adding mass to change driver parameters

[just a guy]

I have never tried OB before and I want to experiment. For free. I have a collection of cheap and not so cheap drivers to use but none are specifically suited to OB (without eq), although some look like they might be close.

The main problem is low qts, as I would like to use little or hopefully no eq. Also, some of the drivers could use a reduction in fs. This brings up the question of adding weight to the driver cone to modify the parameters. I realize you can raise qts by adding a resistor in line but this is not a route I would like to consider unless absolutely necessary.

Here are my questions. If driver parameters are known, is there a formula to predict how weight will affect the modified parameters?

If specs are not known, is it worth it to measure them? Remember I am using some very cheap drivers for experimental purposes. I am wondering if there are loose guidelines based on cone area and estimated parameters.

I would imagine adding a significant amount of weight would raise the qms quite a bit, maybe unnaturally so. Will this affect the sound quality in a bad way?

What is the preferred method to add weight? Is it ok to glue objects like washers or coins in a symetrical fashion on the back of the cone or should a coating of something be added to the entire cone area?

Trial and error is king but most of the cones are paper and some are quite old. If I attach weight using tape that is sticky enough to keep the weight in place it will damage the paper cone when I remove it to permanantly attach the weight with glue.

Let me apologize in advance if the answers are already here somewhere. My internet time is severely restricted lately and I have tried searching but the search terms "add, additional, weight, mass, driver, speaker, cone, change, modify, parameters, qts, fs" in several different combinations gives me literally thousands of results not related to the question.

[Geoff H]

Adding mass will not only lower the fs, it will reduce the freq response, trainsient response, and efficiency. All those things we like about OBs.

I suggest you forget the parameters for now, and try the drivers on a 2x4 baffle. Adaptors can be made to save cutting down a forest.

Old cones can be rejuvinated by applying an even coat of PVA, and some not so old, improved.

Also, have a read of this thread: Old Console drivers in open baffles

Most of all, have fun.

Geoff

[Tenson]

Programs like WinISD can calculate the low end response with added weight but it won't tell you how the higher end of the spectrum will be affected.

You usually don’t need much weight when adding it directly to the cone so I use bitumen damping material cut to small sizes and spaced equal distance around the back of the cone and squash them out over the surface a bit. I find this often helps damp resonance too, but you don’t always want that.

[Ron E]

Try :
adding mass to change woofer parameters

hit #3:
http://www.birotechnology.com/articles/mass.html

[just a guy]

Thanks so much, just what I was looking for. All the math and real world experience as well. I was surprised to find that sensitivity takes such a big hit while only mildly altering the other parameters. This is not how I will proceed. But if special circumstances suggest I should sometime, thanks for the advise on how to add mass.

[Svante]

I think the key to understanding what happens when mass is added is to simulate. Equations might be good, but also misleading.

Here is what happens when a low-Q (Qts=0.38) is put in an open baffle and variuos amounts of mass is added.



It can be seen that the higher Qts that is one result of the added mass affects the slope in the range 30-300 Hz, but also that the sensitivity at 300 Hz is reduced by ~10 dB.

Cms, Sd, Bl, Re and Rms are kept constant between systems as they would in the real world, and Mms is varied.