[Duke]

I have taken a few orders for a new "bigbox" version of the Rhythm Prisms.   As the name implies, the main difference is internal volume, which in this case extends the in-room bass about 1/3 octave deeper.  The larger size makes it feasible to flush-mount the drivers, which looks a bit better, and continuing in that vein the standard finishes will include Walnut veneer as well as Oak veneer.  Visually the closest thing in my current lineup is the twisted bipolar Strato Prism; the dimensions differ a little bit (the bigbox RP will be slightly narrower and a bit deeper). 

The exceptional room-adaptability of the standard Rhythm Prism is retained, but the bigbox version will work better in larger rooms.  In small rooms, or in locations where the speaker is getting a lot of boundary reinforcement, the bigbox version should work just well as the regular version (for one thing when tuned very low, the rolloff is a bit shallower and starts a bit higher up).  Also, the bigbox version will be a better match for low-damping-factor tube amps.

Preliminary specifications:

Type:  Two-way controlled-pattern floor standing bass reflex system

Radiation pattern:  90 degrees in the horizontal plane from about 1.4 kHz on up

Impedance:  8 ohms nominal, tube-friendly

Efficiency:  93 dB/1 watt

Recommended amplifier power:  5 to 400 watts

Thermal compression:  Less than 1 dB at 50 watts (110 dB SPL)

Typical bandwidth, light boundary reinforcement:  41 Hz to 18 kHz

Typical bandwidth, moderate boundary reinforcement:  34 Hz to 18 kHz

Typical bandwidth, strong boundary reinforcement:  27 Hz to 18 kHz

Dimensions:  44" tall by 14" wide; depth along long side = 24"

Estimated weight:  110 pounds

Introductory price:  $2900.00 per pair plus shipping, Oak or Walnut veneer [edit:  the introductory time is up, and the current price is $3300 a pair plus shipping.]

[Russell Dawkins]

This seems like a logical evolution.

I know it doesn't mean much but, to me, some designs just look right and the original Rhythm Prisms looked a tiny bit narrow in width. With the extra width I envisage the backwave from the woofers being able to expand more symmetrically and easily.

I have wondered whether, when a driver is (from the rear perspective) operating into a radically asymmetric volume of air, the physical influence on cone behavior is also asymmetrical and causing un-anticipated and non-linear movements in the cone itself as a result - complex modes involving all moving parts - surrounds, spiders and the cones.

I've always thought that your Jazz Modules were one of those designs that "just looked right".

cheers,

Russell

[Jeffrey Hedback]

Hi Duke,

I heard a rumor of such an update.

You're the best.

[Duke]

...to me, some designs just look right and the original Rhythm Prisms looked a tiny bit narrow in width. With the extra width I envisage the backwave from the woofers being able to expand more symmetrically and easily.

The original Rhythm Prisms take your "looked a tiny bit narrow in width" as a compliment!  Acutally the interior wall that I pay the most attention to is the one right there alongside the cone.   Once that wall is treated with an aggressive acoustic damping material, we're in good shape in the midrange.  After that, I think asymmetry is actually our friend.

I have wondered whether, when a driver is (from the rear perspective) operating into a radically asymmetric volume of air, the physical influence on cone behavior is also asymmetrical and causing un-anticipated and non-linear movements in the cone itself as a result - complex modes involving all moving parts - surrounds, spiders and the cones.

Uneven airload on the back of the cone can result in a rocking mode at low frequencies that can introduce audible distortion at high sound pressure levels.   But it's the port location more than the anything else that contributes to this.  I learned this lesson the hard way in my little bass cab; the prototype had both ports high on the rear panel, and at very high levels there was distortion on some low notes.  The Eminence engineer who designed the woofer suggested relocating the ports to provide a more even airload, and that solved the problem.

I think an uneven airload situation due to port location is more likely to be an issue when the box size is fairly small in relation to how far we're trying to get the cone to travel; in other words, high internal pressure.  Even the original Rhythm Prism has a fairly big box, and the bigbox version even moreso.  With both the high and low ports open, there's decent (though not ideal) pressure symmetry on the back of the cone, and both ports are most likely to be open in a large room where the speaker is asked to play quite loud.  In smaller rooms where less power (less cone movement) is required, we'd probably plug one of the ports but in that situation we won't be demanding such long excursions anyway.

