January 21, 2012

Rythmik dual opposed tapped horn sub

I have always been happy with the sound of my Rythmik subs. I have a pair of sealed 12" servo kits that have been in 70L sealed boxes. They are equal to the best I have heard. I achieve 15 Hz extension in room without any more EQ than is included in the servo system. I have found that I can find their limits thermally with music and have managed to drive them to thermal shutdown twice (since 2006). This is a limitation inherent in nearly all subs. Low efficiency combined with amps intended for home theatre with a lighter duty cycle results in subs that can overheat with music turned up.

I'm very reluctant to let these go so my solution is to start with active woofers running down to 40 Hz. I have 18" pro woofers driven by a pro amp - no chance of running out of steam. The Rythmik subs will then simply add extension and carry a lighter thermal load. Tapped horns will make the box carry more of the load and increase the sensitivity so that instead of around 88 dB from a pair I get around 94 dB.

Here is the concept:

These are around 400L and yes that is big! Boxes will sit under the floor and be attached to it, hence the need for dual opposed mounting. The box is around 400 x 600 x 2250mm. 

1w1m simulated:

94 dB with some sacrifice of anechoic extension to achieve higher efficiency. I need only to get down to 25 Hz and room gain gets me down to 15 Hz with a sealed box. I'm aiming for sensitivity and headroom over extreme extension. 

Max SPL as limited by the amp:


The excursion peaks at 31 Hz (16.7mm) - xmax is 18mm. 

How to avoid over-excursion

The driver will bottom as with a vented enclosure if not carefully protected. The trick is to add a high pass. The amp includes a first order HP which won't be enough when pushed. I will add a 2nd order HP, probably around 20 Hz. I can then test with tones, using a 30 Hz sine wave and noting excursion. Then applying a 10 Hz sine wave with a check on excursion. The excursion below the cut-off should never be able to exceed that above. When that is achieved, the output will be limited only by amplifier clipping and I will be sure that xmax will not be exceeded. This means when unexpected heavy bass hits in a movie, I won't have to worry about damage to subs. I will have set up the system so that the drivers are protected from all conditions. 

Achieving headroom

I don't want to create the impression that Rythmik subs are lacking output. However, asking 12" subs to match the headroom of a high sensitivity horn system isn't realistic. To get down to 20 Hz it is necessary to move a lot of air. This could mean one giant super hero woofer with a very powerful amp, but I prefer a number of more modest subs. So for those who want SQ loud, a number of Rythmik subs is a good way to go. I may in fact build two of these, one in each corner.


  1. Paul, We use & sell & Rythmik subs as well - they are great in the lower octave. Like you, we use (& sell) pro woofers as well, providing higher output & better (hi-impact) sound in the upper bass octave.

    Maybe we can do some work together - we are in the US & online @ HDSound.us.

    When you say Tapped horns will "increase the sensitivity" 6dB, have you confirmed that your sub models are all in the same acoustic load (in free space)? See http://www.diyaudio.com/forums/subwoofers/114340-tapped-horn-dummies-10.html#post1597059

  2. Perhaps we can - if there is anything of interest let me know.

    I had a look at Patrick's post. If one models in free space then the result will not compare to other programs using a more typical half space loading. That is why I prefer to use half space simulations. Even better is to model the same drivers in a sealed box in Hornresp to make a proper comparison. When compared this way, T20 achieves 12 db higher output at 20 Hz compared to a sealed box considering both thermal and excursion limits down to 20 Hz.

    The subjective experience agrees with the simulations. Keeping the gain the same, the windows suddenly start rattling. They do this at 20 Hz but previously it was only ever an issue running test tones at xmax. The difference is dramatic.

    Tapped horns don't always justify themselves. Recendly I was looking at a tapped horn for some 18" pro woofers, hoping to extract more out of them than I could get in a vented box. I was not able to get enough improvement to make it worthwhile. I think their best use as a rule is as narrow band LFE devices. Their main limitation is that they won't effectively cover a wide bandwidth like a vented box. They won't do 20 - 80 Hz for example. 20 - 60 Hz is about as much as you will get and if you tune lower you also give up the top end.

  3. Paul---Do you have and/or sell build plans for these? Can the Rythmik 15" drivers be used in place of the 12's without any change in dimensions (except for the driver cut-out hole, of course)? Thanks---EJ.

  4. Eric, I have just created a custom version of T20 for a customer using different drivers. My own version has been built and measured and it performs well. Now installed under the floor. The design has progressed beyond the crude version shown there and is fully documented. You can't substitute the Rythmik 15" drivers without modifying the design - the box is likely to become larger. The 15" drivers gain about 2 dB in sensitivity, but this comes at the cost of a larger box. It's quite likely that the line would need a larger cross sectional area at the throat and mouth. You can contact me via the contact form link on the top right hand side, just under the blog header.


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