This box doesn't make sense...

Siorus

Member
2013-02-09 8:16 am
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:
 

djk

R.I.P
2001-02-04 4:23 am
USA
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
 
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 .
 

Siorus

Member
2013-02-09 8:16 am
Thanks for the input, I appreciate it.

It may well not be suitable for vented boxes.

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.

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.

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

Member
2011-01-02 10:44 pm
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 :)
 
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Well Done !

>< 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.
 
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just a guy

Member
2006-05-12 6:59 pm
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

Member
2002-06-27 10:41 pm
USA, MN
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.
 
It must be remembered that an un-eq'd f3 of 37Hz sealed is quite respectable. Room gain helps sealed subwoofers a lot.

Here's what I'd do.

Make the box as small as you can while eq'ing the low end (you'll need to simulate a Linkwitz Transform for this) to whatever you like.

Set the cabinet size such that excursion and thermal power handling run out at about the same time.

Power input is just a matter of how hard it is to move the cone at a given frequency. A small cabinet makes it harder, so you'll run out of power handling before excursion.
A big cabinet is more efficient: it lets the cones move more freely, so excursion runs out before power handling.

When both run out simultaneously, you have the smallest cabinet that still allows you to get all the SPL available out of your drivers.

Chris
 

OscarS

Member
2011-01-02 10:44 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.

I'm sure you agree that "acceptable" is in the eye of the beholder. If you listen to heavy metal at ear-melting levels like I do, all of a sudden quick double-bass drum blasts don't sound so distinct if you run a Qtc that high and would sound quite aweful. It all just depends on the listening material and the preference of the user.
 

djk

R.I.P
2001-02-04 4:23 am
USA
In 0.5 cu ft stuffed the Qtc=1.0 F3=65hz, this might be OK in a car.

A smaller BP4 box, 95.3dB/2.83V/1M

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 = 1.5 cu.ft
Fb = 54.23 Hz
QL = 6.589
F3 = 34.77 Hz
Fill = heavy
Chamber 2 - upper-freq.
Vb = 2.113 cu.ft
Fb = 54 Hz
QL = 6.761
F3 = 85.03 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = two flush
Dv = 6.012 in
Lv = 5.929 in
 
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Hmmm.........

A BP4 box with the low pass section bigger volume
than than the high pass section ? not good IM0.

Paper design hardly ever reflects the real build,
the things are so damn sensistive to parameters.

With room gain -3dB at 37Hz is very good for sealed,
though F-6dB actually matters more for a speaker,
and F-10dB (with power handling) for a subwoofer.

rgds, sreten.
 

djk

R.I.P
2001-02-04 4:23 am
USA
"A BP4 box with the low pass section bigger volume
than than the high pass section ? not good IM0."

Tell that to Polk, they sell many subwoofers designed like this. Mathew Polk wrote a technical paper published in Audio magazine a few years back and offered free ($5 to ship and cover the disc cost) software for designing BP4 enclosures.

High mass, high Qts drivers work well in BP4 designs when done as per Polk suggestions..
 

Siorus

Member
2013-02-09 8:16 am
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

Thanks, that makes sense.

To expand on this a bit, WinISD doesn't "want" anything.

"Wants" was a poor choice of words on my part; "suggests as a starting point" would have been more accurate.

Is this for a car or a house?

This is for my living room.

It must be remembered that an un-eq'd f3 of 37Hz sealed is quite respectable. Room gain helps sealed subwoofers a lot.

Objectively speaking, you're absolutely right. a 37Hz f3 isn't half bad. But I'm a little OCD and I have a somewhat (but not completely) arbitrary set of specifications that I want to hit. Mainly an F3 under 25Hz and a theoretical maximum SPL (with the drivers operating at their full RMS input power rating) >125dB, while retaining good sound quality.

