Impedance correction at a compression driver's Fs - How to calculate - Wayne P? [message #18648] |
Tue, 21 March 2006 04:13 |
Peter K
Messages: 17 Registered: May 2009
|
Chancellor |
|
|
Hello! I have a pair of 2” BMS 4592 ND (16 Ohm version) coax compression drivers on their way to be used from about 300 Hz and up on a large 200 Hz exponential horn for my home system. I would like to make an impedance correction curcuit to flatten-out the impedance peak around the Fs of the BMD "mid-section" (app. 360 Hz according to the graphs on the BMS website). There is a link to the driver below. Besides, I have from the manufacturer (BMS) got the following values for the 16 Ohm version: Re (mid) = 8.9 Le= 0.19 (at 10 kHz) If I understand it correctly, an impedance corrections curcuit (Zobel?) can be calculated quite precise when the driver’s Qes and Qms are available. But that is usually not the case when the driver is a compression driver, as in my case. However, at the link given here:
http://www.the12volt.com/caraudio/crosscalc3.asp#zobel there is an online Zobel-calculator that based on the description seems to do what I search for. 1. The calculator asks for the “Nominal resistance”. In my case, is that the 16 Ohm (because it is said to be an 16 Ohm driver), or the Re = 8.9 Ohm provided by BMS, or…?? 2. Is the formula in the link the way to go, or should I rather do something else - and in that case what? I would really appreciate your help – thanks! Regards Peter
|
|
|
|
|
|
|
"Crossover Electronics 101 seminar" [message #18654 is a reply to message #18649] |
Wed, 22 March 2006 00:35 |
Duke
Messages: 297 Registered: May 2009
|
Grand Master |
|
|
I met Wayne at the crossover seminar he gave at the last Midwest Audio Fest a couple of years ago. Can't say I understood everything he said, but enough to suddenly make it possible for me to design a crossover for a horn. Before attending his seminar, all I knew was that my cookbook approach was totally useless. That workshop alone was worth the drive from New Orleans to Lima (Ohio, not Peru) for me. Duke
|
|
|
Re: Impedance correction at a compression driver's Fs - How to calculate - Wayne P? [message #18698 is a reply to message #18649] |
Thu, 06 April 2006 10:46 |
Earl Geddes
Messages: 220 Registered: May 2009
|
Master |
|
|
Wayne While I think that your crossover paper is useful for the novice it is highly over-simplified and if naively applied would not yield the desired results. Specific points are: Driver inductances are never ideal - Leach and Vanderkooy both point this out, and your Spice model simply does not represent the actual impedance variations found in real drivers. This can be quite a pronounced effect. In my computer simulations I use the Leach model which works quite well. Spice models only give electrical performance. In the real world we have to add in the acoustical performance of the drivers which adds a great deal of complexity, but ignoring it is highly erroneous. Thats why I don't use Spice for acoustical problems. For instance, any CD device must have a falling axial response which must be corrected in the crossover. This is not reflected in your designs or discussion. The multiple impedance peaks in a compression driver are caused by reflections in the horn. A properly designed waveguide does not exhibit these higher peaks and as such allows for a much better crossover design as well as a much better directivity control. In short, if you have these peaks in your impedance curve then the horn is poorly designed and no crossover approach is going to fix that.
|
|
|
|
|
Re: Impedance correction at a compression driver's Fs - How to calculate - Wayne P? [message #18701 is a reply to message #18700] |
Thu, 06 April 2006 15:02 |
|
Wayne Parham
Messages: 18782 Registered: January 2001
|
Illuminati (33rd Degree) |
|
|
Actually, one of Leach's most often cited works is a horn model done with Spice.Regardless, I don't use Spice to model acoustics, only electronics. My Spice models are designed to give an indication of what the crossover is doing in the electrical realm only. It's a circuit modeler. Leach uses a virtual circuit to model acoustic properties but that's not what I'm doing.I use Spice to model the crossover, and that's it. My models do take account the transformation of mechanico-acoustic properties into the electrical circuit, but this is purely done to get a better model of the electrical circuit. It's not intended to model things like acoustic radiation pattern. The Spice models I made are used to give a better view of what's happening in the crossover circuit than simplified resistor-load models provide. But again, it's not an acoustic model nor was it ever intended to be one. If you took the time to look at what I've done before replying on it, you would see that.
|
|
|