Ask A Scientist© Engineering Archive

Index Key:   ENG019
Author:      quillen
Subject:     Speaker impedance
Text:        How does one measure the impedance of speakers?  I want to attach 
a set of them to my existing system, but want to be sure that the impedance 
matches.  Also, is there any way to adjust the impedance on speakers to match 
the required impedance on the stereo?  Ex. like adding a resister.

Response #:  1 of 1
Author:      dipper
Text:        This is not a clear cut answer.  This will give you the short 
answer, and if you are interested (and even if you are not) I will write in 
later the more complex answers.  First of all, the impedance will not match --
- and its not that important.  The NOMINAL impedance of a speaker is just that 
-- nominal.  It will change in each room setting, in each enclosure, even with 
the relative humidity.  Unless you have an old tube-type amplifier, and the 
voice coils on your new speakers have one turn (very unlikely), then you can 
generally not worry about it.  However, to be safe, measure the DC resistance 
across the terminals.  If it is greater than about 4 Ohms, you should not have 
a problem with even the most touchy amplifier or receiver.  Impedance of a 
speaker will change with frequency.  The better the speaker (generally) the 
less the nominal impedance will change in any given situation.  In some 
experiments way back in high school (and I do mean way back), I could get the 
impedance of a speaker to change by a factor of 2 by simply positioning it in


different spots in the room.  Actually, I found that the system changed 
response most dramatically by a change in the speaker wire.  For that reason, 
I spend almost as much on good wire as I do for most of the other components 
(outside of amplifiers and speakers).  Changing the impedance of a speaker (or 
more accurately its DC resistance) opens up a whole other can of worms.  There 
are devices produced and marketed at most high-end stereo stores to do just 
that with a minimum of impact on the overall performance of the speaker.  
While you can design and build your own, the math behind it is not pleasant, 
and you will have to know an awful lot about the speakers.