about definition trigger Audio frequency

Using Arduino Audio
#1

i want 20kHz is the trigger Audio frequency(my sensors is LM567),than my sensors receive 20kHz sound, sensors will send Signal .
what did i write in arduino to definition 20kHz is trigger Audio frequency?

#2

Goertzel's algorithm?

#3

i'm not sure ：'（
just tell me your idea,thanks

#4

My idea

#5