DIY Passive Radiators

[Collo]

GDay!

I've got a couple of my favourite 10in drivers that have distorted coil formers due to "excess enthusiasm".

Could I remove the magnets and use them as a pair of PR's?.

If so I would build a single driver / dual PR box.

A frequency sweep should identify the tuning frequency, and thus the apparent mass of the PRs. From there it should be easy to work out how much mass to add to get the desired tuning frequency.

Has anyone been down this road before...?

Collo

[sreten]

Hi,

its certainly possible. I've used 2 blown 8"s with 2 6.5"s.

UK 2p coins hapened to fit perfectly into the voice coil, and superglued
into place near the suspension point were used to adjust mass.

It was a bit rough and ready but one 2p seemed to work well for
a overlarge sealed cabinet the owner had built for a 6.5" driver.

/sreten.

[Cal Weldon]

Yes, I've done it a few times. The trick is to get old woofers with a fair amount of excursion, nice big spider etc.
Paint the backs of the cones with a few coats of PVA glue to stiffen. (Assuming they are paper)
Add weight (can be glue) to the inside of the dust cap from the back. This will also help to seal the cap if not solid.
Plug the hole in the voice coil if it is still attached.

If your old drivers are not high excursion then lower the ratio of active to drone ie: one 8" or 10" active with two 10" drones. It'll work like a charm.

[Collo]

Thanks for those encouraging replies.

When I model PRs in WinISD, it asks for some T/S parameters for the PR. As mass is added, it shows the new Fs

Qms.....same as original driver?
Xmax.....same as original driver..maybe more with the pole piece removed??
Sd.....same as original driver
Vas
Fs
Does anyone know if the Vas and starting Fs (no mass added) are also the same as the original driver, or whether these have to be determined experimentally?

[mod_evil]

Can I use a OLD retified speaker to be a Passive Radiator?

Is this a good idea?

[rabbitz]

Originally posted by Collo

Does anyone know if the Vas and starting Fs (no mass added) are also the same as the original driver, or whether these have to be determined experimentally?

Hi Collo

I'd punt on them being the same as I had a look at the Peerless XLS drivers and PR is identical to the motored woofer but without the motor and it's the added weight 400-425g that has changed the Fs and Vas.... but I could be wrong.

Is the starting mass to add equivalent to the weight of air that would be in a port with the diameter = effective cone diameter? I'd like to know as this looks worthwhile to play with to use up those busted drivers.

[Collo]

Hi there Rabbitz,

Thanks for that comparo on the Peerless gear. I'll push on assuming that the T/S are the same.

The finishing mass (with the mass added) is equivalent to the mass of air in an equivalent port

WinISD pro lets you choose PR's when you're first setting up the project and choose the box type. The parameters for the PR are entered and later you can change the added mass on the PR tab. This saves you having to do the calculations manually.

I'm looking at around 230g to add. One method to add mass, is to use some lead shot mixed with epoxy, and pour it into the coil former - a bit permanent though. For experimenting, a hot glue gun and some lead washers might be a better bet.


Here's my project file using the Jaycar Titanium 10inch drivers:
http://www.users.bigpond.com/bcollis...-pr-design.wpr
(save it to a temporary file and then open it. For some reason it doesn't load correctly direct from the web)

[Collo]

Progress so far - magnet cut off and rough edge cleaned up with some push-on automotive trim

[rabbitz]

Collo

The Peerless PR's have a M5 stud in the centre so you can add weights to your hearts content. Just epoxy in a small plate in the VC with a bolt attached or a tapped hole.

Cheers

[sreten]

Originally posted by Collo

I'm looking at around 230g to add.

Huh ?????? a quarter of a kg ????? Are you sure ?????

If this is the correct mass, no way will a normal speakers suspension cope.

/sreten.