You may have seen recently this kind of statement about a loudspeaker: "89 dB/SPL at 1 meter for 1 watt." It looks cryptic, but it's trying to tell you something known as a sensitivity ratting. It has been used by many speaker companies and it probably will be written into a forthcoming industry standard on loudspeakers.
What it means is that when the speaker is fed with a test signal of one watt amplifier power, the speaker's output level -- as measured at a distance of one meter in front of the speaker -- comes to a sound pressure level (SPL) of 89 decibels (dB).
With some arithmetic you then can estimate how much louder or softer the speaker might sound when fed with more or less power respectively. Here are the ground rules for your calculation: A 2-to-1 change in power comes to 3 dB, which sounds "somewhat" softer. A 10-to-1 increase in power (plus 10 dB) is said to sound "twice as loud." A tenfold decrease (minus 10 dB) sounds "half as loud."
Start with what they tell you, in this case, one watt from the amplifier drives the speaker to an output level of 89 dB. That means if the speaker is to sound twice as loud (99 dB) it needs 100 watts (assuming it can handle that 100 watts without distortion or damage).
Note that doubling the power will not give you double the sound volume. If the speaker produces 99 dB for 20 watts input (plus 3 dB) which -- compared to 99 dB -- is only "somewhat" louder.
Working down from the one-watt sensitivity level, you can estimate the volume at lower power inputs to the speaker in our example, this speaker will produce 79 dB/SPL for an input of 0.5 watt. It will produce 79 dB/SPL for an input of 0.1 watt.
You can make a fairly complete table of dB/SPL levels that correspond to specific amounts of amplifier power for a given speaker, once you know the basic output level for the one-watt input. The table won't be perfect (it may contain some slight numerical discrepancies) but it will be very close in most cases. Note that the numbers apply actually to the one-meter distance. The reason for this short distance is to minimize room effects in the original rating. Doubtless you listen to speakers from a distance greater than one meter and your room is bound to influence the sound you hear at any distance. In view of these factors, the rating system is obviously less than perfect, but at least it makes for a uniform starting point. It's a lot more meaningful than older vague expressions of "speaker efficiency" and "minimum power needed."
Of course, you still must know the safe upper limit of any speaker's power-handling capability. That information can help you avoid driving a speaker beyond its clean or safe output level. It also tells you how loud you can expect the speaker to sound. Suppose, in our example, the upper limit for the speaker was given as 50 watts. That means the most you can expect to get from it would be 106 dB/SPL. That is, of course, plenty loud -- but if you wanted sound levels of 100 dB or more, you would do better with a different speaker. One possibility would be a speaker of the same sensitivity rating but also with a higher sensitivity way, 92 dB/SPL for the one-watt input). Your table then would show that such a speaker could produce 110 dB/SPL for about 65 watts input. Of course, whether it would sound clean at that enormous volume depends on how good it is generally.