Confusion about measuring resonance frequency...

[jaberwocky6669]

Hi!

I searched these forums for about an hour with many different search terms but I didn't find an answer. So, I feel justified in starting a new thread.

Ever since I read about T/S parameters a few years back I have always wanted to measure them for myself because it seems so cool! I found all of these great resources on how to do just this but I am confused over one main point. Well, actually two maybe.

At this website:

http://www.diysubwoofers.org/measure.htm

It says to determine the resonance frequency of a driver, vary the frequency until the voltage across the resistor reaches a minimum. What exactly is meant by this? Is it zero volts that I need or is it 1.0 volt? Also, I think my multimeter may be too limited! There are only two settings for the VAC -- 750v and 200v. Is it sufficient to use the 200v setting? Do I even use AC at all? Or, do I use DC? How many digits does the interface need? I have three zeroes -- 00.0



Questions, comments, theories, or revelations? Anything would be greatly appreciated! Thank you.

[BWRX]

As you know, the impedance of a loudspeaker's voice coil changes with frequency. By putting a resistor (whose impedance doesn't change with frequency) in series with the voice coil and measuring the voltage across that resistor at various frequencies, you can make an impedance plot for your driver.

When the voltage across the fixed resistor reaches a minimum the voltage across the voice coil will be at a maximum because the two are in series (that's why the rms output voltage of the amp should be constant regardless of frequency), which just so happens to be the driver's resonance frequency.

Just set your meter to 200VAC and you should be fine.

[jaberwocky6669]

I'm still not sure that I understand. What exactly is meant by minimum voltage? Is it zero volts?

What I am doing is testing a speaker for which I already have the manufacturer's specs. I am going to compare them against my own to see if I am doing the procedure correctly. The manufacturer says the Fs is 48 Hz and so I set my freq. gen. to 48 and the multimeter just displays 00.0! It actually reaches 00.0 way before 48Hz, somewhere around 65Hz or so.

[BWRX]

Do you just have the frequency generator connected to the resistor and driver? If you do, adjust its output to 1Vrms. You should be able to measure that with your multimeter and the voltage across both the resistor and driver should be 1Vrms (or whatever you set the output to) at all times regardless of frequency.

What "minimum voltage" means is that you will see the voltage across the resistor vary as you change the frequency. When you find the frequency where the voltage across the resistor reaches its lowest value the impedance of the voice coil will be at its maximum and that corresponds to the driver's resonance frequency.

[jaberwocky6669]

My setup is this:

1. PC based frequency generator
2. External amplifier
3. Multimeter
4. 9.7ohm resistor
5. Driver (Seas L12RCY/P h1207)

Ok, so the voltage across the driver and the resistor should stay the same regardless of frequency. And the voltage across the resistor should change with variations in frequency. Sounds impossible to me.

[Ron E]

Originally posted by jaberwocky6669
Ok, so the voltage across the driver and the resistor should stay the same regardless of frequency. And the voltage across the resistor should change with variations in frequency. Sounds impossible to me.

The impedance of the driver changes with frequency, so the voltage drop across the resistor and driver change with frequency, but they always add up to the same amount.

I prefer a different method of measuring, seen at:
http://www.epanorama.net/documents/a...arameters.html

[jaberwocky6669]

Originally posted by Ron E



The impedance of the driver changes with frequency, so the voltage drop across the resistor and driver change with frequency, but they always add up to the same amount.

I prefer a different method of measuring, seen at:
http://www.epanorama.net/documents/a...arameters.html

Oh man that page is confusing to me too!

[jaberwocky6669]

So far no one has said that having only three zeroes on my display is a bad thing, so I am going to assume that this is a sufficient readout display. Is this correct? Also, am I correct to assume that 01.0 means one volt? How would a millivolt display? WOuld it be: 00.9?

[BWRX]

Attached is an impedance plot of an 8ohm nominal 4" driver in free air and in a small sealed enclosure. You can see the impedance peak is about 58ohms at about 106Hz - this is the driver's resonance frequency. Most other impedance curves will look very similar, including the one for the driver you are trying to measure. You can easily see how the impedance of the voice coil changes with the input frequency.

Your multimeter won't display millivolts so it probably isn't accurate enough to measure with.
The number you see on the display is in volts so 0.1 is 100mV.

[hitsware]

Originally posted by jaberwocky6669
I'm still not sure that I understand. What exactly is meant by minimum voltage? Is it zero volts?

What I am doing is testing a speaker for which I already have the manufacturer's specs. I am going to compare them against my own to see if I am doing the procedure correctly. The manufacturer says the Fs is 48 Hz and so I set my freq. gen. to 48 and the multimeter just displays 00.0! It actually reaches 00.0 way before 48Hz, somewhere around 65Hz or so.

When your meter reaches 00.0 switch scales to 750 mv.
If this still don't work, raise the level of the input signal.
I know I have 2 meters and for some reason 1 of them
(the cheaper?) will measure low level ac fine but the other
won't.........Also try the other method (as mentioned) where
you look for a peak reading rather than a null. (1K ohm in
series with driver and put meter accross driver)...........