DIY Ripol Subs

[Jazzman53]

Hi All,

I'm pleased to share my latest speaker project: A pair of 12" Ripol subs.

A Ripol is a special type of dipole sub patented by German speaker builder Axel Ridtahler. The name 'RiPol' designates a Ridtahler dipole, which is basically a compact, folded-baffle dipole with two woofers in opposing push/push arrangement. Its radiation pattern isn't the classic dipolar figure-8, but cardioid shaped with a pronounced frontal lobe, and off-axis nulls.

A unique Ripol characteristic is it's effect on the woofers' fundamental resonance. All woofers have a natural resonance which is typically loud, and always undesirable because it's a distortion that isn't in the music.

Conventional sub enclosures force the woofer's resonance higher than it's resonance in free air (more so in a smaller box, less so in a bigger or ported box, but always up/never down).

A Ripol enclosure forces the woofer's resonance lower by as much 10Hz, which not only allows it to play lower but has a profoundly positive effect on tonal quality.

Ridtahler's patent includes measurements of a woofer with a free-air resonance (Fs) of 25Hz, exhibiting the following resonances in a sealed box versus a Ripol:

- The same woofer in a 150 liter sealed box resonated at 47Hz, which is within the audible bass band, so any music containing energy at that frequency would excite that resonance, and its distortion would be heard.

- The same woofer in a Ripol resonated at 17.5Hz! Let that sink in for a minute and consider the consequences:
Most music contains little or no energy down that low, in which case the woofers' resonance would not be excited so would not even occur. Even if the music were to contain energy that low, the resonance would be infrasonic so you might feel it but wouldn't hear it as a tone.

Another Ripol advantage is its dipolar off-axis nulls that tend not to excite room resonances that can render the bass sluggish and inarticulate, or even produce the dreaded "one note boom".

Dipolar bass isn't very efficient so I always recommend a pair of Ripol's rather than just one, and not everyone likes Ripol's because they don't hit you in the chest like a sledge hammer, but I LOVE them. Their sound rises up from nowhere and recedes back to nowhere-- wonderful for jazz, and simply the cleanest, most unobtrusive bass I've ever experienced.

Modak Akustik in Germany markets a similar Ripol sub that uses the same Peerless SLS woofers.
A Product Review of the Modal Akustik sub by 6-Moons Audio summed up its performance as follows:

"For music-first listeners who prioritize speed, articulation and enunciated clarity, it's the long awaited messiah."
Srajan Ebaen, 6moons.com

I share that sentiment about mine too, and I will share my drawings & parts list with anyone who asks-- just PM me an email address.

I have drawings available for the version shown, and also for simpler version with exposed plywood edges.

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Build info:

The woofers are Peerless SLS 12's and the cabs are 3/4 red oak plywood with oak edge members inserted and rounded over, which looks much better than exposed plywood edges but is a LOT MORE work. The center section is solid brown oak, indexed to the cabs with oak dowel pins. The cabs and center section are held together with all-thread rods and cap nuts.

The light-colored oak cabs were oil stained with a mixture of 1/3 golden oak, 2/3 natural, and a bit of powdered turmeric root added for a yellowish tint. The darker oak center section was oil stained with red oak, for contrast. The finish is sprayed-on coats of clear satin polyurethane with a sanding step between coats.


Enjoy!
Charlie in Savannah, GA

Below: Rabbited plywood cab pieces self-locate and bank together for gluing.


Below: Woofer cab glued and clamped.


Below: Cab edges were notched (rabbitted) on the table saw, to accept the red oak edge members.


Below: Mitered edge members installed and held in place with painter's tape while the glue sets.


Below: Cab with edge members installed and sanded flush.


Below: The 3-piece assembly details.


Below: Both subs temporarily assembled and edges rounded over to 3/8 radius on router table.


Below: Details stained and spray coated with clear satin polyurethane.


Below: Completed subs with woofers installed.


Below: Back view

[Jazzman53]

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