Sound

The speed of sound can change depending on altitude, temperature and the medium of which it travels through.

Echoes
An echoe of sound can be described as a reflection of sound. Some examples of echoes can be the bottom of a well or the walls of an inclosed walls

Resonance
The Resonance is a natural frequency of vibration made by the physical parameters of the vibrating object.

Frequency
Frequency is the number of vibrations that an individual particle makes in a second.

The Doppler effect is maned after an Australian physicist Christian Doppler. The Doppler effect can be compared to the sirens of a fire truck, as the fire truck is close the sirens will be loud and as the truck passes the sound of the sirens would not be as loud.