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does anyone know how to test for impedance per frequency or plot it somehow? i wish to know what the impedance is of a woofers loudest in car frequency so i can plan on what amps to drive it,,or at least what the impedance is for each frequency that the woofer will play hope that made sense Joey
-------------------- Joey Audio Illusions Hickory NC 828-466-1515 Proud Memphis Car Audio Dealer Zero tolerance policy for prefab enclosures =) Posts: 333 | From: nc | Registered: Mar 2003
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Your best bet is to do some searching on "wheatstone bridges" and then come back.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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What I've done is measured the ac voltage from one amp and divide that by the current at each frequency you want to test. With the amps at low power I charted a graph from aprox. 30hz to 80hz, then checked it at full power at a few different locations along the graph; but in my case I first found the amp with the lowest resistance to test.
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thanks guys i thought this might be a feature of termpro or some other software i had heard you could do this with leap or some other program any hints would be appreciated wayne? u there? =) Joey
-------------------- Joey Audio Illusions Hickory NC 828-466-1515 Proud Memphis Car Audio Dealer Zero tolerance policy for prefab enclosures =) Posts: 333 | From: nc | Registered: Mar 2003
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You can do it with nothing more than some test tones, and a DMM. R = V / I. The only feature you really need.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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Alright, if you're truly serious about it and have a few hours to kill, try this setup. First, you'll need: -The sub -a test amp -a cd with tones or any tone generator. -a DMM -a test resistor. 1W 1K does the trick for me.
A few starting notes. Remove the sub from any box, unless you want the in-box impedance. If free-air is what you want, try to hang the sub from a 3 foot wire from the ceiling. The purpose is to avoid reflected waves. Another good setup is a pair of clamps to firmly hold the speaker to a baffle. DO NOT simply leave the sub sitting on a surface while testing.
Now for the good stuff.
**With the speaker clamped, read its DC resistance. 1) Play a sine wave(20-30Hz) at moderate power for a good 10-15 minutes. Once the suspension is nice and loose, we can begin. 2)Test the resistance of the resistor, Rt. It will be say, 1047 ohms. Place it in series with the sub. Chances are you will not hear a single sound from the sub while testing. Which is good, since we are doing a small signal test. 3)Start with the low frequencies, 10 Hz, 12-14...20. Then read every 1 Hz from 20-100. The amount of precision you want is up to you. With a good tone generator, precision to 0.1 Hz is possible. 4)The method for reading impedance is as follow. You have the 1K resistor and the speaker in series. So total power is near nill. Also, the resistor will absorb the majority of the voltage, even if the speaker impedance changes. This constant current is the basis for the measurement. We can adjust the voltage across the resistor by modifying the amp volume. 1V across the test resistor is desireable. If we read 1V across the test resistor and 0.012 V across the speaker, the speaker has an impedance of 12 ohms at that frequency. Each millivolt represents one ohm since the current is constant. 5)For each frequency, f, we measure the speaker voltage, sv, and the resistor voltage, rv. If the resistor has a resistance of Rt, the speaker impedance is I = sv*Rt/rv
6) We can even show how this formula is derived. We know that the same current passes through each component. Since I1=I2, and I = V/R SpeakerV/ speaker impedance = ResistorV / test resistance. Speaker impedance = (voltage across speaker)* (test resistance) / (voltage across resistor). As long as you stick to Volts, and not millivolts, everything works out nicely. The test resistance in our case is 1047 ohms, not 1.047.
Good luck.
Posts: 768 | From: Ottawa, On, Canada | Registered: May 2000
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