What makes a good sealed box subwoofer? What is the ideal?

cvjoint

Member
2011-04-05 6:47 am
Recently I designed this guy:

20160316_225643_zpsc8v80hxf.jpg


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Parts used:
*8 units of Vifa NE 265W08-HS - 10" 8 ohm woofer by Vifa
*2500 watts amplifier @1% THD into 1 ohms
*3/4" MDF braced with 6 rods horizontal, 7 vertical
*Wood glue and drywall screws
*2.5 lbs of polyfill
*active crossover, time delay, 31 band graphic EQ

Design:
*Sealed 2.5 cubic feet net box
*Woofers are arranged in force cancelling push-pull and acoustic push-pull
*Coil are connected in parallel for 1 ohm load
*Intended band 20hz to 63hz 24db/oct slopes


I'm starting to think the Vifa NEs simply are not designed for a 1.1 QTC box. They sounded even boxier after adding more bracing.

What if I replace the 8 subs with only 4 subs that are build specifically for a small box? What do small box subwoofers have that others don't? Stiff suspensions? Why? Etc.
 
Well you've done an awful lot right there. You've got push-pull force cancellation and push-pull acoustic loading for cancellation of even order distortion products.

First of all the box size is most likely far too small for an optimum alignment for that many drivers. You mention a Qtc of 1.1? Usually people aim for a Qtc of 0.5-0.7 for unassisted designs. This would no doubt require a significantly larger box.

The answer to this is to use a linkwitz transform circuit to assist the alignment of the design. This can either be done in DSP or with an analogue filter. With the LT you can transform the high Q, high resonance, small enclosure to something more suitable. This is at the expense of amplifier power mind you, nothing comes for free, but it's a method that lots employ.

The second problem is the wiring of the subs. 1 ohm loads are hardly optimal for any situation unless absolutely required. Amplifiers do not like driving loads as low as this and losses in any cabling start to become significant. Not only this but the inserted series resistance of the cables will act to raise the Qtc of the box even further by affecting the electrical damping of the system.

If the amplifier can provide 2500 watts into 1 ohm then it will give ~1250 into 2 ohms and 625 into 4 ohms. I would have thought ~625 into 4 ohms would be plenty given the situation.
 
Ah so you're going to get lots of cabin gain when the trunk/doors are closed?

In which case you'll most certainly want to apply a LT circuit to get the Qtc down to a more reasonable figure but you'll need to make sure that you target a certain roll off so that you end up with a flat frequency response in car.

The biggest offender when it comes to poor bass is simply high linear distortion, ie a non flat frequency response. Get things flat and most of the troubles go away.

The Vifa drivers aren't terribly suited to sub duty, something from the Peerless XLS/XXLS line would have been a better choice, or something from the Dayton Ultimax line, but you work with what you've got. And all things considered? 8x 10" NE drivers is still a lot of VD (volume displacement) so you should be able to deafen yourself quite easily.
 

cvjoint

Member
2011-04-05 6:47 am
Lots of good input already!

Right, so the frequency response ends up fairly flat to 20hz given that this is dropped in a car. I tried not to mention car in here since you lose some of the best member contributions. Cars tend to scare people away. With the 31 band EQ and 2,500 watts I tried a whole lot of FR shaping. Nothing really takes away the small box sound, even when I boost 20s and 30s over everything else. I know linear distortion is the easiest thing to hear but I think we have something more going on here. A linkwitz transform is just a passive way to shape the FR correct?

Here is the FR summed with the mids joined in at 63 hz 24db/oct slopes and tuned to taste:
Vette%20tuned%20summed_zps84wa1gco.jpg



The box size is basically fixed, since that's all I can fit in there, but I'm open to new drivers or box designs given that they fit.

