DIY drone cone

[Cal Weldon]

I looked at the previous thread, but I have questions still.

I have a set of 10" woofers that I want to to convert to drones. They are a pulp cone and have a paper dust cap. I want to remove the magnets and coils but leave the spiders for a more lineal throw. I was thinking of painting the back of the cone with white glue to stiffen. Maybe a few coats, I don't know. I would also paint the dust cap from the back side. I was thinking of adding lots of glue to the back of the dust cap to add a little weight.
I have done this before, but never got the completed system checked. It sounded just fine.
Now, if I'm going spend a little time doing this I'd like to know if I'm heading in the right direction or not. It won't cost anything to do it, but are passives still used to good effect in 8" two way's?

Thanks for your time
Cal

[leadbelly]

Originally posted by Cal Weldon
II was thinking of painting the back of the cone with white glue to stiffen. Maybe a few coats, I don't know. I would also paint the dust cap from the back side. I was thinking of adding lots of glue to the back of the dust cap to add a little weight.

I'll let the gurus answer the big picture questions, but I would think your biggest problem might come from adding many layers of hand applied glue. With many layers, you'll probably have poor uniformity of thickness and this will unbalance the cone.

[GM]

Assuming they have enough excursion to be of any use, bonding a flat plywood disc to the cone with insulation in the hollow works well, and if you leave the VC attached you can fill it as required to fine tune it.

GM

[Cal Weldon]

OK, I can always go easy on the glue.

Plywood?

Haven't I created a stiff and heavy cone with the glue? Need it be another diaphragm? Would this be panelling thickness I'm guessing?

It sounds like work to me there GM. I was hoping to get by on the lazy side. Besides the finishing on the plywood would more work than it's worth. No?

Cal

[roddyama]

Originally posted by GM
Assuming they have enough excursion to be of any use, ...

This comment by GM is of particular importance. A PR can easily require twice the volume displacement (or more) of the driver depending on the PR tuning frequency. the result of too little excursion will be a compressions effect at higher levels as the PR forces the the suspension to its extremes. One solution is to use two PR's or one PR of a larger diameter. Although it will still work properly at lower levels.

PR's aren't easy. You will need to use mass to tune them as you would change the length and diameter in tuning a port. The problem is you don't have any software to tell you exactly how much mass to add (or remove) to make the PR resonate at the correct frequency. You will have to do this by trial-and-error.

[GM]

>Plywood?

====

Well, it has to be void free and resonate well above the PR's BW. HDF should work if its tuned low enough.

====

>Haven't I created a stiff and heavy cone with the glue?

====

Beats me, I'm not familiar with the driver's construction or how thick a white glue you're using. Since the white glue I'm familiar with isn't very massive and doesn't dry very hard, I doubt it though.

====

>Need it be another diaphragm? Would this be panelling thickness I'm guessing?

====

No, it will need to be at least 1/2" thick, but mostly it depends on how much mass needs adding.

====

>It sounds like work to me there GM. I was hoping to get by on the lazy side.

====

Well, to my way of thinking, gluing a wood disc on IS the lazy/quick way, and why I suggested it. Anybody who knows me well knows I was born lazy and immediately had a relapse.

====

>Besides the finishing on the plywood would more work than it's worth. No?

====

Since when is a fine finish a prerequisite to good performance? Sounds like you're trying to make work for yourself.

GM

[leadbelly]

Originally posted by GM
[B>Besides the finishing on the plywood would more work than it's worth. No?

====

Since when is a fine finish a prerequisite to good performance? Sounds like you're trying to make work for yourself. [/B]

No, I think he meant that bevelling the edge on a round piece of plywood to make it fit the cone tightly is a lot of work. After all, cones are not usually at some common angle like 30 or 45 degrees which can be handled with router bits.

[sreten]

Originally posted by Cal Weldon
I looked at the previous thread, but I have questions still.

I have a set of 10" woofers that I want to to convert to drones. They are a pulp cone and have a paper dust cap. I want to remove the magnets and coils but leave the spiders for a more lineal throw. I was thinking of painting the back of the cone with white glue to stiffen. Maybe a few coats, I don't know. I would also paint the dust cap from the back side. I was thinking of adding lots of glue to the back of the dust cap to add a little weight.
I have done this before, but never got the completed system checked. It sounded just fine.
Now, if I'm going spend a little time doing this I'd like to know if I'm heading in the right direction or not. It won't cost anything to do it, but are passives still used to good effect in 8" two way's?

Thanks for your time
Cal

The simple answer is this can work well but there
are a number of proviso's to be considered :

The 10" driver must be a long throw design, with a larger
suspension excursion (and its larger cone area) it could
work well with an 8" bass/mid driver.

There is the possibility of leaving the magnets in place :
connecting a variable resistor across the terminals will
then give control over the damping of the reflex action.

Removing the magnets is relatively easy, removing the
coil is not, I can't see the point of removing the coil.

Adding mass to the back of the cone is awkward due
to the chassis, adding to the front is a lot easier.
I'd suggest spraying the front via a circular stencil with
car underbody anti chip spray, heavy and good damping.
If you want to stiffen the cone first spray
it with black hammerite or similar paint.

In both cases add in thin layers and allow plenty of time
between layers, and using a proper spraying pattern will
ensure an even coverage of the cone.

Adding mass inside the voice coil is a possibility,
find stainless washers that fit and epoxy into place.
(If you are removing the magnets, I can't see the point)

To answer your final point, a reflex port is far cheaper
and almost as effective is any medium sized and above
8" two way speaker. Generally the extra cost of a passive
radiator is not worth the minor extra performance.

However in a medium sized enclosure ridiculous amounts of
mass will not be needed to be added to the driver, and the
possibility of variable damping is an advantage.

sreten.

[kelticwizard]

Originally posted by roddyama


PR's aren't easy. You will need to use mass to tune them as you would change the length and diameter in tuning a port. The problem is you don't have any software to tell you exactly how much mass to add (or remove) to make the PR resonate at the correct frequency. You will have to do this by trial-and-error.

My understanding is that you simply add mass to the cone until you get the tuning you want.

Software? Bullock and White's Boxmodel will tell you two things:

A) What the Mass of the cone is already just from the following Thiele-Small specs:
Fs,
Vas,
Qes,
Qms,
Re, (DC resistance)
Sd, (Area of cone) Actually, the area of a 10 incher is always around 56 sq inches or 350 sq cm.

B) What the mass required is to tune the box to the desired frequency. You then subtract the mass of the cone you just looked up or calculated from this amount.

I agree with the others that assuming your cone excursion is about equal to your projected active speaker, you should use a speaker no larger than an 8".

Here is the link.
http://www.hal-pc.org/~bwhitejr/

Note: When downloaded, you will get several programs when unzipped, like a program to predict bandpass enclousures, crossovers, and Transmission Lines. Do not use the Transmission Line program-it was an experimental model that Bullock later admitted did not predict results. The other programs are perfectly fine, however.

[Cal Weldon]

It was going to be an 8" two way.

Alas, I feel the wind retreating from my sails.

Perhaps best to acknowledge the difficulties and return to a tuned port, like the rest of the sheep.

Baaah...d news for the old woofers

Thank you for your answers

Cal