I'm doing research on a speaker upgrade (the fun stuff) but I'm getting a headache over speaker impedance ratings. I understand some of the tech about impedance and frequency & loudness but it seems that there are games being played with how speakers are rated vis-à-vis impedance. "Nominal" ratings (wink, wink) seems to be part of this game. Sometimes they will list a minimum but often this minimums seems to belie the nominal (ie a nominal 8 ohm speaker may dip to a 3.2 ohm minimum). Reviews on speakers often use subjective terms like "robust", "quality", or "expensive" to describe amplifier requirements for low ohm speakers. I did see one interview with Andrew Jones where he stated that a speaker minimum should be around 80% of the rating but this guideline certainly doesn't seem to be an industry standard. How do we as consumers deal with this problem ? Specifically, I have a Yamaha Aventage RX-A660 avr this is listed as a 6 or 8 ohm speaker amplifier. Am I stuck with 6 or 8 ohm speakers for the life of the avr or can I drive a 4 ohm center speaker with it in a 5.1 system ?
You have hit on a big problem. Ideally every speaker should have a published impedance curve together with a graph of the phase angle between current and voltage with frequency. So you actually can have an impedance lower then the trace on the impedance curve.
Minimum impedance is a guide if the frequency is stated where this occurs. Usually it is right on the power range of 80 to 600 Hz. If that is so then the rough rule of thumb is that the impedance is minimal impedance plus 10%.
Now the use of subwoofer is totally irrelevant but goes on being repeated. In the sub range this almost always occurs at the twin peaks of impedance in a ported enclosure and the mass loaded TL. It occurs over the single impedance peak for sealed speakers and the traditional non mass loaded TL. In addition there is actually not a lot of power below 80 Hz. People think there is because people want small subs which are inefficient.
So lets get this straight subwoofers offload the bass and bass mid drivers of the other speakers and NOT the receiver.
Unfortunately there are a lot of speakers with adverse load characteristics and you are right the manufacturers usually hide the fact.
I would say that if a speaker has two woofers it is almost certainly 4 ohms or less. If it has more than two woofers don't drive it with a receiver unless you know the phase angles and impedance curve.
I personally think that driving speakers with a receiver unless you play it very quietly is not to e recommended. Good external power amps are a wise investment, unless you are driving bookshelf speakers in a small room. But essentially I regard receiver amps as pretty nasty contraptions, and I don't listen via receiver power amps.
If you are concerned this
woofer tester is strongly recommended. It will test a complete speaker and is very accurate.
Actually if you intend to put together a decent system, and keep it in good order, investment in some sensible test gear should be part of the budget. Members here do not invest in test gear like they should. It would save them a lot of trouble and expense if they did.