Lowering Fs by adding weight, Can't find the thread

[markbark]

Unless I'm dreaming I was reading a thread on adding weight on a sub woofer cone to lower the Fs. If I was dreaming how about adding weight or mass to the cone to lower Fs? I'm going to make the assumption that the added weight will not burden the voice coil. I was thinking about using Mortite ( rope insulation, sorta). Has anyone experimented? I fact, I think I saw this attempted on a project, in of all places Stereophile Can anyone help the lost? Have a great woofin day!

 

[weltersys]

Not a good idea in general, but it can be done. Magnets sandwiched on either side of the cone can be used (balance them axially), Mortite would probably fall off.

http://www.diyaudio.com/forums/multi-way/13463-adding-weight-speaker-lower-fs.html

 

[AndrewT]

I have gone through the newest 200 but could only find these two.
http://www.diyaudio.com/forums/subwoofers/239113-adding-mass-driver.html?highlight=added+mass
http://www.diyaudio.com/forums/multi-way/131938-adding-mass-cone.html?highlight=added+mass

The Thread I was expecting to hit must be a lot older than I remember.

 

[markbark]

Thanks guys! more info please

Thanks Andrew, I saw one of the threads but all too soon the thread was turned into an obscure discourse on how great the manufactures were to sell expensive sub woofers lol. If I had the money I would buy a 18" Bag end and be done with it. Oh I was thinkingof a large glob of Mortite glued to the center dust cap. I don'tthink they make lead tape anymore 🙂

 

[Soldermizer]

sorry can't resist...

I would recommend using items that have more mass, such as fishing sinkers or even washers or spare change. Thread is probably not weighty enough 🙂

 

[GM]

Not sure what you're looking for, but Ron E worked up this simple way to calculate how much it raises Qts, lowers efficiency:

mr = (Fs/Fs')^2

Qes' = Qes*mr^0.5

Qms' = Qms*mr^0.5

then: Qts' = Qes'*Qms'/(Qes'+Qms')

where:

mr = mass ratio
Fs' = new, lower Fs

New, lower effective efficiency:

n0’ = ~ [9.6352*10^-10*Fs’^3*Vas]/Qes’

SPLeff’ = ~ 112.018+10*log10[n0’]

GM

 

[tinitus]

coating is one option is if the cone need stiffening

or use solid core copper thread to build a thick ring, and place/glue it where the cone and voice coil joins

it is indeed very tempting, but I have destroyed enough woofers to know it's no simple matter

one problem may be the tendency to use cheap woofers, with possibly very weak magnet gap, and no control

a woofer with light cone and BIG magnet, yet still with high Qts, are good reasons to be suspicious

and further
you also need good Xmax specs to make any sense doing this
which is often not the case

that said, this one worked out quite well ... but I was very lucky not to destroy it in the process 🙂

 

[abcdmku]

This calculator will be your best friend:
mh-audio.nl - Home

 

[markbark]

Thanks abcdmku, thanks all! great little calculator I find that by adding sixty grams I can lower the Fs from 21 to 18 Hz loosing only 1.5 Dbl. It will bring my Qtc Up to 7.5 but I can live with that as a slight deviation from my flat 0.7 as it gives me about 5hz improvement in extension, close to 15 hz plus or minus.

 

[markbark]

Now, thinking about the best material and where to place it.

 

[AndrewT]

I hammered a piece of lead to form a very thin sheet.
Cut that sheet into strips and hand formed each to fit the cone shape just outside the dust cover.
Used double sided tape to glue the strips onto the cone.

I am sure I reported this years ago.

 

[Djim]

Now, thinking about the best material and where to place it.

Hi markbark,

If you take off the fabric of a mouse pad you have an ideal cheap material that is easy to shape and also increases the cone damping.

 

[GM]

Since I've only done significant mass loading for 'sub' duty, I took the expedient of bonding driver cutout discs to the woofer with some loose fiberglass insulation in the cavity. Cheap, easy and much less chance of causing potentially destructive imbalance, with the downside of course that it's permanent.

For smaller drivers, I formed coils out of lead based solder to fit the 'well' of the dust cap/diaphragm junction.

GM

 

[tinitus]

which method could depend on cone material, coating, etc

but can I ask why, and just curious ...

not producing good lows as is right now, or ... ?

Thanks abcdmku, thanks all! great little calculator I find that by adding sixty grams I can lower the Fs from 21 to 18 Hz loosing only 1.5 Dbl. It will bring my Qtc Up to 7.5 but I can live with that as a slight deviation from my flat 0.7 as it gives me about 5hz improvement in extension, close to 15 hz plus or minus.