Sound waves and the human ear Paul

As we saw, sound is caused by vibration The ear is able to detect these vibrations and convert them into the sounds that we hear The following slides show the construction of the ear and how it works in relation to physical principles

Basic structure of the human ear OUTER MIDDLE INNER

Basic structure of the human ear Pinna – the outer part of the ear - the plural is PINNAE Focuses sound into the ear A sound coming from the left reaches the left ear slightly earlier than the right – this helps you to detectwhich direction the sound is coming from

Basic structure of the human ear Ear Drum – the tympanic membrane A thin piece of skin approximately 10mm in diameter. Held very taut by muscles This seperates the outer ear from the middle ear The middle ear is connected to the throat via the EUSTACHIAN TUBE. In this way there is the same air pressure on both sides of the ear drum Incoming sound moves the ear drum inward

Basic structure of the human ear Ear Drum – the tympanic membrane Higher frequency sound waves move the drum more rapidly, and louder sound (higher amplitude) moves the drum a greater distance The eardrum also protects the inner ear from prolonged exposure to loud, low frequency sound. A reflex occurs at the eardrum to make it more rigid. When this happens, the ear does not pick up as much noise at the low frequencies so the loud noise is dampened.

Amplifying Sound The changes in air pressure are extremely small. They don't apply much force on the eardrum, but the eardrum is so sensitive that this minimal force moves it a good distance. The cochlea in the inner ear conducts sound through a fluid, instead of through air. The small force felt at the eardrum is not strong enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force per unit of area) must be amplified. Pressure = Force Area

The ossicles, a group of tiny bones in the middle ear. The ossicles are actually the smallest bones in your body. They include: The malleus, commonly called the hammer The incus, commonly called the anvil The stapes, commonly called the stirrup

Amplifying Sound The ear amplifies sound in two ways…... 1.The ear drum has a much greater area than the oval window - about 55mm 2 compared to about 3.2mm 2 Since Pressure = Force this gives about 17 times increase Area 2.The malleus, incus and stapes,act as a series of levers The malleus is longer than the incus and therefore moves a greater distance Energy = Force x Distance

Ultimately the pressure on the oval window is about 22 times that on the ear drum

Inside the Cochlea The cochlea is filled with fluid and coiled like a snail. It translates the pressure from the ear drum into electrical impulses that the brain can process

Inside the Cochlea A vibration wave is set up in the fluid which passes along the thousand hairs that line the cochlea Each of these is tuned to a different frequency and the appropriate hairs resonate and pass the information to the brain

Overall the ear is a highly complex series of processes, yet is one of the smallest parts of the body

How do we measure sound qualities? Loudness: Amplitude in units called Decibels 0 dB-threshold of sound Each 10 dB step is X10 greater loudness You feel pain at 120 dB You can damage the ear at 90 dB Pitch is a term for sound frequency that is measured in units called Hertz (Hz) Infrasonic: low frequency=vibrations in the body Ultrasonic: high frequency heard by a dog Repetitive sound exposure and tone deafness is a huge problem in the work place and the legal system