Professor U. Mallik

Sound Propagates as a Wave

 

Waves are energy moving through water, air, or whatever.

 

Sounds are waves of air molecules. The crests of these waves are areas of high density, called compressions. The troughs are called rarefactions.

Your eardrum acts like a trampoline. When air molecules bounce on it, it vibrates and you hear sound.

 

Loudness and Pitch

 

Loudness is measured in decibels. Some common sounds on the decibel scale are listed below.

Threshold of Hearing 0dB Normal Breathing 10dB Rustling Leaves 20dB 30dB Library 40dB 50dB Normal Conversation 60dB 70dB Noisy Office with machines 80dB Heavy Traffic 90dB 100dB Construction Noise (up close) 110dB Threshold of Pain 120dB Rock Concert 130dB 140dB Jet Takeoff (from runway) 150dB

Sound waves are loud if their compressions are dense. This is analogous to high crests.

 

		The sensitivity range for human hearing depends on the loudness and pitch. Noises along each black line would be heard with the same volume.
The higher the frequency, the faster your eardrum vibrates. This is what causes pitch.

 

Interference

 

Two waves of the same frequency overlap to form a single larger wave. This is constructive interference.	If the crests of one wave overlaps the toughs of another, they cancel each other This is destructive interference.

 

Beats

 

Two waves of slightly different frequencies overlap, adding together in some areas, canceling each other in others. The bulges of loud sound (high wave) are called beats.

 

Sonic Technologies

 

F/A-18 Hornet passing through the sound barrier. Photograph taken by Navy Ensign John Gay while aboard the carrier USS Constellation, 07/07/99.	Your first baby picture was taken using sonar.

Where there is no light we can see with sound. Sonar is is used to locate sunken ships.