Strange Speaker Impedance Curve

[e-side]

I just tried to measure the TS parameters of a loudspeaker. Everything was ok, until I had to measure two frequency points (Fl or F1 and Fh or F2, see http://www.epanorama.net/documents/a...arameters.html) at which a certain calculated impedance had to occur. While finding these, I noticed strange impedance values, so I decided to put these into a graph (see attachment).

The impedance curve of the measured loudspeaker, in free air, has two maximum impedance points, in stead of one at the resonance frequency. I'm sure I followed the measurement instructions correctly.

The driver has 2 voice coils, they were wired in series when I measured. I already did some measurements some time ago, before the drivers were put in a bassreflex enclosure with only one coil connected. They had a quite low Qes and Qts values, around 0.25

Could someone tell me what the cause of this strange impedance curve could be?

regards
Ewin

[sreten]

Hi,

Possibly you've wired the coils in anti-phase ?
No idea whats this causes (except poor output)
but I assume measured results would be strange.

/sreten.

[tinitus]

could it be because of "copper short rings", I dont know what its called in english

[KBK]

The shorting ring would not cause that. The best axiom has always been.."the more difficult the problem to solve..the more basic the mistake." (meaning the root source of the issue at hand is buried in fundamental questions, issues, or 'facts')

I would suggest looking at the whole thing, with fresh eyes, from the start, again.

[soongsc]

The impedance level is so high, I do wonder whether they are connected out of phase or not. Perhaps you'd want to show how the imnpedance measurement is hooked up?

[Svante]

Is the driver mounted in a box or is it in free air? If it is mounted in a BR box the impedance will look similar to what you describe, but typically with a deeper dip between the two peaks.

Is there an extra mass load on the cone (like coins or something for measurement purposes)?

Are there any crossover components still connected to the drive?

What voltage do you use for the measurement, is it possible that the driver is overloaded?

[e-side]

Originally posted by sreten
Hi,

Possibly you've wired the coils in anti-phase ?

I checked the connections several times; I'm sure the speaker was connected correctly. Also, I could feel the cone moving when frequencies were applied. I guess it wouldn't move when the coils were wired in anti-phase...

Originally posted by soongsc
The impedance level is so high, I do wonder whether they are connected out of phase or not. Perhaps you'd want to show how the imnpedance measurement is hooked up?

As follows:


Amplifier +

Originally posted by Svante
Is the driver mounted in a box or is it in free air? If it is mounted in a BR box the impedance will look similar to what you describe, but typically with a deeper dip between the two peaks.

Is there an extra mass load on the cone (like coins or something for measurement purposes)?

Are there any crossover components still connected to the drive?

What voltage do you use for the measurement, is it possible that the driver is overloaded?

Originally posted by KBK
The shorting ring would not cause that. The best axiom has always been.."the more difficult the problem to solve..the more basic the mistake." (meaning the root source of the issue at hand is buried in fundamental questions, issues, or 'facts')

I would suggest looking at the whole thing, with fresh eyes, from the start, again.

[e-side]

Originally posted by sreten
Hi,

Possibly you've wired the coils in anti-phase ?

I checked the connections several times; I'm sure the speaker was connected correctly. Also, I could feel the cone moving when frequencies were applied. I guess it wouldn't move when the coils were wired in anti-phase...

Originally posted by soongsc
The impedance level is so high, I do wonder whether they are connected out of phase or not. Perhaps you'd want to show how the imnpedance measurement is hooked up?

As follows:

Amp [+]----- (1K resistor) ----- [+]* (VC1) [- ]------[+] (VC 2) [- ]*------- [- ] Amp

* = connected to multimeter

Originally posted by Svante
Is the driver mounted in a box or is it in free air? If it is mounted in a BR box the impedance will look similar to what you describe, but typically with a deeper dip between the two peaks.

Is there an extra mass load on the cone (like coins or something for measurement purposes)?

Are there any crossover components still connected to the drive?

What voltage do you use for the measurement, is it possible that the driver is overloaded?

- This curve also made me think of a BR enclosure, but the measurement was taken in free-air.

- I didn't use any mass load.

- I only used a 1 KOhm resistor between the amplifier and the driver, no other crossover components

- The voltage across the calibration resistor (which is substituted for the speaker coils in the circuit above) was equal to it's resistance: Rcr = 8 Ohms and Vcs = 8 mV. I used this voltage level (voltage equal to resistance) in previous measurements, and the results were accurate. Nevertheless, would it be better if I used a higher voltage level?

Originally posted by KBK
I would suggest looking at the whole thing, with fresh eyes, from the start, again.

I'll definitely do that.

Thank you all for your replies.

regards
Erwin

[Svante]

So, your system setup sure sounds alright. There is a slight possibility that the level was too low, with 1mV/ohm, but I would not think so.

Another explanation that came to my mind. Was the driver lying on a table or something that could flex a bit? It might be that the mass of the driver (magnet system and all) and the springiness of the table happened to resonance at somewhere in the range 30-80 Hz. That would give you an impedance somewhat like the bass-reflex box, I think.

Yet another thing: did you test the multimeter reading with the calibration resistor with different frequencies? I might be that the multimeter has a funny frequency response.

[soongsc]

Is the driver ferro fluid dampped?