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]

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]

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]

[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]

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]

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

 

[roddyama]

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

Easily said...

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.

What happens when the old suspension compliance value has change over the years (which it likely has). Since the driver is unuseable in the normal sense, you can't make any new T/S measurements. I afraid the software won't be of any real help in that case.

...you should use a speaker no larger than an 8".

Why?

 

[roddyama]

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

The path of least resistance. Cal, your a man after my own heart.

 

[kelticwizard]

What happens when the old suspension compliance value has change over the years (which it likely has). Since the driver is unuseable in the normal sense, you can't make any new T/S measurements. I afraid the software won't be of any real help in that case.

Roddy:

The software is likely to give you a ballpark figure as to how much added weight you are looking at. Some of these PR's seem to require weight added that well exceeds a pound, which might be taxing the suspension of the PR. Besides, if the suspension does loosen up with age, I would think that less mass would need to be added to get the PR tuning lowered to the correct frequency. Of course, you would fine tune the system by doing an impedance test and seeing where the lowest impedance is.

Why?

Assuming the PR's had excursions similar to the drivers he plans to mate them with, a 10" PR has nearly twice the cone area of an 8" speaker. Weems and others recommend that the PR should have twice the cone area of the driver for the reasons you outlined in your first post.

 

[roddyama]

Roddy:

The software is likely to give you a ballpark figure as to how much added weight you are looking at.

KW,

Agreed. This is probably a good starting point.

Some of these PR's seem to require weight added that well exceeds a pound, which might be taxing the suspension of the PR. Besides, if the suspension does loosen up with age, I would think that less mass would need to be added to get the PR tuning lowered to the correct frequency. Of course, you would fine tune the system by doing an impedance test and seeing where the lowest impedance is.

The suspensions will initially loosen with age and use, but stiffen in the long term as the polymerization sets in. Also, you can't necessarily assume that you'll be able to get impedance information from the damaged VC. In theory, I agree, but practically, there will still need to be some level of trial and error to in the end.

Assuming the PR's had excursions similar to the drivers he plans to mate them with, a 10" PR has nearly twice the cone area of an 8" speaker. Weems and others recommend that the PR should have twice the cone area of the driver for the reasons you outlined in your first post.

I get it now.

 

[kelticwizard]

The suspensions will initially loosen with age and use, but stiffen in the long term as the polymerization sets in.

I didn't know that. But if the process is not too drastic, we should still be in the neighborhood.

Also, you can't necessarily assume that you'll be able to get impedance information from the damaged VC.

I meant take an impedance test of the driven speaker, to see if the weight added is the right amount to tune the box to the desired frequency. The impedance of the driven speaker will be at it's lowest point at the tuning frequency.

In theory, I agree, but practically, there will still need to be some level of trial and error to in the end.

I agree completely. I think the impedance test should be used to add or subtract weight to see if the box is tuned to the correct frequency.

 

[leadbelly]

Re: Re: DIY drone cone

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

This post has me interested, as I have a cheapo 8" car sub and 2 ancient 10" woofers just laying around.

How do you remove the magnet? I can't tell just by looking at it what type of bond it is. I have attached a picture. Sorry about the quality.

 

[roddyama]

I didn't know that. But if the process is not too drastic, we should still be in the neighborhood.

The loosening is minor, maybe 5 to10% (a guess) at the most. Essentially the the "breakin". The polyerization is the rubber hardening process with age and/or exposure to elements like sulfur. Over a long enough period this process can be complete. At that time the rubber becomes brittle having lost most of its compliance. Hence, dry rotted tires, or rubber bands that break instead of stretch, and my favorite, the foam in special tool cases (like drafting tools) that turns to dust when you touch it. (Yeah, I still have my board drafting tools, just not in their original cases.)

 

[roddyama]

Re: Re: Re: DIY drone cone

This post has me interested, as I have a cheapo 8" car sub and 2 ancient 10" woofers just laying around.

How do you remove the magnet? I can't tell just by looking at it what type of bond it is. I have attached a picture. Sorry about the quality.

I doubt that its welded. If it is you'll be just about out of luck. Chances are that its gluded. In which case a screwdriver (pry bar since a screwdriver is not a prying tool) should do the trick. Try to work around all sides so you can lift the magnet structure straight off without damaging the voice coil.

Or you can leave the magnets in place because it still work as a PR. If the voice coil is still good, it will allow you to play with damping as was pointed out in a previous post.

 

[sreten]

Re: Re: Re: Re: DIY drone cone

I doubt that its welded. If it is you'll be just about out of luck. Chances are that its gluded. In which case a screwdriver (pry bar since a screwdriver is not a prying tool) should do the trick. Try to work around all sides so you can lift the magnet structure straight off without damaging the voice coil.

Or you can leave the magnets in place because it still work as a PR. If the voice coil is still good, it will allow you to play with damping as was pointed out in a previous post.

I severely doubt that its welded. (Heat causes demagnetisation)
It could be glued but they are most commonly riveted.
(The bottom pole piece is riveted to the chassis before
the cone is fixed from the front.)

Either way its brute force that's required using a lever.
You should be able to get it off without distorting the chassis.

The magnet only needs to be removed for one of two reasons :
1) the coil is damaged and creates rubbing noises
2) airflow is poor, though removing the dust cap could fix this.

sreten.

 

[sreten]

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

All this talk of variable suspension parameters has me confused.

Simply it will not matter as the volume of the box should
dominate the tuning frequency of the PR, variations of
the units Vas will only really effect its free air frequency.
(Assuming 10" Vas is a lot larger than 8" Vas which it should be)

IMO a maximally flat alignment is difficult to achieve as PR tuning
frequency must be spot on, but my favourite overdamped reflex
alignments are much more forgiving regarding tuning frequency.

sreten.