That is covered in the documentation of the project you linked to.
"If using HC-SR04 sensor transducers, be sure to use the emission transducers with a T marked on the back."
Only one will have a T, because it's the transmitter...
That is covered in the documentation of the project you linked to.
"If using HC-SR04 sensor transducers, be sure to use the emission transducers with a T marked on the back."
Only one will have a T, because it's the transmitter...