Old School Polk Audio C4 Subwoofer - Four 6x9's in Isobaric Teardrop Enclosure

BigDWiz

I wasn't sure if this should go in the Vintage Speaker section or the Car Audio section, but since it's car audio, I decided to put it here. I just picked up this MINT 1992'ish Polk C4 subwoofer from a local CL ad. I took off the bottom panel to show off the "guts" and plan on doing a video demo soon.

(info below was found in an older post in the vintage speakers section. I also found an Ad in a 1992 Car Audio & Electronics and a test in a March/April 1992 Car Stereo Review. I'll post these as soon as I get them scanned in)

Here's a Polk Audio C4, circa 1992. Original MSRP was around $500.

I took off the bottom panel so you could see the goodies. I'll do a video demo soon, so stay tuned...

Specs:
Driver complement - Four 6x9 drivers monted in a isobaric push pull configuration
Enclosure type - 4th order bandpass
Frequency response - 25 to 150 Hertz
Sensitivity - 94 db @ 1 Watt @ 1 Meter
Recommended amplification - 25 to 200 Watts per channel
Impedance - 4 Ohm stereo, 2 or 8 Ohm mono
Dimensions - 8" H x 26.25" W x 17.125 D
Weight - 26 pounds

BigDWiz

Mods feel free to move this to the Car Subwoofer section...I should've looked at all of the forum titles before posting :redface:

BigDWiz

I picked it up late yesterday and haven't had a chance to fully test it out in my car. I tested it for functionality only, but I'm planning on doing an entire video demo of the unit and will also give my impression of the subwoofer. I currently have a JL Audio 12w6v2 in my car, so I don't expect that level of output, but clean, hard hitting bass is really what I prefer anyway. I'll be sure to post the video here and any follow up impressions.

BigDWiz

Each 6x9 in the enclosure has a date written on it (Sharpie?) and I've looked as close as I can and it still looks like 12/18/98 to me. It would make more sense to be either 91/92/93, but it looks like an 8 to me. Maybe the 6x9's were replaced or my eyes are deceiving me?

BigDWiz

Here's an Ad from a May 1992 CA&E magazine:

BigDWiz

And the CSR test from March/April 1992 as mentioned in the Ad...

Still confused why my speaker input terminals are on the opposite side of the box?? Can anyone who has one of these confirm which side the terminals are on their box? :question:

MacLeod

Now thats just damn cool, I dont care who ya are.

hoosier21

MacLeod wrote: »

Now thats just damn cool, I dont care who ya are.

That is a neat find

BigDWiz

Thanks guys...I put it in my car yesterday and have put it through a few listening tests powered by around 300w. It sounds VERY nice! Not the window flexing bass output of my JL Audio 12w6v2, but clean and loud enough to feel the kick. It would be a superb upgrade to a factory system to give some extra punch. The teardrop shape makes it easy to fit in many applications. I put together a video overview and demo of the C4 and will post it here when it's been uploaded.

Vital

That IS nice!
I wonder why enclosures/subs like this aren't being sold nowadays. Seems like it puts out a lot bass especially for the size they are which as mentioned fits just about anywhere due to "teardrop" shape..

BigDWiz

Finished up the video today and finally got it uploaded. See below:

Watch on YouTube in 1080P HD or embedded below:

https://www.youtube.com/watch?v=iWNGeePHEeA

IJGN

Wow, I just had to register for this post. Brings back awesome memories! I had 2 of these maybe 8 years ago. These dont really sound all that great in a trunk, but I put one on the rear shelf area of a camaro SS and wow, the hatchback really made it sound like a decent quality 12 was back there, and the SQ was TIGHT.

I will remark that there appeared to be 2 versions of this box. One I suspect was much earlier, with silver frame, paper cone drivers, looked much cheaper inside. The construction of the curvature was different too, and not as good, as it totally "collapsed" rendering the box junk, the area where the slats are doesnt appear to be thick enough. Was powered with 350 watts RMS IIRC, not a problem but I believe the earlier version developed a coil knock/scratch.

I also had a version much like yours with the rubber surrounded black drivers, and I could have sworn the curvature part was totally smooth inside, as if they perfected the construction even better.

Really wish I could find another mint one, just to have. Sold one to a reluctant corvette owner who was more than thrilled with the sound after installing it.

Jstas

I have one of these.

I've posted about it before.

I've used it as a home sub as well for a home theater.

Stellar sub!

-FROG-

Cool sub indeed.

liljimmy

if your ready to sell the c4 im ready to buy. :-)

7tcatvert

I found one and traded for it but it wont fit in the back of my mustang...It also came with a amp and crossover.

