Материал: Андрианова English Thesaurus of Student-Physicist 2013

Characteristics of sound wave

The general motion of elements of air near a vibrating object is back and forth between regions of compression and rarefaction. This back- and-forth motion of elements of the medium in the direction of the disturbance is characteristic of a longitudinal wave. By contrast, in a transverse wave, the vibrations of the elements of the medium are at right angles to the direction of travel of the wave.

Categories of Sound Waves

Sound waves fall into three categories covering different ranges of frequencies. Audible waves are longitudinal waves that lie within the range of sensitivity of the human ear, approximately 20 to 20 000 Hz. Infrasonic waves are longitudinal waves with frequencies below the audible range. Earthquake waves are an example. Ultrasonic waves are longitudinal waves with frequencies above the audible range for humans and are produced by certain types of whistles. Animals such as dogs can hear the waves emitted by these whistles.

Applications of Ultrasound

Ultrasonic waves are sound waves with frequencies greater than 20 kHz. Because of their high frequency and corresponding short wavelengths, ultrasonic waves can be used to produce images of small objects and are currently in wide use in medical applications, both as a diagnostic tool and in certain treatments. Internal organs can be examined via the images produced by the reflection and absorption of ultrasonic waves. Although ultrasonic waves are far safer than x-rays, their images don't always have as much detail. Certain organs, however, such as the liver and the spleen, are invisible to x-rays but can be imaged with ultrasonic waves.

The speed of sound

The speed of a sound wave in a fluid depends on the fluid's compressibility and inertia. The speed of sound is much higher in solids than in gases, because the molecules in a solid interact more strongly with each other than do molecules in a gas. In general, sound travels faster through solids than liquids and faster through liquids than gases, although there are exceptions. The speed of sound also depends on the temperature of the medium.

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Fig. 14. Thesaurus for Unit 14

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Energy and intensity of sound waves

As the tines of a tuning fork move back and forth through the air, they exert a force on a layer of air and cause it to move. In other words, the tines do work on the layer of air. The fact that the fork pours sound energy into the air is one of the reasons the vibration of the fork slowly dies out. Intensity of a wave on a given surface is defined as the rate at which energy flows through the surface. The average intensity of a wave on a given surface is defined as the rate at which energy flows through the surface, divided by the surface area.

The faintest sounds the human ear can detect at a frequency of 1 000 Hz have an intensity of about 10~12W/m2. This intensity is called the threshold of hearing. The loudest sounds the ear can tolerate have an intensity of about 1 W/m2 (the threshold of pain). The relative intensity of a sound is called the intensity level or decibel level.

Spherical and plane waves

If a small spherical object oscillates so that its radius changes periodically with time, a spherical sound wave is produced. The wave moves outward from the source at a constant speed. Because all points on the vibrating sphere behave in the same way, we conclude that the energy in a spherical wave propagates equally in all directions. This means that no one direction is preferred over any other.

The distance between adjacent wave fronts equals the wavelength. The radial lines pointing outward from the source and perpendicular to the arcs are called rays.

Now consider a small portion of a wave front that is at a great distance from the source. In this case, the rays are nearly parallel to each other and the wave fronts are very close to being planes. At distances from the source that are great relative to the wavelength, therefore, we can approximate the wave front with parallel planes, called plane waves. Any small portion of a spherical wave that is far from the source can be considered a plane wave.

The Doppler effect

If a car or truck is moving while its horn is blowing, the frequency of the sound you hear is higher as the vehicle approaches you and lower as it moves away from you. This is one example of the Doppler effect, named for the Austrian physicist Christian Doppler (1803-1853), who

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discovered it. The same effect is heard if you're on a motorcycle and the horn is stationary: the frequency is higher as you approach the source and lower as you move away.

Standing wave

Standing waves can be set up in a stretched string by connecting one end of the string to a stationary clamp and connecting the other end to a vibrating object, such as the end of a tuning fork, or by shaking the hand holding the string up and down at a steady rate. Traveling waves then reflect from the ends and move in both directions on the string. The incident and reflected waves combine according to the superposition principle. If the string vibrates at exactly the right frequency, the wave appears to stand still - hence its name, standing wave.

Forced vibrations and resonance

In Unit 13 we learned that the energy of a damped oscillator decreases over time because of friction. It's possible to compensate for this energy loss by applying an external force that does positive work on the system.

For example, suppose an object-spring system having some natural frequency of vibration is pushed back and forth by a periodic force with frequency. The system vibrates at the frequency of the driving force. This type of motion is referred to as a forced vibration. Its amplitude reaches a maximum when the frequency of the driving force equals the natural frequency of the system called the resonant frequency of the system. Under this condition, the system is said to be in resonance.

1.Explain the process of producing a sound wave.

2.What characteristics of a sound wave do you know?

3.What are the three categories of a sound wave?

4.Give the definition of ultrasonic wave.

5.What do you know about Doppler effect?

6.In what conditions do we observe a standing wave?

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Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

3. Fill in the gaps with the missing words from the list:

sound wave, longitudinal sound wave, intensities, standing longitudinal waves, wave speed

1.We can use a sinusoidal curve to represent a _____.

2.The motion of the elements of the medium in a _____ is back and forth along the direction in which the wave travels.

3.The _____ depends on an elastic property of the medium and on an inertial property of the medium.

4.The loudest tolerable sounds have _____ about 1.0 X 1012 times greater than the faintest detectable sounds.

5._____ can be set up in a tube of air, such as an organ pipe, the result of interference between sound waves traveling in opposite directions.

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