[stevenkelby]

Or, for more functionality:

http://test-tone-generator.fyxm.net/

Dave

Even better, thanks Dave, I'll play with that

Steve.

[sfdoddsy]

Thanks for that link, very useful!

I don't have an SPL meter yet but promise to get one and report back here with the results!

If you have an iPhone you can download an SPL meter. Just make sure it has compensation for bass frequencies.

[stevenkelby]

No iphone, just an old Nokia 6300 that has proven much more reliable that friends iphones I'll buy a meter this Friday, I'm going to the city

[rythmik]

Brian,

I wonder if you might elaborate on this a bit?  Are you saying your objective (with the servo control) is to lower Qts in an open-baffle configuration to something much lower than the "native" Qts of the driver?

The time domain response is determined by the roots of the characteristic function (which is a Laplace Transformation). For Q value of higher than 0.5, the roots of the characteristic function are complex conjugates which lead to time domain ringing.  So what about the Q value of less than 0.5?  The roots become real roots and the 2 roots begin to separate at frequency domain and as Q continues to go lower, the separation is so much so the 12db/oct roll-off looks like a 6db/oct roll-off and that is where we get faster response (which means the role of the 2nd root has become so low that its importance is no longer there).

Say I started with a driver that has Qts = 0.7.  Your objective would be reduce to reduce it 0.7/8 or 0.087?  Or your preference is to start with a driver that has much higher Qts and then reduce it to a more "nominal" value?

As an example, the GR SW-12-16FR driver is spec'd at Qts = 0.748.  Why would you want to reduce Qts on that driver by a factor of eight?

Maybe I'm not understanding your definition of Qts or "effective Qts?"

Cheers,

Dave.

SW12-16FR has been out long time ago.  When I met Danny in person at Fort Worth  in 2006 discussing what we should do with this servo OB driver, I told him that the correct way to make the driver is to make the driver's Qts value high, something like 0.7 to 0.9, so that the output of the OB driver is highest at 20hz.  For instance, a driver with Qts value of only half of SW12-16 will have 6db less output (assuming Fs is same).  These days there is no dispute that only high Qts drivers are suitable for OB for output reason.  Servo (with a low effective Qts value) on a high Qts driver gives us best of both world, high output and fast bass response.

I spend most of my forum time at AVS forum. But I recently just told the member I have an interview question as follows.  Say we want to design a HPF with 10x gain. We can do a gain stage of 10x followed by a HPF, or the other way around. What is the pros and cons between these two arrangement?   A lot of young clueless engineers would tell you here is no difference. But there is.  If we have HPF at the last stage, it filters out the noise below the corner frequency of HPF vs if the gain stage is the last stage, the noise and distortion from the gain stage is fully exposed.  These are very important engineering questions and observation. If an engineer cannot make observation like this, his design will always be mediocre at best. I am in no position to educate or enlighten them.   Now intrinsic high Qts value has the same problem. The high Q value amplifies the noise and distortion coming from the previous stage.  So it always a wise design practice not to allow any part of the signal chain having an effectively high Q value, including the driver. Servo just reduces the Qts in one complete close loop.  There is no separation of driver and amplifier stage. In servo, the driver and amplifier become  a fully integrated component. 

BTW, another related issue is active crossover system vs passive crossover system. People keep reporting those two do not sound alike at all. A lot of audiophiles still prefer the passive crossover systems.  The reason?  As I have reasoned above, these two systems are not even close. Without the passive crossover to filter out outlier signals, the drivers in active systems receive unwanted noise/distortion,etc.  It really shocks me that our members fail seeing these subtle difference.  Com-on, we are smarter than that 

 

 

[rythmik]

I am also skeptical that servo's are needed for competent box-sub implementations either. For the guys that were curious, if you do your own forum searches elsewhere (or Amazon, etc, review sites), you will find plenty of people who have owned or heard Rythmik but felt that some other sub was just as good or better.

Here's a home theater focused comparison that compares 4 different subs in room, including a Rythmik and a HSU. The reviewers felt that out of the four, these two tied for sound quality and accuracy. I spoke to these guys at length later on and they both confirmed it. So it goes to show you that non-servo subs should also be auditioned and/or considered when shopping.

Here is the shocker.  Below is what one of the brothers told me.  I am going to betray him by sharing what he wrote me.  I think after 18 months, it should no longer be kept as a secret.  Here is what one of the brothers wrote me June 29, 2011:

"Hi Brian,

... [snip]

And the other thing that sucks is, to be honest (and this stays between us please), if we *had* decided to rank the 5 subs in the shootout, the FV15 (that is a Rythmik in case you don't know) would've been our top pick. And you wanna improve it - that's awesome. Well if there's anything else we can do for you, don't hesitate to let us know.

