A SOUND TOPIC. The Physics of Sound Sound travels in longitudinal waves

A SOUND TOPIC

The Physics of Sound

Sound travels in longitudinal waves

The Physics of Sound

What is sound?

Sound is a longitudinal or compressional wave which travels through the air in a series of compressions and rarefactions.

What is sound intensity?

Sound intensity is the energy that the sound wave possesses. The greater the intensity of sound the farther the sound will travel and the louder the sound will appear.

Loudness is very closely related to intensity. Loudness is the human perception of the sound intensity. The unit for loudness is decibels.

Sound is Energy

When you crank up the volume, you are changing the amplitude or increasing energy output.

We hear the sound waves as they pass through the air into our ear

sound wave

vibrates ear drum

amplified by bones

converted to nerve impulses in cochlea

Sound must travel through a medium.Sound travels different speeds in different media. Sound typically travels faster in a solid that a liquid and faster in a liquid than a gas.

The denser the medium, the faster sound will travel.

The higher the temperature, the faster the particles of the medium will move and the faster the particles will carry the sound.

What is the Doppler Effect?

The Doppler Effect is the apparent change in frequency detected when the sound is moving relative to the hearer.

Video-car horn http://www.youtube.com/watch?v=ZPJyYaXhuv4

Video-Train animation

What is a sonic boom?

• When an object which puts out sound travels faster than the speed of sound, the crests of the sound wave add up and reach an observer’s ears all at once.

• Since the waves add up, the AMPLITUDE is much larger and the sound is LOUDER. Thus, we call it a sonic BOOM!

Video

Jet fighters breaking the barrier

How is frequency related to pitch?The pitch of a sound wave is directly related to frequency. A high-pitched sound has a high frequency (a screaming girl). A low-pitched sound has a low frequency (a fog-horn).

A healthy human ear can hear frequencies in the range of 20 Hz to 20,000 Hz. Humans cannot hear below 20 Hz. Sounds below this frequency are termed infrasonic.

Sounds above 20,000 Hz are termed ultrasonic. Some animals, such as dogs, can hear frequencies in this range in which humans cannot hear.

Resonance

• vibration at an object’s natural frequency produces resonance

• resonance – a condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system

• the human ear transmits vibrations that cause nerve impulses

= Resonance =• Forced Vibration– when one vibrating object

forces another object to vibrate at the same frequency

– results in a louder sound because a greater surface area is vibrating

– used in guitars, pianos, etc.

= Resonance =

• Resonance– special case of forced

vibration– object is induced to

vibrate at its natural frequency

= Resonance =“Galloping Gertie”

The Tacoma Narrows Bridge Disaster

Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.

{ Harmonics }• Harmonics – occur when standing waves constructively

interfere with each other. One ½ of a wave is all that is needed to produce a “Fundamental” frequency. (2 nodes 1 anti-node) that resonates.

• Fundamental– the lowest natural frequency of an object the 1st harmonic

• Overtones– multiples of the fundamental frequency

Resonance• Beats– variations in sound intensity produced by 2 slightly

different frequencies– both constructive

and destructive interference occur

Acoustics• Acoustics– the study of sound

• Reverberation – echo effect

produced by the reflection of sound

Anechoic chamber - designed to eliminate

reverberation.

What is sonar?

Sonar is a system that uses the reflection of underwater sound waves to detect objects. This has been used to find sunken ships and schools of fish.

“Seeing” with Sound• Ultrasonic waves - above 20,000 Hz can be used to create

sonograms or images of dense materials hidden behind other slightly different dense materials.

Medical Imaging

SONAR“Sound Navigation

Ranging”