rem56 Posted January 1, 2007 Share Posted January 1, 2007 when i deflect the paper cone the resistance changes.help me under stand.thanks Quote Link to comment Share on other sites More sharing options...
JohnA Posted January 1, 2007 Share Posted January 1, 2007 The voice coil resistance doesn't change. Electricmotors produce work by passing an electric current through a magneticfield. Generators make current by moving a coil through amagnetic field. Some motors can function as generators. A volt meter applies a small voltage (and thus a current) to the voice coil to measure the resistance. Movingthe cone causes the voice coil and magnet (the "motor") to generate avoltage and current, called Back EMF (electromotive force). Thechange in voltage causes an error in the reading of your voltmeter. The meter senses a different voltage "drop" across thevoice coil telling it, erroneously, the resistance changed. Depending on the direction of movement the voltage change could add orsubtract resistance. Quote Link to comment Share on other sites More sharing options...
voxr3m Posted January 4, 2007 Share Posted January 4, 2007 If you use an ohm meter to measure the resistance across the speaker's terminals (not the speaker as a whole, just the driver/woofer) you're measuring the DC resistance of the voice coil. Since the voice coil is just a wire, you're essentially measuring the resistance of a wire. But this DC ohm reading isn't what people refer to when they talk about speaker impedance. Impedance is the AC 'resistance' of the voice coil and varies with frequency. While a speaker might be rated 8 ohms, it's actual value may only be 8 ohms at a select few frequencies. At others, it may go below or above that rating. You'll need more than a simple multimeter to measure the impedance as a function of frequency if that's what you're looking for. Quote Link to comment Share on other sites More sharing options...
pauln Posted January 4, 2007 Share Posted January 4, 2007 Should probably mention that to measure a speaker it needs to be disconnected from the network... can't just put leads to the speaker while it's still connected. Quote Link to comment Share on other sites More sharing options...
WMcD Posted January 4, 2007 Share Posted January 4, 2007 Just to add a little to the good comments above. You haven't quite told us for what you're trying to test. "Testing" for you might be different than measuring for the rest of us. At least some of the answers addressed the difficulties of measuring. It seems that you have disconnected the speaker box from the amp and then placed an ohm meter across the input terminals. If you don't move the woofer, the ohm meter is measuring a certain d.c. resistance. There are a lot of components inside the speaker box, specifically the crossover filters and the various drivers. However, it is only the woofer circuit which has a path for direct current (or zero Hertz of frequency) as applied by the ohm meter. Therefore, you are measuring the d.c. resistance of the woofer (somewhere between 3 and 16 ohms) and the d.c. resistance of the inductors (coils of wire wound around a metal core) of the low pass filter which are in series with the woofer. The inductors typically have resistances of less than 0.5 ohms. - - - John A. is of course correct. Assuming you two are talking about the same thing. But let me tell you what I thought when I first read your post. It was that you were testing (maybe) a cranky or partially burnt out woofer. That would show up as a gritty - binding feeling as you push on the woofer diaphragm and no bass. If the remains of the voice coil were getting ground around in the magnetic gap, you'd see variation in resistance. I'm a little prejudiced to think about that because I recently did a post mortem on two woofers which had failed. For the record and the interst of others here: This required cutting apart the diaphagm and spider. The remains of the voice coil were a "slinky toy" in the gap. One slinky showed some charing of insulation on the voice coil insulation) but the other did not. It was interesting to me that the slinky voice coils in each case had totally lost adhesion to the former and there was no evidence of any adhesive. These were SpeakerLab 1's which used a sealed box and 8 inch woofers and a dome tweeter. The owners deny any abusive power levels and I have great trust in their verasity. (But what about the charing? Maybe someone cranked them while the owners were away.) I don't mean to bad mouth SpeakerLab at all. They probably bought from a supplier and could not check adhesive, etc. Best, Gil Quote Link to comment Share on other sites More sharing options...
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