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Old 3rd April 2003, 05:19 PM   #1
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Question zobel and baffle step questions

A couple of questions:

1. Does a zobel have an effect on the frequency response of a driver? If so, what is the effect?
2. Is there a simple formula for calculating baffle step -3db point?
Thanks.
Steve
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Old 3rd April 2003, 06:01 PM   #2
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1. It depends. The raw driver? No. The driver/crossover? Yes.

2. All calculations are only a rough approximation, more misleading than useful. Use a program like BDS (from the FRD Consortium) to get a more accurate model.
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Old 3rd April 2003, 06:18 PM   #3
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Sy,
THanks yet again... I've learned a lot from you lately about speaker stuff!
I was hoping that the zobel wouldn't affect the driver response... I take it that, by affecting the crossover response, you mean that it is changing the impedance the crossover is "looking into" (which is the point of the zobel in the first place, correct?).
Steve
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Old 3rd April 2003, 06:31 PM   #4
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Yes, exactly. The response of a crossover is highly load-dependent (duh!). Change the load, you change the response.

If you don't have one already, I'd highly recommend one of the numerous speaker/crossover CAD programs. You can model stuff like that lickety-split, and you then have the capability to move beyond using that RC circuit as merely a Zobel. I use CALSOD, but it's got an old, clunky user interface. Others will, I'm sure, have better recommendations.
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Old 3rd April 2003, 06:41 PM   #5
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Yes, the Zobel does have an effect on frequency response of the driver - that's why it is used Due to the inductance of a voice coil in larger drivers (i.e. mids and bass), this alters the impedance of the driver. Higher frequencies see a higher impedance than lower frequencies with inductors. "Resistance" (to an electrical engineer) only refers to "true" resistance - which is only half of the picture.

Impedance is broken into two directions - real and imaginary. The real axis is the x-axis and the imaginary is the y-axis. Real impedance represents true resistance - a pure resistor. Imaginary is represented by capacitors (negative imaginary) and inductors (positive imaginary). Generally, negative real resistance isn't referred too... so for simplicity we'll leave it out here.

Now, the high frequencies are "impeded" by inductors, and capacitors are seen as no resistance (or short) by high frequencies. Using design formulas (basic textbook or advanced), one can calculate the values where the impedance of the inductor begins to effect the response of higher frequencies. This is where the Zobel comes in. The capacitor looks like a short circuit in the Zobel network to high frequencies, so they basically see just a resistor (in the Zobel network) as the path to ground. This is normally chosen as the magnitude of the impedance of the woofer (magnitude and angle represent the vector of impedance - real and imaginary axis).

So the high frequencies have the option of taking a high impedance route through the woofer (sometimes up to 30 or 40 ohms of impedance - magnitude) OR they can just go through the low resistance resistor. Because the nature of electrons is to take the simplest path, they go through the Zobel network. This gives the image that the woofer has a flat impedance load for the amplifier.

If the high frequencies went through the woofer (asuming no Zobel), then they would be going through a higher impedance which would cause the magnitude of the signal at high frequencies to be reduced, thus causing lower output of high frequencies through the driver.

The equations for the Zobel are indeed usually oversimplified - experimentation is the best way to go about it, but even using the textbook values will get your crossover a much flatter response. Good luck, and hope this long winded explanation helped -
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Old 3rd April 2003, 07:46 PM   #6
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I guess to rephrase my question, and to clarify:
When designing a crossover and using impedance compensation (zobel) to flatten the impedance the crossover is designed for, does it affect the response (with no crossover- only driver with zobel) of that driver. In other words, do you then have to compensate for anything in your crossover (not counting the flattened impedance curve)
From Sy's explanation, the answer to that is no (and I think this is what Tieftoner was getting at in a longer way).
To rephrase one more time- does the zobel change the voltage sensitivity of the driver? Being that it is a parallel circuit, I would say no. Correct?
Thanks for tolerating my thick-headedness.
Steve
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Old 3rd April 2003, 08:09 PM   #7
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Zobel's will give you a flatter impedance speaker so the crossover will see a more constant impedance and this should be translated to as a more close reactance to the calculated load (speaker) applied on the calculations. More confused? sorry sometimes it's not easy to say it with words.
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Old 3rd April 2003, 08:12 PM   #8
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Sorry, forgot to add that a zobel will mostly affect the higher frecuency impedance on the speaker, and will not affect the impedance at the resonant frequencies.
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Old 3rd April 2003, 08:14 PM   #9
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Sorry - misunderstood you. Now I get it

No, the Zobel won't alter the actual response of the driver. Because the Zobel is in parallel with the woofer, the woofer and the Zobel network will both have the same potential across them... thus, the output response of the driver itself will not change.

In detail: Because the driver will see the same voltage (whether the Zobel is there or not), it will draw the same amount of current through the voice coil as a function of the unchanged impedance of the voice coil. And because this current doesn't change through the coil, the magnetic field that is incurred to move the cone doesn't change, and thus the acoustic output of the driver doesn't change. Wow - hope you can follow that...

Initially, one might think that there will be additional current (power increase) need from the amplifier, because the impedance of the "woofer load" that the crossover sees has changed. There is a lower impedance witht the Zobel, thus drawing more current from the amplifier. Is this true? No. Just wanted to point it out, I guess. The reason the power requirement from the amp won't change is mostly because the crossover will be presenting a high impedance load to the higher frequencies for the woofer network... so, the amplfier really never sees much of the actual woofer impedance - the amp just sees (for the most part) the impedance of the woofer crossover network. The load for high frequencies is represented as the tweeter. Boy, that was kinda confusing - hope I didn't create extra confusion...

nobody special: Hopefully, you'll take my woofer recommendations into consideration from your other post about the P17's... Good luck with this project - and I look forward to seeing/reading the reasults!

Best wishes,
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Old 3rd April 2003, 08:53 PM   #10
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Thanks... I do get it now.
I usually have to talk through these things, and then it becomes clear. As soon as I typed about the zobel being in parallel with the woofer, it all made sense (parallel circuit= voltages the same on both branches= same current through driver with same voltage= same response).
Interesting about the amplifier not needing to put out much more power. I hadn't thought about it, but it makes sense.
Steve
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