KennysFord Posted July 3, 2008 Share Posted July 3, 2008 hi folks. i have a couple of mystery speakers at home.i want to see what Ohmage they are,can you test it with a multi meter. thanks in advance. Quote Link to comment Share on other sites More sharing options...
bnt Posted July 3, 2008 Share Posted July 3, 2008 [quote name='kennyrodg' post='231769' date='Jul 3 2008, 08:45 AM']hi folks. i have a couple of mystery speakers at home.i want to see what Ohmage they are,can you test it with a multi meter. thanks in advance. [/quote] You can get a reading of the DC resistance, but the actual impedance will be a bit higher. I remember trying this, and seeing a 6Ω DC resistance on a speaker rated at 8Ω impedance. (Strictly speaking, impedance is the resistance to sIgnals of a specific frequency e.g. 8Ω @ 40Hz.) Quote Link to comment Share on other sites More sharing options...
KennysFord Posted July 3, 2008 Author Share Posted July 3, 2008 thanks for the reply,i'll give it a shot. Quote Link to comment Share on other sites More sharing options...
BOD2 Posted July 3, 2008 Share Posted July 3, 2008 I'm not sure this will help. The DC ohmeter won't give you the rated value (e.g. 8 ohms or 4 ohms) so you'll get some other value which means you still have to guess what the speaker's impedance actually is ! On the other hand, it won't do any harm and if you have a known speaker you can also check you might be able to work it out by comparison with the known speaker. Quote Link to comment Share on other sites More sharing options...
KennysFord Posted July 3, 2008 Author Share Posted July 3, 2008 thanks for that.I have an ashdown speaker rated at 16 ohm,the multi meter showed it at 12.the one in particular that i wanted to know about came out at 4.2,i think i'd better put it back away in the shed,hehe. thanks guys. Quote Link to comment Share on other sites More sharing options...
bnt Posted July 3, 2008 Share Posted July 3, 2008 Maybe this will help: the Impedance [b]Z[/b] is the vector sum of the Resistance [b]R[/b] (which you measured) and the frequency-dependant [url="http://en.wikipedia.org/wiki/Reactance"]Reactance[/url] [b]X[/b] (which you don't know). Z² = R² + X² , so you can safely say that the Impedance will never be [i]lower[/i] than the Resistance, only [i]higher[/i], so that's probably not a 4Ω speaker. If you'd like to see some more of the Maths on the subject, try [url="http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/impedance.htm"]this[/url]. The rated impedance figure on a speaker is only a guide, because the Reactance changes with the frequency. Exactly how much is down to all kinds of factors, even the cabinet design, so you're not going to get a simple figure like the one you see in the adverts, such as 8Ω. You often see speaker makers measuring the actual Impedance over a frequency range, and providing a chart like this: Quote Link to comment Share on other sites More sharing options...
alexclaber Posted July 3, 2008 Share Posted July 3, 2008 At least the DC resistance cannot be higher than the nominal impedance and is rarely less than half the nominal impedance. That plot shown above is quite useful - shows why obessing over 4 vs 8 ohm nominal impedance to get max power is so futile, as the region where you need the power is where the impedance has the big peaks. Alex Quote Link to comment Share on other sites More sharing options...
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