Speaker impedance

Discussion in 'General Electronics Chat' started by joshwhitesub, Jan 19, 2010.

1. joshwhitesub Thread Starter New Member

Jan 19, 2010
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0
Can anyone help me understand, ohms in speakers I think its about resistance but if it is than a whats the difference between a 4 ohms sub and a 2 ohm sub, more sound, does it require more power to get the same sound out of the 4 ohm and if thats the case why would you get a low ohm subwoofer?

2. Audioguru Expert

Dec 20, 2007
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1,216
Impedance is resistance to an AC signal.
A 2 ohm speaker can use double the current of a 4 ohm speaker. Double the current is double the power. The amplifier must be able to provide double the current.

3. Wendy Moderator

Mar 24, 2008
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BTW, this usually translates into higher voltages. One goes with the other.

4. Audioguru Expert

Dec 20, 2007
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If the amplifier doubles the voltage to a speaker then the current also doubles so the power in the speaker is 4 times as much.

5. Mike33 AAC Fanatic!

Feb 4, 2005
349
25
Speaker ohms are a measure of impedance. Impedance equals the DC resistance PLUS the AC resistance of a component. So a spkr. may measure 6 ohms with a DMM, but be rated at 8 ohms, etc.

A 4 ohm speaker will draw 2x the power of an 8 ohm, as stated above. Which is why it is often 'dangerous' (to the speaker/output network) to try to drive a lower impedance speaker from a designated tap...i.e., the amp wants to 'see' an 8 ohm impedance, but you use 4 ohms; you are asking the amp for 2x the current...can damage your amplifier.

6. dsp_redux Active Member

Apr 11, 2009
182
5
The impedance of a component is expressed as: $X=R+jI$ where X is the impedance, R is the real component of the impedance (resistive) and I is the imaginary/complex component. By convention, if I is positive, you have an inductive component, if I is negative, you have a capacitve component.

7. KL7AJ AAC Fanatic!

Nov 4, 2008
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415
All other things being equal (not always a good assumption) a lower impedance speaker will have a "flatter" impedance curve over a wider frequency range.... but at a sacrifice of efficiency.

Fewer turns of the voice coil winding gives you less reactance (a good thing), but fewer ampere turns...hence less force for a given current (a less good thing).

Speaker impedance is actually a rather complex thing, including both electrical and mechanical resonances which are TRANSLATED into additional electrical "virtual" components. Some of the best information on this is the ancient tome, "The RCA Radiotron designer's Handbook".

Beg, borrow, or steal this book.

Eric

8. Audioguru Expert

Dec 20, 2007
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Many speakers are made as 4 ohms and as 8 ohms. Their efficiencies and impedance curves are exactly the same.

A simple car radio operates from the 13.8V battery so an 8 ohm speaker gets 1.8W, a 4 ohm speaker gets 3.4W and a 2 ohm speaker gets 6.4W.
The amplifiers are bridged to effectively double the output swing so an 8 ohm speaker gets 6.8W, a 4 ohm speaker gets 12.8W and a 2 ohm speaker blows up the amplifier or gets about 20W.