What is Sound

What is sound? Sound is the vibrations of molecules in the air as the energy is passed in waves of compression and rarefactions. It can be pictured similar to when a stone is dropped in a lake, and the radiation of the waves from that point.

The five main components of sound are: Frequency, Amplitude, Velocity, Wavelength, and Phase.

Frequency is the number of cycles a wave completes in one second, and is measured in Hertz (Hz). For example, if 500 cycles per second are completed, the frequency is 500Hz.

Pitch is how humans psychologically perceive frequency.

Amplitude is the wave's maximum height (crest) and minimum (trough). This is perceived as loudness. Loudness is measured in decibels, which is on a logarithmic scale in powers of 10. In order to double loudness, an increase of 6 dB-SPL is needed.

Velocity is the speed of a sound wave. Generally this is 1130 ft/sec at 70 degrees F at sea level through air. Thicker materials slow down the molecules - 4800 ft/sec in water, 11700 ft/sec in wood. Also, as air warms up, the speed of sound increases, and the inverse is also true.

The wavelength is exactly what is sounds like - the distance a sound wave travels to complete one cycle. Longer wavelengths correlate to lower frequencies, while higher frequencies have shorter wavelengths.

Sound movement examples: http://www.fearofphysics.com/Sound/dist.html
Decibel vs. loudness charts: http://www.gcaudio.com/resources/howtos/loudness.html
Reflection examples: http://www.kettering.edu/~drussell/Demos/reflect/reflect.html

Questions:
1. What is timbre? How does it define different instruments?
2. How does the inner ear convert sound waves to electrical pulses for the brain to understand?