Sound Waves

Mayank Sharma
8 Min To Read
November 18, 2023
- Sound waves

Sound Waves are longitudinal mechanical waves. When they hit our hear drum, they produce in us a sensation of hearing. Generally, sound waves have low frequency and high wavelength.

Sound Waves cannot travel in vacuum. It needs a medium to travel from one point to another. That’s why we cannot hear sound in space, as there is no air there.

Table of Contents

Speed of Sound
Types of Sound Waves
Characteristics of Sound Wave (Musical Sound)

Speed of Sound

The speed of sound depends upon multiple factors. Let’s list them down.

Elasticity of medium

More the elasticity of a medium, the more will be the speed of sound in it. The increasing order of elasticity of gases, liquids and solids is:

Gases < Liquids < Solids

As solids are the most elastic, the speed of the sound is maximum in solids. It is the minimum in gases.

If V_s, V_l and V_g are speed of sound waves in solid, liquid and gases, then:

V_s > V_l > V_g.

Note

When sound enters from one medium to another medium, its frequency remains unchanged. Only its speed and wavelength change.

Density of medium

Less the density of a medium, more is the speed of sound.

For example, the density of humid air is less than the density of dry air. That’s why the speed of sound is more in humid air than in dry air. You must have noticed that we hear the siren of a train up to a far distance in rainy season than that in summer season.

Note

Speed of sound waves is more in solids and liquids than in gases. As solids and liquids are denser than gases, one may assume that a wave will find it harder to move across such mediums. But solids and liquids are more difficult to compress too, i.e. they have a higher values of bulk modulus. It more than compensates for their higher densities than gases.

Note

Though the speed of sound is dependent on the density of medium, it is independent of pressure, i.e. the speed of sound remains unchanged by the increase or decrease of pressure.

Temperature

More the temperature of a medium, more is the speed of sound. For every 1°C rise in temperature, the speed of sound in air increases by 0.61 m/s.

The relation of speed of sound wave with temperature is given by the formula:
v = k√T, where, v = speed of sound, T = temperature.

Effect of the speed of the medium

If a medium is speeded up then:

the speed of sound increases in the same direction, and
the speed of sound decreases in the opposite direction.

Speed of Sound Vs. Speed of Light

The speed of sound is pretty slower as compared to the speed of light. That’s why we hear the thunder of a cloud a little later after we see the flash of lighting.

Shock Waves

When the speed of a body becomes more than the speed of sound in air, it is called supersonic speed. This is the case with many modern age jet planes.

Such a fast-moving body leaves behind it a conical region of disturbance which spreads continuously. Such a disturbance is called shock wave.

These shock waves carry huge energy and may crack the window panes in the vicinity, or even damage a building.

Mach number

Mach number is the ratio of the speed of the sound source and speed of sound in air.

So, Mach-Number = (v/v_s) , where v = velocity of sound source and v_s = velocity of sound,
If Mach-Number (v/v_s) > 1, then the velocity of the sound source is called Supersonic.
If Mach-Number (v/v_s) > 5, then the velocity of the sound source is called Hypersonic.

Types of Sound Waves

Sound Waves may be of various kinds depending on their frequency.

Audible waves: These sound waves lie in the frequency range of 20 Hz to 20000 Hz. Human ear is sensitive to these sound waves.
Infrasonic waves: These sound waves have frequencies less than 20 Hz. These waves are produced by big-size sources, such as earthquakes, volcanic eruptions, ocean waves, and by big animals such as elephants and whales.
Ultrasonic waves: These sound waves have frequencies greater than 20000 Hz. They were produced for the first time by Galton. Human ear cannot detect these high-frequency waves. Though certain animals can detect these waves, such as dogs, bats, mosquitos.

