Which is the most important TS parameter?

[Wombat2]

I repaired a pair of 20+ year old Pioneer 12" drivers (I was told they were used in their mid to high end equipment in their day so figured it was worth the trouble and challenge) All they needed was new surrounds and one needed it's magnet reglueing (with epoxy). After repairs they measured almost identical in Speaker Workshop and sounded fine when music was played through them.

I then rebuilt the enclosures and remodelled them using several computer modelling programs. - they sound fantastic!!

This was 12 months ago and I've just got around to hooking them into the system and the one with the reglued magnet has its voice coil rubbing and therefore distorts loud low frequency signals. I think it may have sagged with standing.

I've yet to pull it apart and check it but if it is beyond help and I decided to replace the 12" drivers which TS parameters are more important in trying to match the drivers to the enclosures without too much alteration (probably limited to retuning the ports)

[leonrocks]

Look for a driver with similar Vas, Qts, Re and Fs.

The further any of these are from the original, the more you will have to adjust the box or crossover.

Remember you can always take volume away from the box too by adding solid material. It may also be possible to increase the volume by adding stuffing material but this can have other unwanted effects.

[rabbitz]

Further to what leonrocks has said, you can always check out Fs/Qts and Vas x Fs².

If the new driver comes close to original using these, then you're not too far off the money. If they are part of a multi-way speaker then you also have to consider the impedance plot for the crossover.

[Wombat2]

Originally posted by rabbitz
Further to what leonrocks has said, you can always check out Fs/Qts and Vas x Fs².

If the new driver comes close to original using these, then you're not too far off the money. If they are part of a multi-way speaker then you also have to consider the impedance plot for the crossover.

Thanks for that - they are part of an active crossover system so don't have to worry about the impedance plot - just tweek the crossover to suit.

[myhrrhleine]

Vas Fs and Qts are like three corners of a triangle. Change the angles and you'll still have a triangle, but it'll look different.

Each is important in its own way.
If you want the final to sound like the original, match all three.

[sreten]

Hi,

The Fs, Qts and Vas can be very different and still give very
near the same result when the driver is put into a box, as
it is usually the box volume that dominates.

that is : Vas can be very large, consequently Fs is very low
and Qts is very low can give the same result as Vas Medium,
Fs medium and Qts medium.

What we are really talking about is cone mass and effective
piston area and the magnet system driving it. If this is the
same for two drivers results in a box are pretty much set.
(Unless Vas is stupidly low, Fs and Qts stupidly high)

For a bass reflex ideally you want driver Vas to be ~ 2 x box volume.
If the above, you want Qts to be around 0.35 to 0.38.
(For Vas >> box volume, Qts needs to be lower)
Given the above the lower the Fs of the driver, the lower the
efficiency of the driver will be but you'll get more extended bass.
I like port tuning at Fs or fairly near above it. (Fs near 30Hz)

If you want to get near the current alignment model the Qtc of
a sealed box with the Pioneer driver and your box volume. Any
driver that models the same Qtc is that box volume will reflex
with the same alignment as the Pioneer.

As said before the actual port tuning will depend on efficiency.

/sreten.

[Svante]

I recently posted the picture below on another forum, it shows the effects of altering Cms +/- 30% in a standard butterworth alignment bass-reflex box. Doing so gives fairly large effects on fs, Vas and Qts, but very little effect on the response. Changing just one of fs, Vas or Qts would give a larger effect on the response.

I guess that this supports what streten among others have said in this thread.


------------black blue red
fs [Hz]----: 35 - 40 - 31
Vas [liter]: 33 - 25 - 42
Qts -------: 0.38 0.43 0.34

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[Wombat2]

Thanks for the info everyone - fortunately I was able to fix the problem. When I pulled it out I could feel the piston binding when gently pressed with the driver vertical but when horizontal it was all but gone so I figure it was a very fine contact. Close examination revealed the piston stighlty off centre with the closer distance near the folded seam in the metal sleave. I wedged a business card in the gap and gave it some sideways force which appears to moved it enough. I remounted the driver with the seam down (was up before) so if there is any sagg under gravity it will open the gap more. Hooked it up and played a couple of heavy Bach organ pieces and the result is perfect - not a hit of distortion.