That's a fairly tall order, and I know it. Power won't be an issue, though; I've got a pile of amps that are credibly capable of shoving 1000wpc (if not comfortably more than that) RMS/continuous into 2 or 4 ohm loads with both channels driven. The Crown CE4000 on the bottom of the pile will actually do 3,600w into 4 ohms if I need it to, and I've been looking for an excuse to pick up one (or several :rolleyes: ) MA-5002VZs or big Crests or something anyhow. Just a matter of finding drivers and a couple of boxes. :D

And I've always considered "room gain" a cop-out. It's like running nitrous on an otherwise mildly built car. It'll get the job done but it's a halfassed way of doing it. If you want a f3 of, say, 30Hz, build the system for 30Hz. Whatever you get from room gain is a bonus. Just how I see it. *shrug*

Set the cabinet size such that excursion and thermal power handling run out at about the same time.
(...)
When both run out simultaneously, you have the smallest cabinet that still allows you to get all the SPL available out of your drivers.

Chris

How do I go about determining that, though? :confused: Is there some formula that can be applied to calculate whether a given speaker/enclosure combination is going to hit the driver's xmax before it hits its max. power handling or vice/versa? OscarS mentioned that some enclosure modeling software has the ability to predict cone excursion, but that's not something that I see either WinISD beta or WinISD Pro alpha being capable of; is there some other software that's worth considering? I know PE carries a couple different programs but the reviews of them seem to be pretty mixed.
 
How do I go about determining that, though? :confused: Is there some formula that can be applied to calculate whether a given speaker/enclosure combination is going to hit the driver's xmax before it hits its max. power handling or vice/versa? OscarS mentioned that some enclosure modeling software has the ability to predict cone excursion, but that's not something that I see either WinISD beta or WinISD Pro alpha being capable of; is there some other software that's worth considering? I know PE carries a couple different programs but the reviews of them seem to be pretty mixed.

Its a bit trial-and-error.

I'll run some numbers and see what happens.

...
Remember that WinISD provides (IIRC) groundplane simulations: I think you can gain 6 & 12dB of boost from wall- and corner-loading respectively.

I can tell you immediately that 4x12" sealed won't hit 125dB @25Hz. The displacement-limited SPL is ~108dB at 25Hz.

Ported box required.

Ported can pretty much get you into the 120s, without any boost from walls etc. 0.9 cubic metres (900L), tuned ~22Hz.
Maximum SPL drops to around 117dB at 28Hz, as cone excursion becomes a problem above port tuning as well as below. Infrasonic filter definitely needed at these power levels: the ports will give very little resistance to <20Hz stuff, and the drivers will bottom out immediately.


Given correct eq, you could get (corner loaded) 120dB from 25Hz upwards from a 100L sealed cabinet.
You could stick all 1400w into it, and the drivers would hit 9mm one-way travel around 18Hz.
You'll need an infrasonic filter, or <20Hz stuff will take up all the headroom: a Linkwitz Transform set for f3=25Hz, Q=0.5 is used (the lower Q reduces the boost needed, and integrates better with room gain).

The Linkwitz Transform is essential.

HTH

Chris
 

djk

R.I.P
2001-02-04 4:23 am
USA
123.8dB/1KW/1M in 2Pi, no room gain.

Driver Properties
Name: Polk 12
Type: Standard one-way driver
No. of Drivers = 4
Mounting = Standard
Wiring = Series-Parallel
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 = 11.6 cu.ft
Fb = 41.83 Hz
QL = 5
F3 = 24 Hz
F6 = 21 Hz
Fill = heavy
Chamber 2 - upper-freq.
Vb = 8.453 cu.ft
Fb = 41.83 Hz
QL = 5.378
F3 = 73.62 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = two flush
Dv = 12 in
Lv = 7.493 in

This is in 2Pi, if you push it up against a solid wall you could pick up 3dB~5dB or so. Will handle 1KW down to 30hz without exceeding 8mm on sine, much more on real program material. It could be made as a 34" cube (external), but I would make one dimension 30" so it could fit through a standard home doorway (roughly 36x36x30). The drivers could be mounted push-pull pairs (not isobaric) for lowest distortion, and perhaps a triple chamber design would be best, or two separate sub cabinets.
 
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