Have you ever taken time to consider how you are able to detect sounds? Specifically, have you ever thought about the manner in which sound travels from one person or object so that it can be detected by your own ears?
A. Sound Waves. Characteristics
Sound is a mechanical wave. The physics of waves helps to explain the process by which sound is produced, travels, and is received. Sound is a wave that is produced by objects that are vibrating. It travels through a medium from one point, A, to another point, B.
As is true of all types of waves, specific behaviors, properties, and characteristics apply to sound waves.
Similar to the slinky wave described in the previous chapter, a sound wave carries a disturbance (vibration) from one location (point) to another. For the most part the medium through which it travels is air, although sound waves can just as readily travel though water or metal materials.
There must be a source of the wave, some type of vibrating object that is capable of setting into motion the entire chain of events for the disturbance. For sound waves to sound waves, an originating source might be a pair of vocal chords, a stereo speaker, or a tree falling in the forest.
A long-held philosophical discussion has surrounded the question, "If a tree falls in the forest and no one is around to hear it, does it really make a sound?" The point of this debate is that although a sound wave is produced, if there is no receiver is the wave really considered to be a complete sound?
Certainly the sound could travel because what is known as "particle interaction" allows the vibrating wave to be transported from one location to another. However, ultimately, the mechanical sound waves need a receiver in order to complete their journey. And although there may be a body present to receive the transmitted waves, because they are vibrating at such a high frequency they are virtually undetectable to the unaided ear.
B. Sound Waves. Mechanical versus Electromagnetic
To differentiate mechanical waves and electromagnetic waves, we will first point out several major characteristics of electromagnetic waves.
- Electromagnetic waves have both an electric and a magnetic nature.
- Electromagnetic waves are capable of traveling through a vacuum.
- Electromagnetic waves do not require a medium to transport their energy.
And, now to contrast the two, we will identify several primary traits of mechanical waves.
- Mechanical waves do require a medium in order to transport their energy from one location to another.
- Mechanical waves are unable to travel through a vacuum. Because mechanical waves depend upon particle interaction as a means for transporting their energy, they are unable to travel through regions of space devoid of particles.
The second trait helps to explain why sound, a mechanical wave, does not transmit when sent through tunnels, underwater, and all other examples of vacuous structures.
C. Sound Waves. Longitudinal
As we stated previously, sound is a mechanical wave. It is created by a vibrating motion that travels through a conductive (non-vacuous) medium. Sound results from the longitudinal motion of the particles of the medium through which the mechanical sound wave travels.
Should the sound wave move from left to right through air, a byproduct will be the displacement of air particles as the energy of the sound wave passes. The motion of the particles exists in both parallel and non-parallel forms to the direction in which energy is being transported. Hence, this is the trait that characterizes sound as a Longitudinal Wave.
An example of the sound wave moving in a longitudinal direction is a vibrating tuning fork. As the tines of the fork vibrate back and forth, they exert pressure upon adjacent air particles.
Because of this phenomenon, sound is a combination of pressure variances. Due to the longitudinal motion of the air particles, there are pockets where the air particles are pressed together (compressions) and other regions where the air particles are spread apart (rarefactions, or rarifications). The compressions are regions in which high air pressure has clustered (condensation) whereas rarefactions are regions comprised of low air pressure (dilation).
D. Sound Waves. Measurement Methods
Waves can be measured in a range of different ways: by their amplitude, wavelength, frequency, speed, and, at times, their phase.
Amplitude is a metric method associated with hearing. It is commonly grouped with intensity, loudness, and (or) volume.
The wavelength is easy enough to detect, you simply note, during one complete wave cycle, the distance a disturbance travels through the medium in one complete wave cycle.
Once every wave cycle, a wave will repeat its pattern. For this reason, the wavelength is sometimes referred to as the length of the repeating pattern or the length of one complete cycle.
In the case of transverse waves, 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. However, because longitudinal waves do not have crests or troughs, their wavelengths must be measured in a different manner.
Thus, because longitudinal waves are comprised of repeating patterns of compressions and rarefactions, their wavelengths can commonly be measured as the distance from one compression to the next compression, or the distance from one rarefaction to the next rarefaction.
Sometimes referred to as pressure waves, sound waves encompass repeating patterns of alternating high and low pressure regions moving through a medium. Typically, in a sound or pressure wave, these alternating fluctuations in pressure occur at regular, set times.
If each fluctuation were to be plotted on a grid with pressure and time continuums, the final markings would indicate a sine curve, defined as random x plotted points supported by the consistency of y markings.
Lastly, the speed of sound depends upon the type of medium and its state. It is generally affected by two things: elasticity (ease with which molecules move or degree to which molecules move away from their neutral position when disturbed) and inertia (the denser the air or medium, the more inertia the sound wave has).
E. Sound Waves. Summary
A sound wave is not a transverse wave with crests and troughs, but rather a longitudinal wave with compressions and rarefactions. These regions of high pressure and low pressure, known respectively as compressions and rarefactions, are established as the result of the vibrations of the sound source.
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