Subwoofer impedance question

[Bent]

I'll be ready to test the tuning of my DIY sub - the one I'm calling the Heretic.

I have access to a very cabable frequency generator (a Doble unit with variable frequency output, adjustable current outputor adjustable voltage ), and can also put out huge power while doing any freq.

I understand that with the driver installed and the box sealed, I would apply a frequency of, say 20 hz, at a fixed voltage and measure the current with a true RMS ammeter, plot the current vs. freq, then repeat at 20.5 hz, 21 hz, etc.

The tuning frequency is where the system impedance is the highest which should be indicated by the lowest current reading, am I on the right track?

[ThomasW]

Read the 4th post in this thread.

https://www.htguide.com/forum/showthread.php?t=11959

[Bent]

Excellent, in his case, the resitor is a shunt resistor to allow measuring the voltage drop throuught the cct. which would manifest itself through the increasing impedance of the driver-in-box. very much a simialr variation on what I was thinking.

My Doble power supply will allow such a thing, plus give me a chance to really work the driver too.

[ThomasW]

People don't really need to burn in (= wear out) drivers. Just play them and let nature take it's course

[taz13]

Let me know how the Doble works out, please. :T :T
I wonder if a recorder would be useful?

[Bent]

I have to get ahold of a device that doble calles the "minicontroller", (I know of two..., I trained in one of the locations, and our inst. services dept. has the other).
The doble unit without the minicontroller only allows frequencies output in multiples of 60, but fully variable in increments of 0.1 Hz when it is included, via a rotary dial in real time...and with calibrated accuracy. Too bad the test equipment has to cost 20 times more that the sub, huh?

[Dennis H]

Just double checking that you caught it from the other thread....

A ported box will have two impedance peaks down low with a dip in between. The tuning frequency is the bottom of the dip.

[Bent]

Actually I didn't...

I'll read closer, 'cause I think I may have mis-understood something here.

[Dennis H]

Here's an impedance curve of a Blueprint 1803 in a box tuned to 20 Hz.

[Bent]

I think I get it - I don't know why I never did get a grasp on this.

Lets see if I pass this time - Am I correct that at the tuning freq, the driver will be moving the least and the port will be flowing air the most?
And if so, it would stand to reason that since the cone isn't moving much, that this would be where the least amount of "back EMF" would be generated by the driver's motor structure?
Building on that, the apparent resistance (reactance) of the driver would be at it's lowest due to the least amount of back EMF being generated at this frequency, allowing the current flowing through it to be at it's highest, resulting in the highest reading of voltage across the "test shunt"?

[Dennis H]

The resistor is in series, not a shunt. +wire to resistor to +terminal of the speaker. Voltage is measured between the + and - terminals of the speaker, not across the resistor. Measured voltage will be lowest at the impedance minimum. Reread Thomas's posting in the other thread. It's all there.

[Bent]

Thanks Dennis,
you are saying the same thing I am, but differently.
The shunt I refer to is a "meter shunt" which in my case has a thermally stabilized low insertion resistance (I think 100 micro ohms, but that's not relevent), and a set of terminals in which to connect a voltmeter to. It allows the use of a voltmeter to take acurate current readings by measuring voltage drop across itself proportional to current flow through itself. And yes, the resistor is inserted in series with the speaker.

I set my doble power supply for a constant output voltage (regulated) across a variable freq. range, connected to my driver mounted in the completed box, and measure the voltage "across" the shunt, graphing the voltage across the shunt (in series with the driver) vs. frequency will give me the impedance curve.

much like this diagram shows.