altering T/S parameters?

[AndrewT]

Hi,
I want to alter the Q and Fs of a bass speaker to make it more suitable for sub-bass use.

If one adds mass to the cone to lower Fs, what other Thiel Small parameters change with the added mass. eg. No.(efficiency).
Please post formulae for these other changes?

If a series resistor is added to the driver to raise the Qts, do any other T/S parameters change?

Again formulae would help.

[sreten]

Hi,

play around with defining drivers in winISDpro with autocalculate, increasing Mms.

The series resistor changes Qe, Re, and BL only I think.

sreten.

[Puggie]

I asked this a while ago, do a search for threads started by me, I think it was in the subwoofer section.

[Dirk]

Hi,

It should be possible to change the TS parameters (especially Qes and Qts) of a dual voice coil driver by putting a variable resistor over one of the voice coils (only one voice coil is driven). This should allow changing parameters continuously; haven't got the equations, though :-(

Regards,
Dirk

[ocool_15]

adding cone mass will make a big difference to Vas. To make the VAS smaller you would have to tighten the suspension(maybe a DVC with the 2nd coil shorted togetheer would help?) Stiffening up the suspension will increase the fs again.

i would think halving fs(double mms) should square the Vas. The easiest way to play with this it to put a small magnet on each side of the cone and have them evenly distributed around the center.

[richie00boy]

Adding mass lowers Fs and increases Vas. By how much depends on the original cone mass. I think it's a square law in both cases.

Adding resistance raises Qes, thereby raising Qts. No other parameters change. I'm just going to try and find the link for how you can work out the change.

EDIT: here is the formula from Weems.

R = [(Qrequired / Qmeasured)Re] - Re

[clem_o]

I had the formulas way back, have to find the reference. One thing I am sure of - adding mass to the cone drops Fs and increases Qm, thereby increasing Qt. Series resistor shouldn't change BL (I think)...

Cheers

Clem

[AndrewT]

Hi all,
so far we have

added mass lowers Fs, increases Vas, raises Qm leading to raised Qts, increases BL,

Added resistance raises Re => Qe => Qts.

No change to BL,

Any more?

Richie thanks for the formula. You have confirmed the calc that comes out of Bullock.

[Ron E]

There is a lot of incorrect information so far in this thread.
Most notably, adding mass does not change Vas or Bl at all.

Adding mass causes the following changes:
Because Cms=1/((2*pi*Fs)^2*Mmt)

Fsm/Fs=sqrt(Mmt/(Mmt+Madd))

Fs is the original resonant frequency
Fsm is the new Fs with added mass
Mmt is the original moving mass (including air load)
Madd is the amount of mass added.

Then:
Qes'=2*pi*Fsm*(Mmt+Madd)*Re/Bl^2
Qms'=2*pi*Fsm*(Mmt+Madd)/Rms

These can be simplified but I am lazy this AM.

Sensitivity is reduced by 20*log10(Mmt/(Mmt+Madd))

-------------------------

Adding series resistance increases Qes and reduces sensitivity by 20*log10(Re/(Re+Radd))

Qes'=Qes*(Re+Radd)/Re

Series resistance does not affect BL or Vas or anything but Qes (and of course Qts).

----------------------------------

I am sure I have posted all of this in the past, as well as the effects of shorting or resistively damping a dual voice coil woofer. Search for RDO (Resistively Damped Operation)

[AndrewT]

Hi Ron,
thanks for that, I'll do another search. So far that has been unsuccessfull. Even used google to search DIYaudio as suggested by a couple of posters.

Surely there is something wrong with our search engine if we have to use google!

Is that all that changes?