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What is sound Sounds starts with the vibration of a medium. This vibration can come from striking a surface such as a drum or the plucking of a string. Air molecules around the medium are normally evenly distributed in a state of equilibrium. When the medium vibrates the air molecules create areas of compression and rarefaction. Air molecules are closer together during compression and farther apart during rarefaction The amount of pressure that is created by air molecules during this process is called the sound pressure level or SPL for short. The above graph shows the sound pressure level in a wave form. The top of the wave is called the crest and the bottom is the trough. The line through the center is the equilibrium and represents the steady state of air molecules before they are excited. The amplitude is the distance from the crest or trough from the equilibrium and the wavelength is the distance between two crests or troughs. The time that it takes for a wave to make one full cycle is called a period and the amount of periods or oscillations per second is called the frequency and is measured in herts (hz) www.shedthemusic.com How do we hear sound The ear is made of three parts; the outer ear, middle ear, and the inner ear. When sound comes through the outer ear (the part that you can see on your head) it is channel through the ear canal and comes in contact with the tympanum or eardrum. The tympanum vibrates with the areas of compression and rarefaction causing the three bones in the middle ear (hammer, anvil, and stirrup) to vibrate. The inner ear is filled with fluid and sound travels at a different speed in fluid than it does in air. The bones in the middle ear make up for this change of speed by enacting a mechanical ratio between the two speeds. The cochlea is contained in the inner ear. It is shaped like a shell, filled with fluid, and has thousands of tiny hairs in it called cilia. Each hair vibrates at a certain frequency. When the hair vibrates, a message is sent through the auditory nerve to the brain and it registers as a sound. Frequency and pitch Pitch is the brain’s interpretation of a certain frequency. When the frequency is higher, the pitch goes up and when the frequency is lower the pitch goes down. If you double the frequency of a pitch, the resulting pitch will be one octave above the other. www.shedthemusic.com