[djn]

Hi all.  Can some explain or point me to a paper that detail the diff between a servo sub and a regular sub.

[Jonathon Janusz]

http://www.rythmikaudio.com/technology.html

This is the tech page for the company that makes Danny's servo sub drivers explaining what the servo control does and how it works.

[jtwrace]

http://www.rythmikaudio.com/technology.html

This is the tech page for the company that makes Danny's servo sub drivers explaining what the servo control does and how it works.

Brian Ding is the designer of all the Rythmik Servo Subs.  He is the man! 

[Early B.]

Here's a non-technical response --

Generally, a servo driver in a box is a step up from a regular sub, but both sound like "bass in a box." The magic of servo lies in open baffle.

[rajacat]

Here's a non-technical response --

Generally, a servo driver in a box is a step up from a regular sub, but both sound like "bass in a box." The magic of servo lies in open baffle.

Doesn't this depend on how many boxes? I bet that 4 boxed servo subs in a swarm configuration would sound great and largely eliminate most box-like coloration and room mode problems.

[jtwrace]

Doesn't this depend on how many boxes? I bet that 4 boxed servo subs in a swarm configuration would sound great and largely eliminate most box-like coloration and room mode problems.

Yes, that's correct.  In fact, when using multiple subs the type doesn't matter if properly implemented and setup.  I use two Geddes Bandpass subs along with the two custom built aluminum Rythmik Audio Servo Subs. 

[Early B.]

Doesn't this depend on how many boxes? I bet that 4 boxed servo subs in a swarm configuration would sound great and largely eliminate most box-like coloration and room mode problems.

The sound of a swarm is still coming from boxes. How do you eliminate "box-like coloration" unless you get rid of the box?     

[rajacat]

The sound of a swarm is still coming from boxes. How do you eliminate "box-like coloration" unless you get rid of the box?   

It's not a given that all boxes have coloration. I suspect that all open baffle subs don't sound exactly the same therefore is it coloration that causes this difference?

[Danny Richie]

Guy, not all boxes add coloration, but most of them do to some degree. Just take a look at the size of any un-braced panels or how thick the panels are and you'll have a good idea of how much coloration to expect.

Open baffle designs are not immune to this problem as side panels in H and W frames can be excited and resonant. They are less likely to resonate like a boxed speaker and it is easily controlled.

However, changes in box pressure also causes delayed movements of the cone that are not damped away by the suspension of the driver. The servo control system knows that this secondary movement is not part of the input signal and adds additional damping control to eliminate it. So the additional of servo control is a big factor regardless of whether you are using a swarm configuration or not.

See illustration: http://www.rythmikaudio.com/re-radiation.html

[JLM]

The re-radiation shown in Danny's link is generated by the back and forth motion of all drivers and comes out into the room from most speaker designs.  Cones, domes, ribbons, etc. all are thin (therefore acoustically transparent) enough to allow the back-wave to pass through and smear the sound.  In the case of dipoles (including open baffles) 99.9% of the back-wave doesn't have to squeeze through a port, structure wall, or driver material and so is much more pronounced.  Note that all back-waves are out of phase and that the typical diagram used to demonstrate cancellation of front and back waves incorrectly show the cancellation occurring at the baffle plan (which ignores the time needed for the back-wave to travel to the front wall and return.

Notice I said "most speaker designs".  In theory the back-wave from properly designed, closed end transmission lines would fully absorb the back-wave.  A near example of that would be the B&W Nautilus speaker.  Other good examples of absorbed back-wave designs would be any cabinet built into a perfectly rigid wall with the back space being outdoors.

[Danny Richie]

Even a well designed transmission line doesn't do away with the pressure changes in the box. Air pressure changes move more slowly through the box and the positive and negative pressure changes can easily get out of phase with the woofer movement. If the frequency of the woofers output changes slightly then it is out of phase from the internal pressure changes of the movement that happened right before it. The servo control system will damp that out also.

Also all sealed box, transmission lines, or ported subs still play in an omni directional frequency range. So there is just as much back wave output or output at the front wall as with an open baffle speaker. And since they are omni that pressure is also an issue to the sides of the speaker and can cause some room loading in some areas that even a swarm configuration doesn't effect. Open baffle speakers cancel the output at 90 degrees off axis and does not have this room loading issue to the sides of the woofers.

[Early B.]

A quote from the link Danny provided:

The term boxy sound has been misunderstood as the sound from a closed box. It is actually more of a box with one wall open so that the reflection/standing waves inside can escape. Therefore a 100% sturdy enclosure cannot get rid of a boxy sound because the internal reflection can still escape. If the cone is not well controlled, the energy can move the cone and produces additional acoustic waves. The result is a loss of definition.

[mlundy57]

I have both types of direct servo subs, a sealed sub with a single 12" driver and two different OB versions, one has three 8" drivers in an H-Frame and the other has two 12" drivers in a wedge (U-Frame). I can't articulate all the technical details that differentiate the two (I'm a social sciences & health care professional, not an engineer) but I can certainly hear the differences. The OB subs are fuller, more expansive, blend seamlessly with the OB midrange and tweeter (sound the same just lower frequency) and don't over-pressurize the room (the walls don't rattle and the chair doesn't shake).

The sealed version is not as expansive so while it blends well with my transmission line speakers it is not as seamless with the OB speakers and it does pressurize the room (chest thumping, wall rattling and couch shaking).

Whether one is better than the other is, to me, a matter of application. With boxed mains (sealed, ported or TL) I prefer a sealed sub. With open baffle mains I want open baffle subs. If I am going to be watching movies as well as listening to music then even with OB mains and subs I still want a sealed sub connected to the LFE channel because when the jet flies over or the bombs explode I want the walls to rattle, the chair to shake and the chest thump.

Mike

[Rocket_Ronny]

Great explanation. 

Rocket_Ronny