SOUND AND THE DOPPLER EFFECT

UNIT 9: WAVES AND SOUND

SOUND WAVES

• Sound waves are longitudinal waves that travel through a medium
• Any SOLID, LIQUID OR GAS
• Energy is traveling through the medium - the medium is not moving
• All sounds are produced by the vibrations of an object

SOUND WAVES

• When you speak or shout, your vocal cords vibrate
• These vibrations travel through the air as waves
• When the waves reach our ears, they make our eardrums vibrate, so we can hear the words

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FREQUENCY

• The frequency of the vibrating source equals the frequency of the sound wave
• Measured in Hertz (Hz)

• Pitch refers to how high or low sound frequencies are heard
• High frequency = high pitch (siren)
• Low frequency = low pitch (fog horn)

FREQUENCY

• Humans can detect sounds from 20 Hz to 20,000 Hz
• Sounds below 20 Hz are called infrasonic
• Earthquakes, volcanic eruptions, ocean waves, etc.
• Sounds above 20,000 Hz are called ultrasonic
• Dog whistles, dolphins, bats, ultrasounds, etc.

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HOW GOOD IS YOUR HEARING?

• https://decibelhearing.com/hearing-loss-overview/high-frequency-hearing-loss/

AMPLITUDE

• The amplitude of a sound wave is the strength of the vibrations
• We perceive this as loudness
• Measured in decibels (dB)

WHICH STATE OF SUBSTANCE WOULD SOUND TRAVEL THROUGH FASTER?

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THE SPEED OF SOUND

• Most sounds we hear are transmitted through air
• Sound travels fastest through a solid because the energy takes less time to pass to each particle because they are closer
• Sound travels slowest through a gas because the particles are more spread out
• Sound cannot travel through a vacuum (empty space)

THE SPEED OF SOUND

THE SPEED OF SOUND

THE SPEED OF SOUND

• Sound travels much slower than light
• The speed of sound in dry air at 20°C (room temperature) is 343 m/s
• The speed of sound changes with:
• Temperature: warmer temperature = greater speed – the particles are moving faster
• Water vapor (humidity): more water vapor = greater speed

EXAMPLE

An automatic focus camera focuses on objects by use of an ultrasonic sound wave. The camera sends out sound waves that reflect off distant objects and return to the camera. A sensor detects the time it takes for the waves to return and then determines the distance an object is from the camera. If a sound wave (speed = 343 m/s) returns to the camera 0.115 seconds after leaving the camera, how far away is the object?

v = 343 m/s

2t = 0.115 s

v = d/t

343 = d/0.0575

d = 19.7 m

EXAMPLE 2

A male vocalist with a bass voice can sing as low as 85 Hz. Given that the speed of sound is 343 m/s, what is the wavelength of the sound waves?

v = 343 m/s

f = 85 Hz

v = fλ

343 = 85λ

λ = 4.04 m

THE DOPPLER EFFECT

THE DOPPLER EFFECT?

• The apparent change in frequency due to the motion of the source (or receiver)
• The greater the speed of the source, the greater the effect

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THE DOPPLER EFFECT

Imagine a bug jiggling its legs and bobbing up and down in the middle of a quiet puddle

THE DOPPLER EFFECT

What would happen to the waves if the bug began to swim forward, toward B?

THE DOPPLER EFFECT

What does that do to the frequency of the waves, in front of the bug and behind the bug?

• In front of the bug…the frequency ↑
• Behind the bug…the frequency ↓

= lower

pitch

= higher

pitch

Waves spread further apart, increasing wavelength

Waves are compressed together, decreasing wavelength

EXAMPLE

1. From which truck will the guy on the street hear a higher pitch?
2. From which truck will the guy on the street hear a lower pitch?

Truck A

Truck B

THE DOPPLER EFFECT EQUATION

EXAMPLE 3

A police car is traveling towards a stationary observer at a speed of 30 m/s. The siren on the police car emits a sound at a frequency of 700 Hz. The speed of sound in air is 343 m/s.

1. Calculate the frequency experienced by the stationary observer.

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• After passing the observer, the police car continues to travel at the same speed. Calculate the frequency of the sound heard by the observer as the police car moves away.

v sound = 343 m/s

v_s = 30 m/s

v_o = 0 m/s

f = 700 Hz

EXAMPLE 3

A police car is traveling towards a stationary observer at a speed of 30 m/s. The siren on the police car emits a sound at a frequency of 700 Hz. The speed of sound in air is 343 m/s.

3. How do the frequencies heard by the observer at each point compare to the actual frequency of the police car? Why?

The frequency as the police car approaches you is higher that the actual frequency because the wavelengths are squished together, resulting in a shorter wavelength, which equals a higher frequency/higher pitch.

The frequency as the police car leaves you is lower than the actual frequency because the waves are getting stretched out, so the wavelengths are longer, which results in a lower frequency/lower pitch.

SONIC BOOM

• A sonic boom occurs when an object travels faster than the speed of sound
• The “boom” is the sudden onset and release of pressure from the shock waves

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9.3 LESSON CHECK