Understanding Impedance of CM5 Loudspeakers

[BrettMendes]

Can I assume from the Audioholics review of the B&W CM8 loudspeakers that my CM5 bookshelf speakers are also a 4 ohm speaker (even though B&W lists them as 8 ohm on their webpage)? How does this affect which amplifier I should be using to drive them? Currently I have them bi-amped with a Marantz SR6006 and am considering switching to a separate processor/amplifier combination or integrated amplifier but am unsure as to whether I will achieve the kind of performance gain that warrants spending all of the additional money.

Thanks,
Brett

 

[fmw]

I haven't read the review but I'm guessing they are probably rated at a nominal 8 ohms but do dip to 4 ohms at some frequencies. That would be quite normal. Not at all unusual. Impedance ratings are guidelines and averages. They don't mean that the speakers have the same impedance all the time or at every frequency. Generally, if the nominal impedance rating is 8 ohms then it is OK to treat them as 8 ohm speakers in terms of amplification. In my experience you probably wouldn't gain anything by the amplification change you suggest but I don't know how big the room is or how loud you play it. The receiver you have is of good quality and powerful enough to handle 99% of home theater applications.

 

[Pyrrho]

Can I assume from the Audioholics review of the B&W CM8 loudspeakers that my CM5 bookshelf speakers are also a 4 ohm speaker (even though B&W lists them as 8 ohm on their webpage)?

No. First of all, the Audioholics review does not involve an actual measurement of the speaker; it is noticing that the CM8 has two woofers that are marked 8 ohms that are wired in parallel. If those markings on the woofers are accurate, that means that the overall impedance of the woofer section is 4 ohms.* When B&W rates them as 8 ohms, they are basically lying; such a speaker should be rated as 4 ohms (assuming that the impedance markings on the woofers are accurate). However, your bookshelf speakers have only one woofer, so this is not an issue with your speakers. But it does mean that you cannot trust B&W to accurately tell you the impedance of your speakers, and should look for a review in which the impedance curve is actually measured. They do, however, mention that the minimum impedance of your speaker is 3.7 ohms. Unless that is at a very high frequency,** it means that the 8 ohm nominal rating on your speaker is basically a lie as well.

To learn more about impedance, this will get you started:

Loudspeaker Sensitivity Specifications & Measurements Explained | Audioholics

How does this affect which amplifier I should be using to drive them? Currently I have them bi-amped with a Marantz SR6006 and am considering switching to a separate processor/amplifier combination or integrated amplifier but am unsure as to whether I will achieve the kind of performance gain that warrants spending all of the additional money.

Thanks,
Brett

You probably won't gain anything from spending your money that way. You are already pretty well at the limits of the power your speakers can handle (assuming that isn't a lie as well). If you are unhappy with the sound, buy different speakers.


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* Also, the midrange and tweeter are marked 4 ohms, so the rest of the frequency spectrum is also 4 ohms (forgetting, for the moment, about the possibility of crossover affects).

** The reason a high frequency would matter far less is that there is far less power used for high frequencies in reproducing music.

 

[fmw]

You can see Stereophile magazines measurements here. Basically they confirm the 8 ohm nominal impedance rating and state that the low point (3.7 ohms) occurs at the top end of the spectrum where you wouldn't encounter it very often if at all. It appears that my assumptions were correct. You can deal with this speaker as an 8 ohm speaker without further worry. In other words, go enjoy your system and stop worrying.