Yep, as Rob says you need to dig a little - Google is your friend! If an amplifier is "100W amplifier" you need to know if that is 100W RMS or 100W peak. You then need to know the sensitivity and impedence of the speakers being used. A cut and paste from the other thread:
"We use decibels (db) to measure loudness/volume and changes in this or whatever. 1db is just about perceptible and most people find a 3db rise in sound or so a noticeable increase in volume. This however can vary from ear to ear! 10db is a doubling of volume. What do dbs sound like? When listening to live music an orchestral crecendo/climax reaches around 100-105db whereas a loud rock concert can easily reach 120/130db or more, everyday background noise is about 30-40db. Note that the scale is not linear but logarithmic - curve not straight line.
We measure power in watts (W). The relationship between amplifier power (in watts) and perceived volume is also logarithmic – it takes 10x the power to double the volume ie a 100W amp will be twice as loud as a 10W, 1000 2x as loud as a 100W amp and so on – all other things being equal.
Speaker sensitivity and efficiency next – this is a measure of how well the loudspeaker converts the watts (electrical power) from your amplifier into dbs (the sound you hear). Sensitivity is measured by giving the loudspeaker to be tested a test signal of 1 watt (or 2.83V) and measuring the output in decibels1 metre away. The next thing to affect perceived volume is listening distance from the speaker. This is governed by the inverse square law (remember your school physics!). If you have a light on in your garden at night as you walk away from it the decrease in illumination provided is not linear, again it's logarithmic, the same applies to sound and speakers, when you move from 1m to 2m away from the speaker you need 6db more for the same volume, from 2m to 4m another 6db increase and so on.
Something else to think about is the load the speaker presents to the amplifier – this impedence is expressed in ohms, 4 or 8 being typical for hi-fi speakers (and is why valve amps sometimes have 4 or 8ohm taps on their transformer outputs). The lower impedence (4 ohms) needing more oomph for the same volume level (2.83 Volts into an 8 ohm load is 1 watt. 2.83 Volts into 4 ohms is 2 watts) – hence my Dynaudio kit at 4ohms needs a lot more current up 'em than my Merlins do"
