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-   -   This box doesn't make sense... (http://www.diyaudio.com/forums/subwoofers/229658-box-doesnt-make-sense.html)

Siorus 9th February 2013 09:38 AM

This box doesn't make sense...
 
Hi guys. I'm not new to audio, but I've never built an enclosure before and I don't have a fantastic understanding of the impact of some T/s parameters on driver performance.

I've found some new/surplus Polk drivers that I'd like to use in a project, but the numbers that I'm getting out of WinISD seem a little off to me.

These are the only specs that I have for the subs:
  • 4 Ohm rated
  • 250wrms/400w peak
  • 2" voice coil
  • 91dB @ 1w/1M
  • 20Hz-1500Hz rated
  • FS - 23.8926 Hz.
  • Revc - 3.2038
  • Zmax - 21.2296 ohms
  • Qes - 0.5382
  • Qms - 3.0342
  • Qts - 0.4571
  • Le - 2.0306 mH (at 1 kHz)
  • Vas - 176.2302 L

WinISD is showing that 4 of them in a 25ft^3 enclosure tuned to 25Hz should perform very well.

Supposedly, that should net a f3 around 24Hz and a maximum SPL of around 130dB at 38Hz-ish with 1600w input power. But 6.25ft^3 (per woofer) seems really, really large for 12" woofers and I'm worried that there's something that WinISD isn't accounting for. Their simulated performance in a much smaller enclosure (say 2.5ft^2 per sub) is very poor, though; much worse than I would expect out of a sub with a Fs of 23Hz-and left to its own devices, WinISD auto-calculates a ported enclosure volume of almost 10ft^3 for one driver.

So I'm very confused. I plugged in some rough guesstimates for the missing parameters (xmax, Sd, BL) and the results didn't change much. It just seems like too much box volume for a bunch of 12" woofers. Can someone help me understand this? :confused:

wintermute 9th February 2013 11:02 AM

Unibox recommends a closed box for it. Vented comes out at 317L (for one). sealed about 110L. It may well not be suitable for vented boxes.

Tony.

djk 9th February 2013 11:52 AM

Driver Properties
Name: Polk 12
Type: Standard one-way driver
No. of Drivers = 1
Fs = 23.9 Hz
Qms = 3.034
Vas = 176.2 liters
Cms = 0.442 mm/N
Mms = 100.4 g
Rms = 4.968 kg/s
Xmax = 8 mm
Xmech = 12 mm
P-Dia = 259.8 mm
Sd = 530 sq.cm
P-Vd = 0.424 liters
Qes = 0.538
Re = 3.204 ohms
Le = 2 mH
Z = 3.845 ohms
BL = 9.473 Tm
Pe = 250 watts
Qts = 0.457
no = 0.431 %
1-W SPL = 88.49 dB
2.83-V SPL = 92.47 dB
-----------------------------------------
Box Properties
Name:
Type: Bandpass Single-Tuned Box
Shape: Prism, Bandpass
with two chambers
Chamber 1 - lower-freq.
Vb = 2.901 cu.ft
Fb = 41.83 Hz
QL = 6.589
F3 = 24.99 Hz
Fill = minimal
Chamber 2 - upper-freq.
Vb = 2.113 cu.ft
Fb = 41.83 Hz
QL = 6.761
F3 = 72.03 Hz
Fill = minimal
No. of Vents = 2
Vent shape = round
Vent ends = two flush
Dv = 4 in
Lv = 13.53 in

Alternate vent

No. of Vents = 1
Vent shape = round
Vent ends = two flush
Dv = 5.999 in
Lv = 14.46 in

JMFahey 9th February 2013 01:58 PM

Quote:

It just seems like too much box volume for a bunch of 12" woofers. Can someone help me understand this?
Without even simulating a box, does 55 Liters per 12" woofer seem too much?
They have 170 Liters Vas to begin with, not exactly designed for shoebox sized cabinets.
And you are trying to reach 25Hz, no surprise it comes out with a relatively large box.
If, besides that, you use 4 of them .... well .... that means 4 x the original volume .

sreten 9th February 2013 09:26 PM

Hi,

Vented will always come out very large compared to Vas
for a driver with Qts=0.45, which clearly places it in the
"best used sealed" category.

rgds, sreten.

Isobaric is a possibility for a pair of small sealed subwoofers.

Siorus 10th February 2013 02:24 AM

Thanks for the input, I appreciate it.

Quote:

Originally Posted by wintermute (Post 3362373)
It may well not be suitable for vented boxes.

Quote:

Originally Posted by sreten (Post 3363070)
Vented will always come out very large compared to Vas for a driver with Qts=0.45, which clearly places it in the
"best used sealed" category.

Yeah, that's basically what WinISD had to say about this whole thing too; it's giving me an EBP of 44.4, which is apparently pretty heavily weighted toward the 'sealed box' side of things.

