Timbre
In general, a musical instrument will produce sounds that are made of combinations of the available frequencies. The lowest frequency in a complex sound wave is called the fundamental and the other frequencies are called overtones . Since all the overtones are integral multiples of the fundamental, they are also called harmonics .
The complex shape of the sound wave means that different instruments playing the same note will have a different tone quality. You can recognize your favorite singer by the timbre of his voice.
Human Ear
The physiology of the ear is detailed in the text. Please read it.
A brief overview of the ear: http://www.youtube.com/watch?v=p3Oy4lodZU4

The perception of loudness depends on frequency.
Pitch is the perception of frequency. Higher pitch means higher frequency (and shorter wavelengths).
Beats occurs when two sound waves are close in frequency. It is very useful for tuning instruments. The beat frequency is the difference in the two frequencies

f beat
 f
1
 f
2
The Doppler Effect
Probably best explained in http://www.youtube.com/watch?v=Y5KaeCZ_AaY
Or not! Maybe this will work? http://www.youtube.com/watch?v=yWIMWqkcRDU
The Doppler effect is the change in observed frequency (pitch) resulting from the motion of the sound source and/or sound observer. It can be used to measure the speed of a moving car
(or baseball).
In front of the moving source, the wave crests are closer together and the frequency is higher. Behind the moving source, the wave crests are further apart and the frequency is lower. f
O



1

1 v
S
/ v

 f
S
Important: v
S
> 0 for a source moving in the direction of the wave.
An observer moving relative to a stationary source will experience a different frequency.
An observer moving towards the source will experience a higher frequency and an observer moving away from the source will experience a lower frequency.

f
O

( 1
 v
O
/ v ) f
S
Important: v
O
> 0 for an observer moving in the direction of the wave.
If both the source and observer move f
O



1
1

 v
O v
S
/ v
/ v

 f
S
The sign convention is given for the individual cases. You must get the signs right to get the correct answer!!
Shock Waves
When the source moves faster than the speed of sound, the wave crests pile on top of each other and a large amplitude wave occurs. For airplanes, this is called a sonic boom.
Another video: http://www.youtube.com/watch?v=-d9A2oq1N38

Echolocation and Medical Imaging
Sound and echoes are used to locate objects. Here is an old video explaining sonar: http://www.youtube.com/watch?v=Hxvo1rmj9tg
Ultrasound can be used to image structures inside the body.
High frequency sound is used since higher frequencies will have shorter wavelengths. Short wavelengths diffract less around small obstacles.
And now from Kazakhstan , the amazing dombra.