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Old 26th October 2002, 04:44 PM   #1
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Default Having a problem with impedance measurement.

Hello all,

I measured the impedance using Speaker Workshop using two 2" ports in a 1547 cu. in (25.? liter) box in order to tune the ports to the right length. I got about 3.31 to 3.37 inches (also from WinISD). Am I supposed to get a saddle point at around 62Hz which is the box resonance freq? A couple of projects I have seen have two resonance points (two humps in the impedance graph) but mine has the one resonance point at driver resonance and a small hump below the supposed box resonance freq. I do not get the pronounced saddle point for some reason and so I cannot find the box resonance accurately. I would like to know what I am doing wrong if I am.

Another problem is that I reduced the port lengths to 3" and still dont see an appreciable shift to the right box resonance. I am trying to tune it to 62 Hz but cannot go beyond 58 Hz (if what I am seeing is indeed the saddle point which isnt very clear from the graph) even for 3" port length.

I am using the Vifa P13-WH in an MTM config. I hope my question makes sense. I know it is hard to see what I am saying without a graph.

I would appreciate some help from people who can answer this.

thanks.
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Old 26th October 2002, 06:20 PM   #2
Wizard of Kelts
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By "saddle point" I suppose you mean a point of low impedance in between two peaks of high impedance.

Yes, that is exactly what you are supposed to get. It depends on the various setups, (called "alignments" in the Thiele-Small world), but those two impedance peaks do not have to be equal in amplitude, and they can vary in distance from the resonance frequency. But that is what you are supposed to get.

You say you are not getting it? Hmmm. You are not putting stuffing in your ports are you? Are the ports inside free and clear of any obstruction, such as stuffing?

PS: I checked your numbers on WinISD. You are not using the Vifa P13WH-10, are you? The P13WH-8 conforms to the numbers you are giving.

If, in a vented box, you come up with an impedance peak at what you think is resonance, and another peak below that, then there is an excellent chance that your box is tuned to the impedance valley between those peaks, not to the frequency you think it is. Why is anybody's guess.
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Old 26th October 2002, 06:39 PM   #3
Wizard of Kelts
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Just a hunch. The situation you describe would make sense if only one two inch diameter port, 3" long, was functioning. This would tune the box to 45 Hz, and the impedance peaks would be as you say they now are. One of your ports is not blocked shut, is it?
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Old 27th October 2002, 01:42 PM   #4
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Quote:
Originally posted by kelticwizard
Just a hunch. The situation you describe would make sense if only one two inch diameter port, 3" long, was functioning. This would tune the box to 45 Hz, and the impedance peaks would be as you say they now are. One of your ports is not blocked shut, is it?
No, both ports are clear of any obstruction. They are 2" in dia each. Two Vifa P13WH-00-08 5" mid/bass drivers are being used in an MTM config. I will try and include the graph if possible, from SW to clarify. It seems that there are two peaks, one is very pronounced most probably from the driver resonance and the other is very small around 58 Hz or so but I am not sure what it is.

By the way, I am using lightly filled polyfill as a lining material at the back and sides of the box.
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Old 27th October 2002, 01:53 PM   #5
Wizard of Kelts
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One inch of stuffing around the walls and back of the enclosure is considered normal in a ported box. So that is not the problem.

I'll try to think of something, but I have to admit right now I'm stumped.
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Old 29th October 2002, 06:54 AM   #6
Wizard of Kelts
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Old 29th October 2002, 02:00 PM   #7
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kelticwizard,

thanks. I will try this tonight and get back. According to this formula, the resonance frequency will be shifted from 82.75Hz to
93.6 Hz. I will see if I am in the ballpark.
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Old 29th October 2002, 03:56 PM   #8
Wizard of Kelts
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Prash:

The P13 seems to have an Fs of 60 and a Vas of 10 liters. Your box is 25 liters.

Therefore, if the specs are holding, your resonance point in the closed box should be around 71.

Closed boxes that are larger than the Vas of the speaker don't raise the resonance frequency that much.
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Old 30th October 2002, 12:38 PM   #9
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Quote:
Originally posted by kelticwizard
Prash:

The P13 seems to have an Fs of 60 and a Vas of 10 liters. Your box is 25 liters.

Therefore, if the specs are holding, your resonance point in the closed box should be around 71.

Closed boxes that are larger than the Vas of the speaker don't raise the resonance frequency that much.

As expected there was only one resonance pt at about 106 Hz. The orig *measured* resonance in free air was about 83 and Vas was 7.11 liters.

This new measurement is a slightly high number however.
With the ports unobstructed, I get the "dip point" around 59.7Hz for the two 3" ports. Whereas I should get it at 62 Hz for a port length of about 3.25" for the two ports.
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Old 30th October 2002, 01:42 PM   #10
Wizard of Kelts
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Prash:

I think I am getting a little lost here. Let me retrace.

If I follow you correctly, you measured the parameters of the unenclosed Vifa P13 and it came out to:
Fs=83
Vas=7.11 liters.

Okay, we will use those numbers instead of the publiched specs.

Your box is 25 liters, am I right?

With the ports closed, your Fc, (resonance frequency of the speaker in a closed box) should be, according to the formula, 94 Hz. Yet you seem to have expected, and gotten, a number of 106 Hz-which is not that far off, by the way. That is only a sixth of an octave.

Now, back to the ports. You want to tune the box to 62 Hz. Right now, it is at 60, (59.7) Hz.

First. let me just say that 60 Hz is close enough to 62 Hz for me. Basically, you might wish to consider stopping right there.

However, I want to just clear up something. How long are the ports when the box is tuned to 60 Hz? Are they 3" long or so, or did you remove the tubes completely and that 60 Hz tuning is for just two 2" diameter holes in the wall of the cabinet?

If your ports are in the 3" to 3.5" long range, I would say you already have approximately what you were aiming for.
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