# Free Air Frequency

Discussion in 'General Electronics Chat' started by Cretin, Apr 18, 2013.

1. ### Cretin Thread Starter Member

Dec 13, 2012
68
1
Hey all, I'm trying to understand the significance of free air frequency, and doing a test to measure it for a speaker. I found this sample "test" online, but I need a bit of clarification it says:

Hold up or suspend the woofer away from all nearby surfaces. Sweep the frequency on the sine wave geneator down, watching the AC voltage reading across the resistor with the multimeter. Sweep all the way down to zero and back up. At the frequency where you get the lowest voltage reading, you have the highest speaker impedance. Note the frequency on the sine wave generator: that is your free air resonance frequency.

why does it say the lowest voltage represents resonance. I don't understand that whatsoever, and I can't seem to find any links as to the physics/electrical understanding of it.

2. ### #12 Expert

Nov 30, 2010
16,661
7,305
Let's check the logic. At resonance, the speaker will use the least amount of energy to keep it in motion, compared to other frequencies, so less current through a resistor will cause less voltage drop across the resistor.

Yep. Seems right to me.
Even if I'm wrong, you will see an obvious peak, either higher or lower when you find resonance. It's not all that sneaky.

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I believe speaker impedance is maximum at resonance, so the voltage should be maximum at that frequency. A little Googling seems to confirm that.
You apparently got your info from Ehow. I don't trust Ehow anyhow.

EDIT: My apologies to Ehow. See post #6.

Last edited: Apr 18, 2013
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4. ### #12 Expert

Nov 30, 2010
16,661
7,305
This agrees with my assessment.

5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,300
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Everything is explained here:

http://sound.westhost.com/tsp.htm

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
After reading that, I re-read the Ehow instructions. I had missed the part about the series resistor.
The voltage across the resistor will be lowest at resonance.

7. ### JMac3108 Active Member

Aug 16, 2010
349
66
I remember doing this exact same experiement in physics class in college. Once we found the speakers resonant frequency as you described we did a second experiement.

Connect the speaker directly to an oscilloscope (no signal generator or anything else driving the speaker). Tap the speaker to get it vibrate. On the scope you'll see a decaying waveform. If you measure the frequency of that waveform it will be the same resonant frequency you found in the first experiement. This might seem obvious now, but it was amazing and enlightening at the time