Old School Polk Audio C4 Subwoofer - Fourth Order Bandpass Enclosure

Polk Audio C4 Subwoofer - Fourth Order Bandpass Enclosure with Four 6x9 Subwoofers in Isobaric Configuration

Wow, a true survivor here! An early 90's Polk Audio C4 subwoofer. This enclosure is one of the most unique factory car audio subwoofers of all time. It uses four 6x9 subwoofers mounted in a compound isobaric configuration. The center chamber is sealed, while both outer chambers are vented with a 3" x 5" port. Dual binding posts offer either 4 ohm stereo, 8 ohm mono or 2 ohm mono configuration.

Here are the technical specs provided by Polk:

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

Bottom Panel of C4 Removed

Now, looking cool is nice, but sound quality and bass output is what's really important. After reviewing some  of the magazines in the OldSchoolStereo library, I found a test of this subwoofer by Car Stereo Review (March/April 1992). Since I don't have the equipment to compile technical measurements of speakers, I was intrigued to see the reviewer was able to get 119dB out of this unit. I've been using a JL Audio 12w6v2 for the past few weeks and although I haven't had my system measured, I feel pretty confident I'm pushing 125dB with only 300w. I guess what I'm alluding to is right off the bat, I wasn't expecting to obtain the output I'd become accustomed to over the past few weeks. However, I have been using a single Kicker Solobaric s10c the past year or so and it has been sufficient for my tastes so the C4 may be enough for my needs.

Now, I've only tested out the C4 for an hour or so total, so I won't leave my final impression until I've been able to give it a few weeks in my car. For now, I'll just say my initial impression is the C4 is quite impressive. It won't flex your windows or take your breathe away, but it will provide clean, hard-hitting, dynamic bass good enough for those looking to add some low end to their modest systems. I'll update the article in a few weeks with my overall impression. I've posted the video overview including demo below. Enjoy!


Make sure to Subscribe to my YouTube Channel to stay updated of my latest videos. You can also follow me on Twitter @oldschoolstereo.


See the video on YouTube in 1080p HD or embedded below:

Old School Car Audio Bookshelf Speakers? DIY Audiophile Project

Custom Rockford Fosgate AUDIOphile Bookshelf Speakers

Ok, I admit it....I'm an old school car audio junkie! Much of my equipment is in mint condition and actually using this gear is really cool. You guys may recall my Rockford Fosgate AUDIOphile RFA-414 component system from the Old School Boombox article. I was going to need the 4" mids in enclosures anyway, so why not use some small bookshelf cabinets? Exactly! I just needed some patience, a few tools and wallah...my first set of car audio based bookshelf speakers.

The donor bookshelf speaker cabinets were taken from an early 90's surround sound system by a company called Universal. The particular cabinets I used for this project had a 4" speaker, but no tweeter. No problem. A quick trip to the hardware store to get a 2 1/8" hole saw (per Rockford's RFA manual) and the mounting hole for the tweeter was done. It is interesting Rockford recommends a 2 1/8" hole as I found it slightly large....oh well, I digress.

In addition to cutting holes for the tweeters, I also had to line the interior of the cabinets with sound deadening material. The metal enclosures without the dampening material would cause some unwanted coloration (is that a word?). Anyway, I think you know what I mean. After adding the deadening material, the enclosures had a "thud" instead of a "ting". Still not as dead sounding an MDF based enclosure, but much lighter and smaller.

As with many inexpensive speakers, the donor Universal's had tiny 22ga internal speaker wire. This was easily remedied by de-soldering the factory wire and upgrading to some 16ga Phoenix Gold OFC speaker wire. I also used hook and loop tape to mount the AUDIOphile crossovers. The flush mount tweeter pods were mounted using silicon sealant and some custom brackets. Everything came together very well and I'm happy with the results.

Now, I can write all day about the process, how they sound, etc., but if a picture is worth 1000 words, then a video is worth 10,000. I created a video build log of this project and also give a sound demo of the speakers. They are very transparent, but as expected, somewhat weak on the bass output. This component system was designed to be used with a subwoofer and maybe even a midbass. My plans are to cross them over at 150Hz and other plans TBD...


Make sure to Subscribe to my YouTube Channel to stay updated of my latest videos. You can also follow me on Twitter @oldschoolstereo.



See the video on YouTube in 1080p HD or embedded below: