Sensor/transducer for obtaining signal information

I have been planning to do a rubber characterization system using STM32-F401RE, signal processing and then classification modeling.

The transducer (or) sensor sends the pulse into the rubber scrap and based on its physical properties the signal echoes back. The ToF, signal attenuation, waveform shape, etc.. info are obtained and then using signal algorithm necessary features are extracted and then fed into the classification model which is pre-trained by me for identifying the type of the rubber scrap.

After that it is classified for recycling based on the model's output.

AI said that ultrasonic transducer can be useful for this but i'm new to it and there aren't more resources for it.

Which transducer/sensor i can use to send acoustic signals and extract the info after recieving the echo signal? Please suggest a cheaper component under 25$/ 2000rs.

(Note: This is not my own idea i got this from chatgpt but the method of approach is my own input. Sorry if i have made some mistakes in approach of asking question)