We hear an echo due to

An echo is a distinct sound heard when a sound wave is reflected back to the listener after hitting a surface [2]. This phenomenon occurs when sound waves encounter a rigid or smooth obstacle and bounce back into the original medium [1]. For an echo to be perceived as a separate sound, the reflecting surface must be far enough away from the source to create a sufficient time delay [2]. While sound waves can also undergo refraction (bending when changing mediums) and diffraction (bending around corners), these processes do not produce the distinct repetition of sound characteristic of an echo [1]. Resonance, on the other hand, involves an object vibrating at its natural frequency when exposed to a matching sound wave, which is different from the simple reflection mechanism that causes echoes [2].

Sources

1. [1] https://sathee.iitk.ac.in/article/physics/physics-sound-waves/
2. [2] https://satheeneet.iitk.ac.in/article/physics/physics-sound-waves/