What Is Sound? Vibration and the Propagation of Sound

Have you ever used a musical instrument like a guitar, drum or violin? When we strike the strings of a guitar, we hear a sound. Same with the drum, when we hit a drum we hear a ‘thump’ sound. Also, how is one instrument able to create a wide variety of sounds? How does this happen? What is sound? How do we make a sound? How is the sound produced and about the propagation of sound?

Table of Contents

What Is Sound?

A sound is a form of energy, just like electricity, heat or light. Sound is one of the important senses of the human body. Some sounds are pleasant, and some are annoying. We are subjected to various types of sound all time. Sound waves are the result of the vibration of objects. Let’s examine some sources of sounds like a bell. When you strike a bell, it makes a loud ringing noise. Now, instead of just listening to the bell, put your finger on the bell after you have struck it. Can you feel it vibrating? This is the key to sound. It is even more evident in guitars and drums. You can see the wires vibrating every time you pluck it. When the bell or the guitar stops vibrating, the sound also stops.

The to and fro motion of the body is termed vibration. You can see examples of vibrations everywhere. Vibrating objects produce sound. Some vibrations are visible; some aren’t. If you pull and then release a stretched rubber band, the band moves to and fro about the central axis and while doing, so it also produces a sound. The sound moves through a medium by alternately contracting and expanding parts of the medium it is travelling through.

In physics, the sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.


Read More: Forms of Energy

Watch the video and learn about the fundamentals of sound

Sound Wave Characteristics

After understanding what is sound, let us study the characteristics of the sound wave. The distance between two consecutive peaks or troughs is termed as the wavelength of the wave or the period. The number of cycles per unit time is termed as the frequency of the sound. Frequency is measured in cycles per second or Hertz.

The faster an object vibrates, i.e. the higher the frequency, the higher the pitch of the sound. The difference between the voices of a man and a woman must be clearly evident to you. The voice of a man has a lower frequency which contributes to the deepness of the bass in the voice. Women, in contrast, have a voice with higher frequency resulting in a higher shrillness or pitch.

Propagation Of Sound

Closing Our Ears When We Hear Loud Noise

If you hear a very loud sound, what do you do? You cover your ears. How do you think that helps? When you cover your ears, you shut off the air inside your ears from the rest of the atmosphere. The sound waves travelling around you are now unable to get through to your ear or the intensity of the sound you hear is greatly reduced. Blocking your ears creates a discontinuity in the medium due to which the flow of sound energy is disturbed. Through this, we can make a very important observation; Sound waves rely on the medium for propagation. The propagation of the sound wave is not possible through the vacuum. The medium here can be gas, liquid or solid. The speed of sound when it is travelling through a medium depends on the type of medium. The speed of sound when travelling through air is 343 m/s or 1,235 km/h.

Sound Fun Facts

Speed of Sound

The speed of a sound wave is affected by the type of medium through which it travels. Sound waves travel the fastest in solids due to the proximity of molecules. Likewise, sound waves travel slowest in gases because gases are spread far apart from one another. The state of the medium through which sound travels is not the only factor that affects a sound’s speed. The speed of a sound wave can also be affected by the density, temperature, and elasticity of the medium through which the sound waves travel. Below is a table, we have listed the speed of sound in various materials.

Speed of Sound

Can Sound Travel in Space?

A medium is essential for the propagation of sound. Sound cannot travel through a vacuum because there are no molecules that can be compressed and expanded in space. Our voice is produced by the vibration of strings known as the vocal cords which are inside Adam’s apple. When you make a sound, its vibration travels through the air, and when it reaches your brain through your ears, it is interpreted as sound. In this case propagation of sound takes place through the air medium. How your brain and ear decode pressure variation in sound waves into sound is fascinating!

Question: An electric bell is placed inside a transparent glass jar. The bell can be turned on and off using a switch on the outside of the jar. A vacuum is created inside the jar by sucking out the air. Then the bell is rung using the switch. What will we see and hear?


Answer: In this case, we can see the bell ringing, but we cannot hear it. This is because sound needs a medium to travel and it cannot travel in a vacuum. As a result, we do not hear the bell ringing.

Human Hearing and Speech

Humans can hear sounds ranging from 20 Hz to 20 kHz. Sounds with frequencies above the range of human hearing are called ultrasound. Sounds with frequencies below the range of human hearing are called infrasound. The typical sound produced by human speech has frequencies in the order of 100 to 1,000 Hz.

Characteristics of Sound Waves

Reflection of Sound

Frequently Asked Questions – FAQs


List physical factors that affect sound propagation

Following are a few factors that affect the propagation of sound:
  • 1. Atmospheric Turbulence: If the atmosphere in which the sound wave is travelling is turbulent, sound waves would scatter due to velocity fluctuations of the medium.
  • 2. Wind Gradient: Sound propagating along the wind would bend downwards while sound propagating against the wind would bend upwards.
  • 3. Temperature Gradient: Sound waves travel faster in a warm atmosphere near the surface of the earth. Here, there is upward refraction of sound waves. In case of a decrease in temperature at higher altitudes, the refraction would be downwards.
  • Q2

    Which property of sound is affected by the change in temperature?

    The wavelength of the sound is affected by the change in temperature.

    What waves are used in Sonography?

    Ultrasonic Waves are used in Sonography.

    What do you mean by an echo?

    An echo is a reflection of sound that arrives at the listener with a delay after the direct sound.

    Which property of sound is affected by the change in temperature?

    The wavelength of the sound is affected by the change in temperature.

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