Chapter 9 Sound
  1. Sound Waves
  2. Measuring Sound Waves
  3. Interference of waves
  4. Doppler Effect
  5. Chapter Quiz
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  Section Section 1. Sound waves


Sound waves are longitudinal waves produced by variations in air pressure. A vibrating source pushes molecules in air back and forth, creating areas of compression and rarefaction. When a molecule moves, it collides with the next one and makes it move too. The energy of a sound wave travels away from the source trough a series of molecule collisions parallel to the direction of the wave. Sound cannot travel through a vacuum.
rarefaction compression
Sound waves

Sound waves can also travel trough liquids and solids. The velocity of a sound wave depends on the temperature of the medium and its elasticity (more elasticity means that molecules will move easily). Through air, sound waves travel at 343 m/s. Actually, sound waves move faster through liquids and solids than through gases.


  Section Section 2. Measuring sound waves


The frequency of a sound wave is called pitch. In music, different pitches (C, D, E, etc.) are represented by notes.


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The human ear is able to feel frequencies between 20 Hz to 15 000 Hz, depending on the age of the person. Sound waves with a frequency above 20 000 Hz are called ultrasonic waves.



The amplitude or volume of a sound wave is the amount of pressure exerted by a sound source to air molecules. The higher the pressure, the harder the molecules will collide and the farther the wave will travel.

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Scientists measure the amplitude in atmospheres. Humans can detect from less than a billionth of an atmosphere to values one million times higher. However, it is hard to deal with this huge range of different values. Instead, the pressure is measured by the intensity of the sound. The quietest sound corresponds to a value of zero decibels (unit of sound intensity) and a value above a hundred corresponds to annoying sounds.

A sound wave with a wavelength of 1.24 m is moving at 343 m/s. What is the frequency of this wave?
wl = wavelength
wl = 1.24 m
v = 343 m/s

v = (wl)f
f = v/wl = (343 m/s)/(1.24 m) = 276.61 Hz

QUESTION: The frequency of a sound wave is 25 Hz, and the velocity is 343 m/s. What is its wavelength?


  Section Section 3. Interference of waves

When two or more waves with the same frequency reach the ear, the ear interprets these waves as one wave with amplitude as big as the sum or difference of the initial waves.

Constructive Destructive
Constructive interference Destructive interference




When two waves with different frequencies reach the ear, the resulting sound will be pleasant if the frequencies have ratios that are small whole numbers (2:1, 3:2, 4:3). This is called consonance.

Consonance (2:1) Octave

On the other hand, an unpleasant sound or noise will be produced if the frequencies have ratios others than small whole numbers. This is called dissonance.


  Section Section 4. Doppler effect
Doppler Effect In this example, the sound produced by the source is always the same; however, John is receiving more oscillations per second because the source is moving toward him.

The sound that reaches John's ear has a higher pitch than the sound that reaches Bob's ear. This effect is called Doppler effect.



Section 5. Chapter 9 Quiz

Now it's time to demonstrate you became a genius. Try chapter 9 quiz!

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