Sound wavesSound waves

Sound is a longitudinal wave, like this compression wave on a Slinky®.

What is sound?What is sound?

The difference: it is AIR that is being compressed.

• Sound waves are traveling oscillations of air pressure.

• Sound waves can interfere.

• Sound obeys a wavelength and frequency relationship like a wave.

Why is sound a wave?Why is sound a wave?

The loudness of a sound wave depends on its amplitude.

Louder sounds waves have larger amplitude pressure variations.

A stereo’s speakers move back and forth a greater distance when producing a loud sound than when producing a soft sound.

Loudness and amplitudeLoudness and amplitude

Humans can hear sound frequencies from about 20 Hz to 20,000 Hz.

Pitch and frequencyPitch and frequency

The pitch of a sound depends on the frequency of the sound wave:

•Low-pitched sounds have lower

frequencies.•High-pitched sounds have higher

frequencies.

Most sounds contain multiple frequencies at the same time.

Musical instruments produce a fundamental frequency and many overtones (additional frequencies).

Overtones give the sound its timbre, its “piano-ness” or “guitar-ness”.

Timbre and overtonesTimbre and overtones

Amplitude on the graph below represents pressure, NOT distance!

Visualizing sound wavesVisualizing sound waves

Sound has a huge frequency range.

• Humans can hear sounds in this frequency range: 20 Hz < f < 20,000 Hz.

• By middle age, most people can only hear sounds less than about 12,000 Hz.

• Click on this Sound wave generator on page 440 to test your own hearing range.

FrequencyFrequency

Some animals can hear higher and lower frequencies than humans:

Audible frequenciesAudible frequencies

Medical ultrasound technology uses very high frequency sound waves.

Differences in tissue density reflect ultrasound waves back to a detector and allow sophisticated imaging without harm to the patient.

Ultrasound technologyUltrasound technology

Sound waves are fast.

The speed of sound in air is 343 m/s (767 mph!)

Many military jets are capable of supersonic flights.

SpeedSpeed

Sound travels even faster in water, or ice, or steel. The stiffer the medium, the faster the sound speed tends to be.

When sound passes from one medium to another . . .

•speed and wavelength change

•frequency stays the same

Speed in various materialsSpeed in various materials

A 1000 Hz sound in …

AIR has a speed of 343 m/s and a wavelength of 34 cm.

WATER has a speed of 1480 m/s and a wavelength of 1.5 m.

ICE has a speed of 3500 m/s and a wavelength of 3.5 m.

Speed in various materialsSpeed in various materials

Sound can’t travel in a vacuum.

The loud explosions from space battles in science fiction movies are not realistic.

If you were actually watching a space battle from a distant space ship, you would hear total silence.

No sound in a vacuumNo sound in a vacuum

Sound waves have small amplitudes.

AmplitudeAmplitude

BUT our ears are extremely sensitive and can easily detect these tiny pressure oscillations.

Typically the variation in pressure is about 0.0001 atmospheres, far below our ability to detect through our sense of touch.

The amplitude of a sound wave determines its loudness. Larger amplitude means louder sound.

BUT, to a human ear, frequency also matters.

A high amplitude sound at a frequency of 40,000 Hz is silent to a human ear but quite loud to a bat!

Amplitude and loudnessAmplitude and loudness

The Equal Loudness Curve shows how sounds of different frequencies compare in perceived loudness to an average human ear.

Examine the graph. Which frequencies do we hear the best?

Loudness and frequencyLoudness and frequency

Our ears can detect an enormous range of pressures.

For this reason, the logarithmic decibel (dB) scale is used to measure loudness.

On the decibel scale, an increase of 20 dB means the wave has 10 times greater amplitude (and 100 times greater power).

The decibel scaleThe decibel scale

AssessmentAssessment1. Based on this graph:

a) What is the frequency of the sound wave?

b) Is this a transverse or longitudinal wave, and why?

c) What can you say about the consistency of the loudness of

this sound?

1. Based on this graph:

AssessmentAssessment

a) What is the frequency of the sound wave? 400 Hz

b) Is this a transverse or longitudinal wave, and why? longitudinal

(sound)

c) What can you say about the loudness of this sound? It is constant.

2. Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water?

A. 19.7 m/s

B. 40 m/s

C. 1500 m/s

D. 2990 m/s

AssessmentAssessment

2. Sound waves with a frequency of 172 Hz have a wavelength of 2.0 meters in air. When these waves enter water, their wavelengths change to 8.7 meters. What is the speed of sound in water?

A. 19.7 m/s

B. 40 m/s

C. 1500 m/s

D. 2990 m/s

AssessmentAssessment

The frequency stays the same.

The speed of sound and the Doppler effect

The speed of sound and the Doppler effect

Brainstorm Brainstorm Have you ever listened to a siren on an emergency vehicle as it speeds towards you, and then away?

The sound of the siren changes pitch as the vehicle passes by.

Why? Is it the speed of sound that’s changing?

The speed of soundThe speed of soundSound travels at 343 m/s (767 mph) in air (at 20o C and atmospheric pressure).

The speed of sound in air is constant: it doesn’t change, even if the sound source is moving.

But the frequency and wavelength of sound do change.

The siren on a fire engine operates at a frequency of about 2000 Hz. How long are these wavelengths?

Finding the wavelengthFinding the wavelength

Asked: λ Given: v

Relationships:

Solution:

Asked: wavelength λ Given: v = 343 m/s, f = 2000 Hz

Relationships:

Solution:

The siren on a fire engine operates at a frequency of about 2000 Hz. How long are these wavelengths?

Finding the wavelengthFinding the wavelength

Asked: wavelength λ Given: v = 343 m/s, f = 2000 Hz

Relationships:

Solution:

The siren on a fire engine operates at a frequency of about 2000 Hz. How long are these wavelengths?