What would something be worth on the open market?

Ocelot

Has anyone looked at this thread in the last recent while? I just registered and was hoping to discuss this little C4 auto subwoofer. I recall hearing one in a showroom back in the mid '90's and being very impressed.
I've been trying to reverse engineer this on paper for the last little while and am considering trying a DIY of it. Tang-Band has had a bare 6x9 "subwoofer" for a while whose specs look like they'd work in a 4th order coumpound loaded bandpass box. I'm having some trouble getting my head around the tuning frequency for the vented chambers. My quick calculations, assuming vented volume of 1/3 ft^3 (my best guess) say a 3"x5" round port should land me at about 85 hz. Is it reasonable that there could be a +40 hz and - 40 hz bandwidth from this box and that in-cab cabin gain allows extra extension?
I get that the isobaric or compound loading should allow the drivers to work in a box volume that is half of what I'd calculate as optimal, and I think I can guess that that applies to the vented chamber as well as how I can see how it would apply to the sealed chamber.
Do you think my guess at the net volume of the vented portion being approx 1/3 ft^3 and the sealed portion being approx 0.4 to 0.5 ft^3 seems reasonable?

gp4jesus

I'd love to hear that driver config w/out ported chambers - essentially a 4 driver isobaric alignment similar to 2 drivers sharing space.

crazyndn49

I know this is an OLD post, but I just picked up a perfect working one for $50.

Jstas

I didn't notice this from January 2018 'cause there's no traffic here anymore. But I'm going to address it because there's multiple posts here asking similar info.

The tuning frequency of the chamber is going to be dependent on the drivers themselves. The Polk C4's specs will not help you here as the drivers are different from the Tang Band subwoofer.

https://www.parts-express.com/tang-band-w69-1042j-6x9-subwoofer--264-837

What you first need to do is figure out your crossover network. There's plenty of tools online. The 12volt is actually a very good resource for this kind of stuff:

https://www.the12volt.com/caraudio/subwoofer-wiring-diagrams.asp

For the C4 to work properly, the isobaric configuration needs to be created in a certain way. The teardrop shape is not necessarily consequential to the system.

The C4 is not dual reflex, but this is the configuration of the C4's drivers.



Your wiring of the drivers is basically going to turn each pair of drivers into a single unit. The two drivers facing each other need to be wired out of phase from each other. This is simple to do, you wire it the same way you would in-phase, you just assume that one speaker's leads are reversed. This post of the12volt's forum shows it simply.

https://www.the12volt.com/installbay/forum_posts.asp?tid=79428

The difference that Polk did, though, was the center chamber was sealed. So when you are constructing your enclosure, you need to make the sealed center section appropriately sized for not just one woofer but both woofers that will fire in to it. So if the woofer needs .62 cubic feet of air space in a sealer enclosure, you need to make the enclosure 1.24 cubic feet. Unless you are going dual reflex then those numbers will change and you need to figure out the port tuning too. You also want to make sure that the two woofers that are mounted in the sealed chamber are wired with the same polarity or they will cancel each other out.

The next thing you have to do is for the woofers that are in the ported enclosures at either end, they get wired out of phase from the sealed woofers but, you also need to know what the size of the enclosure would need to be for the ported enclosure. For that Tang Band woofer, Parts Express lists that as .65 cubic feet. But ported enclosures need to be tuned too. Unless you're willing to do some complicated math, you should enlist the help of a calculator to help you determine the length and width of your port.

The12volt to the rescue again!
https://www.the12volt.com/caraudio/boxcalcs.asp

Now, your port for your vented enclosure matters. It's designed and tuned to emphasize the point where your speaker's performance drops off significantly. You need the port to be long enough so that the air column inside compliments the driver's movement when it's being moved by the woofer's backwave. If you get it off, the port will work against you and sound terrible.

So, for this Tang Band woofer, the F3 frequency is 51 Hz. F3 is the point where the response curves drops 3 dB down and you start losing SPL significantly. However, the resonant frequency is 35 Hz. The resonant frequency is the point at which below that frequency, the driver cannot reproduce sounds that would be audible. In large subs, this is often much closer to what the F3 frequency is because cone and surround construction is heavier. So if you're going for an SPL competition, you want to tune for the resonant frequency because the harmonics produced will give you a boost in the SPL from the port performance. You're also, often using a slot port. So sound quality doesn't matter, what matters is how loud your burp is. But you don't want to necessarily go for that here because an isobaric band-pass isn't about volume, it's about packaging and sound quality.

