Resonance, Sound Waves and The Ear

1. Resonance, Sound Waves and The Ear

2. What does the natural frequency depend upon? • The naturalfrequency depends on many factors, such as the tightness, length, or weight of a string. • We can change the naturalfrequency of a system by changing any of the factors that affect the size, inertia, or forces in the system. • For example, tuning a guitar changes the naturalfrequency of a string by changing its tension.

3. Resonance • You can think of resonance as having the naturalfrequency of the system exactly in tune with your force. Each cycle of your force exactly matches each cycle of the system. • As a result, each push adds to the next one and the amplitude of the oscillation grows. • Two tuning forks with the same natural frequency—one vibrating nearby will cause the other to vibrate—the forks are connected by air molecules.

4. Natural Frequency--Forced Vibration--Resonance • Natural frequency: the frequency (or frequencies) at which an object naturally vibrates when disturbed. • Forced Vibration: a vibrating object in contact with another object causes it to vibrate at the same frequency. • Resonance: Condition occurring when the frequency of a vibration of one object matches the natural frequency on another object and causes it to dramatically increase in amplitude • Resonance can occur whenever successive impulses are applied to a vibrating object in rhythm with its natural frequency. (pushing someone on a swing) • http://www.youtube.com/watch?v=WDZmjzxaxhs

5. 26.6Natural Frequency vs. Forced Vibration When any object composed of an elastic material is disturbed, it vibrates at its own special set of frequencies, which together form its special sound. We speak of an object’s natural frequency, the frequency at which an object vibrates when it is disturbed. A forced vibration occurs when an object is made to vibrate by another vibrating object that is nearby. A natural frequency is one at which minimum energy is required to produce forced vibrations and the least amount of energy is required to continue this vibration.

6. 26.8Resonance An object resonates when there is a force to pull it back to its starting position and enough energy to keep it vibrating. It is the frequency or frequencies at which an object will most easily vibrate.

7. Examples of Resonance • Sympathetic vibrations • speakers buzzing • cilia in cochlea of inner ear vibrating • Building up amplitude • girl pushed on swing gains height • one tuning fork causes another to vibrate without direct physical contact • Tacoma Narrows Bridge Collapse

8. 26.8Resonance If the frequency of a forced vibration matches an object’s natural frequency, resonance dramatically increases the amplitude. You pump a swing in rhythm with the swing’s natural frequency. Timing is more important than the force with which you pump. Even small pumps or pushes in rhythm with the natural frequency of the swinging motion produce large amplitudes.

9. 26.8Resonance • The first compression gives the fork a tiny push.

10. 26.8Resonance • The first compression gives the fork a tiny push. • The fork bends.

11. 26.8Resonance • The first compression gives the fork a tiny push. • The fork bends. • The fork returns to its initial position.

12. 26.8Resonance • The first compression gives the fork a tiny push. • The fork bends. • The fork returns to its initial position. • It keeps moving and overshoots in the opposite direction.

13. 26.8Resonance • The first compression gives the fork a tiny push. • The fork bends. • The fork returns to its initial position. • It keeps moving and overshoots in the opposite direction. • When it returns to its initial position, the next compression arrives to repeat the cycle.

14. Tacoma Narrows Bridge • http://www.youtube.com/watch?v=3mclp9QmCGs now Then (July 1940)

16. Sound waves and the Ear What is the audible range of frequencies for a human? 20 -20,000 Hz Relative intensity of sound wave is volume and is measured in decibels (dB) What type of wave is a sound wave? Mechanical and Longitudinal The frequency of a sound wave is called pitch. Like all mechanical waves, sound waves can only travel through matter

17. BASIC FUNCTION OF THE EAR • The ear converts changes in air pressure due to sound waves to nerve impulses that signal the brain Eardrum vibrates at same frequency as tuning fork and with a certain intensity compression 256 Hz 256 Hz rarefaction

18. The Ear Semicircular canals (for balance) 3 tiny bones (hammer, anvil and stirrup) cochlea Pinna

19. Cochlea of the Human Ear—cilia and nerves in different regions in the cochlea resonate to specific frequencies of sound waves. The region that resonates at 256 Hz “lights up” and signals your brain via the nerves. 256 Hz

20. Hearing Problems • conductive hearing loss (interferes with the transfer of sound vibrations) • sensory hearing loss (affects the cochlea’s ability to resonate from 20 - 20,000 Hz.) • neural hearing loss (affects the connection between the cochlea and the brain.)

21. Hearing Corrections • Repairs to the conductive parts of the ear • Cochlear implants (addresses frequency deficiencies) • Hearing aids (increase amplification)

22. Main Parts of the Ear • Inner Ear • Middle Ear • Outer Ear outer inner middle

23. Outer Ear • Structures: the pinna, ear canal and eardrum. • Purpose: to receive, focus (or amplify) and transmit sound vibrations to the middle ear. Eardrum vibrates at same frequency as tuning fork 256 Hz 256 Hz

24. Middle Ear • Structure: Ear bones (hammer, anvil and stirrup are the three tiniest bones in the human body) • Purpose: To transmit sound vibrations from the eardrum to the inner ear.

25. Inner Ear • Structure: Cochlea • Purpose: The fluid in the cochlea receives sound vibrations from the stirrup, causing tiny hairs inside the cochlea to vibrate, which stimulates auditory nerves connected to the brain. 256 Hz