How to measure loudspeaker frequency response and use SPL as vertical axis?

To do this the measurement input chain has to be calibrated. See the calibration chapter on how to do this.

In addition, we have to set up the plotting so that it plots in scope mode. This will explain why: a frequency response of a system is found by measuring the input and the output of the system. The system gain will NOT change if we increase the signal we feed the system with (e.g. if we have a linear amplifier with gain 30 dB, the gain will be 30 dB independent on the gain of the input signal). But if we want to find the SPL of a loudspeaker, the SPL is obviously dependent on the gain of the signal we feed the loudspeaker with. Therefore we have to plot in scope mode, since scope mode does not consider the loudspeaker output signal.

In Plot->Advanced Plot Settings..., make sure is checked.

Then we must set the scope mode to get the correct scaling. In Plot->General Frequency Domain Settings..., make sure the scope mode is set to Transient signal as shown