| EM waves | Sound waves |
They are both forms of energy.
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They show wave properties
through reflection, refraction and diffraction.
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The formula speed = frequency x
wavelength applies to them.
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Differences
| EM waves | Sound waves |
| They do not require a material medium to travel. They can propagate through vacuo. | They require a material medium to travel as they are mechanical waves. They cannot travel in vacuo. |
| In air, they travel at 3 x 108 m/s. | In air, they travel at 330 m/s. |
| They are transverse waves, meaning the vibration is perpendicular to the direction of travel. | They are longitudinal waves, meaning that the vibration is parallel to the direction of travel. |
| They consist of a vibrating electric field and a vibrating magnetic field coupled together. The electric field and magnetic field are vibrating perpendicular to each other. | They consist of a series of compressions (regions in which pressure is higher than normal) and rarefactions (regions in which pressure is lower than normal). |
Attempt these questions before you refer to the answers that follow.
- In outer space, we can see an explosion but cannot hear it.
Why?
- We can calculate the distance a storm is away from us by
measuring the time between seeing a lightning and hearing the
thunder. What other value is needed? What assumption is being made?
- A tuning fork vibrates with a frequency of 423.5 Hz. How many
compressions pass through a point in one second when the sound from
the tuning fork passes by?
- The audible range of frequency for humans is 20-20,000 Hz. Calculate the shortest wavelength and longest wavelength that can be heard by humans. Take the speed of sound be 330 m/s.
Answers:
- Light can travel through vacuo but sound cannot.
- The speed of the sound in air is needed. Assumption:
- 423.5 compressions pass through the point in one second.
- Shortest wavelength = speed / highest frequency = 330 / 20000
= 0.0165 m. Longest wavelength = speed / lowest frequency = 330 / 20
= 16.5 m
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