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Old 11th April 2013, 05:53 PM   #1
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Default understanding Impedance graph.

I had been using impedande and impedance phase graph for tuning the port. My understanding about the graph is very limited. All I know is if you see a third hump then its resonance because of box, and it might be because of leakage.

But while tryng out with WinISD, I noticed that the peak of the two humps are always equal.Is it important or what it says? Also,what more does this graph speaks.
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Old 11th April 2013, 07:47 PM   #2
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If you are literally measuring the impedance curve of the woofers in the box that is the true real world impedance curve. WinISD offers an approximate reading through simulation.

Why are you concerned about the amount humps offered in your bin?
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Old 12th April 2013, 12:26 AM   #3
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Originally Posted by Aucosticraft View Post
I had been using impedande and impedance phase graph for tuning the port. My understanding about the graph is very limited. All I know is if you see a third hump then its resonance because of box, and it might be because of leakage.

But while tryng out with WinISD, I noticed that the peak of the two humps are always equal.Is it important or what it says? Also,what more does this graph speaks.

It's a decent question. The impedance graph can tell you a lot of things about your built speaker.

For example, if the "humps" are in the right position, but the higher one seems to be much lower than predicted, and there are some ripples in the response around it, then this suggests that the box is not properly braced.

I'm sure the other experts here can add their own observations .

The impedance response is usually the FIRST measurement I take of any of my DIY systems.
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Old 12th April 2013, 02:59 AM   #4
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As far as I know, the impedance curve closely reveals the motion of the cone (at least below maybe 1kHz). And that's that. And that is why motional feedback using the voice coil achieves such wonders.

However, the output of a speaker can be more than the motion of the cone. I'm prepared to say it is a mistake to build any speaker where there is much disjoint between the cone motion and the sound.

The worst example of that would be the bass-reflex box with TH taking second-bad place.

Traditional tuning of BR is to have the box resonance poison the exact resonance of the driver-in-that-box. Which, in the traditional theory, leaves a two-humped camel-like impedance curve with a gap in cone motion right at that resonance frequency. (Nobody says you must adhere to the tuning prescribed by the traditional theory, but that's the theory.)

Sometimes you follow models and yet are off in your guess of where the resonance of the box or the driver-in-the-box turns out to be. So much for models. When you have the camel-curve, you have a traditionally tuned BR.

Hope that helps, even if it hurts the feelings of BR and TH enthusiasts.

Ben
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Last edited by bentoronto; 12th April 2013 at 03:07 AM.
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Old 12th April 2013, 03:48 AM   #5
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Originally Posted by bentoronto View Post
As far as I know, the impedance curve closely reveals the motion of the cone (at least below maybe 1kHz).
Not really. Otherwise the impedance curve for a sealed alignment would not have a peak. It would have a curve that increases as frequency decreases.
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Old 12th April 2013, 09:48 AM   #6
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Not really. Otherwise the impedance curve for a sealed alignment would not have a peak. It would have a curve that increases as frequency decreases.
It does have a peak. Right at system resonance. Smaller in size and higher in frequency than in free air.

B.
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Old 12th April 2013, 10:28 AM   #7
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Thanks for the reply,

Well, The box's bad bracing. This should affect the resonance graph of Port as well the driver resonance, because the the port resonance and driver resonance are generated in combination with the box. So can we say, the box resonance shows its effect in the captured graph.

Is there any thing like a "Q" of the port tube? that is reflected on the resonance graph of port? I mean if the port width is varied (of-course the length too needed for tuning to the required frequency) .
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Old 12th April 2013, 11:17 AM   #8
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I built kls3 with adjustable port length.
I tuned it so that the double hump of the relex ended up with the same impedance/height.

It did not sound right.
I shortened the ports slightly to make the lower frequency impedance lower than the higher frequency impedance.

It now sounded "more right".
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Old 12th April 2013, 11:29 AM   #9
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Originally Posted by bentoronto View Post
It does have a peak. Right at system resonance. Smaller in size and higher in frequency than in free air.

B.
Model a driver in a sealed alignment Qb=0.5
Have a look at the driver excursion
Have a look at the impedance response

One has a peak. The other does not.
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Old 12th April 2013, 11:58 AM   #10
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Originally Posted by Brian Steele View Post
Model a driver in a sealed alignment Qb=0.5
Have a look at the driver excursion
Have a look at the impedance response

One has a peak. The other does not.
Ummm, you gotta fix your model, eh.

B.
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