You may use one tweeter as transmitter and receiver.
1 – Find if the tweeter has a resonance frequency. If you find one use that frequency to transmit. If you don’t find it use something like 20kHz.
2 – Transmit a square pulse with half period of the frequency used.
3 – The signal received will be a low level underdamped sinusoidal wave so you will have to amplify that AC signal and rectify it until a reasonable level to trigger next circuit; a Schmitt trigger. The necessary gain will be a function of your transmitted power, the distance, the reflective index of your target, its area and the pressure to voltage conversion of your tweeter. Your receiver may be a tunable one to increase signal to noise relation.
It may be necessary to learn some theory about ultrasound and sonar.
MOR_AL