Sound and Wave Interactions

Sound

and

Wave Interactions

Wave Interaction

Wave interaction happens when a wave meets another object or wave

OR

​passes through another medium.

Examples: reflection, diffraction, refraction, interference, resonance​

When a wave strikes an object that it cannot pass through.

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The ray will bounce off the surface of the object so that the angle of reflection is equal to the angle of incident.

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Reflected light in a mirror is how you see your beautiful face every morning!

Reflection

Refraction is a bending of waves as it passes from one medium to another.

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This bending is caused by a change in speed.​

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Slower (more dense)

• light bends towards the normal

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Faster (less dense)

• light bends away from the normal​

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​Refraction

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The pencil looks like its broken because the light wave bends as it enters a new medium (air to water).

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Fun fact: Refraction of light can causes mirages, like the appearance of water on a hot highway or desert!

Refraction​

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Multiple Choice

Is the image an example of reflection or refraction?

• Refraction

• Reflection

Diffraction

Diffraction is the bending of a wave as it moves around an obstacle or through a narrow opening.

Some text here about the topic of discussion

Multiple Choice

What type of wave interaction is shown in the picture?

• reflection

• refraction

• diffraction

• sound

Sound

Sound is a created as a pressure wave. (Longitudinal wave)

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​The air compresses in an area of high pressure (compressions) and is followed by areas of low pressure (rarefactions)

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• sound wave is generated

• ​

• ​sound wave vibrates ear drum

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• ​amplified by ear bones

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• ​converted to nerve impulses in cochlea

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Sitting close to loudspeakers at concerts (which can reach about 120 decibels) can damage your hearing in just 7.5 minutes!

Human Hearing

​Sound

The speed of sound depends on:

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The temperature of the medium:​

• 344 m/s in air at 20°C

• Travels faster through higher temperatures​

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The type of medium:

• ​Travels better through solids than liquids

• Can't travel through a vacuum. ​

Fun fact: No one can hear you scream in Space!

Multiple Choice

Which medium does sound travel through the fastest?

• gases

• solids

• liquids

• clouds

Pitch

​Pitch refers to the highness or lowness of a sound and depends on the frequency of a sound wave.

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Human hearing range:

• ​20 - 20,000 Hz

Fun Fact: The frequency of a dog whistle is between 23-54 kHz. To a human, it sounds like a quiet hissing!

Intensity

Intensity refers to the volume of a sound.

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Measured in decibels (dB), it depends on the energy (amplitude) of the sound wave.

160 dB will cause instant perforation of the ear drum.

Medical imaging

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Sonar

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Sound Navigation​ Ranging

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​Seeing with Sound:

Ultrasonic waves: above 20,000 Hz​

Multiple Choice

The highness or lowness of a sound refers to a sound's

• velocity

• pitch

• melody

• color

​Doppler Effect

​The doppler effect is an apparent shift in frequency due to a moving source of sound with respect to an observer.

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Ex: A siren on a passing fire truck.

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It is important to realize the frequency is not actually changing.​

The Doppler effect is used in studying the motion of stars!

Multiple Choice

If an ambulance is zooming away from you, will the perceived frequency of the siren be higher or lower than the actual frequency of the siren?

• higher

• lower

• the same

​Wavelength gets smaller as the source gets closer to observer.

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• frequency gets higher

• pitch gets higher​

Source: Moving towards observer

Observer: Stationary​

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​Wavelength gets larger as the source gets closer to observer.

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• frequency gets lower

• pitch gets lower

Source: Moving away from observer

Observer: Stationary​

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​Wavelength gets larger as the observer gets further from the source.

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• frequency gets lower

• pitch gets lower

Source: Stationary

Observer: Moving away from source

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​Wavelength gets smaller as the observer gets closer to the source.

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• frequency gets higher

• pitch gets higher

Source: Stationary

Observer: Moving towards source

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Multiple Choice

A car horn emits a frequency of 400 Hz in 20 °C air, which has a speed of sound of 343 m/s. If the car is moving towards a stationary observer at 20 m/s, what is the frequency that is observed?

• 400 Hz

• 389.2 Hz

• 424.4 Hz

• 465.3 Hz

Multiple Choice

A tug boat horn emits a frequency of 250 Hz in 18 °C air, which has a speed of sound of 342 m/s. If the tug boat is moving away from a stationary observer at 15 m/s, what is the frequency that is observed?

• 239.5 Hz

• 250 Hz

• 267.1 Hz

• 200.4 Hz