The efficiency of a loudspeaker depends on its size and rated power. A speaker rated at 100 Watts RMS will be less efficient at converting power into sound than a speaker rated at 50 Watts RMS.
So, if you have an amplifier which produces a maximum output power of 30 Watts RMS, will you get higher sound volume from the 50 Watt speaker or the 100 Watt speaker?
You'll get higher volume from the 50 Watt speaker because it is more efficient than the 100 Watt speaker. The 100 Watt speaker is designed to handle 100 Watts and will be of more robust construction than the 50 Watt speaker. It will waste more of the input power as heat.
So, for reliability, it's a good idea to buy a speaker of the same or slightly higher RMS rating than the amplifier needs. It's better to match the speakers and amplifier.
If you have a 100 Watt amplifier, you'll usually get more volume by sharing the power among four 30 Watt speakers than by two 50/60 Watt speakers or one 100 Watt speaker.
However, there's another consideration. If you drive a loudspeaker too close to its rated power level, you'll get a lot more distortion and it may fail sooner. So, choosing a loudspeaker is always a tradeoff between maximum volume, minimum distortion and maximum reliability.
In addition, if you run an amplifier above about 80 percent of its rated output, you'll usually get more distortion and poor reliability. So, for example, an amplifier rated at 100 Watts RMS will probably give best quality sound and reliability into four 30 Watt speakers at 80 percent of maximum output. Pumping up the volume beyond that will inevitably pump up the cost in the long term.
Finally, an amplifier that is turned up so high that it "clips" will put high frequency energy into the speaker, which it may not be able to handle. This energy will heat up the voice coil, causing it to fail. Use your brain and your ears. If it sounds distorted, fuzzy or "scratchy" then you are probably damaging the speakers.
These are not rules "set in stone" but are good for general guidance.
What is loudspeaker impedance?
A loudspeaker usually has an "impedance" of 8 Ohms. If you measure it with a resistance meter, your reading will be lower than this. The reason is that the "impedance" measurement is for a specific frequency or range of frequencies whereas your resistance meter is measuring DC resistance (zero frequency).
The amplifier output impedance should be less than the combined impedance of all loudspeakers connected to it. So, for example, an amplifier with an output impedance of 2 Ohms per channel can happily drive four 8 Ohm loudspeakers connected in parallel to each output channel.
If the amplifier output impedance was four Ohms then you'd have to use a different connection method. For each channel connect two pairs of speakers in series (making 16 Ohms for each pair) then connect these pairs in parallel (making 8 Ohms in total for each set of four). That's confused you, hasn't it! Take a look at these pictures...
