Agreed. Room nodes are also vertically oriented when it comes to bass waves. Interested in the trade-off of six subs stacked in two columns versus six subs placed all around the room.
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Off hand, I would say that the room is “excited” differently in different locations. So…it might not be about a trade off so much as it is about different potential impacts in different locations.
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Chris, what are tour thoughts on Dirac ART and Trinnov’s waveforming tech? Looks like big changes on low frequency performance are coming soon. If it is as good as people say, be a good time to jump in the subwoofer market.
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I had suspected something.
Thanks for the information about it. 
How about an aluminum dust grille like on the D215s?
It’s aesthetically pleasing, but I have no idea if it might create any turbulence.
I totally agree with you.
Yes exactly.
I also read up on the KEF one, but never before had I seen it with this opposite-frontal approach.
It sounds like the principle of the Procella V18.
The YG Acoustics Invincible 21.2 also uses this approach.
Of course, I don’t think a double 21" is suitable for home use.
True, I agree with you.
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Thanks to you Chris, as I told you we will “consume” this discussion.
We could make a video where Paul explains why PS Audio has chosen a similar approach for its future subwoofer.
It wouldn’t be a bad idea at all. 
@Chris_Brunhaver I find the “totally isobaric” approach of the Wilson Benesch very fascinating, even if aesthetically the opposed driver is not very nice.
I also bring you a very interesting study by Wilson Benesch.
Why is Stiffness so important in Low Frequency Sound Reproduction?
Sound is a waveform of energy. To understand how sound waves move, the air around us can be considered like a liquid. Low frequency sound is perceived by the ear when it receives sound pressure in the form of long wave forms and the ear perceives high frequency sound when it receives short wave forms. A dynamic drive unit creates sound by converting electrical energy from the amplifier into kinetic energy moving the diaphragm of the drive unit to push and compress the air in front of the diaphragm of the drive unit to create a wave form. To produce high frequency sound, the diaphragm need only move a small amount of air to produce a small sound wave, but to create low frequency sound, the diaphragm must move large amounts of air to produce a large sound wave. Going back to another analogy, imagine if we stand in a perfectly still lake and try to create a ripple in the water with our hand, to create many small ripples is relatively easy, it can be done in fact with by casting a small pebble into the water, but in order to create large ripples or a wake, one must use not only one hand but in fact probably two in sync and lots of muscle power to form a wake in the water, indeed a large rock when lifted and thrown into the water will create a large wake and not a ripple like the pebble. The challenges are no different for a dynamic drive unit, when creating low frequency sound such as required by a subwoofer, the drive unit diaphragm must move large amounts of air to create low frequency sound waves. To do this accurately and effectively, the diaphragm must be stiff and it must be driven with lots of power. Going back to our analogy in the lake, imagine having a large wooden board, all of a suddenthe power in not just your hands and arms can be used, but in your entire body can be used to create a large wake. If that wooden board was flexible, the task of creating a wake would be much harder or even impossible as energy created by your body is lost as the flexible board bends and deforms in the water. Thus the diaphragm in a subwoofer should be as stiff as possible to convert as much of the energy from the amplifier into kinetic energy and push the air in front of the woofer as efficiently as possible. Carbon Fibre is one of the stiffest materials known to man. Weight for weight, carbon fibre is ten times stiffer than steel, whilst also being significantly lower in mass than aluminium.
Why is a Low Mass, Lightweight Diaphragm so Important?
If fidelity is accuracy, then within a system that is recreating a recorded piece of music, timing is fidelity. Any delay in the drive unit diaphragm transient response to the signal that it is delivered by the amplifier is a loss of fidelity. If we then consider a higher mass diaphragm and a low mass diaphragm, it is easy to consider why a low mass diaphragm offers advantages in terms of speed and therefore accuracy and fidelity. This is physics at its most basic level, if we take the analogy of a stationary car and imagine that our task is to record the fastest time possible in accelerating down a straight road from 0-60mph as quickly as possible. If that car was full of heavy luggage, the first thing we would instinctually do is remove the luggage and then record the fastest possible acceleration. What we are doing is reducing the mass and therefore inertia of the car that the engine must overcome to accelerate the car. In a subwoofer design, the challenge is much the same. In this instance the engine is the amplifier and the diaphragm is the car. By producing the lowest mass, lightest 18” dynamic drive unit cone in existence today, Wilson Benesch have created the ideal diaphragm for low frequency sound generations.
IGx - Infrasonic Generator: Because it is Not a Subwoofer
At the heart of the IGx there is no dynamic drive unit in the conventional sense. A conventional dynamic drive unit places a diaphragm inside a basket with a voice coil that is connected to a stiff spider at the centre of the basket structure that controls the position of the diaphragm with a rubber surround that connects to the basket perimeter. Conversely the IGx does not have a basket at all, during playback the position of the lightweight diaphragm is dictated to and controlled by two coils and two high power magnets that operate in a push-pull formation. The two magnets and coils in turn reside around a huge steel core that is placed in the centre of the IGx in the vertical axis. A dual motor push/pull drive consisting of two sets of 82mm diameter voice coils that are deployed in front of and behind the diaphragm around a massive 16kg precision machined central core containing high energy rare-earth magnets. The coils use non-commercially available but incredibly high heat conducting boron nitride, and the core conducts all the heat and structural borne resonant energy directly to the ground bypassing the cabinet.
