[srb]

Imagine the kick and slam you get from a box sub married to the clarity and detail you get from a traditional OB sub and you've got a pretty good idea of what makes the GR/Rythmik solution so special.  As I said previously, sometimes things are just flat out better.  This is one of those cases.

So why doesn't Rythmik offer an assembled OB model?

Steve

[bdp24]

This is the GR Research Forum. Questions about Rythmik products are better asked on their forum. But Brian Ding (Rythmik owner and designer) has stated that he doesn't want to get into the IB/OB field.

[mojave]

I look at this only from an audiophile standpoint. You don't gain anything. You just loose performance. The moving mass of the 12 is much lower and offers better control and stopping ability. The 15 doesn't play any lower. It just adds moving mass and less control.

There are a several of things to consider here. The motor strength of a subwoofer driver is so strong that moving mass is almost insignificant. I have heard a subwoofer designer say comparing the Mms (moving mass) of two subwoofer drivers to cars "would only be accurate if it was comparing a 2000 pound and a 4000 pound car, both driven by 10,000 HP turbofan jet engines. That would be a fair comparison, as the Mms relative to the force provided by the driver motor is miniscule." The motor force of a subwoofer operates near the speed of light since it is electro-magnetic force. This makes its ability to overcome driver inertia happen so fast that it can't be measured in audible levels. Adire Audio had a paper years ago that showed that Mms does not have an effect on woofer speed.

To compare Mms between two drivers, you have to take the the weight (Mms) divided by the area (Sd) to see which driver provides the lightest moving mass per square centimeter. The 15" driver has an Mms of .321g per sq cm while the 12" driver has an Mms of .289g per sq cm. Considering that the 15" driver has higher relative motor strength, I think it should be able to start and stop faster than a 12" driver if Mms was related to this. The 15" drivers I use have an Mms of .265g per sq cm which is even lower than the GR-Research 12" drivers.

Even if Mms is important, a better comparison between drivers would be travel distance. To reach 95 dB at 20 Hz with the Rythmik 15" it only has to travel 7 mm. With the 12" driver it has to travel 10.8 mm. This means for a single 20 Hz note, the round trip of of the 12" driver is 7.6 mm further or 33% more. It is actually 33% more at any given frequency that the subwoofer will play. Given the stronger motor strength of the 15" driver and the much shorter distance it has to travel for the same output, one can see that a larger driver isn't at any disadvantage.

If you need to move more air than one 12 then use two.

This doubles the Mms now making the moving mass heavier than a single 15. So is the "speed" of two 12" drivers now half that of a single 12" driver? Of course not. They won't have to move as far and they have more motor strength which is my point above regarding a 15" driver to a 12" driver.

Having listened to over 30 subwoofer drivers in the past few years it seems that my preference is based on low Mms per sq cm, high sensitivity, and low L/R Time Constant. The L/R Time Constant is how long it takes the current in a driver to reach its maximum value. It is calculated by dividing the inductance (Le) by the resistance (Re). The best driver I've heard so far is the Acoustic Elegance TD18H+. It is an 18" driver with an Mms of 189.2 grams (only .16g per sq cm!), a sensitivity of 98.15 dB/1w, and an L/R Time Constant of .08ms. By comparison, some subwoofer drivers get up over 3.00ms for the L/R Time Constant.

[Danny Richie]

mojave,

Moving the mass or putting into motion is not at all the issue. It's about stopping it. Settling time is where it's at. 

"would only be accurate if it was comparing a 2000 pound and a 4000 pound car, both driven by 10,000 HP turbofan jet engines.

Yeah, but they both have the same brakes. And one will take a LOT longer to stop.

[mojave]

Yeah, but they both have the same brakes. And one will take a LOT longer to stop.

Cars have brakes. I thought subwoofers have the same electro-magentic motor to start/stop the driver. Larger drivers and multiple drivers have a shorter distance to stop. I guess it comes down to whether one thinks a 33% greater travel distance (12" driver) or an 11% higher moving mass (15" driver) is more important considering that the 15" driver also has a 40% higher motor/braking force for starting/stopping.

A passive radiator can have a large Mms of over 1500g. The passive radiator is producing all of the sound in a subwoofer at the tuning frequency. Even thought they have a large moving mass, they can react instantaneously to the internal box air pressure due to the soft compliance of their suspension. Just something to think about.

Just for fun I looked up the moving mass of my LS-6 drivers. Wow, an Mms each of 18.9g or only .138g per sq cm!

