Impedance and Tuning Frequency

[internova]

I did an impedance sweep on the woofer mounted in a ported cabinet. The valley of the impedance chart confirms the tuning frequency is on spec with what I modeled (41Hz).

I then did a near port measurement - putting the mic right in front of the port. The peak of the frequency response is around 50Hz.

So the impedance says my tuning frequency is 41Hz and the measurement suggests it is 50Hz? Or did I interpret the measurement incorrect?

If I use 50Hz as the tuning frequency in the UniBox, I will have a similar F3 but with a 2 dB peak in the bass region. It is no big deal in this application but was wondering how to interpret the data.

[billfitzmaurice]

Re: Impedance and Tuning Frequency

So the impedance says my tuning frequency is 41Hz and the measurement suggests it is 50Hz? Or did I interpret the measurement incorrect?

Chances are the front wave is corrupting the port measurement. Believe the impedance chart.

[internova]

Re: Impedance and Tuning Frequency

Chances are the front wave is corrupting the port measurement. Believe the impedance chart.

Thank you! It's fun to try these experiment.

[rone]

Re: Impedance and Tuning Frequency

do a nearfield on the woofer and look for the dip - more accurate than the impedance.

[killersoundz]

Re: Impedance and Tuning Frequency

do a nearfield on the woofer and look for the dip - more accurate than the impedance.

Do a close frequency response measurement? A dip? I'm not sure there would be a clear 'dip' in the response enough to determine the Fb...

[jmb]

Re: Impedance and Tuning Frequency

The impedance measurement is, in my opinion (and I am not an EE type), the least likely measure to be corrupted, but the nearfield measurement should agree. My measurements have always demonstrated enough of a nearfield dip to assess this, so I agree with Rone that this is a good measure but I would be inclined to use it as a confirmatory measure.

[Bob Brines]

Re: Impedance and Tuning Frequency

The "dip" will be in the DRIVER nearfield FR. It should correspond to the peak in the impedance curve. The port nearfield FR should show a peak at the same place, but it will be broad and may not correspond exactly.

Bob

[billfitzmaurice]

Re: Impedance and Tuning Frequency

You can always do the sugar test. Put the speaker facing up, put sugar on the cone, do a sweep with a tone generator. The sugar will dance above and below Fb but won't at Fb, where cone excursion is maximally damped. This used to be a commonly used method before impedance sweep gear was affordable.

[killersoundz]

Re: Impedance and Tuning Frequency

You can always do the sugar test. Put the speaker facing up, put sugar on the cone, do a sweep with a tone generator. The sugar will dance above and below Fb but won't at Fb, where cone excursion is maximally damped. This used to be a commonly used method before impedance sweep gear was affordable.

Sweet, I'm going to need to try this and the close speaker frequency measurement since I haven't purchased a woofer tester yet

[rone]

Re: Impedance and Tuning Frequency

The "dip" will be in the DRIVER nearfield FR. It should correspond to the peak in the impedance curve. The port nearfield FR should show a peak at the same place, but it will be broad and may not correspond exactly.

Bob

It corresponds with the trough (not a peak) because the peaks are caused by diaphragm velocity, and there is a minimum at tuning. The port fr can have many shapes and is not a good indicator of tuning unless you are interested in secondary resonances. VC inductance can change the shape of the imp curve, as can damping material.

[Bob Brines]

Re: Impedance and Tuning Frequency

It corresponds with the trough (not a peak) because the peaks are caused by diaphragm velocity, and there is a minimum at tuning. The port fr can have many shapes and is not a good indicator of tuning unless you are interested in secondary resonances. VC inductance can change the shape of the imp curve, as can damping material.

Right. I knew that. Too late and too much scotch.

[billfitzmaurice]

Re: Impedance and Tuning Frequency

Right. I knew that. Too late and too much scotch.

There's never too much Scotch, just not enough time.