't normally an issue since it will decay to below audibility by the time it gets up high enough in frequency to be audible to the average person. Also, once it rotates around to where its slope is 6dB/octave (a true EBS alignment) it will have lower group delay in this BW than a sealed Q = 0.5 (critically damped) alignment, and why it's my fave. The tradeoff is lower efficiency so more drivers and/or linear displacement may be required.
====
>I think I'll *try* to buy four drivers and mount them in the corners of my room (I'm building it, so I'll make it perfectly symmetrical and built to avoid resonant room modes).
====
You can't avoid eigenmodes (room), all you can do is minimize their impact on the FR by averaging them out, which requires that all the sub drivers be in one corner. If the XO point is higher than the first mode you may be able to localize where the sub is though rather than it sounding omni-directional.

If the four drivers are in each corner then all modes will be driven to their max, not a good plan IMO.
====
> All things considered, I'd rather go with Cerwin-Vega instead of the other brands, which seem to be focused on midbass punch.
====
Sounds like a plan if you don't want to spend more $$ on drivers/amps specifically designed for max LF extension.

GM
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#18 of 33 Javier_Huerta

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Posted November 21 2002 - 02:22 PM

Greg, I meant I'd mount the drivers in the *front* corners of the room. Still, you answered my question: I guess I should mount whatever I build in one of the corners exclusively.

Anyway, I'm tired of looking for a semi-decent driver around here. I'm going to try to buy a Tempest and have it sent here. I guess that if I build the 4th order "boombox" alignment I'll get all the bass I'll ever need (the enclosure that is described at the Adire web site).

That is, I understand the Adire driver is, by far, way better than 2 of the subs I intend to buy. Of course, I could be mistaken Posted Image

Thanks a lot for your help!!! Posted Image As I said, I'm learning a lot.

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#19 of 33 Greg Monfort

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Posted November 21 2002 - 03:14 PM

OK. Yes, you will still get the high Q peaks/nulls if 'stereo' subs are used.

FWIW, for a given power/Vb, the two C-Vs should out SPL a single Tempest enough to notice if you could compare them.

Anyway, good luck with whatever you build. Posted Image

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#20 of 33 Javier_Huerta

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Posted November 22 2002 - 11:59 AM

Greg, thanks again for your help!!!

I think I found the driver of my dreams. Is it a Tempest equivalent? Check it out:

Cerwin-Vega Vega 154
Diameter: 15"
Voice coils: 1
Sensitivity: 96 dB
Power handling: 500 Wrms
Nominal impedance: 20-500 Hz
Voice coil diameter: 3.0"
Mounting diameter: 13.75"
Mounting depth: 6.5"
Free air resonance: 23 Hz

Qts: 0.359
Qms: 4.42
Qes: 0.390
Vas: 6.40 ft3
Xmax: 38.2 mm (+-19.10 mm)
Revc: 3.4 ohms
Vol. displacement: 0.26 ft3

Now, I might be wrong, but I think *this* is the sub to buy for $150 around here. Is it a better choice than my first option of buying twin 12"? I'd only be able to buy one of these, but it looks *a lot* better than my first choice.

Thanks!

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