Sound

When something makes a noise, it sends vibrations, or sound waves, through the air.
 * Sound **

The human eardrum is a stretched membrane, like the skin of a drum. When the sound waves hit your eardrum, it vibrates and the brain interprets these vibrations as sound.
 * Eardrum**

In dry air at 20 °C (68 °F), the speed of sound is 343 meters per second (1,125 ft/s). This equates to 1,236 kilometers per hour (768 mph), or about one kilometer in three seconds and about one mile in five seconds.
 * Speed of sound**

Higher frequency in front and lower frequency behind.
 * Doppler effect**

How far do the wave travel apart.
 * Wavelength**




 * Amplitude: The height from the line to the top of the wave**

A standing wave (black) depicted as the sum of two propagating waves traveling in opposite directions (red and blue).

Materials that can be used in sound proof room are cavity walls, double glazing windows, door with draught excluder. Room that should be sound proofed is kind of room that people used to pratice their instruments or pratice sing, like a music room.
 * Sound proofing: Sound can not go through**

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing.
 * Ultrasound**

Noise cancelling headphones reduce unwanted ambient sounds by means of active noise controll, noise cancellation makes it possible to enjoy music without raising the volume so high.
 * Noise cancelling earphone**