Sound is a Pressure Wave

Sound is a Pressure Wave

Sound is a mechanical wave which results from the longitudinal motion of the particles of the medium through which the sound wave is moving. If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. The motion of the particles parallel (and anti-parallel) to the direction of the energy transport is what characterizes sound as a longitudinal wave.

A vibrating tuning fork is capable of creating such a longitudinal wave. As the tines of the fork vibrate back and forth, they push on neighboring air particles. The forward motion of a tine pushes air molecules horizontally to the right and the backward retraction of the tine creates a low pressure area allowing the air particles to move back to the left. Because of the longitudinal motion of the air particles, there are regions in the air where the air particles are compressed together and other regions where the air particles are spread apart. These regions are known as compressions and rarefactions respectively. The compressions are regions of high air pressure while the rarefactions are regions of low air pressure. The diagram below depicts a sound wave created by a tuning fork and propagated through the air in an open tube. The compressions and rarefactions are labeled.

The wavelength of a wave is merely the distance which a disturbance travels along the medium in one complete wave cycle. Since a wave repeats its pattern once every wave cycle, the wavelength is sometimes referred to as the length of the repeating pattern - the length of one complete wave. For a transverse wave, this length is commonly measured from one wave crest to the next adjacent wave crest, or from one wave trough to the next adjacent wave trough. Since a longitudinal wave does not contain crests and troughs, its wavelength must be measured differently. A longitudinal wave consists of a repeating pattern of compressions and rarefactions. Thus, the wavelength is commonly measured as the distance from one compression to the next adjacent compression or the distance from one rarefaction to the next adjacent rarefaction.

Since a sound wave consists of a repeating pattern of high pressure and low pressure regions moving through a medium, it is sometimes referred to as a pressure wave. If a detector, whether it be the human ear or a man-made instrument, is used to detect a sound wave, it would detect fluctuations in pressure as the sound wave impinges upon the detecting device. At one instant in time, the detector would detect a high pressure; this would correspond to the arrival of a compression at the detector site. At the next instant in time, the detector might detect normal pressure. And then finally a low pressure would be detected, corresponding to the arrival of a rarefaction at the detector site. Since the fluctuations in pressure as detected by the detector occur at periodic and regular time intervals, a plot of pressure vs. time would appear as a sine curve. The crests of the sine curve correspond to compressions; the troughs correspond to rarefactions; and the "zero point" corresponds to the pressure which the air would have if there were no disturbance moving through it. The diagram below depicts the correspondence between the longitudinal nature of a sound wave and the pressure-time fluctuations which it creates.

The above diagram can be somewhat misleading if you are not careful. The representation of sound by a sine wave is merely an attempt to illustrate the sinusoidal nature of the pressure-time fluctuations. Do not conclude that sound is a transverse wave which has crests and troughs. Sound is indeed a longitudinal wave with compressions and rarefactions. As sound passes through a medium, the particles of that medium do not vibrate in a transverse manner. Do not be misled - sound is a longitudinal wave.

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The Nature of a Sound Wave

Sound is a Longitudinal Wave

In the first part of Lesson 1, it was mentioned that sound is a mechanical wave which is created by a vibrating object. The vibrations of the object set particles in the surrounding medium in vibrational motion, thus transporting energy through the medium. The vibrations of the particles are best described as longitudinal. Longitudinal waves are waves in which the motion of the individual particles of the medium is in a direction which is parallel to the direction of energy transport. A longitudinal wave can be created in a slinky if the slinky is stretched out in a horizontal direction and the first coils of the slinky are vibrated horizontally. In such a case, each individual coil of the medium is set into vibrational motion in directions parallel to the direction which the energy is transported.

Sound waves are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction which the sound moves. A vibrating string can create longitudinal waves as depicted in the animation below. As the vibrating string moves in the forward direction, it begins to push upon surrounding air molecules, moving them to the right towards their nearest neighbor. This causes the air molecules to the right of the string to be compressed into a small region of space. As the vibrating string moves in the reverse direction (leftward), it lowers the pressure of the air immediately to its right, thus causing air molecules to move back leftward. The lower pressure to the right of the string causes air molecules in that region immediately to the right of the string to expand into a large region of space. The back and forth vibration of the string causes individual air molecules (or a layer of air molecules) in the region immediately to the right of the string to continually move back and forth horizontally; the molecules move rightward as the string moves rightward and then leftward as the string moves leftward. These back and forth vibrations are imparted to adjacent neighbors by particle interaction; thus, other surrounding particles begin to move rightward and leftward, thus sending a wave to the right. Since air molecules (the particles of the medium) are moving in a direction which is parallel to the direction which the wave moves, the sound wave is referred to as a longitudinal wave. The result of such longitudinal vibrations is the creation of compressions and rarefactions within the air.

Regardless of the source of the sound wave - whether it be a vibrating string or the vibrating tines of a tuning fork - sound is a longitudinal wave. And the essential characteristic of a longitudinal wave which distinguishes it from other types of waves is that the particles of the medium move in a direction parallel to the direction of energy transport.