-Take care,
Exxxx"

I was pretty upset when I read this email.  It was typical forum politics. Do you still believe what you heard? 

[sfdoddsy]

No iphone, just an old Nokia 6300 that has proven much more reliable that friends iphones I'll buy a meter this Friday, I'm going to the city

It probably goes without saying, but don't forget to keep the volume down at first. My servo sub simply refuses to play any louder when it starts to get over-stressed and I assume the GR ones do too, but you never know. I've blown bass drivers in my OB bass section from over-exuberant use.

[Danny Richie]

Okay, I had a Super-V on hand. So I set up my mic and took some measurements. Keep in mind that these were designed for open baffle applications so the servo amp produces an inverse of the the roll off in an open baffle that is missed from a near field measurement. So in the near field the response is higher the lower you go in frequency. Out away from the cabinet and back to the listening distance the response is flat.



The room was not as quiet as when I normally take measurements. The wife and kids are home. Need I say more? So there are a few little squiggles that is related to room noise. Those fluctuated a bit with each measurement.

The red line is with the 14Hz extension filter. Note that this setting produces a response that is flat to below 20Hz in room. The purple line is using the 20Hz setting and the blue line is using the 28Hz setting.

So with servo control you can basically get as much low end extension out of it as you want.

Several of you seemed to either doubt that this is possible or do not understand how it works. I can understand the doubts or not understanding, but don't accuse me of being disingenuous. I will always tell you the truth.

I was also amused at those that doubted my claim of hitting over 100db levels at 20Hz in my room at RMAF several years back. I kept waiting for someone (anyone) to ask the more relevant questions, but none did. What you should have been asking was how big was the room? Or what was the measuring distance? I already told you that I was at listening distance so there should have been some idea there, but not one question about the room.

Contrary to what you guys may think, OB bass response can load up or pressurize the room. Dave even mentioned it and still no one asked...

The room we were in was 13' wide by 19' long. And yes we were really loading that room. And so hitting those high SPL way down low was not a problem.

We did not hit those same SPL's at those really low frequency ranges in the double sized room the next year with the same speakers.  We were however still hitting some really good SPL levels way down low. We did have the best sounding bass response at the show, says everybody.

[Danny Richie]

Here are three more measurements with the extension filter set to 14Hz and with high, medium, and low damping settings.



This changes the overall Q of the system and changes the control factor.

Again, the small squiggles are room noise. Sorry about that. I think the heater was running in the house too.

[JohnR]

Can you do impulse or step responses? And with the servo turned off?

[Danny Richie]

Hopefully we'll see some 8" servo data too as they are supposedly capable of 20Hz.   

Here you go man.

These are the measurements that I took of our 8" servo subs. These are taking on a flat baffle (not an H frame) like the ones picture in my circle. There were three of them on a baffle. These are the near field measurements using the three extension filters on the new test amp.



There you go fellows. An 8 inch woofer that plays to a -3db of 20Hz on an open baffle.

I think the extension filters were set at 20Hz, 26Hz, and 34Hz. We decided that the production units will be more like 20Hz, 30Hz and 40Hz or so.

And again, the servo system produces an inverse of the natural roll off in an open baffle. In room, the response is flat.

[Danny Richie]

Can you do impulse or step responses? And with the servo turned off?

I'll go give the step response and impulse response a try shortly.

I tried disconnecting the sensing coil and shooting a response. Guys that own these can forget trying that. The amp was still looking for the feedback. The response was erratic in some areas and the output shot up quite a bit.

So I used a separate amp instead.



The green line is the standard amp. Not bad actually. You can tell it is a high Q driver with a low Fs. I would imagine it to be fairly rolled off in an in room response much like any other woofer in an OB.

[Davey]

Danny, you simply can't break the laws of physics.  A driver "X" has physical limits that no amount of servo control can overcome.  Getting "as much low end extension as you want" is simply equalization.  Nothing more.

I hope you're clear on those points.....you seem fuzzy.

Regarding pressurization of a room.  You can obviously create localized pressure and flutter pant legs, but a larger space simply can not be pressurized by an open baffle configuration in the same way a closed-box system can.  This should be intuitively obvious to all.

Cheers,

Dave.

[Danny Richie]

Danny, you simply can't break the laws of physics.  A driver "X" has physical limits that no amount of servo control can overcome.  Getting "as much low end extension as you want" is simply equalization.  Nothing more.

No one is claiming any laws of physics are being broken. I don't know where you guys come up with this stuff? I am showing you the measured responses. What else do you want to see? And of coarse the servo system ads gain to maintain the linearity. But if you add that kind of gain to most woofers to get that kind of extension then the woofer looses control easily and even bottoms out.