I haven't thought about the electrical damping of the system as being changed by ohm loads or cables. This is new to me! Now I have another perspective to explain why my tested QTC of 1.11 is much higher than the predicted QTC of .98 in WinIsd modeling. :cheers: Not sure I want to give up my 2,500w at 1 ohm output, power is all I have going for me in this design. :eek:

This statement here is the golden nugget: "The Vifa drivers aren't terribly suited to sub duty." Ok so what is about the speaker choice that I can improve? What do the XXLS drivers have that the NEs do not? SPL is plenty, I just want better sound. Should I look for the stiffest cone, stiffest suspension, smallest spider woofer for a small box? I got the Vifas because they had soft suspension parts and that worked well for me in the past with infinite baffle.

Small boxes seem to put a lot of pressure on suspension parts. I want to know what modern speaker design can do to counteract the loads in the box.

The only thing that helps is opening up the car windows.
 
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I was told that in a car you need to aim for a box-Q of around 0.5 because of cabin gain. If that's true then I wouldn't go any higher than around Q 0.7. Fiberglass is probably the best material for lowering box Q. Polyfill will do next to nothing.

EDIT: The Peerless subwoofer drivers are probably the most commonly used drivers because they work well in very small, sealed boxes.
 
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cvjoint

Member
2011-04-05 6:47 am
I was told that in a car you need to aim for a box-Q of around 0.5 because of cabin gain. If that's true then I wouldn't go any higher than around Q 0.7. Fiberglass is probably the best material for lowering box Q. Polyfill will do next to nothing.

EDIT: The Peerless subwoofer drivers are probably the most commonly used drivers because they work well in very small, sealed boxes.

I see Alpine and several other car manufacturers aim for QTC of about .8 in their white sheets. But in general actual recommended boxes yield QTCs all over the map, I presume because some companies want to prevent coil damage and returns (so they recommend high Q) while others may optimize output or extension. Not sure what they aim for, car audio is a bit ghetto at times.

Any link to the fiberglass filler?

I found the original Peerless car subwoofer application sheet.

Peerless claims "With the high BL available, Peerless has been able to design drivers with a high moving mass and low fs but without high Qt and low sensitivity." Not much else I could find about small box specific construction.

http://www.tymphany.com/wp-content/uploads/2014/11/Appnote-XLScar.pdf
 

cvjoint

Member
2011-04-05 6:47 am
Can you measure just the subs, run full-range, no EQ?
I suspect you've got a couple of peaks in the 100s of Hz range that show up and get really annoying.
How are you crossing them over?

Chris

Ok, I will do that tonight. The amplifier has a subsonic at 10hz. From playing test tones it seems like there is very little output below 10hz, it must be very steep. On the top end I use a LPF of 63hz 24db/oct. I believe it is a Butterworth design.
 

Windforce85

Member
2014-11-08 11:59 am
You need the driver with following parameters:

Low Qts <0.22
Low Vas <70 dm3
Low fs <20 Hz
High Xmax
High Bi
High power handling

This is recipe for good driver in small closed cabinet. There are only a few on the market these days becouse market is all for boomy BR's...
 
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That might be playing a part but I would be surprised if it was the main culprit, afterall there's nothing unusual about this design, many high quality subs use small cabinets.

The fact that the problem got worse with bracing isn't a surprise as it would have reduced the cabinet volume even more making the poor alignment even worse.

Adding a Linkwitz Transform filter would sort this out.
 

cvjoint

Member
2011-04-05 6:47 am
You need the driver with following parameters:

Low Qts <0.22
Low Vas <70 dm3
Low fs <20 Hz
High Xmax
High Bi
High power handling

This is recipe for good driver in small closed cabinet. There are only a few on the market these days becouse market is all for boomy BR's...

Curiously it looks like the XLS parameters. :D

I can see either the Peerless XLS, Dayton HO, or Seas L26R04Y working well! The Seas has a substantial xmax/xmech and BL advantage. It's also twice the money.

Read this: BK XLS200

Bk subs are very popular over here in the UK.

The woofer seems quite expensive over in your part of the world though: Peerless 830452

I see what they did. They went with a known brand and common choice for box Q. I don't see any deeper insights. Thanks though, it means I'm on the right track!