The Nature of a Sound Wave

Lesson 1: The Nature of a Sound Wave

Sound is a Mechanical Wave

Sound and music are parts of our everyday sensory experience. Just as humans have eyes for the detection of light and color, so we are equipped with ears for the detection of sound. We seldom take the time to ponder the characteristics and behaviors of sound and the mechanisms by which sounds are produced, propagated, and detected. The basis for an understanding of sound, music and hearing is the physics of waves. Sound is a wave which is created by vibrating objects and propagated through a medium from one location to another. In this unit, we will investigate the nature, properties and behaviors of sound waves and apply basic wave principles towards an understanding of music.

As discussed in the previous unit of The Physics Classroom, a wave can be described as a disturbance that travels through a medium, transporting energy from one location to another location. The medium is simply the material through which the disturbance is moving; it can be thought of as a series of interacting particles. The example of a slinky wave is often used to illustrate the nature of a wave. A disturbance is typically created within the slinky by the back and forth movement of the first coil of the slinky. The first coil becomes disturbed and begins to push or pull on the second coil; this push or pull on the second coil will displace the second coil from its equilibrium position. As the second coil becomes displaced, it begins to push or pull on the third coil; the push or pull on the third coil displaces it from its equilibrium position. As the third coil becomes displaced, it begins to push or pull on the fourth coil. This process continues in consecutive fashion, each individual particle acting to displace the adjacent particle; subsequently the disturbance travels through the slinky. As the disturbance moves from coil to coil, the energy which was originally introduced into the first coil is transported along the medium from one location to another.

A sound wave is similar in nature to a slinky wave for a variety of reasons. First, there is a medium which carries the disturbance from one location to another. Typically, this medium is air; though it could be any material such as water or steel. The medium is simply a series of interconnected and interacting particles. Second, there is an original source of the wave, some vibrating object capable of disturbing the first particle of the medium. The vibrating object which creates the disturbance could be the vocal chords of a person, the vibrating string and sound board of a guitar or violin, the vibrating tines of a tuning fork, or the vibrating diaphragm of a radio speaker. Third, the sound wave is transported from one location to another by means of the particle interaction. If the sound wave is moving through air, then as one air particle is displaced from its equilibrium position, it exerts a push or pull on its nearest neighbors, causing them to be displaced from their equilibrium position. This particle interaction continues throughout the entire medium, with each particle interacting and causing a disturbance of its nearest neighbors. Since a sound wave is a disturbance which is transported through a medium via the mechanism of particle interaction, a sound wave is characterized as a mechanical wave.

The creation and propagation of sound waves are often demonstrated in class through the use of a tuning fork. A tuning fork is a metal object consisting of two tines capable of vibrating if struck by a rubber hammer or mallet. As the tines of the tuning forks vibrate back and forth, they begin to disturb surrounding air molecules. These disturbances are passed on to adjacent air molecules by the mechanism of particle interaction. The motion of the disturbance, originating at the tines of the tuning fork and traveling through the medium (in this case, air) is what is referred to as a sound wave. The generation and propagation of a sound wave is demonstrated in the animation below.

In some class demonstrations, the tuning fork is mounted on a sound board. In such instances, the vibrating tuning fork, being connected to the sound board, sets the sound board into vibrational motion. In turn, the sound board, being connected to the air inside of it, sets the air inside of the sound board into vibrational motion. As the tines of the tuning fork, the structure of the sound board, and the inside of the sound board begin vibrating at the same frequency, a louder sound is produced. In fact, the more particles which can be made to vibrate, the louder or more amplified the sound. This concept was also demonstrated by the placement of the vibrating tuning fork against the glass panel of the overhead projector; the vibrating tuning fork set the glass panel into vibrational motion and resulted in an amplified sound.

In the tuning fork demonstrations, we know that the tuning fork is vibrating because we hear the sound which is produced by their vibration. Nonetheless, we do not actually visibly detect any vibrations of the tines. This is because the tines are vibrating at a very high frequency. If the tuning fork which is being used corresponds to middle C on the piano keyboard, then the tines are vibrating at a frequency of 256 Hz - 256 vibrations per second. We are unable to detect vibrations of such high frequency. But perhaps you recall the demonstration in which a high frequency strobe light was used to slow down the vibrations. If he strobe light puts out a flash of light at a frequency of 512 Hz (two times the frequency of the tuning fork), then the tuning fork can be observed to be moving in a back and forth motion. With the room darkened, the strobe allows us to view the position of the tines two times during their vibrational cycle. Thus we see the tines when they are displaced far to the left and again when they are displaced far to the right. This is convincing proof that the tines of the tuning fork are indeed vibrating to produce sound.

In a previous unit of The Physics Classroom, a distinction was made between two categories of waves: mechanical waves and electromagnetic waves. Electromagnetic waves are waves which have an electric and magnetic nature and are capable of traveling through a vacuum. Electromagnetic waves do not require a medium in order to transport their energy. Mechanical waves are waves which require a medium in order to transport their energy from one location to another. Because mechanical waves rely on particle interaction in order to transport their energy, they cannot travel through regions of space which are devoid of particles. That is, mechanical waves cannot travel through a vacuum. This feature of mechanical waves was demonstrated in class using a segment from a laser disc. A ringing bell was placed in a jar and air was evacuated from the jar. Once air was removed from the jar, the sound of the ringing bell could no longer be heard. The clapper could be seen striking the bell. but the sound which it produced could not be heard because there were no particles inside of the jar to transport the disturbance through the vacuum. Sound is a mechanical wave and cannot travel through a vacuum.