Zp acpr.rho*acpr.c*(R1+i*X1) Piston surface impedance
R1 1-2*besselj(1,2*ka)/(2*ka) Piston function R1
X1 2*K1/(2*ka) Piston function X1
K1 2/pi-besselj(0,2*ka)+0.5*(-5+16/pi)*sin(2*ka)/(ka)+(12-36/pi)*(1-cos(2*ka))/(2*ka)^2 Struve function (approximation)
W_ana intop_pt(pi*a^2*(u0^2)/2*real(Zp)) Radiated power (analytical)
W intop_out(acpr.Iz*acpr.nz+acpr.Ir*acpr.nr) Radiated power