Sound WavesCharacteristic Review and Propagation

Sound Waves

• What are they?

– Longitudinal– Require medium

Production

• Compression- region of a longitudinal wave in which the density and pressure are greater than normal (crests)

• Rarefaction- region of a longitudinal wave in which the density and pressure are less than normal (troughs)

Characteristics of Sound Waves

• Frequency- the # of cycles per unit of time– Audible: 20 – 20,000 Hz– Infrasonic: less than 20 Hz– Ultrasonic: more than 20,000 Hz

Characteristics of Sound Waves

• Frequency determines pitch– Frequency is a quantity of sound that can be

measured– Pitch is how different frequency are perceived by

our ears– Increase in frequency = Increase in pitch

Click below to watch the Visual Concept.

Visual Concept

Resonance (Frequency)

Speed of Sound• Sound waves travel though solids, liquids and

gases.– In which would the speed generally be

greatest? Why?• Solids. Because the molecules are more

closely packed, the particles respond more rapidly to compressions.

– How might the temperature of air affect the speed of sound waves? Why?• Higher temperature increases the speed of

the waves because the particles are moving faster and colliding more often.

Speed of Sound

Characteristics of Sound Waves

• Propagation– Travel in all three dimensions– Wave Fronts- indicate center areas of

compression– Rays- indicate direction of waves, perpendicular to

the wave fronts– Plane waves

Spherical Wave

• Sound propagates in three dimensions.

• The diagram shows:– Crests or wave fronts (blue circles)– Wavelength ()– Rays (red arrows)

• Rays indicate the direction of propagation.

• How would these wave fronts appear different if they were much farther from the source?

Spherical Wave

• Wave fronts and rays become more nearly parallel at great distances.

• Plane waves are simply very small segments of a spherical wave a long distance from the source.

Doppler Effect

• Motion creates a change in frequency– Why?

Doppler Effect

• Why are the waves closer together on the left?– Waves are closer because the vehicle moves to the left

along with the previous wave.

Click below to watch the Visual Concept.

Visual Concept

Doppler Effect and Sound

Question 1

• What is the difference between pitch and frequency?

• Frequency measures particle vibration and pitch is subjective to the listener

Question 2

• Why does the speed of sound depend on the temperature of the medium. Why is this temperature dependence more noticeable in a gas that in a solid or liquid?

• Molecules that have more motion (higher temps) transfer their vibrations more easily. This is less noticeable in liquids and solids because the particles are closer together.

Question 3

• A bat flying towards a wall emits a chirp at 40 kHz. Is the frequency of the echo received by the bat greater, less than, or equal to 40 kHz? Why?

• Greater than