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THE IB SAYS
THE IB SAYS
Understandings: The nature of sound waves
Applications and skills: Investigating the speed of sound experimentally
Data booklet: c = f λ
My vodcast
My vodcast
The nature of sound waves
The nature of sound waves
Sound waves are a form of pressure wave. Vibrations or sudden expansion causes compression waves to pass through materials. Like all pressure waves these are longitudinal waves, with particles oscillating backwards and forwards around a fixed position, in a direction parallel to the direction of propagation of the waves. The high pressure compressions are separated by low pressure rarefactions.
Animation (c) Dan Russell. See more excellent wave animations here.
Speed of sound
Speed of sound
Unlike EM radiation, sound waves require a physical medium through which to pass. The speed of the sound wave depends on the nature of the medium - both phase (solid, liquid or gas), substance (e.g. air, helium) and temperature. The speed of sound in air is about 330 m/s at zero celsius, and it is this figure the IB uses in most of their questions.
Experimental determination of the speed of sound
Experimental determination of the speed of sound
This can be done simply (though inaccurately) by finding a place near a large wall from which an echo can be detected. Stand a measured distance from the surface (a large exterior wall for example), make a noise and time how long it takes to hear the echo.
Earthquakes, moonquakes and starquakes
Earthquakes, moonquakes and starquakes
Other examples of pressure waves are found in physics apart from sound (although these other fields are also sometimes referred to as acoustics).
The energy from an earthquake is transmitted through the Earth by S-waves and P-waves. S-waves (Shear waves) are transverse, but the faster P-waves (Pressure waves) are longitudinal. My measuring the time they take to arrive at different points on the Earth, and knowing where they were created, we can use the information to determine the internal structure of the Earth.
Similar investigations have been done on the moon. Pressure was have also been observed on the Sun, in stars and even in the left-over radiation from the Big Bang.
Resources from textbooks
Resources from textbooks
Oxford Physics: pages 129 - 130 Examines the experiments to determine speed of sound, but not a lot else.
Hamper HL (2014): pages 177 - 178. Hamper takes a slightly different path through the topic although the same content is covered in the end.
Relevant questions from Dot Point Physics (4 Oscillations and Waves)
Relevant questions from Dot Point Physics (4 Oscillations and Waves)
There isn't really a dedicated section, but sound related questions are found on pages 193 and 197
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