So yes, asymmetrical airload on the back of the cone can be an issue, but the suspension is stiff enough (and the excursions small enough) at midrange frequencies that it's not an issue up there.  It's down in the bass region where uneven airload can theoretically cause audible distortion at high SPLs.  In the type of application where large bass excursions are most likely to be demanded (loud music in a big room), the Rhythm Prisms are likely to less be susceptible to cone rocking than most because they will have open ports both above and below the woofer cone.     

I heard a rumor of such an update.

Yeah, well... I heard a rumor that a certain pair of custom Rhythm Prisms were going to be finished weeks ago... so don't believe everything you hear!

Seriously, thanks for your patience. 

[JLM]

Location of ports in long cabinets can turn them into acoustical pipes, as you observed.  I recommend you take advantage of the concept, embrace Martin King's MathCad application, and consider yourself welcomed into the wonderful world of transmission line design (I world that I've lived in since the 70s).

[Duke]

Location of ports in long cabinets can turn them into acoustical pipes, as you observed.  I recommend you take advantage of the concept, embrace Martin King's MathCad application, and consider yourself welcomed into the wonderful world of transmission line design (I world that I've lived in since the 70s).

I built transmission lines for many years as an amateur, and published an article on a system that used transmission line loading for the woofer section and for a wideband ribbon in SpeakerBuilder magazine back in 1986.  In my experience a well-designed vented box is superior.  The problem is, a transmission line tends to have a significant cancellation dip at the frequency where the line length is equal to one wavelength.  There are techniques that minimize this, but in the end the tradeoffs are too great in my opinion.

My understanding of Martin King's work is that he is addressing the long-dimension resonance of a tall vented box speaker through judicious woofer and port placement.  I would not call his approach a "transmission line", but perhaps that's just semantics.  I choose box height and port location(s) based on my own approach to addressing that top-to-bottom internal standing wave.  I doubt that anyone has noticed, but my different floorstanding models vary in height over a range of about two inches, and that has to do with how I juggle woofer and port locations relative to the internal height.  Different formats dictate slightly different approaches (the round waveguide I sometimes use takes up more vertical space than the rectangular horn, pushing the woofer location down lower), but the general principle remains the same.  Type and placement of damping material also plays a role.

[JLM]

Duke,

Which of the Prisms would you recommend for a 8 ft x 13 ft x 21 ft room (Cardas Golden Cubiode)? 

BTW my current speakers are excellent (the original pair of Bob Brines FTA-2000's) with in room bass response below 30 Hz (I'm not a basshead, but fully believe that bass is foundational) and have added EnABLing thanks to Bud himself, but aren't as dynamic or with the controlled dispersion of yours.

TIA

[Duke]

Either one would work fine.  The bigbox version has a little bit more low-end extension, and can be tuned for a little bit wider variety of amplifier compatibility. 

[Norris Wilson]

All of us attending LSAF in May are looking forward to listening to your creations.  Either it be the standard Rhythm Prisms, or the new big box version.

Diana and I will only be able to make it on Saturday until early afternoon this year.

Say hello to Lori for us?

Drive safe, and enjoy the ride.

Norris

[satfrat]

Duke, I'd love to see a comparison picture of the "bigbox" Prisms next to or compared with a picture of the Rhythm Prisms so to get a better idea of the size increase of the 2 loudspeakers for a given living room setting.
 
Here's a picture of the Rhythm Prisms from your original thread:
 

 
Cheers,
Robin

[Duke]

Satfrat, I will get a side-by-side photo in a week or two.  They're pretty similar. 

Norris, I don't yet know what I'll be bringing, but it will be something that I haven't shown before.   

[Duke]

It now looks like the bigbox speaker will make its debut at Lone Star 2011 after all.  The design has evolved somewhat in the past few weeks, such that it's no longer just a "Rhythm Prism in a bigger box".  It's now it's own thing, so to speak, though clearly a close relative of the Rhythm Prism.  Once I have photos, it will formally get its own name and introductory thread.   

The original Jazz Modules retained certain theoretical advantages over the Rhythm Prisms, but that gap is significantly narrowed (and spanned in a few areas) by this new generation Prism-type speaker.