Hi Len, You don't have a Butterworth filter, it is a Bessel.
Calculating from the RC values, its response is down -6dB at 7092Hz.
The difference between a Bessel's -6dB response and a -3dB response for a 2nd order filter is 0.6434 so the -3dB frequency is 4563Hz. SwitcherCad is correct. :lol:
It is the filter used in a Linkwitz-Riley crossover network for a speaker system. Since it has an even number of poles, it causes a null at the crossover frequency so one driver is inverted. If a Butterworth filter is used then with a driver inverted the frequency response has a +3dB peak at the crossover frequency.