Isobaric subs have an advantage over normal band-pass. Because of the multiple enclosures and complimentary drivers, they can extend their frequency response to a sizable curve rather than having a fairly flat, low SPL curve with a hump around a certain frequency. That's why they are called band-pass. Only a certain band of frequencies are boosted. They are boosted above what is normal for the loaded driver so the driver performs a bit better at the tuned frequency. You tune the band-pass to the driver's sweet spot which varies. You can often calculate it well enough but it's far better to use a response curve graph from the manufacturer 'cause they got that with a testing apparatus.

So, anyway, an isobaric band-pass, especially one with a sealed center section is basically a physical crossover network. It limits and boost performance of the drivers by using physical constraints to control frequency response. Because of the sealed center section in the Polk C4, the frequency response curve gets extended to more of a plateau instead of a hump or spike. It also tends to have a stronger bass-reflex effect (i.e.: port boost) so it can play louder on lower levels of power.

Now this is where it gets heady. The C4 is not an eight order bass reflex box like what I posted above. But I needed to tell you all of the above to tell you this.

---<continued in next post>---

Jstas

The C4 is essentially a pair of of single reflex isobaric enclosures like this:



It's closest approximation is a this 4th order isobaric 3 chamber single reflex bandpass enclosure.

https://www.the12volt.com/12voltimages/isobpass3cham.gif

But instead of having the shared chamber vented, the other two chambers are vented. So, it's a fourth order band pass and is essentially two bandpass boxes together. The shared sealed chamber is what makes it a 4th order bandpass and turns the unit into a single sub.

Now, when you wire this up, you have options. What Polk did was give you two sets of terminals, one for each set of drivers and the drivers were wired to present a single load on the terminal. So when wiring the C4 you treat each set of drivers like you would a dual voice coil sub. Doing it the way Polk did it gives you the most flexibility in installation which is good for them because it fits the most number of possibilities.

But if you're going to duplicate it, you can wire it any way you like as long as you are happy being stuck with that wiring set up.

So, for these Tang Band drivers, you need to make a 3 chambered box with the following volumes:

- .5 cubic foot center section
- .65 cubic foot end sections x 2

The chambers can be any shape as long as you have space for the driver's necessary clearances inside and in the case of the vented sides, enough room for the ports.

The ports should be 2 inches wide and you'll want to tune for the 51 Hz F3 in this case so they would need to be about 3.5447999999999995 inches long. Or, about 3 and 9/16ths of an inch long. It should actually be about 35/64ths but who has a tape measure that measures that?

Depending on how you wire it up and what load you want presented, use a calculator to wire the the subs up to present the load you want. Unfortunately, because those Tang Band woofers are 8 ohms impedance, you're stuck with 2 ohms or 8 ohms. In the 90's when the C4 came out, that mattered because not much was 2-ohm stable. But then monoblock, D-class sub amps became a thing and some were 1.5 Ohm stable. So now, it shouldn't matter. But, you can do like Polk and use a terminal cup pair, wire the speaker pairs in parallel to the terminal cups to present 4 Ohm loads at each terminal set. Then you can wire those in parallel to present a 2 ohm load or wire them in series to present an 8 ohm load or use a 2 channel sub amp with a 4 ohm load on each channel.

When you build this box, every joint needs to be sealed. Including sealing the speakers to their mounting baffles and the ports to their mounting baffles.

Things I would recommend:
- Make one common box, it will be more rigid and less chances for air leaks.
- Make one long side of the enclosure removable so you can get all this set up without being a contortionist.
- Use nuts and bolts through pre-drilled holes and neoprene washers between the flat washers and the speaker baskets to mount the drivers to the mounting baffle.
- Use neoprene gaskets between the driver mounts and the mounting baffle to not only seal but isolate the baskets from the baffle.
- The Xmax on the Tang Band drive is 7.3mm so that means the maximum physical excursion of the cone from the basket is 7.3mm. Since you are mounting them facing each other, you need at least 7.4mm of space between them so the cone/surround doesn't impact the face of the opposing speaker.
- Seal everything, including the holes drilled for wiring. This box design works on pressure differences between enclosures, having them vented to each other by just the teeniest bit will make this not work. That goes for any speaker enclosure but more so for a type of enclosure like this because, like I said, you are making a physical crossover network here and just like electrical, if those network components short each other out, they will not work.

gp4jesus

+10000 on the sealing details. Dont be afraid to over engineer the closure- internal bracing. Further, don’t skimp on wire SIZE. Your amp is dealing w/a very reactive load - gauge will make a HUGE dif!

