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-   -   Impedance curve (http://www.diyaudio.com/forums/multi-way/2523-impedance-curve.html)

masterp2 28th February 2002 04:45 PM

Impedance curve
 
Would somebody here provide an explanation of the sealed box impedance curve (what is significant about the spike)? I can see dramatic increase at Fb. What is the significance of this, should it me minimized/maximized? Is it a good idea to design your bandwidth around this spike, or stay away from it?

In other words, if I want a sealed box from 20-50HZ, should the Fb be 35 or 55?

mrfeedback 28th February 2002 05:08 PM

Hiya, you need to connect a series RLC network across the bass driver to null the resonance impedence hump, thereby giving relatively flat impedence/frequency load characteristic to the driving amplifier.
Also need series RC network across the bass driver to null the rising impedence characteristic.
This will give valid load for predicted crossover network, and clean load for amplifier - this is strongly audible.
These parameters are tunable and interactive.
Understand tuning, and tune it how you like it.

Good reading is "The Loudspeaker Design Cookbook"
Seeya.

mrfeedback 28th February 2002 05:21 PM

Hiya, you need to connect a series RLC network across the bass driver to null the resonance impedence hump, thereby giving relatively flat impedence/frequency load characteristic to the driving amplifier.
Also need series RC network across the bass driver to null the rising impedence characteristic.
This will give valid load for predicted crossover network, and clean load for amplifier - this is strongly audible.
These parameters are tunable and interactive.
Understand tuning, and tune it how you like it.

Good reading is "The Loudspeaker Design Cookbook"
Seeya.

masterp2 28th February 2002 05:41 PM

I'm sorry, I didn't understand any of that. A simpler explanation to my original question please, anybody? Help a newby.

Ryder 2nd March 2002 11:59 PM

Impedence
 
Go to the Madisound.com forum and look at a thread started by me (Ryder). Most of the responses are 3/1/02. Very good explanation by smart folk. If you do not get their drift post here again because we have been looking at the same question and I am getting a handle on it. Generally, the Madisound.com forum is the best for speaker advice. Don't you love the engineers who bury you with shi* instead of simply explaining something!

Cheers,

CHRIS8 3rd March 2002 12:30 AM

This impedance peak is perfectly normal, and unavoidable in bass alignments. Their are two individual peaks in a ported alignment btw. Cetainly these peaks are of significant importance, as in how you deal with them will affect final transient decay of the woofer at an around these points. The peaks are a result of stored energy, after all. However, where these peaks occur and how much energy is stored depends on specific bass alignment that you choose. Also note that as you approach resonance, group delay(linear phase angle change) is effectively increased regardless. Though these things may be relatively unimportant to you at the moment, you did ask.

You should not design outside of this phenomena in a bass alignment. Now, resonance of a tweeter or sealed back midrange is usually beneficial to dampen, as it will be to close to the crossover points. This is accomplished with a notch filter, design formulas of which are available on many websites(search engine time). It is possible to dampen a sealed bass alignment too, but this is of no benefit, and would reqire rather massive passive crossover component values.

As a previous poster commented, it is important to flatten the rising impedance curve of the driver in order to more easily design the crossover. This is called a zobel network, and consists of a resitor and capacitor in series, and connected parallel to the woofer circuit. A web search will find the formula for you.

-Chris

kelticwizard 3rd March 2002 12:17 PM

In a previous post, Chris8 wrote: "It is possible to dampen a sealed bass alignment too, but this is of no benefit, and would reqire rather massive passive crossover component values."

I would just like to agree. The only reason to worry about impedance peaks is for crossover purposes. At Fb, you aren't going to cross over, so why worry about it?

At the upper end of the woofer's range, where it crosses over to the midrange/tweeter, you might want to deal with impedance peaks. Not at the lower range, though.

Assuming your frequency response is okay at the range in question, (no dips), an impedance peak means your woofer is working most efficiently, drawing very little power from the amp. Saves strain on your amplifier.

AudioFreak 3rd March 2002 12:29 PM

just one more point ..... any passive x/over etc will shift with temperature because the driver load varies dramatically over it's typical operating temperature range.

masterp2 3rd March 2002 01:34 PM

Thanks all once again.


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