Quantity   Component name
1 × Ultrasonic Transducer Available from eBay for under $20.
1 × 100mm of 6061 aluminum rod To make a tuned aluminum horn. About $10 on eBay.
2 × ATX power supply One supply for power, one to scavenge the SMPS transformer
1 × Arduino nano Control board interface
1 × UC3525 Pulse width modulator IC
1 × MCP4161 10K Digital potentiometer
1 × MCP4131 50K Digital potentiometer
1 × AD9833 Sin/Square/Triangle generator, About $6 on eBay
1 × ACS712 Hall effect current sensor
1 × PCB, assorted passives Standard fare for an electronics project
1 × Transducer mount Cutouts from anything you have at hand. Acrylic or wood will work