Using the graphical interface, specifications were entered for an elliptic filter with passband of 20kHz, 1dB passband ripple, stop-band starting at 21kHz, and a minimum stop-band attenuation of 40dB. Here is some output:

Magnitude Plot

Magnitude Plot (Zoom of Passband)

Pole zero plot

Transfer Function

ratfn:  tx
gainCoefficient:
        413.7118284
order:
        13
numerator roots( p(s) ):
        0 - 15489.4232j
        0 + 15489.4232j
        0 - 10811.32195j
        0 + 10811.32195j
        0 - 9979.235354j
        0 + 9979.235354j
        0 - 9799.235138j
        0 + 9799.235138j
        0 - 9758.835819j
        0 + 9758.835819j
        0 - 9750.117308j
        0 + 9750.117308j
        0.0 - Infinityj
denominator roots( e(s) ):
        -3408.749395 + 0j
        -1963.902163 + 6565.412502j
        -1963.902163 - 6565.412502j
        -580.5189482 + 8970.119641j
        -580.5189482 - 8970.119641j
        -140.5390099 + 9567.776532j
        -140.5390099 - 9567.776532j
        -32.41908411 + 9705.925353j
        -32.41908411 - 9705.925353j
        -7.318245434 + 9737.385075j
        -7.318245434 - 9737.385075j
        -1.302603428 + 9744.196368j
        -1.302603428 - 9744.196368j