Originally Posted by bemoore
Once the signal reaches the driver, and becomes a sound wave, I don't know how it works.
I do. This is how voltages combine in audio/loudspeaker circuits:
If you run a 3v signal at 100Hz this is what you get:
If you run a 3v signal at every frequency this is what you get:
That’s 3 volts at every frequency, not 3+3+3+3 etc.
This relates to SPL in this fashion, first showing a 2.83v signal at 100Hz:
Expand that same 2.83v signal to every frequency and you get this:
The reason you don’t get a uniform SPL across the spectrum is the differing sensitivity of the speaker to each frequency. But the voltage of the input signal would be uniform. If you split that chart at, say, 3kHz, with the left portion being the woofer, the right portion being the tweeter, the voltage required for each driver to reach the given SPL remains 2.83 v, whether the source is one amplifier with the signal divided passively by an internal passive crossover, or two amplifiers, with the signal divided actively prior to being amplified.
As a side note, when you take a FR measurement you’re not measuring decibels, you’re measuring the voltage output of the measurement microphone at each individual frequency point of the sweep. That voltage is extrapolated to arrive at the SPL.
Look at the signals on an oscilloscope.
We almost never look at an oscilloscope in the loudspeaker design field, as it's for the most part irrelevant. Judging by your posts you seem to have a strong EE background. That's a good stepping stone to the more specialized field of Acoustical Engineering, but they're not the same thing, and not everything taught in EE necessarily applies to AE.