Back in the 90s I traveled this road w/a pair of 3 ohm seal isobaric subs. Read carefully as what follows is no stretch of the truth:
Using 10ga*, I “upgraded” from a single 10’ run* to a DOUBLE 5’ run just for fun - not expecting a change. Guess what? Had to turn ‘em down - Louder subs for free!

I can see the mud and tomatoes being loaded in the torpedo tubes already. Either that or some one running to check their BS meter.

Since joining this forum I believe I’m loudest, most frequent “mo’ bigga, mo’ betta” wire gauge advocate, not to mention internal speaker wire upgrades. All 5 of my HT speakers’ mids are internally soldered w/10 ga Blue Jean cable. My LR’s woofers, 8 ga, soldered. Further I upgraded my LCR BPs w/much bigger Daytons.

Here’s a thread from several years ago:

https://forum.polkaudio.com/discussion/172939/why-did-my-subs-get-louder#latest

Read it to draw your own conclusions.

A few in the Polk 2 ch & HT circles will $pend 1000s between their amps or AVRs and speakers all the while ignoring the CRAP ~18 stuff inside.

I’ve sufficiently “beaten the dead horse.”

Tony

krazypolk

Just buy this instead, https://atlanta.craigslist.org/eat/ele/d/lilburn-rare-polk-audio-c4-bandpass/6987408221.html

CH46E

I know its a old thread but is @BigDWiz the one and the same Williston Audio Labs. Big D on YourTube?

"You big dummy!"

Hahahah no way!

krazypolk

CH46E wrote: »

I know its a old thread but is @BigDWiz the one and the same Williston Audio Labs. Big D on YourTube?

"You big dummy!"

Hahahah no way!

I seen his you tube videos. Looks like he didnt stick around long in here.

BigDWiz

Big Dummy is still around

CH46E

In case anyone wondered who he is? This is his YT channel.

https://www.youtube.com/c/bigdwiz

Funny commentary and cool amp dynos.

BigDWiz

la2vegas wrote: »

The man, the myth, the legend.
To what do we owe the honor?

1 of those 3 for sure, lol. I made the video about this Polk C4 subwoofer back in 2012, it was considered old then, almost ancient now. Awesome to still see interest in the oldies! At one time this was my most popular video. You may not be able to tell by the video itself, but it took 15-18 hours total to shoot and edit this video. Again, it makes it all worth the work when people are still checking it out so many years later!

ZLTFUL

BigDWiz wrote: »

la2vegas wrote: »

The man, the myth, the legend.
To what do we owe the honor?

1 of those 3 for sure, lol. I made the video about this Polk C4 subwoofer back in 2012, it was considered old then, almost ancient now. Awesome to still see interest in the oldies! At one time this was my most popular video. You may not be able to tell by the video itself, but it took 15-18 hours total to shoot and edit this video. Again, it makes it all worth the work when people are still checking it out so many years later!

I enjoy watching your old gear dyno tests. I still remember going to the car shows in Minneapolis/Chicago/KC when I was still in HS in the early 90s and seeing things like the Rockford Fosgate Astro van with the rack of Power 1000Cs in the back.

gp4jesus

BigDWiz wrote: »

la2vegas wrote: »

The man, the myth, the legend.
To what do we owe the honor?

1 of those 3 for sure, lol. I made the video about this Polk C4 subwoofer back in 2012, it was considered old then, almost ancient now. Awesome to still see interest in the oldies! At one time this was my most popular video. You may not be able to tell by the video itself, but it took 15-18 hours total to shoot and edit this video. Again, it makes it all worth the work when people are still checking it out so many years later!

@bigdwiz I’ve since watched several of your equipment test/reviews - you ROCK!

As a former car audio installer/tech and someone who has watched quite a few YT vids on many different topics, you are one of the best. Your videos are ALL about what’s in front of you, not you. You provide just enough dialogue to fill in what the visual can’t tell the viewer and I feel like you’re talkin TO me, not down to me.

I’ve watched others, on cars for example, more for the entertainment than tech. While they do some really wild stuff, their vids would be 20-30% shorter if they’d shut up and get busy! Another who, while big on my hot button (sub design & construction*), again could take a lesson from you, because of his “colorful” language. To his credit he Is hard at work the whole time talking nearly non stop. Still another has grossly repetitive dialogue habits that add to video length w/out adding worthwhile content
* much more for the Home Theatre

You and Steve Meade: keep doin’ wat y’all doin’, dudes!

Tony

la2vegas

It does appear to be in very nice condition indeed. however, some may disagree with your lack of etiquette with regards to your disregard for the selling guidelines.