Resonance, Sound Waves and The Ear

http://www.phys.unsw.edu.au/jw/hearing.html

What does the natural frequency depend upon? The natural frequency depends on many

factors, such as the tightness, length, or weight of a string.

We can change the natural frequency 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 natural frequency of a string by changing its tension.

Resonance You can think of resonance as having the natural

frequency 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.

Natural Frequency--Forced Vibration--Resonance Forced Vibration: a vibrating object in contact with

another object causes it to vibrate at the same frequency. Natural frequency: the frequency (or frequencies) at which

an object naturally vibrates when receiving a disturbance. 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

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.

26.6 Natural Frequency vs. Forced Vibration

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.

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.

26.8 Resonance

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

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.

26.8 Resonance

a. The first compression gives the fork a tiny push.

26.8 Resonance

a. The first compression gives the fork a tiny push.

b. The fork bends.

26.8 Resonance

a. The first compression gives the fork a tiny push.

b. The fork bends.

c. The fork returns to its initial position.

26.8 Resonance

a. The first compression gives the fork a tiny push.

b. The fork bends.

c. The fork returns to its initial position.

d. It keeps moving and overshoots in the opposite direction.

26.8 Resonance

a. The first compression gives the fork a tiny push.

b. The fork bends.

c. The fork returns to its initial position.

d. It keeps moving and overshoots in the opposite direction.

e. When it returns to its initial position, the next compression arrives to repeat the cycle.

26.8 Resonance

Tacoma Narrows Bridge http://www.youtube.com/watch?

v=3mclp9QmCGs

Then (July 1940)

now

Sound waves and the EarWhat is the audible range of frequencies for a human?

20 -20,000 Hz

What type of wave is a sound wave?

A Compressional wave

Relative intensity of sound wave is volume and is measured in decibels (dB)

The frequency of a sound wave is called pitch.

Like all mechanical waves, sound waves can only travel through matter

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

256 Hz

256 Hz

compression

rarefaction

The Ear

cochlea

3 tiny bones (hammer, anvil and stirrup)

Pinna

Semicircular canals (for balance)

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

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.)

Hearing Corrections

Repairs to the conductive parts of the ear Cochlear implants (addresses frequency

deficiencies) Hearing aids (increase amplification)

Main Parts of the Ear

Inner Ear Middle Ear Outer Ear

outermiddle

inner

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

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.

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

http://www.thinkreliability.com/CM-Tacoma.aspx

http://www.cedengineering.com/upload/Ethical%20Issues%20Tacoma%20Narrows.pdf