16.0 - Introduction
Sounds are so commonplace that it is easy to take them for granted, but they are a
central part of the human experience. When you think of sound, you may think of
your favorite song or an alarm clock that goes off early and loud. To a physicist,
though, both a pleasant song and a shrill alarm are mechanical longitudinal waves
consisting of regions of high and low pressure. The physics of sound waves is the
topic of this chapter.
Sounds can be classified as audible, infrasonic or ultrasonic. Audible sounds are in
the frequency range that can be heard by humans. Infrasonic sounds are at
frequencies too low to be heard by humans, but animals such as elephants and
whales use them to communicate over great distances. Ultrasonic sounds are at
frequencies too high to be perceived by humans. They are used by bats for sonar
and by doctors to see inside the human body.
You may have used the speed of sound in air to estimate the distance to a
thunderstorm. The flash from the lightning reaches you almost instantaneously,
while the sound from the thunder takes more time. Sound travels at approximately
343 m/s in air at 20°C, so for every third of a kilometer of distance to the lightning,
the sound of the thunder lags the flash of light by about one second.
Sound travels slowly enough in air that manmade objects such as airplanes can catch and pass their own sound waves. You may have heard
the result when a plane is flying faster than the speed of sound: a sonic boom. A small sonic boom is also the cause of the "crack" of a whip, as
the tip of the whip travels faster than the speed of sound.
You may begin your study of sound with the simulation to the right, which allows you to experiment with a loudspeaker that causes sound
waves to travel through a tube filled with air particles. One set of particles is colored red to emphasize that all the particles just oscillate back
and forth; they do not travel along with the wave.Sound waves can be described with the same parameters that are used to describe transverse mechanical waves: amplitude, frequency and
wavelength. Recognizing these parameters in a longitudinal wave may require some practice.
When you open the simulation, press GO to send a sound wave through the air. You will see the loudspeaker's diaphragm vibrate horizontally.
This causes the nearby air particles to vibrate and a longitudinal wave to travel from left to right along the length of the tube.
Observe the differences between this wave and the transverse waves you saw in strings. You should be able to see how the particles of the
medium (air) oscillate parallel to the direction the wave travels in a longitudinal wave, as opposed to the perpendicular motion of the particles
in a transverse wave.
The simulation lets you control the loudspeaker to determine the amplitude and frequency of the wave. As with any wave, the amplitude is the
maximum displacement of a particle from its rest position, and the frequency is the number of cycles per second. You can vary these
parameters and observe changes in the motion of the loudspeaker and in the properties of the sound wave. You can also observe how the
wavelength changes when you alter the frequency.
Humans can identify different sound waves by pitch, which is related to frequency. If you have audio on your computer, turn it on and listen to
the pitch created by a particular wave. Then increase the frequency and hear how the pitch changes. The loudness of a sound wave is related
to its amplitude. Increase the amplitude of the wave in the simulation and note what you hear.16.1 - Sound waves
Sound waves are longitudinal mechanical waves in a medium like air generated by
vibrations such as the plucking of a guitar string or the oscillations of a loudspeaker.
Sound waves are caused by alternating compression and decompression of a medium.
In Concept 1, a loudspeaker is shown. As the loudspeaker diaphragm moves forward, it
compresses the air in front of it, causing the air particles there to be closer together.
This region of compressed air is called a condensation. The pressure and density of
particles is greater in a region of condensation. This compressed region travels away
from the loudspeaker at the speed of sound in air.
The diaphragm then pulls back, creating a region in which there are fewer particles.
This region is a rarefaction, and the pressure there is lower. The rarefaction also travels
away from the loudspeaker at the speed of sound. The velocity of the wave is indicated
with the orange vector v in the diagram. The back-and-forth motion of an individual
particle is indicated with the black arrows.
The illustration for Concept 2 also shows the alternating regions of condensation and
rarefaction as the loudspeaker oscillates back and forth. The wavelength is the distance between two successive areas of maximum
condensation or rarefaction. As with transverse waves, the wavelength is measured along the direction of travel. The wavelength can be
readily visualized as the distance between the midpoints of the two regions of condensation shown in the diagram.Sound waves
Are longitudinal
(^308) Copyright Kinetic Books Co. 2000-2007 Chapter 16