Finding the wavelengthFinding the wavelength

If the siren on a police car has a wavelength of 10 centimeters, what is the frequency of the sound wave?

Finding the frequencyFinding the frequency

Asked: f

Given: λ

Relationships:

Solution:

Asked: frequency f

Given: v = 343 m/s, wavelength λ = 0.10 m

Relationships:

Solution:

Finding the frequencyFinding the frequencyIf the siren on a police car has a wavelength of 10 centimeters, what is the frequency of the sound wave?

Asked: frequency f

Given: v = 343 m/s, wavelength λ = 0.10 m

Relationships:

Solution:

Finding the frequencyFinding the frequencyIf the siren on a police car has a wavelength of 10 centimeters, what is the frequency of the sound wave?

Doppler effectDoppler effect

When a siren is at rest, all observers (A, B, C, D) hear the same frequency.

Sheldon dresses as the Doppler Effect…

When a siren is at rest, all observers (A, B, C, D) hear the same frequency.

But what happens if the source begins to move?

Doppler effectDoppler effect

When the siren is at rest, all observers (A, B, C, D) hear the same frequency.

What changes as the source begins to move?

Doppler effectDoppler effect

Doppler effectDoppler effect

•Person A hears a higher frequency sound.

•Person C hears a lower frequency sound.

If the speaker moves towards you, the wavefronts bunch up.

The wavelength is shorter so the frequency is higher.

Why does frequency change?Why does frequency change?

If the speaker moves away from you, the wavefronts spread out.

The wavelength is longer so the frequency is lower.

Why does frequency change?Why does frequency change?

If a source moves with velocity v TOWARDS an observer at rest, the observed frequency is:

f  = observed frequency (Hz)f0  = frequency of source (Hz)v  = velocity of the source (m/s) vs  = speed of sound (m/s)

Calculating the frequencyCalculating the frequency

Calculating the frequencyCalculating the frequencyA siren with a frequency of 2000 Hz is mounted on a fire engine. If the fire engine heads towards you at 25.0 m/s, what frequency do you hear?

Asked: f

Given: vs

Relationships:

Solution:

A siren with a frequency of 2000 Hz is mounted on a fire engine. If the fire engine heads towards you at 25.0 m/s, what frequency do you hear?

Asked: frequency, f

Given: f0 = 2000 Hz, v = 25.0 m/s, vs= 343 m/s

Relationships:

Solution:

Calculating the frequencyCalculating the frequency

A siren with a frequency of 2000 Hz is mounted on a fire engine. If the fire engine heads towards you at 25.0 m/s, what frequency do you hear?

Asked: frequency, f

Given: f0 = 2000 Hz, v = 25.0 m/s, vs= 343 m/s

Relationships:

Solution:

Calculating the frequencyCalculating the frequency

If the source is moving AWAY from the observer, the same equation applies:

BUT the velocity v of the sound source will be a negative number. The resulting frequency will always be lower than f0.

Calculating the frequencyCalculating the frequency

A siren with a frequency of 2000 Hz is mounted on a fire engine. If the fire engine is moving AWAY from you at 25.0 m/s, what frequency do you hear?

Asked: frequency, f

Given: fo = 2000 Hz, v = -25.0 m/s, vs= 343 m/s

Relationships:

Solution:

Calculating the frequencyCalculating the frequency

A siren with a frequency of 2000 Hz is mounted on a fire engine. If the fire engine is moving AWAY from you at 25.0 m/s, what frequency do you hear?

Asked: frequency, f

Given: fo = 2000 Hz, v = -25.0 m/s, vs= 343 m/s

Relationships:

Solution:

Calculating the frequencyCalculating the frequency

Supersonic describes motion at speeds higher than the speed of sound.

What happens when a supersonic aircraft moves faster than the sound it makes?

What do you hear?

Supersonic flightSupersonic flight

When an aircraft moves FASTER than the speed of sound, the Doppler equation no longer applies!

Supersonic flightSupersonic flight

Shock wavesShock wavesA shock wave forms at the nose and leading edges of a supersonic aircraft. The shock wave is a result of the constructive interference of the wavefronts.

A person standing on the ground will hear a sonic boom as the shock wave passes by.

Shock waves can be powerful enough to break windows.

Shock wavesShock waves

The wake of a boat (or even of a duck!) is an everyday example of a shock wave.

When an object travels faster than the speed of water waves, a wake is formed by the constructive interference of the waves.

Shock wavesShock waves

AssessmentAssessment

1. Calculate the period and frequency of the sound waves depicted in graphs A and B. How do they compare with one another?

1. Calculate the period and frequency of the sound waves depicted in graphs A and B. How do they compare with one another?

Graph A: T = 0.02 s, f = 50 Hz Graph B: T = 0.04 s, f = 25 Hz. B’s period is twice as long as A’s and B’s frequency is half of A’s.

AssessmentAssessment

2. An ice cream truck is traveling very fast as it plays its song. As it approaches, the song sounds higher in frequency to the stationary observer. Why is this?

AssessmentAssessment

A. The crests of the sound waves are closer together.

B. The truck's speed gives the sound more momentum.

C. The wavelength of the sound increases, increasing the

frequency.

D. The ice cream man increases the frequency of his music to get

the attention of a potential costumer.

AssessmentAssessment

A. The crests of the sound waves are closer together.

B. The truck's speed gives the sound more momentum.

C. The wavelength of the sound increases, increasing the

frequency.

D. The ice cream man increases the frequency of his music to get

the attention of a potential costumer.

2. An ice cream truck is traveling very fast as it plays its song. As it approaches, the song sounds higher in frequency to the stationary observer. Why is this?

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