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01trublugt 
Copper - Posts: 112
Copper spacespace
Joined: January 08, 2007
Location: United States
Posted: April 02, 2010 at 12:40 AM / IP Logged  
I was looking through some specs on a few speakers. In the specs they show impendance with a value usually 4 or 8 ohms. After that they show Re: xx ohms. For example I was looking at a Morel tweeter, it shows impendance as 8 ohms then Re 5.2 ohms.
What is the Re ohms and when building passive crossovers or figuring out ohm loads in multiple speaker connections do you need to use the Re ohm spec or the normal 4 or 8 ohm they show for the impendance??
Steven Kephart 
Platinum - Posts: 1,737
Platinum spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spaceThis member consistently provides reliable informationspace
Joined: November 06, 2003
Location: Oregon, United States
Posted: April 02, 2010 at 1:19 AM / IP Logged  
Re is the DC resistance of the speaker. If you put a multimeter on it you would read 5.2 ohms as long as the voice coil isn't moving. The impedance is a combination of the resistance, inductance, and capacitance of a speaker in an AC circuit and varies with frequency. The 4 or 8 ohm rating is a nominal rating and is not a measurement. It represents the lowest impedance within the pass band and is usually rounded off at 1,2,4,8, or 16 ohms.
Since the nominal impedance is made up, you will find that speakers with the same nominal rating will have differing Re values. IIRC the most accurate nominal impedance to use would be 1.707 of the Re. When comparing amplifier ratings at a given impedance you can use the nominal impedance as most of the information given is generalized anyway. With crossovers however accuracy is important so you want to know the impedance curve of the speakers being used and design accordingly.
stevdart 
Platinum - Posts: 5,816
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Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: April 02, 2010 at 6:41 AM / IP Logged  

If you are interested in learning passive crossover design, here are a couple of links that will give you a great assortment of tools:

http://www.speakerworkshop.com/

http://www.pvconsultants.com/audio/frdgroup.htm

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

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