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I'm acquainted with the concept of impedance matching in audio applications, but there's something that eludes me. First, a snippet from a home theater site:

Loudspeakers have impedances of 8 ohms, 6 ohms or 4 ohms (those are "nominal" or approximate values, because the impedance of a speaker changes all the time with the different frequencies of music). A 4-ohm speaker draws more electric current through your AV receiver's output transistors, and since more current equals greater power, 4-ohm speakers tend to have greater dynamic range and play louder more easily than 8-ohm speakers.

If a lower impedance loudspeaker generally has better dynamic range and can achieve louder volumes, why do we still see 6, 8 or even 16 ohm speakers? Shouldn't we be "progressing" toward more efficient speakers with lower impedance? Are higher impedance speakers merely a product of backward-compatibility?

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Cost. An amplifier that can deliver current to a 4 ohm load must deliver 4x the current compared to a 16 ohm load. The difference between sourcing 2 amps and 8 amps is substantial from a design and cost perspective. –  Nate Dec 23 '12 at 8:14
    
The statement about 4 Ohms having more dynamic range is misleading. Look closely at the assumptions it makes. You then misinterpret that further to somehow relate that to efficiency, which was never stated and is not the case. –  Olin Lathrop Dec 23 '12 at 14:28
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You use different speakers in different systems and even different serial and parallel configurations; that's one of the reasons there are different impedance values. 8 Ohm speakers are the standard for home, while 4 ohm are usually found in car audio systems.

If a lower impedance loudspeaker generally has better dynamic range and can achieve louder volumes, why do we still see 6, 8 or even 16 ohm speakers? Shouldn't we be "progressing" toward more efficient speakers with lower impedance?

While yes, the benefit of 4 ohm speakers is that the increased current means they have a wider dynamic range, they will be harder on the amplifier (if the amp is made for 8 ohm) and at higher volumes they will have larger THD (total harmonic distortion.) Essentially the output voltage will be unstable during high power application as the Amp will struggle to supply enough current to drive the load.

Using a 4 ohm speaker on a generic home amplifier that is made for an 8 ohm speaker will draw twice as much power and can cause the amplifier to go into protect mode or even overheat and break.

why do we still see 6, 8 or even 16 ohm speakers?

Different serial and parallel configurations are used to change the load on an amplifier. Foe example, you can have two 4 ohm speakers in series so that the load will be 8 ohms. Or (common in custom car systems,) you may have two 4 ohm speakers connected in parallel so the load on the amplifier is only 2 ohms, thus doubling the current.

Or in this case, four 8 ohm speakers are connected in series parallel so that the total impedance is only 8 ohms.

4 8 ohm speakers in series parallel

Are higher impedance speakers merely a product of backward-compatibility?

No they have their place, from allowing different speaker configurations, or less wear and tear on amplifiers to improved sound quality.

Using speaker with higher than minimum impedance may improve quality as the Amp will generate more stable voltage and current. Hence the THD will remain lower at higher impedance while the maximum power output by the Amp will be reduced due to higher load impedance.

Loudspeakers have impedances of 8 ohms, 6 ohms or 4 ohms (those are "nominal" or approximate values, because the impedance of a speaker changes all the time with the different frequencies of music

They are also referred to as at rest values, and if you connect a ohm meter to the speaker it should read 4 or 8 etc. ohms. Then if you gently move the speaker that reading will change. If you measure a speaker and it shows a different value than what it is supposed to then it may be defective of blown or at least slightly damaged.

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Is it accurate at all to presume that a 4 ohm speaker could perform just as loud as an 8 ohm speaker at roughly half the power? –  JYelton Dec 23 '12 at 9:44
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@JYelton no because power vs decibels are not related linearly, meaning that the difference between 50-watt and 100-watts is only about 3 decibels. And an increase of 10 db is perceived to be twice as loud. Think of it as a 4 ohm speaker will draw twice as much power as a 8 ohm speaker, and be louder but not twice as loud. –  Garrett Fogerlie Dec 23 '12 at 10:35
    
+1, nicely explained. –  Anindo Ghosh Dec 23 '12 at 10:35
    
Thanks, I don't think I did that good of a job though, @David Kessner could probably explain it better, it's been a while since I've been in the audio industry. –  Garrett Fogerlie Dec 23 '12 at 10:42
    
@GarrettFogerlie I think that you did a great job answering the question! +1 –  user3624 Dec 23 '12 at 16:37
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