Physics 121: Physics of Music
Griffioen; 12 September 1995
Quiz 1
- 1. [8 pts] Define the following prefixes (e.g. femto = 10^-15 ):
- giga (10^9)
- nano (10^-9)
- deci (10^-1)
- mega (10^6)
- kilo (10^3)
- micro (10^-6)
- centi (10^-2)
- milli (10^-3)
- 2. [8 pts] Carefully describe a sound wave. What is it? How does it move? Use your vocabulary
words.
A vibrating object pushes periodically on the surrounding air, causing regions of
higher-than-normal pressure (compressions) and corresponding regions of lower-than-normal pressure
(rarefactions) in between, which travel away from the object at the speed of sound (344 m/s
at 20 degrees
Centigrade). The pressure changes in the air vary along the direction of wave propagation, making
sound a longitudinal wave. The separation between consecutive compressions (or rarefactions) is the
wavelength of the sound wave. The frequency of vibration, which is perceived as pitch, is related
to the wavelength and speed by (frequency*wavelength=speed). Air is non-dispersive, so all frequencies
travel at the same speed.
- 3. [8 pts] Explain the difference between longitudinal and transverse waves. Give two examples
of each.
Longitudinal: the disturbance in the medium is parallel to the direction of wave motion.
The slinky, sound and the side-stepping people chain are examples of longitudinal waves.
Transverse: the disturbance in the medium is perpendicular to the direction of wave motion.
The wave machine shown in class, light, water waves (for the most part), waves on a
string and the standing-and-squatting people chain are examples of transverse waves.
Extra Credit:
a. [2 pts] How many decibels equal one bel? (10)
b. [2 pts] What is the consequence of air being a non-dispersive medium for sound?
All sound waves in air (at constant temperature) travel at the same speed, regardless
of the frequency of the sound.