I've heard both GR-Research open baffle subs (Super V) and a Rythmik sealed sub. They both sounded great. 

[skalos]

Gentlemen,

Isn't there another issue? 

The larger the woofer the more difficult it is to keep the surface from flexing. Starting (motor) and stopping (servo) a woofer that flexes will have a muddled sound. 

All other things being equal, a 15" woofer will flex more than a 12" woofer and will not sound as clean.  Making a 15" or larger woofer stiff enough to compete with a 12" woofer may be possible but may not be practical or cost effective.

As I have seen in Danny's posts, the 8" woofer is probably cleaner than the 12" but the tradeoff is SPL. 

For me, 8" and 12" are the sweet spot for small and large rooms respectively.

[Danny Richie]

Cars have brakes. I thought subwoofers have the same electro-magentic motor to start/stop the driver. Larger drivers and multiple drivers have a shorter distance to stop. I guess it comes down to whether one thinks a 33% greater travel distance (12" driver) or an 11% higher moving mass (15" driver) is more important considering that the 15" driver also has a 40% higher motor/braking force for starting/stopping.

The electrical Q is the same for both drivers. So the larger one has no more stopping force than the smaller one.

A passive radiator can have a large Mms of over 1500g. The passive radiator is producing all of the sound in a subwoofer at the tuning frequency. Even thought they have a large moving mass, they can react instantaneously to the internal box air pressure due to the soft compliance of their suspension. Just something to think about.

Actually a heavy PR is really slow to recover and has a LOT of stored energy. The speed is easy to see in this regard. Just look how limited the upper range is. If it reacted instantaneously (or even near that) then it would play to 20kHz. But they are really slow and can hardly play above their FS.

[Dynky]

Sorry to kick up this older thread, but I just read a remark on AVS regarding Brian from Rythmik apparently working on a paper cone 18" OB driver, due to be released in about three months.
Danny, were you in any way involved in this? In other words, will this be another GR-series driver when it goes to market?

I'm actually pretty surprised to read this, as I always understood that the negatives by far outweighed the positives when considering anything larger than 12" in OB.  That argument (I always assumed) was also the reason Rythmik hasn't got a 15" OB driver on offer, so now going straight to 18" OB is somewhat puzzling to me.

Maybe due to some new technologies/findings this has changed...who knows? 

[Danny Richie]

Sorry to kick up this older thread, but I just read a remark on AVS regarding Brian from Rythmik apparently working on a paper cone 18" OB driver, due to be released in about three months.
Danny, were you in any way involved in this? In other words, will this be another GR-series driver when it goes to market?

I'm actually pretty surprised to read this, as I always understood that the negatives by far outweighed the positives when considering anything larger than 12" in OB.  That argument (I always assumed) was also the reason Rythmik hasn't got a 15" OB driver on offer, so now going straight to 18" OB is somewhat puzzling to me.

Maybe due to some new technologies/findings this has changed...who knows? 

Probably leaning towards giving people what they want rather than teaching them what they need.

I'm all about quality myself. And a smaller or lighter weight driver leans more that way, and with servo control it gives up nothing on low frequency extension. But if you can only use a single driver and SPL is your target then bigger is better.

[Early B.]

Yeah, and once you go beyond 12", the practicality of larger drivers is dramatically reduced for most people. An 18" woofer takes up a lot of real estate, and you'd need at least two of them for optimal stereo listening.

[Captainhemo]

Yup,  I'd guess  one wouold be looking at  a  22" w x 22"h  frame for a  single  18"  woofer (assuming they plan to put it in a modded (half)  "h-frame".   Cut 6" off the width of that, add it to the  height, and  one is almost at the size of a  dual 12'  h-frame. Close to the same volume (254 for the single 18", 226 for the dual 12's) but splitting the work between the 2 smaller,  lighter woofers should produce better results.
As Danny mentioned,  the 12's already play flat down to 20 Hz and the 18's   take up more  floor space 
jay

[jtwrace]

Sorry to kick up this older thread, but I just read a remark on AVS regarding Brian from Rythmik apparently working on a paper cone 18" OB driver, due to be released in about three months.

Can you provide that link?   

[Dynky]

sure ...post #27412 

[jtwrace]

sure ...post #27412

Ah thanks.  No specifics though.  Guess we'll find out in 3 months! 

[Danny Richie]

I developed a 15" servo woofer for the OB application for a specific client. He wanted it to match with some 15" coaxial drivers that he was using. But when it came down to having them made, he couldn't afford the up front cost of the production run. So they weren't produced beyond the samples.

Here are some pics of one of them.