Do compressions and rarefactions of a longitudinal sound travel in the same or opposite directions?

One important characteristic of sound waves is that they are mechanical waves. This means that they travel through a medium. Sound waves can travel through all sorts of mediums. Normally, we hear sound waves that have traveled through air, but sound can also travel through water, wood, the Earth, and many other substances. Sound cannot travel through a vacuum like outer space, however.

The source of sound waves is something vibrating. This vibration causes a disturbance in the molecules around the source. The energy of the wave is transferred from molecule to molecule within the medium.

Longitudinal Waves

Another characteristic of sound waves is that they are longitudinal waves. This means that the disturbance of the wave travels in the same direction as the wave. As the molecules vibrate and transfer energy to each other they cause a wave that moves in the direction of the vibration.

The longitudinal characteristic of sound waves can be seen the picture below. Here you can see how the molecules move in a left to right motion causing the wave and the disturbance to move in the same direction. In some areas of the wave the molecules get bunched together. This is called compression. In other areas the molecules become spread out. This is called rarefaction.

What is the wavelength of a sound wave?

We studied how the wavelength of a transverse wave is measured from crest to crest or trough to trough. This is fairly easy to see when looking at a graph. However, sound waves are different as they are longitudinal. To determine the wavelength of a sound wave you measure from compression to compression or rarefaction to rarefaction.

Pressure Waves

Sound waves can also be thought of as pressure waves. This is because the compressions and rarefactions that move through sound waves have different pressures. The compressions are areas of high pressure while the rarefactions are areas of low pressure.

What is the amplitude of a sound wave?

Sometimes you will see a graph of a sound wave that looks like a sine wave (see below). This is different from the graph of a transverse wave. The peaks and valleys of this wave graph the changes in pressure that occur in the wave. From this graph we can determine the amplitude of the sound wave. The amplitude is the peak of the compression or rarefaction on the graph.

Intensity of a Sound Wave

Sound waves are sometimes measured using a quantity called intensity. The intensity of a sound wave (I) is equal to the sound power (P) over the area (A):

As seen in Figure 9.2, there are regions where the medium is compressed and other regions where the medium is spread out in a longitudinal wave.

The region where the medium is compressed is known as a compression and the region where the medium is spread out is known as a rarefaction.

(ii) For hearing sound, there must be (a) a vibrating body, (b) a material medium for its propagation, and (c) a receiver, such as the human ear.

2. Propagation of Sound:

(i) Sound energy does not propagate through a vacuum.

(ii) When the particles of a medium, oscillate in the same direction, in which wave is being propagated, such a wave is called a longitudinal wave.

(iii) While the transverse wave is being propagated in a medium, the particles of the medium oscillate at right angles to the direction of wave propagation.

(iv) Transverse waves can be produced in solids and liquids, but not in gases.

(v) For a transverse wave, the highest point on the elevation or hump is called the crest, whereas the lowest point on the depression or the hollow is called a trough.

(vi) Compression is a region in a longitudinal wave, where the particles of the medium are crowded together. It is a region of high pressure and high density.

(vii) Rarefaction is a region in a longitudinal wave, where the particles of the medium are spread wide apart. It is a region of low pressure and low density.

(viii) The change in density of a medium from a maximum value to the minimum value and again to a maximum value, in case of a longitudinal wave, is called one oscillation.

(ix) The number of compressions and rarefactions (taken together) passing through a point in one second, is called frequency ν. Its SI unit is hertz (Hz).

(x) The time taken by two consecutive compressions or rarefactions to cross a point is called time period T.

(xi) The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength λ.

(xii) The magnitude of maximum displacement of a vibrating particle about its mean position is called amplitude.

(xiii) The pitch of a sound is determined by its frequency, i.e., the higher the frequency, the more is the pitch hence shriller is the sound.

(xiv) The loudness of sound is determined by the amplitude, i.e., the more the amplitude, the louder is the sound.

(xv) The property by virtue of which the note of same pitch and same frequency can be distinguished is called timbre or quality of sound.

(xvi) Sound travels fastest in solids, slower in liquids and slowest in gases.

(xvii) Some useful relationships (a) ν=1T, (b) v=λν where v is the speed of wave.

3. Reflection of Sound:

(i) A conical tube commonly used for addressing a small group of people, is called megaphone.

(ii) The phenomenon due to which repetition of sound is heard after reflection from a distant object after the original sound from a given source dies out is called an echo.

(iii) For hearing an echo, the minimum distance between the source of sound and reflecting body should be 17 m.

(iv) A sound created in a big hall will persist by repeated reflection from the walls until it is reduced to a value where it is no longer audible. The repeated reflection that results in this persistence of sound is called reverberation.

4. Range of Hearing

(i) Vibrations within the frequency range of 0-20 Hz are called infrasonic vibrations. Humans cannot hear them.

(ii) Vibrations within the frequency range of 20 Hz to 20,000 Hz are called sonic vibrations. They can be heard by human beings.

(iii) Vibrations above 20,000 Hz frequency range, are ultrasonic vibrations. Humans cannot hear them.

5. Application of Ultrasound:

(i) The SONAR technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarines, icebergs, sunken ships etc.

How do the compressions and rarefactions travel in a longitudinal wave?

Do compressions and rarefactions travel in the same speed?

What is the difference between compressions and rarefactions in longitudinal waves?

Is rarefaction the opposite of compression?