Applications of Ultrasonic Waves

Ultrasonic waves are used in SONAR (Sound Navigation And Ranging). SONAR is used to measure the depth of a sea, to locate enemy submarines and shipwrecks.
Ultrasonic waves are used for sending signals.
Ultrasonic waves have many medical uses, e.g. diagnostics, surgery and cure: They are used in ultrasonography. They are used to detect tumors and teeth cavity etc. Ultrasonics are used in bloodless surgical operations. Ultrasonic radiation is used to cure various neurological diseases and arthritis.
Ultrasonic waves are also used for cleaning purposes, e.g. for cleaning cloths, aeroplanes, machinery part of clocks and removing lamp-shoot from the chimney of factories.
Ultrasonics are also used to coagulate the dust particles in winter season. This helps in diminishing the mists and fogs at the airports, which facilitates the landing of aircrafts.
Ultrasonics also has sterilizing properties. If a liquid is exposed to ultrasonics, the bacteria present in it get destroyed. For example, in many countries contaminated milk is purified by passing it through ultrasonics.

Ultrasonic waves are not only used by humans, but also by some animals. Some animals such as bat can not only detect but also produce ultrasonic sounds, and navigate based on their echoes.

Characteristics of Sound Waves (Musical Sound)

Now, let’s understand about the characteristics of sound waves which are very useful in the study of music.

Loudness and Intensity

Loudness depends on intensity of sound. In layman’s terms, more intense sound is louder and less intense sound is weaker.

In technical terms, Intensity of sound at any point in space is the amount of energy passing normally per unit area held around that point per unit time.

The SI unit of intensity is Joule/sec-meter² or watt/meter². Unit of loudness is bel.

Loudness of a sound is measured using the concept of Sound level (β).
β= 10 log₁₀ (I/I₀) dB
Where, I is intensity of the source producing sound waves, and I₀ is the reference intensity which is equal to 10^-12 W/m²

Note

As per World Health Organisation (WHO):

Sound upto 45 db is safe for the human ears.
Sound more than 75 db is assumed to be dangerous, and
Sound more than 150 db can severely affect the mental health of human beings (may make them mad).

Now, let’s see the various factors on which the intensity of a sound depends on.

The intensity is of musical sound is directly proportional to:

the square of the amplitude of vibration of the source,
the square of the frequency of the wave,
the wave speed,
the density of the medium,
the elasticity of the medium, and
the size of the source: a larger source has larger intensity and vice-versa.

The intensity is of musical sound is inversely proportional to:

the square of the distance of the source.

Beats

Beats is a phenomenon wherein two sound waves of nearly equal frequency interfere, which leads to the intensity of resultant wave increasing and decreasing with time.

Beat frequency is equal to difference of the frequencies of the two waves.

Pitch

Concept of pitch is related to the shrillness and base of the voice. It distinguishes a shrill note from a grave note.

The pitch of a sound depends on the frequency of the source. More the frequency of vibration of a source, higher is the pitch of the note emitted and vice-versa.

Higher the frequency of vibration of a source, the higher is the value of pitch, and shriller is the sound. This is the case with the voice of children and women.
Lower the frequency of vibration of a source, the lower is the value of pitch, and graver is the sound. This is the case with the voice of men.

Quality or Timber

Quality or Timber of sound waves (or musical note) depend on the form of the sound wave (not on its frequency). The form of sound waves in turn is reshaped by the presence of different harmonics (The harmonics present do not affect the frequency of the fundamental tone).

So, even the sounds produced by two sources having the same intensity and pitch may appear different. Different harmonics present in the voice of a person or a musical instrument reshape the wave form, which in turn causes the quality of different notes to differ. We can distinguish between various sounds due to the difference in their quality. This is how we identify and detect the voices of well-known persons and various musical instruments.

Other Characteristics

Sound waves have many other kinds of characteristics too, e.g. reflection, refraction and diffraction (just like light waves).

These have been listed below:

Reflection of sound: It is the rebounding back of sound when it strikes a hard surface. The working of megaphone, sound board and ear trumpet is based on the phenomenon of reflection of sound.
Diffraction of sound: Just like light waves, even sound waves get diffracted. Though in comparison the sound waves are diffracted more broadly. That’s why we can hear the voice of a person even sitting out of sight.

Echo

Multiple reflections of sound waves may cause the sound to repeat back again and again. This phenomenon is called Echo.

The essential condition for the formation of an echo is that the reflected wave must be clearly distinguished from the direct wave by a human ear. A human ear cannot distinguish between two sounds arriving within 0.1 seconds of each other. This is possible only when the surface reflecting the sound wave is atleast 16.6 m away from the original sound source.