You need to look at the db rating of a speaker. A speaker that is rated at 89db (decibles) will take twice as much power as a speaker rated at 92db to sound just as loud. You will find that some 3 watt amps with the right speaker will sound louder than some 100 watt amps with a much less efficient speaker.
ie
85db @ 1 watt This is a fairly inefficient speaker
88db@ 2 watts
91db@ 4 watts
94db@ 8 watts
97db@ 16 watts
100db@ 32 watts
103db@ 64 watts and so on
For every 10db increase, you will get a perceived doubling of volume. For that reason, when comparing a 40 watt amp to a 50 watt amp, you shouldn't be able to hear a volume difference.
In light of the decible ratings above, a 7 watt 300B tube amp running a pair of 102db speakers can put out close to 110db!
Final note: Most solid state amps will double output power if impedance goes from 8 ohms to 4 ohms. Most tube amps will now double the output power. If you are looking at receivers, many home theater receivers are not even able to run 5/7 4ohm speakers. The impedance is too low.
ie
85db @ 1 watt This is a fairly inefficient speaker
88db@ 2 watts
91db@ 4 watts
94db@ 8 watts
97db@ 16 watts
100db@ 32 watts
103db@ 64 watts and so on
For every 10db increase, you will get a perceived doubling of volume. For that reason, when comparing a 40 watt amp to a 50 watt amp, you shouldn't be able to hear a volume difference.
In light of the decible ratings above, a 7 watt 300B tube amp running a pair of 102db speakers can put out close to 110db!
Final note: Most solid state amps will double output power if impedance goes from 8 ohms to 4 ohms. Most tube amps will now double the output power. If you are looking at receivers, many home theater receivers are not even able to run 5/7 4ohm speakers. The impedance is too low.