what does "Re" impedance actually mean?

[orangecounty1 10+ year member]

I'm curious, ive always thought, and been told, that the Re is the impedance the sub will actually read on a DMM, and thats roughly what the amp will see as well. reason i ask, im looking for some subs for my room, and i need a 1.5 ohm load for my receiver, and these subs say "dual 8 ohm" but the Re says 3 ohms. does that mean 3 ohms per coil, or when u have them in parallel itll be closer to 3 ohms rather than 4, or what?

if i need a 1.5, i could get 4 of these if theyre dual 3 per coil, and 4 of them would make 1.5...

if theyre dual 8, i could get 3, and wire each in parallel, then wire all 3 to each other in parallel as well, and get a 1.3333333333333333333333333333333333333 (lol) load, which would be fine too.

subs: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-490

this is what PV audio told me in a PM.

Re is the DC impedance and is only a good measure of measuring impedance when giving low frequency signals. All that it actually is is the resistance of the voice coil wire, nothing more.
What you should care about more is the Z, or nominal impedance. That is the impedance when your speaker is moving. As it is a large coil with current running through it, a voice coil is merely an inductor. An inductor's impedance is equal to Zl = Rl + j*w*L where Rl is the Re value that you have, and jwL is what's called the reactive element of the impedance. //content.invisioncic.com/y282845/emoticons/cool.gif.3bcaf8f141236c00f8044d07150e34f7.gif

What that is is the fact that when current is flowing through a wire, the wire has what's called an inductance and tries to resist or impede a change in current flowing through it. Think about it like this: if you have a pipe, and you're flowing water through it in one direction, that is your wire with DC current running through it and its impedance is just R. Now, take the same pipe but have feeds on both ends of the pipe that start flowing water in at different times, thus changing the direction of the flow. How difficult it is for the water to change directions in that pipe is the reactive element. Couple that with the DC value and you have your nominal impedance. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

The reason you care about those values is because you need to know the impedance at all frequencies, and as the frequency drops in a voice coil, it nears a short circuit or begins to act just like a piece of wire, aka, the reactive portion goes to zero. All that's left is the baseline resistance of the wire. //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif

When looking for speaker impedance values, you need to always look at the Z value instead because unless you plan on just having your speakers sit there hooked up to a 9V battery, Re is useless for calculating impedance loads while the speakers are playing. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Hope that helps. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

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[Scoobydoo 5,000+ posts]

WTF type of "receiver" do you have that is stable at 1.5ohm?

 

[orangecounty1 10+ year member]

http://esupport.sony.com/US/perl/model-home.pl?mdl=DAVHDX576WF&LOC=3

says stock sub is 1.5 ohm. i DMM'ed mine and its reading 3.7 tho......i did buy it as a floor model, so im assuming that could explain it...it still gets down for a 6.5" but i wanna get a few 12s and ima build a box and put it at the end of my bed. wanna run budget subs though. 50 a sub aint bad IMO, and those subs have great reviews.

285w rms to play with, so about 71 w per sub if i run 4. should get loud //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[Scoobydoo 5,000+ posts]

So you going to hook up 4 subs with a combined final load of 1.5ohm to a "home theater in a box" receiver?

Please video tape the results

 

[orangecounty1 10+ year member]

whats the difference? its a subout put, and the stock sub is 1.5 ohm........

 

[ciaonzo 5,000+ posts]

I'm curious, ive always thought, and been told, that the Re is the impedance the sub will actually read on a DMM, and thats roughly what the amp will see as well. reason i ask, im looking for some subs for my room, and i need a 1.5 ohm load for my receiver, and these subs say "dual 8 ohm" but the Re says 3 ohms. does that mean 3 ohms per coil, or when u have them in parallel itll be closer to 3 ohms rather than 4, or what?
if i need a 1.5, i could get 4 of these if theyre dual 3 per coil, and 4 of them would make 1.5...

if theyre dual 8, i could get 3, and wire each in parallel, then wire all 3 to each other in parallel as well, and get a 1.3333333333333333333333333333333333333 (lol) load, which would be fine too.

subs: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-490

this is what PV audio told me in a PM.

You should focus on learning the distinction between resistance versus impedance, then things should be a bit clearer for you.

If you're looking at a driver that is said to have dual 8 ohm coils and the Re is measured as 3ohms, it just means that they had the coils in parallel when the T/S parameters were extracted. Re is usually less than Znom.

 

[orangecounty1 10+ year member]

so if i buy one of those, and i wire it in parallel, my amp will see 3 ohms, not 4?

 

[ciaonzo 5,000+ posts]

Well, the Re might measure as 3ohms but the amplifier will see something very different under dynamic conditions.

 

[orangecounty1 10+ year member]

roughly what would it be wired at, not calculating box rise and all that other shit.

 

[orangecounty1 10+ year member]

lmao what if i bought 4 of these http://www.sonicelectronix.com/item_9357_Hifonics+HFi+12D4.html

 

[ciaonzo 5,000+ posts]

Yes, it will be 3ohm if that's the Re spec but I'm trying to prompt a discussion about the difference between Re and impedance as it was the question you posed in the title.

Re is the simple DC resistive measure of the coil. The signal you send through that coil is AC so you're introducing reactive elements that change the apparent resistance dynamically, with changing frequency. The resulting profile is the impedance curve.

 

[orangecounty1 10+ year member]

gotcha. so its a little different in a car becuase you arent giving it AC power in a car

 

[ciaonzo 5,000+ posts]

No, the power you're feeding to your drivers from your amplifier is AC. Otherwise the cone wouldn't move back and forth. Doesn't matter if it's in your home or your vehicle.

 

[orangecounty1 10+ year member]

oh. lol.

ok all this aside, which subs u think i should run? im not looking for killer SQ or anything obviously, its for music, and i have a receiver than is ~285w rms @ 1.5 ohm. just looking for alot more bass. want something CHEAP. you ever messed with those dayton 12s? or should i run some cheap budget CA subs instead? like i said this is just in my room, not an HT setup or anything..

 

[ciaonzo 5,000+ posts]

I think I'll let you decide what drivers to pick, I just saw the title to your thread and wanted to help in that area.