Transmission of Sound Waves (OCR GCSE Physics A (Gateway))
Revision Note
Transmission of Sound Waves
Sound waves are longitudinal waves
They transfer energy by the molecules vibrating and knocking into neighbouring molecules
The more molecules that are present the faster the wave can transfer energy, therefore:
Sound waves travel fastest in solids
Sound waves travel slowest in gases
When sound waves move from one medium to another, there will be changes to its:
Wave speed
Frequency
Wavelength
The relationship between the wave speed, wavelength and frequency can be determined using the wave equation
This change in velocity can also result in a change of direction of the sound wave
This phenomenon is also known as refraction
Refraction of Sound
When sound moves from a denser medium to a less dense medium:
The wavelength of the sound wave decreases
The frequency of the sound wave stays the same
The velocity of the sound wave decreases
When sound moves from a less dense medium to a denser medium:
The wavelength of the sound wave increases
The frequency of the sound wave stays the same
The velocity of the sound wave increases
The speed of sound in air is also affected by temperature
On warm days, air molecules move faster, hence they carry sound waves faster, increasing the speed of sound
On cold days, air molecules move at a slower pace, so they carry sound waves at a slower pace, decreasing the speed of sound
Worked Example
A man shouts across an empty field to a friend during the day, and then again during the night. He finds his friend can hear him more clearly at night because the sound travels further at night.
Explain why sound travels further at night than during the day.
Answer:
Step 1: Identify the difference in conditions between the day and the night
During the day, it is warmer near the ground and cooler in the atmosphere
At night, it is cooler near the ground and warmer in the atmosphere
Step 2: Consider the effect of sound waves travelling between the two air temperatures
Sound waves can travel faster in hot air and slower in cold air
This change in temperature will cause the sound waves to refract
Step 3: Explain the motion of the waves during the day and the night
During the day, the sound wave will travel faster in the warm air and refract towards the sky as the wave slows down
At night, the sound wave will travel slower in the cool air and refract towards the ground as the wave speeds up
Step 4: Relate the explanation back to the question
Sound travels further at night because its speed increases as it moves from a denser medium (cold air) to a less dense medium (warm air)
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