Sound is a waveform made from vibrating matter. The sound wave travels through matter--especially air--in a straight line. When the wave hits a different material, some of it is reflected, absorbed and transmitted through the material. In the case of a sound wave in air hitting a solid wall, most of the sound is reflected.

If the wall is relatively flat, perpendicular to the source of the sound, ...

Sound is a waveform made from vibrating matter. The sound wave travels through matter--especially air--in a straight line. When the wave hits a different material, some of it is reflected, absorbed and transmitted through the material. In the case of a sound wave in air hitting a solid wall, most of the sound is reflected.

If the wall is relatively flat, perpendicular to the source of the sound, and far enough away (but not too far), then you can hear the reflected waveform or echo. If the sound comes back in about 0.1 second or longer, you can readily distinguish the echo.

Since sound travels at approximately 1000 feet per second (or about 300 meters per second) and if the wall was 50 feet (or 15 meters away), the sound would return in 0.1 second. This can be seen from the relationship:

d = v*t or t = d/v
where
* d = the distance the sound wave traveled back and forth,
* v = velocity of sound, and
* t = the time it takes the sound to go back and forth.
t = 30 m / 300 m/s = 0.1 sec.
(Note that the distance was doubled to show the back and forth motion of the sound.)
That is enough time to be able to distinguish between the noises you made and the reflected sound.