Drive level

The voltage Baffle drives the design with. Everything on the SPL plot is the output at this level, measured at 1 meter.

Why 2.83 volts

It is the convention, and it exists because of an old ambiguity. Sensitivity used to be quoted per watt, but a watt depends on impedance, and loudspeakers are not resistors. 2.83 V happens to be exactly 1 watt into 8 ohms (P = V^2/R = 8/8), so for an 8-ohm driver "2.83 V at 1 m" and "1 W at 1 m" agree.

For a 4-ohm driver they do not. The same 2.83 V delivers 2 watts, so a 4-ohm driver looks 3 dB more sensitive on a voltage basis than on a power basis. This is worth knowing when comparing datasheets: a manufacturer quoting "90 dB, 1 W / 1 m" for a 4-ohm driver is making a quieter claim than one quoting "90 dB, 2.83 V / 1 m".

Baffle uses 2.83 V throughout, for every driver, because amplifiers are voltage sources and that is what actually reaches your speaker.

What changing it does

Raising the drive raises the whole SPL curve by exactly 20\log_{10} of the voltage ratio: double the voltage is always 6.02 dB, never more or less. It does not change the shape at all.

What it does change is excursion, which rises in proportion, and the maximum-SPL curves. Turn the drive up and watch the excursion plot: that is where a design stops being theoretical.

Using it to check headroom

Set the drive to the voltage your amplifier actually delivers and look at the excursion plot against the Xmax line. That answers a question no tonal curve can: whether this design survives the level you intend to play it at.

For a rough figure, the voltage at a given power into the driver's nominal impedance is V = \sqrt{P \cdot Z}, so 100 W into 8 ohms is 28.3 V.