Geometry in IGx
It is widely recognised that highly optimised structures exhibit geometrically optimal elements. In pursuit of performance nature has already rejected less than optimal designs and by looking at forms found in nature one can learn many lessons in design. The Wilson Benesch font was designed on pure geometrical forms and it’s the ethos that has underpinned every design for three decades. The shell shown in the figure is one of the finest examples of such a structure. The mollusc shell provides protection to the soft flesh of the mollusc residing inside, therefore this structure needs to be strong. The shell has evolved into a curved shape since this geometric form is one of the strongest forms, distributing weight and pressure evenly to the entire structure. The IGx enclosure and indeed the toroidal form of the diaphragm can be seen to use a curved form to add to stiffness across the structure, it is the optimal form and therefore function for a low frequency generator.
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It’s basically a version of double bass array.
This has been around for quite a while but they are automating it. Basically, you place subwoofers in sub a way (1/4 of the way from each boundary) so that you create a planar wave front from either side of a rectangular room and then you can essentially cancel the room modes in the entire room.
This is particularly useful in home theater applications to keep the seat to seat variations in bass to a minimum. It isn’t needed in hifi with only 1-2 listening seats (though you could still do it, it’s a lot of subs that are really only contributing to smoothness and not higher output).
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the stacking really only increases output. Yes, placing one or more subs off the ground can smooth the vertical mode (around the 100-150 Hz range) you get in a typical room, but I would still put them in different locations (unless they are smaller subs with not enough output, where stacking might make sense).
Earl Geddes did his PhD thesis on this and has some good tips about this here and elsewhere on the internet (and some old youtube videos).
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Thank you, Chris.
Great info.
Scott
Thanks Chris! Familiar with DBA, just always thought it would be a nightmare to integrate. Happy to see an automatic process coming.
Well, there is a lot of marketing speak in there but most companies (ours included) do a fair amount of that too.
There is nothing magic about isobaric and there is a reason why it is rarely done anymore. You’re just making a driver with twice the mass, half the compliance and higher motor force and higher power handling. You could do all of this with a single magnet structure easily and invert half the drivers if you want the 2nd harmonic distortion cancelling effect of the push pull mounting (like what perlisten/M&K do). You can then afford a much better single structure (rather than these double motors) for a given unit.
It looks like they’re using an underhung motor, which has lower motor force and mass than optimal for their box alignment, so I could understand why they do it that way.
Those neo motors are very pretty but we use an even taller top plate (20 mm thick) but split it into two magnetic gaps, giving as linear a motor force but with higher flux utilization (about 60% versus their <50% and room for a thick faraday ring at the voicecoil rest position (which isn’t possible in an underhung design like this, unless you interweave steel and copper like JBL did for their everest woofer).
The purifi approach of using the variable voice coil winding and pre-charged neo (to remove steel from the circuit) and heavy copper rings through the full travel of the coil is the other direction I might want to try in a midwoofer someday. However, we’re using planars for midrange duties (which inherently don’t have the inductance and hysteresis distortion that this dynamic driver approach is addressing). Basically, a lot of the benefits of this approach is in the midrange (where we wouldn’t be benefiting from it).
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I haven’t seen any pictures of the subwoofer internals and what they are doing for the driver (other than the cone material being mentioned). Having two bands of analog PEQ is good and covers most situations pretty adequately (subs are only playing a couple of octaves at most) but it’s nothing unique.
The main issue I see if that placing the subs right next to main speakers is usually not the best location . One of the benefits of a separate subwoofer is that you have disaggregated the system where you can put your main speakers where it’s best for imaging, balance etc. and you can put the subs in separate locations to combat the room modes and get a smooth/even response.
I have always enjoyed the sound of MBL systems at shows. A lot of their smaller passive systems use bandpass woofer loading with a passive capacitor loading that leads to a pretty peaky/rich bass alignment I’m not always a fan of.
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Somewhere there was a mention of putting a single sub up roughly halfway vertically in the room (on a stand like a stand mount speaker would normally be). It was to help reduce floor bounce or change the bounce angle of the long waves if I remember correctly.
Edit:
I should have read the rest of the replies before posting mine. 
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I don’t mind the metal grilles but this open space occupied by the magnet, grilles etc is not seen by the driver as enclosure volume, it’s all there to hide the woofer hanging out the bottom. As such, the subwoofer is much larger (nearly a foot taller) than a subwoofer of equivalent volume without the inverted driver. They must feel like that compromise is worthwhile.
Don’t get me wrong, I think they have a good product but that’s just some of the potential issues and design constraints with this approach.
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I believe it is right to use advertising to make your product known, as long as it is not abused.
Without advertising, I would never have had the chance to learn about your fantastic company.
You’re absolutely right, it was just my curiosity.
I also believe that the M&K/Perlisten approach, as well as that of KEF with its Uni-Core, is an excellent approach to try to eliminate vibration and distortion.
Exactly, this is exactly what I wanted to get to.
A professionally derived woofer applied to domestic use.
What do you think?
My thoughts go to the JBL PRO 5628.
It uses two 2269H drivers.
If I remember correctly, it is the same woofer used in the (much more expensive) JBL SUB 18.
Fantastic!
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Exactly, I would really like to be able to do a direct A/B comparison to see if it’s really worth it.
Thank you very much, Chris. 
I agree with avoiding the close placement of the subs to the mains if feasible. The MBL towers are now designed with electronics (Q and delay factors) that allow the subs to be up to 6 meters away from the main speakers. I am contemplating moving my towers further away (currently around 5 ft)…but the significant other might not like the placement and the look. 
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Change your significant other… 
You have a stratospheric system!
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