Regarding pressurization of a room.  You can obviously create localized pressure, but a larger space simply can not be pressurized by an open baffle configuration.  This should be intuitively obvious to all.

Davey, it was a small room. It was easily pressurized. I was there. I measured it. Others witnessed it. You weren't there and I don't care. You can believe whatever you want.

[Davey]

Your "measured responses" are meaningless......if trying to illustrate the servo-control aspect of the system.  You're showing us equalization changes.

You're answering questions that aren't being asked by the fellas.

My goodness.

Dave.

[Danny Richie]

Can you do impulse or step responses? And with the servo turned off?

Hey John,

I can't turn the servo on and off with the servo amp.

Impulse and step responses look a little different on these really long wave lengths and they take a long time to pass the mic. Changing damping settings can easily be seen though. Here are high and low damping settings.

Impulse:



And step:

[Danny Richie]

Your "measured responses" are meaningless......if trying to illustrate the servo-control aspect of the system.  You're showing us equalization changes.

You're answering questions that aren't being asked by the fellas.

My goodness.

Dave.

Davey, There was a question regarding their ability to play flat to 20Hz. Yes they do. My goodness.

[rythmik]

Danny, you simply can't break the laws of physics.  A driver "X" has physical limits that no amount of servo control can overcome.  Getting "as much low end extension as you want" is simply equalization.  Nothing more.

Again, servo is not simple EQ. Simply EQ does not adapt. Servo does adapt.  This is the difference betwen conventional dumb bomb vs modern days laser guided bomb.  In the old days, the bomber needs to calculate the wind speed etc to be as accurate.  How effective is that type of forward estimation?   Servo is an adaptive EQ with a close loop feedback from the end of the system that can adapt to the nonlinearity and variability of driver's parameters, and even the en ironmental hardship.  If you try to use your hands to block the woofer, see how much force the driver has to push your hand back. Again, in a servo system, it pushes back more than the nonservo version.  In servo system, the EQ is accomplished in the process as a by product. But it does more than that.  If you push the cone of a servo controlled driver in free air, you get the sense of cone pushing you back more than nonservo. That is the physical demonstration of Qts value.  High Qts driver is easy to push. Low Qts driver is harder to push.  But the most important thing is you think deeper into what does this Qts actually translate into real world benefit.  It means if the system has ever gone strayed (like a gusty wind blows our missile off its track), how fast it can return and recover to the path-supposed-to-be? That is related to the intrinsic response capability and it is determined by the effective Qts value.  With high Qts value, the missile will swing left and right around the path-supposed-to-be constantly (remember the ringing of high Qts value). That makes aiming target far more difficult.  Now what happen if it had been with low Qts value? You can imagine the missile returns to the supposed path faster and with far less swing.  Doesn't that make you more confident the missile will actually hit the target?   Now back to speaker cone. What can cause our cone to be strayed? Distortion, external acoustic interference, and etc.  In the case of distortion, the servo system makes it recover faster (because its low Qts value) vs the nonservo high Qts value.   In the latter case, the servo can exert larger force to counter the external interference. 

 

[Danny Richie]

Can you do impulse or step responses? And with the servo turned off?

Hey John,

I gave it a shot. I measured the same woofer using a standard amp (black) and overlaid it on the servo amp impulse response (red). The standard amp sent a full range signal so there is a ton of shorter lines in there. Sorry about that. I'll need to figure out a better way to compare impulse responses. Scaling of the two may be way off as well.



Maybe Brian can post a servo and non servo impulse or step response using the same amp or at least send the same signal via same crossover control settings.

[Danny Richie]

Sale?  That's mighty presumptuous of you.  It would likely be much more along the line of give-away.  And, my daily driver is hardly a "drag car".  Let me know if you reconsider and decide to drive to winter CES.  They have a nice drag strip not far off the strip. 

I am not interested in the 22 hour drive. I have been in and around drag racing all my life. I've been a crew member on teams that set national records to a crew chief at a national event. If I wanted my daily driver to run in the 9's that wouldn't be a problem. I am just not that interested in doing that all over again. I was also recently offered some seat time in a new dragster though. I think I may take a few passes in it. It is capable of mid 7's in the 1/4. Should be fun... So far I have only hit 160 mph in the 1/4. The dragster is capable of speeds a little over 200.

[Danny Richie]

and wire, caps, connectors....

It's all about design and implementation.  ODAC proves this wtih real objective data hence the name.

Young man, I'd love to have you down here for a few days. You'd leave saying, man, those caps, wire, and connectors really make a difference.