I've seen this question asked many times in the past, and twice already today. It's been answered a fair share before too, so this thread is probably useless, but still, I wanted to try to make the concept of how this works, a bit easier to understand for newbies. You've probably heard the analogy before, but that's because it just plain works. No need to reinvent the wheel with new analogies or complex details here, the goal of this thread is just to make things a bit easier to understand.
First let's examine what "ohms" are. Ohms themselves are a unit of measuring electrical resistance. In Alternating Current (AC) circuits, this is actually impedance, not resistance. Although speakers can have a measurable DC resistance (Re) value, we don't feed them Direct Current unless we want to melt them. Fun, but not very productive. Thus, loudspeakers are AC circuits. You have probably read that your speakers are x ohms, and that's why you're looking at this thread to find out if it'll match with your receiver..
For example, let's examine a popular loudspeaker, the dirt-cheap, Polk Monitor 50 which people buy on newegg almost everyday even though we tell them to buy $2000 KEFs, Salks, JTR, and Philharmonics so that they can watch the latest episode of Hell's Kitchen with the highest resolution.
The spec sheet of this speaker has three specifications, which pertain to your receiver's ability to drive it.
Power Rating - 20-150W
Nominal Impedance - 8 ohm
Sensitivity - 89db
So what does this tell us? Well, maybe plenty, maybe not. The truth is, there's a lot of information being hidden from us in the above spec sheet. It might be information that has no bearing on the end result, and it could very well be crucial information. I'll get to it in a bit.
First let's realize one thing. There is, if you think about it, no such thing as a nominal impedance. Think of what the word nominal even means. It basically means "This is what we'll name the impedance of this speaker that we're trying to sell, so that people will have an idea of what to expect". But what if that is, a lie? Impedance will be two things for sure:
Frequency dependant
Temperature dependant
In this thread, I will not really look at the effect of temperature on impedance (although it can be a problem, especially with certain metal-cone speakers). What you need to know, is that, in the audible passband from 20hz to 20khz, impedance is very likely, to be changing. At 20hz, it might be 50 ohms, at 200hz, it might be 8 ohms, and at 10khz, it might be 3 ohms. So what is the nominal impedance of this speaker? Most likely, you would look at the frequency region, where "most" audio content will have power demands situated. Generally, this is the bass, and lower midrange. Maybe from 50hz to 400hz or so. So I guess you could maybe call the above speaker 8 ohm nominal... right? Maybe, maybe not ;P
A better idea, for us to get a better idea of what's going on, is for manufacturers to not use nominal impedances. They are better off, giving us complex impedance - a Z-chart - here is an example of what the impedance of one 8 ohm nominal speaker looks like:
"But it's close to 5 ohms in that critical power region you just mentioned!"
Yep. Another thing you;ll notice in that region, is the graph right underneath - Electrical Phase. It stays pretty close to zero +/-30deg in the power region where the impedance is under 5 ohms which is important. If the phase angle is not crazy, amplifiers can usually "survive" impedance lower than what it might be rated for. At frequencies where the phase angle does start to get a bit difficult, you'll notice that the impedance luckily happens to rise.
So now we've established what the impedance of one "Nominal 8 ohm" loudspeaker, ACTUALLY looks like. What does this have to do with receivers and amplifiers? "My receiver is 6 ohms, not 5 or 8" right?
Wrong. Your receiver, itself, is closer to 0.00002 ohms or something absurdly small like that, unless it's a low feedback tube or 1970s era class D amp. But only silly people called audiophiles use those silly silly things. As for the 6 ohm switch,
here is an article on the matter, for you to read after you finish reading what I'm saying. The only real source of impedance in the circuit your amp sees, is your loadspeaker. What it sees, is exactly the above impedance chart, with reference to the spectrum of frequencies being used in the audio content you are actually listening to.
So how does a loudspeaker convert the watts of your receiver to acoustic energy? Not very efficiently!!! It creates heat, which is what the power rating of your loudspeakers is - a way for manufacturers to tell you it was your fault that you wrecked the speaker. It's also barely accurate. You might be able to blow a 200w speaker, with only a 50w amp at the right frequencies and test content, and a 100w speaker, might survive a 400w amp perfectly healthy with the right content and usage. Use it as a guideline, but don't dwell on this spec as much.
