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RCAM641 : Can someone explain the implications of 4ohm vs 8ohm impedence on speakers? My reciever allows me to set this, and states that for surround sound, all the speakers should have the same impedence.
Does impedence have a relationship to speaker sensitivity, for example, the lower the impedance the higher the sensitivity (and resulting loudness)?
What would be the result of hooking up, say 4 ohm mains, and 8 ohm rears?
Thanks.
Since I'm not familiar with your receiver, I can't answer your questions about the switch settings. Better to ask the manufacturer, and follow their advice. What I can do is clarify the impedance issue a little.
The nominal impedance of a loudspeaker or driver is the "typical" impedance throughout most of its operating range. As lord helmet pointed out, the actual impedance varies with frequency, and can be substantially higher, or lower, than the nominal impedance at certain frequencies.
Impedance is specified in ohms, and resists (impedes) current flow. The impedance to current flow (impedance) can either be frequency dependent, or frequency independent. If it is frequency independent, it is called resistance. If it is frequency dependent, it is called reactance. Further, there are two kinds of reactance, one called inductance, and the other called capacitance. A loudspeakers nominal impedance specification includes both (the sum of) the DC resistance, and the nominal impedance.
At a given frequency (recall that impedance is frequency dependent), the current flowing in a circuit can be calculated using the equation:
I = E / Z - or Current equals Voltage divided by Impedance to current flow. Think of the voltage (E) as pushing against some resisting force (Z), and producing a certain amount of movement or flow (I). If you push harder (more voltage), or you reduce the resistance to the flow (lower Z), then you increase the amount of movement or flow (I). It is a simple, linear relationship (but only at a single frequency, because Z is frequency dependent).
The amount of power is a function of how much current is flowing, and the applied voltage. Think of it this way. If you push harder (more voltage) you have more movement of the same amount of whatever you're pushing (current). The relationship of power to the push (voltage) and flow (current) in an electrical circuit has the relationship:
P = I * E - Or power (work done) equals current (flow) times voltage (push).
The more you push, and the harder you push it, the more work you get done.
An audio amplifier does work (ultimately generating sound) by swinging a voltage across an impedance (pushing current across an impedance) in a "pattern" that is determined by its input, and its transfer function. The transfer function defines the changes to the signal as it passes through the amp, and for the moment, we will consider the amplifier to have a linear transfer function, meaning that it's only going to amplify the signal, and will not distort it in any way. The total power the amplifier can deliver depends on the voltage swing and the amount of current it can deliver. Since P = I * E, when you increase I or E, P will also increase.
From this we can see that the maximum power available will be limited either by the voltage (E) or the current (I) which the amplifier can swing (voltage) or deliver (current).
With a given amplifier, the maximum voltage swing is determined by the power supply design, and the maximum current available depends on a number of factors, including the current carrying capacity of the output devices. ASSUMING that the amp has enough current available, a lower impedance load will allow more current to flow, and more current means more power. Deriving the relationship between Z (impedance), P (power), and V (voltage) requires the use of both the equations given earlier. To calculate the power delivered to the speaker at a given voltage we have three knows, the impedance, and the voltage. From this, we must calculate the current, using I = E / Z. If the amplifier can swing 24 volts, and the impedance is 8-ohms, then I = E/Z = 24/8 = 3. Now, given that I = 3, we can calculate the power, P = I * E = 3 * 24 = 72 watts. To make life easier, we can combine the two equations into one, that will compute power directly, given voltage and impedance.
P = I * E
Since I = E/Z, we can replace I with E/Z in the above equation, giving us...
P = (E/Z) * E.
Now let's quickly calculate the power that 24 volts will push through an impedance of both 8 and 4 ohms.
E = 24.
If Z = 8 then (E/Z) * E = (24/8) * 24 = 72 watts (same as we got before).
If Z = 4 then (E/Z) * E = (24/4) * 24 = 144 watts (which is twice the power delivered through the 8-ohm impedance.
Note that we got twice as much power, because the same voltage could move twice as much current, because the impedance to that movement was cut in half. If you had a 2-ohm speaker at 24 volts you'd double the power again, to 288 watts, and if the speaker had only one ohm at 24 volts we'd have 576 watts. There is a problem though. With only 1 ohm of impedance, the current flow (I = E/Z) is now 24/1, or a full 24 amps. That's a lot of current, and would require output devices and a power supply that could deliver the current required without damage.
The main thing you have to worry about (and I wouldn't worry about it too much) is that a speaker with a very low impedance can cause an amplifier problems, because the amp may not be able to deliver the current required. You get more power out with lower impedance, but if you go too low, you can damage the amp. This is almost never a problem with modern amps and loudspeakers (though it can be, depending on the way the gear is designed). Most amps will shut down, or distort noticeably, or blow a fuse, before they'll die from being asked to deliver too much current (but some amps will hurt themselves trying to deliver the current). You will know if the impedance of the speakers is too low, because you'll either hear a problem, or you'll damage your amp. Any modern amp should handle 4-ohms (or less) without any problem. Low powered amps will always be clipping a little, but they won't eat their own lunch.
The issues can get complex, so when it doubt, it is always a good bet that the manufacturer can provide good advice. If you follow their advice, and the amp smokes, I'd expect them to fix it under warranty.
The things lord helmet pointed out are way too complex to worry about, unless you're designing loudspeakers. At resonance, the impedance of a driver increases, so the power from the amplifier at that frequency decreases, and you might think that the decrease in power flowing through the speaker would result in less output. However, just the opposite is true. Even though less power flows at the resonant frequency, the output is higher. At higher frequencies, the output of a driver may fall off as the impedance rises. Better to let the designers worry about this. They will give their speakers a nominal impedance rating that will be conservative enough to keep you from blowing up too many amplifiers, and that's all you should be concerned about.
Bottom line: It’s probably not critical at all, but if you are concerned, you should ask the manufacturer of your receiver.
Hope this helps,
Chuck</font>
