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The sensitivity (SPL rating) of a particular driver is a measure of its efficiency. The higher the number, the less power will be required to drive it to a particular volume level. This is not directly related to power handling or xmax. All of the parameters of the driver come into play. A driver rated at 85 dB 1W/1m means that it will produce a measured volume level of 85dB with 1 watt of power input at 1 meter distance. The measured volume level will increase with power input and decrease with distance from the driver.

From my experience, Xmax is the limiting factor for playing any driver at low frequencies. At the low end of the frequency range, most drivers will be Xmax limited, not power limited. A well designed ported sub can mitigate that problem quite a bit.

Power input is directly related to volume level. Double the power, double the volume for a given speaker. Each doubling of power creates a 3 dB raise in volume (measured SPL, not the driver rating). However, a driver that can handle more power can only provide higher volume levels if it doesn't have a low SPL rating or limited Xmax.

Here' how it all ties together. All things being equal, a driver with a 3dB higher SPL rating will play twice as loud for a given input power. Again, all things being equal, would you rather pay for 1000W amp and an 85dB1W/m driver or a 500W amp and an 88dB1W/1m driver. Furthermore, that 85dB1W/1m driver will have to handle higher excursions to produce that same volume level and therefore reaching its Xmax limit at low frequencies before the 88dB1W/1m driver. So, a low SPL rating means you need a higher Xmax and more powerful amps to produce peak volume levels.

None of these ratings are more important that the others, you just need to understand how they all interact to choose an adequate driver for your purpose.
 

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This is not a true statement. A 94db rated driver and an 87db rated driver with the same SD in the same sealed box both at 10mm excursion will produce the same SPL. The power required to make each driver produce that excursion will be different.
I did some modelling of different drivers, but entered the same Sd and he's right. I retract the statement and get skoold.
 
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