Physical ScienceWaves and Sound

Lincoln High School

Mr. Lowery

Earth Science

2007-2008

(slightly modified for Integrated Science: Ms. Gall 2011)

When something moves back and forth, up and down, or side to side we say it vibrates.

A vibration is a wiggle. When this wiggle moves through space and time it is a wave.

Light and sound are both vibrations that move through space as waves.

Sound is the movement of vibrations of matter – through solids, liquids, or gases

Sound can not travel through a vacuum. It must have matter to vibrate.

Light needs no matter to vibrate, and can travel through a vacuum such as space.

(a vacuum is a space without matter)

Light is a vibration of electric and magnetic fields.

Example: We see light from the sun.

Amplitude is the distance from the midpoint to the crest or trough of the wave.

Wavelength is the distance from one crest to the next one or one trough to the next one.

Frequency is the number of complete vibrations that a wave makes in a given period of time, usually one second.

The unit we use to measure frequency is hertz (Hz)

The period of a wave is the time it takes for one complete vibration.

Formulas:

Freq = 1/period

Period = 1/freq

Wave Motion – Transporting Energy

• Wave speed – Speed is related to frequency and wavelength.

Formulas:

Wave speed = freq x wavelength

Types of Waves

• Transverse–Vibrations that are at right angles to the direction of wave travel

–The direction of wave travel is perpendicular to direction of the vibrating source

• Longitudinal–Direction of wave travel is along the direction in which the source vibrates

–Vibrations are parallel to the direction of energy transfer

–Area where waves are close together is a compression

–The stretched region, between compressions, is called a rarefaction

–Both compressions and rarefactions together make up the longitudinal wave

Sound travels in Longitudinal waves

• The wavelength of a sound wave is the distance between successive rarefactions

• The molecules in the air vibrates

• Because sound travels by making the molecules vibrate, sound can not travel through a vacuum such as space. It needs a medium. Otherwise, there are no molecules to compress and stretch

• Our subjective impression about the frequency of sound is described as pitch. A high pitch sound like from a tiny bell has a high vibration frequency

• Sound from a large bell has low pitch b/c its vibrations are a low frequency

• The human ear can normally hear pitches from 20 – 20,000 hertz

• As we age, this range shrinks

• Sound frequencies below 20 hertz are called infrasonic

• Frequencies above 20,000 are called ultrasonic.

• What are some ways we use ultrasonic waves?

• Humans can not hear infrasonic or ultrasonic waves, but dogs and other animals can

• Sound can travel through air, solid, liquid or gas

• Many solids and liquids conduct sound better than air

• Examples: sound underwater, ear to a railroad track

Speed of sound

• Does vary by medium

• In air, varies by temp and conditions of the air

• Travels approx 330 m/sec in dry air at 0 degrees C

• Speed increases with temp – travels faster in warmer air

• In water speed is 4 times as fast as in air

• In steel, speed is 15 times as fast as in air

Reflection

• Reflected sound is called an echo.

• Rigid and smooth surfaces reflect large amount of energy

• Sort, irregular surfaces echo less

• The study of sound characteristics is called acoustics

• A room with “good acoustics” means the sound reflects well to all areas of the room

Refraction

• Sound waves bend when parts of the wave travel at different speeds.

• This bending of sound is called refraction

Quiz.

Give your name, date, and hour.

Draw a diagram of a wave.

Label the crest, trough, amplitude and wavelength.