Converting a woofer into a passive radiator

[lowerFE]

Has anyone tried to convert a woofer into a passive radiator? I want a very high excursion 5'' passive radiator, and the Peerless 5.25'' just won't do it. 

 

The Tang Band W5-1138SMF looks like a driver that can do over an inch of excursion, which is what I need. It has around 3-4 times higher Mms than a typical 5'' woofer at 29g, which means the suspension should be capable of handling the 40-50g of additional mass I want to put on. 

 

Good idea? Anything I should watch out for? If it is, any tips on how to do this? The first question would be finding a way to remove the magnet. 

 

Thanks

[Kyle]

Might be hard beacuse you'll need to remove the motor, often times they glue the frame to the gap plate on the smaller drivers which makes it a pita to remove. You might need to just cut the basket off the motor with a Dremel. Once you remove it you need to add weight to the moving mass and the best way to do that is to fill the inside of the voice coil with glue + metal shot. You will also need a way to figure out the right mass. The compliance of the driver won't change so you can take the original TSPs of the woofer, and start changing the mass until you model where you want it in software. Use the Sd and Cms of the original driver and figure out the mass for your target tuning.  Add mass until you hit your target  total mass = new mass + original mass.

 

Having said all that, the softparts of an active driver may not be suitable for a PR. PRs have a lot more mass and it might add a little too much force on the spider or surround. You'll have to just try it

 

edit, also the screws are only accessible on most drivers before the cone is glued on so that means you'll really need to cut the frame off with a  cutting tool. It should be doable.

[lowerFE]

Might be hard beacuse you'll need to remove the motor, often times they glue the frame to the gap plate on the smaller drivers which makes it a pita to remove. You might need to just cut the basket off the motor with a Dremel. Once you remove it you need to add weight to the moving mass and the best way to do that is to fill the inside of the voice coil with glue + metal shot. You will also need a way to figure out the right mass. The compliance of the driver won't change so you can take the original TSPs of the woofer, and start changing the mass until you model where you want it in software. Use the Sd and Cms of the original driver and figure out the mass for your target tuning.  Add mass until you hit your target  total mass = new mass + original mass.

 

Having said all that, the softparts of an active driver may not be suitable for a PR. PRs have a lot more mass and it might add a little too much force on the spider or surround. You'll have to just try it

 

edit, also the screws are only accessible on most drivers before the cone is glued on so that means you'll really need to cut the frame off with a  cutting tool. It should be doable.

 

 

Thank you. I'll try it out. Any way to increase the Qms on the drivers? It seems like even with Peerless PR's 7.4 Qms, the passive radiator is moving much less than simulations show. The passive radiator should be at full excursion when the woofer is doing around half of its xmax of excursion at tuning. However, I need to drive the woofer over its xmax at tune to reach full excursion from the PR's when they should have been blown to pieces at that output level. 

[Contrasseur]

Qms= Mms/(Cms*Rms^2)

 

As I understand it, the compliance is almost completely defined by the elasticity of the surround and the spider only has small influence.

 

Moving mass can be added of course, but that affects your desired tuning.

 

Rms is the only option, and this parameter has to do with how lossy, or damped, your surround is, and can only be influenced if the stretchy parts are replaced with less lossy versions. 7.4 Qms is on the high side and not many options will likely have better performance. If your PR isn't moving far enough (per volt of input power) despite the low loss surround, you need a bigger box. Increase the size of your box and decrease the PR's moving mass to increase sensitivity at tuning.

 

Edit: Furthermore, why are you looking for a higher excursion PR when it's your driver running out of excursion?

[Contrasseur]

Possible reasons why your simulations don't match up:

 

-Xmax on your driver can be quite different from manufacturer spec. Furthermore, it varies depending on frequency and impedance. See the Xmax Investigation thread for more.

 

-Xmax on your PR can vary from manufacturer spec in practice. I think Peerless releases generally conservative specs. You may find more usable performance beyond their advertised limits.

 

-Damping material in the box will reduce PR excursion and system efficiency.

 

-Cms is very hard to control, and often lands +-15-20% from the same drivers in the same batch. Same applies to PRs.

 

-Simulations have limited reliability, especially at excursion extremes.

 

-Have you measured impedance to determine exact tuning? Your system might not have ended up at the tuning your simulation predicted for the same reasons listed above.

[lowerFE]

Qms= Mms/(Cms*Rms^2)

 

As I understand it, the compliance is almost completely defined by the elasticity of the surround and the spider only has small influence.

 

Moving mass can be added of course, but that affects your desired tuning.

 

Rms is the only option, and this parameter has to do with how lossy, or damped, your surround is, and can only be influenced if the stretchy parts are replaced with less lossy versions. 7.4 Qms is on the high side and not many options will likely have better performance. If your PR isn't moving far enough (per volt of input power) despite the low loss surround, you need a bigger box. Increase the size of your box and decrease the PR's moving mass to increase sensitivity at tuning.

 

Edit: Furthermore, why are you looking for a higher excursion PR when it's your driver running out of excursion?

 

If I were to convert the TB woofer into a PR, the woofer's Qms is 3.56, which is only half of the Qms of a dedicated PR like the Peerless one. I asked because even when I used the Peerless PR with the high Qms, the PR wasn't moving very much compared to the simulation, leading me to think that the lower Qms of the TB woofer will result in even more mechanical losses. 

 

I am looking for a higher excursion PR because this is going to be used for a different build where there is only enough space for one 5.25'' PR with a 5'' woofer. Since I can't double up on the PR, then I need to double up on the excursion. 

[SME]

Out of curiosity, I looked at the specs of the passive radiators that Acoustic Elegance sold (still sells?) and they have Qms closer to 70 than 7.  What I wonder though, is doesn't your simulation account for Qms?  I figure you'd see poor performance in the simulation if Qms were the issue.  But there could be other issues with the simulation versus reality that are leading to unexpected results.