marik,
There are three factors with regard to how loud a speaker plays:
(1) Efficiency -- Usually measured as dB/W (in-room or anechoic), it dictates how much sound pressure the speaker produces at the distance of 1 meter in front of it when driven with 1W of power.
(2) Maximum sound level -- The absolute maximum sound pressure playable by the speaker within a given limit on distortions (for example, 110dB at 1kHz at 2% total distortions).
(3) Maximum power handling -- The maximum amount of power input the speaker can handle without breaking itself (regardless of how loud it plays at that level of power).
An important thing to remember here is that, in the vast majority of cases, (3) is irrelevant because (2) is reached way before (3). For example, the M80 is a very efficient speaker with 95dB/W of rated in-room efficiency. If you do the math under the assumption that there is no limitation (2) above, then with 100W of input, the M80 will play at 115dB; and with 400W, 121dB. But in reality, I do not think the M80 (or any ordinary non-horn-loaded speakers) will go much beyond 115dB or so, perhaps much less if you are talking about a “clean” reproduction (there is no published specs of max sound level available). This is why (3) above is irrelevant, other than it gives you some peace of mind.
kcarlile, let me pick nits.
This is a very common misconception, but in fact, there is no such a thing as “RMS (root-mean-square) power.” RMS is a measure of AC voltage or current: To calculate RMS, you square the instantaneous voltage (or current) first, time-average it over one cycle or more, then take the square root of the average. Since the power (W) has the dimension of [voltage (V) x amperage (A)] with always-positive values, you do not do the above. In order to get “average power,” you simply time-average the power.
