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Old 14th June 2006, 07:28 AM   #1
diyAudio Member
Join Date: Jun 2006
Default help speakers wrong ohm rating

Hi All
I have a set of wharfdale speakers that say 8 ohm on the name plate. When I use a fluke meter they read 6 ohms and 5.8 ohms.

Can someone tell me what to look for that could cause this.

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Old 14th June 2006, 07:41 AM   #2
sreten is offline sreten  United Kingdom
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Join Date: Nov 2003
Location: Brighton UK

It is normal for an 8ohm driver to measure 6R for DC resisitance.

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Old 14th June 2006, 07:59 AM   #3
Jennice is offline Jennice  Denmark
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Join Date: Jun 2003
Location: Planet Earth

A little more explanation. There are more details to this subject, but the simple (not complete) explanation is:

Your speaker drivers have a magnetic coil inside them. This coil generates part of the magnetism which moves the membrane.

These coils are inductors, which gives them an "inductance", which is a sort of resistance that increases with frequency. Ignoring the influences from the driver mechanics (which influence the measured inductance), your measured resistance Z would be like Z = R_fluke + R_inductance, where R_inductance is K*frequency
(K would consist of a number of factors, but ignore them for now).

When you measure with the fluke meter, you measure at DC, or frequency=0.
When playing music, frequency is higher than 0.
Therefore, your amplifier sees a higher resistance than what you measure with your fluke meter.

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