What is a bit more useful in determining loudness? The sensitivity spec might helps. it won't tell you what the mechanical limits of the speaker are, but it tells you how much a specific voltage will drive the speaker to what SPL. Some speakers get way louder, with the same voltage, than other speakers might. The Polks above, are rated at 89db sensitive. One thing to be careful about is, units. The Polk specifications, actually give no units. The easiest mistake made, is to confuse efficiency, with sensitivity. An efficiency spec, is given in units of /1w/m while sensitivity is /v/m, usually /2.83v/m. Into 8 ohms, 1w = 2.83v so it might seem negligible, but into 4 ohms, 2w = 2.83v, and into 6 ohms, 1.3w = 2.83v so you can't get confused.
Now we're going to finally get to that analogy. A loudspeaker circuit, is like hundreds of side-by-side water pipes, and an amp (the section of your receiver interacting with the speaker) is like a water pump and river/reservoir. The water pipes, are not all the same diameter - some are very wide (like a low impedance) and some are very narrow (like a high impedance), and it might not have anything to do with the destination. The other end of the pipes, representing the acoustic response, is like a button designed to be pushed in with enough pressure.
Now if you think about the above, the narrowest pipes will reach the farthest, right? But unfortunately we're stuck with what we're stuck with, the speaker we purchased.
The widest pipes, can carry the most water - the highest flow rate. Think of that as current.
The pumps are putting
pressure on the water to move fast. Think of that as voltage.
So what are watts then? Simple, power (watts) = I(current) x V(voltage) - so the more pressure and the higher the low rate, the more power the speaker is getting.
The only thing you need to add to the above, because it's an AC circuit, is power factor (phase angles as I mentioned earlier) - and it can be significant:
http://en.wikipedia.org/wiki/Power_factor
So it's simple right? Get a high current, high voltage amp, and we're set to go? If only! A high current, high voltage amp is NOT cheap. Cheap amps, are NOT high current, and maybe not even high voltage. Current, in electrical terms, is a source of HEAT, and heat needs heatSINKING which costs $$$. To have current, you also need lots of "water" in the reservoir (reserve power). If there's no water to push, it doesn't matter how much pressure (voltage) you can push it with into the pressure-sensitive buttons (loudspeakers)!!
"my receiver is 100w, so it's high current right"?
Maybe, but probably not. It might say "100w @ 8 ohm". Then when the impedance drops to 4 ohm (IE a frequency where we have a wide pipe), it all of a sudden can't deliver the current and just loses stability. On paper, that 100w amp, should be able to do 200w into 4 ohm, to deliver the same water button push (pressure) into the wide pipe, that it delivered into the narrow pipe. But as soon as you current is limited, your voltage becomes limited, and the SPL of the loudspeaker, is limited by the current reserves. Some 100w amps, might only do 125w into 4 ohms, and thus if you listen loudly enough, it's a bit easier to strain and even shut down that amplifier. Some 80w amps, might do 145w into 4 ohms and so can actually get louder with some speakers!
So what to look for in an amp/speaker combo? tough to say. high sensitivity, high impedance (only to the point where your voltage doesn't become a limiting factor itself, of course) speakers can work well with lesser amps, and high current, high voltage amps can work decently with low sensitivity, low impedance amps(although the low efficiency, means LOTS of heat can be created and there are other drawbacks to that). If you're buying speakers based on how they sound, it's tough to "pick n choose" their electrical behaviour, - although there's certainly some fine High Efficiency Speakers out there. When in doubt, just don't push your system so loud to the point where it's shutting down. Perhaps get an SPL meter, and mess around with an online SPL Calculator (remembering that the speaker's sensitivity should be in units of /2.83v/m and the input watts should be referenced to the max voltage the amp is capable of producing into the speakers' low impedance high output frequencies, converted into watts @ 8 ohm)
If I made any mistakes in the above, feel free to correct me. Hopefully this helped someone.
