We have probably all heard echoes - when our voices repeat in large rooms, canyons, or tunnels. But what are echoes? How are they created? Echoes are reflections. Sound bounces off a surface such as the canyon walls and returns so you hear it again. Just as a mirror reflects light, hard surfaces such as canyon walls reflect sound.
If you look at a mirror, you can see images of objects without looking directly at the objects. A car's rearview mirror allows drivers to see objects behind them without having to turn around. How does that work? Well, the trick is that the angle of incidence is equal to the angle of reflection. The following picture explains this angle:
The reflector can be any boundary between two media that changes the direction of an incoming wave, such as a flat mirror. The wave moving towards the reflector is called the incident wave. It hits the reflector at an angle, and its path of movement is redirected. It moves away from the reflector at an angle, and is called the reflected wave. A line perpendicular to the reflector, called a normal line, splits the angle between the incident wave and the reflected wave into two equal angles, the angle of incidence, depicted in this figure as "∠I", and the angle of reflection, depicted in this figure as "∠R".
A flat mirror is a nearly perfect reflector for light - the amount of energy in the reflected wave is equal to the amount of energy in the incident wave. The underside of the ocean surface is a nearly perfect reflector for sound under calm conditions, particularly at low frequencies. When the incident wave hits other types of boundaries, such as the seafloor, a portion of the acoustic energy is reflected back into the original medium (the ocean), and a portion of the energy is transmitted into the second medium (the seafloor). The amount of sound that is reflected and transmitted depends on the acoustic properties of the two media and on the angle of incidence. The more similar the acoustic properties of the two media, the less reflection and the more transmission occurs. The less similar the two media, the more reflection and the less transmission occurs. A seafloor of hard bedrock is a better reflector than watery sand. Therefore, sound waves will bounce better off the hard bottom.