In the previous article, Model Validation, I specified the expression to be inserted in the LTSpice graphic post-processor to obtain the frequency response at one meter, which I report below:
SPL=20*log10[59000*frequency*I(VSYS)]
I emphasize that the display of the vertical axis must be set on a linear scale. Although the expression can be saved as a setting (Save Plot Settings), during the design it is necessary to graphically display also other quantities, often in full screen, and to return to the SPL display it is necessary to re-transcribe all the text. It might also be useful to show the response at different distances. Using the properties of the inductors, I have developed three test points that allow you to quickly view, normalized to the desired distance, the response of the woofer, of the opening and the complete system.
The sound pressure of the system at the distance r is given by: