Well, I try to make clear what all this means in English. Perhaps I took one too many shortcuts. I'll try to fix that here. I want you and anyone else to be comfortable with the language used by the manufacturers.
Here's the deal. The efficiency of the NL210 is rated at 100 dB SPL @ 1 watt @ 1 meter. This means that when the amplifier is driving the speaker such that it is supplying one-watt of power (2.83 volts for an 8-ohm cab, 2 volts for a 4-ohm cab), the sound-pressure level measured 1 meter away from the center of the driver will be 100 dB SPL.
The 127 dB SPL spec means the following. When you drive the speaker to its maximum rated power, which in the case of the NL210 is 500 watts, then if you measure 1 meter away from the center of the driver, you will measure 127 dB SPL.
Be aware that these specs usually apply to one specific frequency region, usually around 1 kHz.
Now, for the geek-minded, here's how you get to 127 dB SPL. We know that when the amp supplies one watt, we will get 100 dB SPL out of the speaker. The manufacturer says that the maximum power-handling capacity of the speaker is 500 watts. So, how many dB above one watt is 500 watts? Here's the answer:
10*LOG(500/1)=26.9897 or, for friends, just plain 27.
Okay, if the wattage by being supplied goes up by 27 dB, so must the output (as long as we have not exceeded the speaker's limitations). Add the 27 to the 100 and you get 127.
One more example. Suppose all the manufacturer tells you is the following:
Efficiency: 90 dB SPL @ 1 watt @ 1 meter.
Maximum Power handling: 700 watts
What's the maximum you can get from this speaker?
Suppose I go from 1 watt to the 700-watt maximum. I will be going up 28.5 dB [10*LOG(700/1)=28.45].
I add this to the 90 dB SPL that I get with one watt. (90 +28.5=118.5)
So, at the 700-watt maximum, you can achieve 118.5 dB SPL
Cool?