The weird thing is, the simulated performance in a sealed enclosure is absolute garbage (f3 of 37Hz). Here's the listing for the drivers; apparently they were destined for Eosone 912 subs, which are-based on a quick google search-ported. :boggled: Who knows why Polk chose to use these drivers in that application.

Quote:

Originally Posted by JMFahey (Post 3362527)
Without even simulating a box, does 55 Liters per 12" woofer seem too much?
They have 170 Liters Vas to begin with, not exactly designed for shoebox sized cabinets.
And you are trying to reach 25Hz, no surprise it comes out with a relatively large box.
If, besides that, you use 4 of them .... well .... that means 4 x the original volume .

55L? No, that works out to about 2 cubic feet, which is about what I'd expect for a 12" sub. That's actually right in the ballpark of what the guy selling these things recommends, too. But according to WinISD, these guys will perform very poorly with 55L per sub-a single sub in a 55L ported box in WinISD has a f3 of 37.6Hz. WinISD actually wants them to have a box volume of 266L per woofer, which is why I'm confused. The final box I came up with for 4 of them is a bit over 700L (or about 175L per driver).

I'm fine with the enclosure taking up that amount of space, but what I'm hung up on is that, in my (extremely limited) understanding of how these things work, if an enclosure is too big and/or tuned too low, you can end up with a situation where it no longer supports the driver, and you can easily exceed the mechanical limits of the speaker at well below its rated power input because the air in the box is no longer acting like a "spring", so to speak. And it just seems like 175L per woofer for a 12" is going to cause that to happen.

But I don't understand Qts, Qms, or Qes at all and while I "get" Vas, Sd, and Fs, I don't really understand how they all interoperate to dictate driver performance in a given enclosure, so I don't know if my suspicion that 175L per woofer is "too much" is valid or not.

Quote:

Originally Posted by djk (Post 3362415)
Type: Bandpass Single-Tuned Box
Shape: Prism, Bandpass
with two chambers

I hadn't considered a bandpass enclosure, that's not a bad idea. I've also been toying with a couple other drivers; the Polks seem to be a pretty good deal, but less so after shipping is factored in. They're still not bad, but there are some decent new drivers that are probably of somewhat higher quality-the Infinity Reference 12s, for instance-that would probably perform similarly or better for about the same total cost if I managed to get them with free shipping.

OscarS 10th February 2013 02:59 AM

What WinISD "wants" is just ONE possible alignment out of an infinite number of alignments. It is likely a maximally flat response, which as was said, gets out of hand quickly with a driver that is compliant as that one with a Vas of 177L.

The large-ish box might or might not compromise mechanical power handling with your particular power input. That is why modern enclosure modeling software can predict Cone Excursion, so you can investigate this phenomenon yourself to see if it's performance/SPL output will be hindered in your particular alignment.

So there is nothing to be confused about; a speaker with a relatively large Vas and an average Qts, will naturally tend to give a maximally flat response in a large-ish box at or near it's reference sensitivity, easy as that. Compromise that by making the box smaller, and it's [likely] high sensitivity will be compromised (slightly lower) and you're gonna have to eat that higher F3 like a wandering nomad that has been crawling through the desert and hasn't eaten in weeks and just got a glimpse of some food---you just gotta take it if that's the route you're going for with that particular driver.

Happy Modeling :)

blakkshepeaudio 10th February 2013 07:06 AM

Well Done !
 
Quote:

Originally Posted by OscarS (Post 3363403)
>< snip

Compromise that by making the box smaller, and it's [likely] high sensitivity will be compromised (slightly lower) and you're gonna have to eat that higher F3 like a wandering nomad that has been crawling through the desert and hasn't eaten in weeks and just got a glimpse of some food---you just gotta take it if that's the route you're going for with that particular driver.

Happy Modeling :)

One seldom finds such eloquent imagery in DIYAudio posts.

Bravo!



blakktop

p.s. Perhaps the driver was originally designed for an EQ-ed alignment?
Not uncommon for commercial designs.

just a guy 10th February 2013 08:14 AM

Quote:

Originally Posted by OscarS (Post 3363403)
What WinISD "wants" is just ONE possible alignment out of an infinite number of alignments.

To expand on this a bit, WinISD doesn't "want" anything. It gives a few popular classically defined alignment options for you to choose from and then it calculates the one YOU choose. From that point it's up to you to decide whether that alignment makes any sense in practical terms and for your intended purposes and goals. Most of the time, none of the predefined alignment choices will line up with your goals. Fortunately it's very easy to adjust the results to suit your needs. As mentioned, it's almost infinitely variable, but as you will soon learn, sometimes the driver itself just can't be forced to operate within the parameters you'd like it to. In that case you can modify the driver (but most mods will require an even larger box) or choose a different driver.

Ron E 10th February 2013 03:25 PM

Is this for a car or a house? If a Car, probably best to just use a smallish sealed box - I have heard subs with Qtc up to 1.3 sound quite acceptable in a car.
176/((1.3/.45)^2-1) gives about 24liters per woofer for a punchy sound - although about 55 liters per woofer is probably going to sound more neutral.


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