Double 15 Reflex (Push Pull Cavity loaded)

Double 15 reflex for Eminence Kappalite 3015LF

Have ported this design over from the old speakerplans site to give an example of alternate driver loading for lowering distortion in a reflex.

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Speakerplans user DJK originally posted.....

The reductions in 2nd harmonic distortion from Push Pull mounting on a flat baffle are well known, the non-linearities are equal and opposite and thus the 2nd harmonic is canceled. Because of the driver spacing it only happens in the bottom octaves. The cheaper the driver, the bigger the improvement. I have measured as much as a 20dB change.
By mounting in a plenum several things change. The spacing becomes much tighter so the cancellation will work at higher frequencies, and a cavity is formed.

The cavity does several things, in no particular order:

The size, especially the depth, acts like a low pass filter. This can reduce 3rd harmonic distortion products near the top end of the passband, the effect is small, but audible. The cavity sizes I use have to be crossed in the 150hz~250hz region. It gives a slope of roughly 12dB/oct that must be taken into consideration in the design of the crossover. There is a small peak about two octaves above this point, ~800hz on many that I have done. By varying the Q of the 12dB/oct lowpass filter a LR4 I can get a LR4 transfer function. This filter also reduces the out-of-band peak to -30dB or better. There is also a big reduction in FMD, on the order of 6dB. The air mass at the exit of the plenum seems to act like a point source with no doppler, the cones are moving back-and-forth sideways rather than towards-and-away from you. At first glance this would seem to eliminate the FMD, in practice the reduction is on the order of 4dB~10dB depending on the frequencies.

I mounted a pair of drivers in another cabinet in a face-to-face, push-push mode. The box became very 'thick' sounding and had a broad peak centered at 160hz, just like the EV MTL4. In room the response of the push-pull was 6dB smoother in the 100hz~200hz octave.

Being compared side-by-side with the Klipschorn and Cornwall was interesting. At first the push-pull sounded 'funny'. Couldn't put my finger on it. Went back to the push-push cabinet. The push-push cabinet had that 'hi-fi' sound, similar to the Cornwall, but a bit 'thicker'. Klipschorn, much cleaner sounding. Back to the push-pull.

The reason the push-pull sounded 'funny' was the total lack of distortion. Much less than the Klipschorn.

I have not built a basshorn since.

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Taking this info on board and being particularly impressed with the lowering of distortion figures, I started to think about a push pull reflex design mounted in a cavity.

Doing some prelim Bassbox models I came up with the following

A double 15 reflex design push pull cavity loaded, using Eminence Kappalite 3015LF a fairly cheap and lightweight driver with a good Xmax capability.

Box volume 200L tuned to 43Hz.

Here is a sim in Bassbox of the above design, notice the drivers do not exceed Xmax even when driven with their full program rating.



You can work out the box dimensions for yourself bear in mind that the drivers have to be loaded into a cavity similar to this (only not at an angle as illustrated)



I came up with this box within a box design for my own purposes and to cut down on storage space, but just make the box fit your stack.



StuEdited by: Strapping Young Stu

Stu, have you built these yet?

Thinking of something similar.

Any pics?

Podman.

No mate probably not happening now, TBH havent had to take the bins out in the last few months, always going through in house rigs or using different PA so its been put in the can for now.

Maybe someday. There is no reason why the full size box wouldnt work, DJK has working models using slightly different drivers. As to the box idea I posted above..... well it needs some bracing, how much and how to get it in there, I never worked out.

Stu