Hmm, substantial 2nd order distortion there. Is that at all power levels or higher with SPL?

The Rythmic website suggests there are time domain distortions as well as meaningful higher order non linear distortions. This calculation then is only a small glimpse at the distortion generated.

I believe my design would also have additional significant distortion effects from the opposed subwoofer design where the cones are working against eachother in the box. :censored:

That might be playing a part but I would be surprised if it was the main culprit, afterall there's nothing unusual about this design, many high quality subs use small cabinets.

The fact that the problem got worse with bracing isn't a surprise as it would have reduced the cabinet volume even more making the poor alignment even worse.

Adding a Linkwitz Transform filter would sort this out.

But are we still talking about linear distortion here? I did boost the output in the 20s and 30s after adding bracing and the boxy sound is still there despite the FR being bottom heavy.
 

cvjoint

Member
2011-04-05 6:47 am

I calculate the distortion at 2,500 watts as only 2%. So this cannot be the driving factor. Where does that constant come from? CMS doesn't even play into this equation even though manufacturers specifically go with a stiffer suspension for small boxes. The theory must not be complete. It probably assumes an infinitely strong cone and suspension, like a pure piston.

Oops that's with 4 drivers. With 8 the distortion goes up to 4.78%. Ok, pretty high, and we're not even accounting for suspension distortions due to the air pressure.
 
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Where does that constant come from?

Which constant are you referring to?

CMS doesn't even play into this equation even though manufacturers specifically go with a stiffer suspension for small boxes. The theory must not be complete. It probably assumes an infinitely strong cone and suspension, like a pure piston.

These equations just describe the non-linearity of the air compliance. That's nothing to do with the driver(s). Total distortion will need to take into account the distortion of the driver and electronics as well.

But I don't think distortion alone would cause a "boxy" sound. Can you measure the impedance response of the subwoofer and post that here?
 
That just seems like an awfully small box for 8 subs. The most delicious bass I ever made was with a single 10" in 2.5 cubic feet. F3 was below 40 Hz before cabin gain, so while it wasn't at all flat, it wasn't boomy, just thick deep solid bass. Q was above 0.7, since I'd designed the box for 2 of those woofers isobaric, then decided to try it with one temporarily. So, I'm a bit skeptical about using high f3 and counting on cabin gain to make it flat.
 

bass thump

Banned
2016-04-19 12:43 pm
seeing that you've already chosen sealed, you're already on the path to superior SQ. i don't have thiel small tech saavy to add to your search, but the worst thing that could happen is that you have too little internal volume and suffer bass rolloff, but ANYTHING'S better than the multiple forms of distortion you get from ports.
 

infinia

Member
2005-05-15 9:51 am
SoCal
aiming for 1 ohm speaker just because an amplifier claims its max power at that point is playing folly. ( what about batteries and charging? ) i'd aim for 2 ohms max and settle for 1000W peaks:) banana jacks /plugs are rated for heat rise at ~ 20A and that's just for the best ones. Get a good 4 wire ohm meter check all the wires and connections i'd bet dollars to donuts you'd be lucky to measure 1000W steady state with your set up. (do it... I dare you)
its good to design with plenty of power headroom even with conservative specs but car audio is well known for ambitious power numbers de-rate stuff by 1/2 to 1/4 . it's not easy figuring Q accurately with low ohms, everything is higher resistance than you thought ( when you measure it correctly) heck I recently bought some "high quality" silicone wire and it measured double what the claimed copper AWG tables said.
get a couple of low ohms test resistors <1% and / calibrate your testing , then re adjust Q with wiring added ( even the amplifier is not zero ohms at its connectors)
 
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infinia

Member
2005-05-15 9:51 am
SoCal
Id target lower Q for high cabin gains, normal rooms sound fine with closed box Q~ 0.9/40-50Hz, cars maybe not. I think choosing cross over frequencies and gains for the best SQ (for the types music you listen to) in you car is best done by ear, IME Crossing too low means the subs only come alive for home theater effects.
you have a great start, just a matter of fine tuning
 
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