Modelling compression drivers

[thosuk]

I’ve been playing with the example compression driver described in the Akabak manual.
It looks like it should be reasonably straightforward to “adjust” the parameters to simulate say a B&C DE250. My approach is to match the SPL and Impedance curves given by B&C for a DE250 mounted on a ME45 waveguide. Assuming the ME45 is a true exponential waveguide it is possible to add this to the compression driver model and use Akabak to generate SPL and Impedance curves for the DE250/ME45 combination.
I would appreciate guidance from others more expert than I on whether they think I am wasting my time, or whether it is best to assume a simple cone type driver paired with the waveguide for a first stab, and then to rely on practical measurements to resolve crossover issues etc
Is it sufficient to match the SPL and Impedance curves when it is possible to achieve this using many wrongs to make a right? As B&C only quote Re and L values the Akabak model requires many others to be adjusted using the graphical fit technique. My instinct is that it should be possible to get a reasonable model if other values such as BL and Mms were known, plus other geometric measurements taken on a partially stripped compression driver. I have sought more data from B&C but no luck so far.
The DIY fraternity now have many super tools to evaluate speaker designs but the modeling of compression drivers remains a real challenge, hence my question.

[gedlee]

I've done a lot of compression driver modeling. Here is how I get the parameters. Measure the impedance curve withoput a horn and model this. Measure the impedance curve with the front port blocked off - model this. If possible run the impedance curve with the back off and model this. With these two or three measurements run the model with the known values fixed while varying the unknown values until you get a good match to all of the impedance curves simultaneously. When you get this as good as you can then you have the proper set of parameters for the driver. The advantage of this approach is that you don't have to destroy the device to get the values.

[thosuk]

Earl,
Thankyou for the advice. I had hoped to be able to screen compression drivers from published data, plus maybe some additional info from others. At least I know that hands on measurement is the way to go, which will no doubt save me much wasted time and anguish.
I purchased SPEAK a few years ago because it covered a full speaker design i.e. multi drivers, full frequency range, crossover and waveguides. I would like to know if you use SPEAK to model compression drivers (on waveguides + crossover) or is there a special unreleased version to do this. Wouldn't blame you if there were as ultimate refinements represent your livlihood. The guidance you freely give is always very relevant and saves many of us from fruitless ventures.
Thanks again for your very prompt reply.
Tom

[gedlee]

I still do rough designs with SPEAK, but the more accurate approachs are proprietary. These later approachs involve a combination of measurements and models which I have found to be quite accurate. Accurate enough that I can use the models with confidence that the final design will be right on once its built.

[Jmmlc]

Hello,

For what is worth, you may find the Hornresp model I designed for a TAD TD2001 compression driver, in Audio Asylum message :

http://db.audioasylum.com/cgi/m.mpl?forum=hug&n=128658

(better to use Ang = 2,0 Pi)

Best regards from Paris, France

Jean-Michel Le Cléac'h

Originally posted by thosuk
I’ve been playing with the example compression driver described in the Akabak manual.
...
The DIY fraternity now have many super tools to evaluate speaker designs but the modeling of compression drivers remains a real challenge, hence my question.

[thosuk]

Thanks Jean-Michel,
I shall experiment with the values you suggest and try to adjust in due course for the DE250.
I became interested in the modelling of compression drivers as I could not see a way of modelling compression driver / waveguide / crossover interactions in a single package except using Akabak. In theory I hoped this would produce an accurate model - provided of course that suitable parameter values could be obtained for the Akabak compression driver model.
Earl suggested a way to refine the Akabak model values but I would love to know if this is essential for accurate modelling, or whether the simplified model you suggest supplemented by practical experimention is just as good. My feeling is that accurate modelling should minimise the trial and error experimental stage.
Tom

[David McBean]

Originally posted by thosuk
Earl suggested a way to refine the AkAbak model values but I would love to know if this is essential for accurate modelling, or whether the simplified model you suggest supplemented by practical experimention is just as good.

Hi Tom,

You might be interested to know that Hornresp has been used by the P.Audio System Company of Thailand, to model horn systems with compression drivers.

Kind regards,

David

[Jmmlc]

Hello Thosuk,

Let's take as an example the TAD TD2001 compression driver modelisation.

Akabak will allow you to add the effect of the cavity inside which the voice coil moves (responsible of a bump in the electrical impedance curve and a hole in the response curve around 1600Hz) and eventually few other minor resonances above 10kHz ... Is this useful, in the initial design process of a horn or a waveguide I doubt ...

More complex modelisation integrating the coupling between, mechanical, thermal, acoustics ... need more complex software suites (FEM...).

But for a first step in the design of a horn or a waveguide, if you don't need to link the crossover (is it so useful), Hornresp will allow you to do the job more more easily and more rapidly.

Best regards from Paris, France

Jean-Michel Le Cléac'h

Originally posted by thosuk
Thanks Jean-Michel,
I shall experiment with the values you suggest and try to adjust in due course for the DE250.
I became interested in the modelling of compression drivers as I could not see a way of modelling compression driver / waveguide / crossover interactions in a single package except using Akabak. In theory I hoped this would produce an accurate model - provided of course that suitable parameter values could be obtained for the Akabak compression driver model.
Earl suggested a way to refine the Akabak model values but I would love to know if this is essential for accurate modelling, or whether the simplified model you suggest supplemented by practical experimention is just as good. My feeling is that accurate modelling should minimise the trial and error experimental stage.
Tom

[JLH]

Originally posted by David McBean


Hi Tom,

You might be interested to know that Hornresp has been used by the P.Audio System Company of Thailand, to model horn systems with compression drivers.

Kind regards,

David

Any chance you might be able to get a hold of some input parameters from them? Several different models would be a bonus. Had to ask.

Rgs, JLH

[thosuk]

Thanks David and thanks to all.
I'm very happy to take all the advice as clearly you've been there before. I shall compare simplified models with the limited published data for real compression drivers as I'm curious about how many parameters are real and how many are contrived.
Its pretty clear how to approach first estimate modelling and I shall no doubt find out in due course about the interactions when crossovers are introduced. An inductor can improve low frequency response, and can be modelled in Horn Response, so clearly there must in general be an influence. It may well be, as Jean-Michel implies, that the subtleties are small and acceptable for DIYers. I just wanted to find out how to address the problems for this aspect of speaker design, which is not well documented. Only those who have been there will know whether going to a lot more trouble is worth it.
Tom