Do Thiele-Small numbers trump construction?

[BillEpstein]

I have a pair of 12" Utah woofers that sound fabulous in a 2.5 cu ft sealed box. They're too old for published specs, apparently (I've googled).

I'm wondering if it would be worthwhile to buy Woofer Tester to get the specs and then see about some Eminence or B&C or whatever drivers to make some more for my kids home theatre?

How close can I get w/o the same voice coil, spider construction, etc?

[gfiandy]

Hi,

In my experience the T-S numbers only tell you how to align the driver i.e how to set up the frequency response with a cabinet. The sound quality is much more about the construction, how the cone breaks up, the motor construction etc.

So even if you fins a driver with equivalent T-S parameters it won't sound the same.

Regards,
Andrew

[LineArray]

Thiele & Small parameters are used for prediction of
low bass behavior in the pistonic range of the driver.

Given a sufficiently rigid cabinet and an internal
shape/size free of "box" resonances, which is usually the
case for low frequencies, boxes having equal parameters
will sound virtually the same.

Having a port which is as free from ventilation noise as
possible is important, as well to control Qb, which is due
to the mechanical loss of the cabinet itself.

Estimation can be done by impedance measurement.


Kind Regards

[gfiandy]

Line Array - I believe your description is extreemly misleading.

This may be partly true in the piston operation region but as soon as the cone breaks up this is completely untrue. So unless the intention is only to use this driver as a subwoofer, which is not mentioned, your advice is incorrect.

Also many drivers create considerbale distortion at lower frequencies and this significantly affects the sound. The cabinet resonance and port resonance can also significantly affect the sound.

I have tried to duplicate a driver by creating one with the same T-S parameters and even just getting the parameters the same is very difficult. Having achived this it sounded nothing like the origonal.

Regards,
Andrew

[BobEllis]

T-S are small signal parameters. Once you get to a signal level where enclosure stiffness and port noise become an issue a lot of other things have changed. The voice coil is hot, increasing its resistance affecting overall Q, you are potentially out of the pistonic range, and excursion potentially creates intermodulation distortion effects, and so forth.

That said, IIRC, Utah made drivers intended for PA use, fairly high efficiency paper cone accordion surround. Looking for something of similar construction, sensitivity and T-S parameters that work in a similarly tuned box should sound fairly similar. You're probably on the right track looking at Eminence and B&C if you are looking to replicate that sound.

[gfiandy]

Bob, pistonic range is determined by the frequency of break up of the cone. This will occur at any signal level it is the frequency at which the entire cone cannot move at the same speed. The middle attached to the voice coil effectively becomes detached from the outside of the cone which responds later and often in unpredictable ways, such as oscillating like a bell arround the outside of the cone. (alot of the tone of a speaker is determined by these characterisitcs)

I guess it comes down to how much like the origonal speaker you expect it to be.

Get the same T-S parameters and box and the low frequency should be vaguely similar. However if you plan to run it wide range (i.e above 500Hz) then I suspect the tone will mostly be determined by other factors.

Regards,
Andrew

[BillEpstein]

I appreciate the info. I run the tweeter down to 2000Hz first order with just a 1/2 henry coil on the woofer so theres a lot of woofer tone involved. Just have to keep looking at E-Bay, I guess.

[GM]

Which Utah model? I have measured specs done in the early '70s of some older models.

Frankly, all of them have the typical huge Vas, low Fs and Qts of their Altec, etc. competitors of the day, so if driven with a very low output impedance, 2.5 ft^3 for sealed is about right. Still, if you're going to experiment with other cab alignments and/or DIY speaker building as a hobby, then the WT will save you a lot of trial n' error.

GM

[LineArray]

Originally Posted by gfiandy

Line Array - I believe your description is extreemly misleading.

This may be partly true in the piston operation region but as soon as the cone breaks up this is completely untrue. So unless the intention is only to use this driver as a subwoofer, which is not mentioned, your advice is incorrect.

...

My post was indeed restricted to "subwoofer" operation, and in that
context in no way misleading ...

Different quality due to cone breakup and cabinet resonances is
simply out of scope of the Thiele & Small parameters and
proposed 'alignments'.

Driver/Box Systems with same alignment show the same frequency
response and the same impulse response. To exclude the effetcs
occuring in midrange indeed low pass filtering would be necessary
for comparison of real objects.

'Qb' is one of the more hidden parameters for DIY'ers , so i mentioned
it's significance.

Distortion is another story as the parameters are valid for small signals
and the range of linear cone excursion, BxLxI = driving force being
independent from excursion, velocity of air moving in the port being
very small in comparison to the speed of sound etc. etc.

It is of great value in the research done by Thiele and Small to have an
abstraction from all those effects which affect sound quality in case
of cone breakup, nonlinear excursion ...

If someone wants to use TS parameters, he is responsible himself for
dimensioning the system in a way to operate close to those
idealizied conditions, thereby considering the "displaced volume" needed
dependent from frequency.


But even a Midwoofer in a two way, which is used for reproduction
of the midrange as well, can be described correctly by TS Parametrs
in the frequency range around resonance frequency.

Midrange behaviour where breakup and beaming occurs as well
as cabinet resonances is out of scope of TS ...

But i think it is a strength of a theory or a framework to be restricted
to a well defined scope ...

It is not that TS in not applicable to our 2-Way, it is simply not
applicable to the whole usable frequency range ... and it is not applicable
when entering (seriously) nonlinear conditions.

If you want to describe "the sound quality" of a driver in breakup mode
and nonlinear excursions and the VC leaving the homogenous area of the
magnetic field and the cabinet radiating due to vibrational modes and
coincidence you cannot use TS Parameters to predict or describe the
effects.

Does that in any way affect validity of TS Parameters ?

Is TS useless because "we always have (some) nonlinearity" ?

You simply have to choose the appropriate theory/framework
for the operating conditions you want to examine or predict.

Systems with same TS Parameters are equvalent within the
limits of the framework.


Kind Regards

[gfiandy]

Since the question asked was not about a sub-woofer, I stand by my comment your post was misleading, it didn't make it clear you were talking only about a sub-woofer.

The question was are T-S parameters enough to make a speaker that sounds the same and they are not.


Regards,
Andrew