Vented Impedance

[richie00boy]

I am curious if anybody has found that the vent tuning frequency was not the point of minimum impedance. Has anybody ever found that the minimum point occured e.g. just after the second peak (i.e. at it's base)?

I always though that the tuning frequency was the impedance minima, but I am currently debating with somebody who claims differently.

[pinkmouse]

It all depends on what you're tuning for. Like most things in audio, it's a variable...

[simon5]

Well, if you look in WinISD Pro, the minimum impedance is higher in frequency than the second peak. I didn't look if it's always true, but I've never seen the contrary.

The tuning frequency is a "local" minimum, but not the true minimum.

[Ron E]

Play around with the Ql setting, you will find that higher Ql's have a deeper minimum and higher peaks. Box absorption affects the upper peak and Port resistance affects the lower peak.

More interesting than whether or not the actual minimum is at the port tuning frequency or the minimum above resonance is whether or not the port tuning frequency is actually at the "tuning" minimum or slightly shifted.

IMO the lowest minimum in impedance is at DC. That's the one you can win a silly debate with.

[richie00boy]

Yes the measured minimum impedance and the true tuning frequency is what has sparked off the debate -- I was aware of the possibility of phase shift affecting the reading.

Thanks everybody for your answers. It does seem as though it is possible for minimum impedance to happen above vent tuning, after the second peak.

[filgor]

I'm thinking the short answer is No, Z min does not reliably occur at fB

In an ideal world the tuning frequency is the minima between the two peaks but other varying impedances which add to form the total impedance will swing the measured minima away from fB.

Impedance minima and maxima occur when the phase difference between reactive elements cross phase at 0 or 180 degrees. When you factor in the contribution of other frequency dependant factors such as frequency dependant resistance and reactance from the voice coil, they add a slight slope to these peaks and dips which moves the apex slightly higher or lower. Port resistance will shift it lower as will increasing resistance (at least I think) which occurs through transformer like coupling between the voice coil and the pole piece.

I read about it somewhere.. Might have to consult the library!

[filgor]

As an after note; Ron E is dead-set right!

Theoretically the impedance below the low frequency peak trails off at unimaginably low frequencies until it reaches the absolute minimum of the dc resistance.

P.S. Does winISD pro account for vc inductance and Freq. dependant resistance?

[simon5]

Yes you can use the Le parameter in WinISD Pro.

Some are saying the model used is not accurate enough on the other hand, so when entering Le you should take the results with a grain of salt.

For myself, I think the model is accurate enough, and I didn't have any problems with it.