Another concept that one must study when going through this chapter is the propagation of sound. In simple words, the traveling of sound is referred to as sound propagation. When the particles in a medium move in a to and fro motion, the sound is propagated. When an object starts vibrating, the particles around the medium also start vibrating. The particles that come in contact with the vibrating object get displaced from their equilibrium position. 

It becomes a kind of chain reaction when each particle disturbs the other particle in contact with it. This disturbance travels from the source to the listener. Only the disturbance created by the body in vibration travels through the medium, and particles themselves do not move forward.

By medium, we mean a substance or matter through which transmission of sound occurs. The medium can be gas, liquid, or solid and is necessary for the propagation of sound.

Let us now focus on the nature of sound waves. It is prudent to understand that a sound wave is a longitudinal, mechanical wave. Since sound cannot travel in a vacuum and needs a medium to travel through, the sound is absent in outer space. 

Moreover, the sound is pressure variation. An increased pressure region on a sound wave is referred to as condensation or compression. A decreased pressure region on a sound wave is referred to as dilation or rarefaction. Sound waves have several characteristics that help in measuring them. These characteristics are intensity, amplitude, volume, and loudness.

The factors affecting the speed of sound are-

• The Medium Temperature- If the temperature of the medium is higher, the speed of sound in the medium is also higher.
• The Medium Density- We know that medium is necessary for sound to travel. If the density of the medium is higher, sound travels at a faster pace through the medium. Conversely, if the density is low, the speed of sound propagation is slower.

Also referred to as the infrasound, the infrasonic sound is below 20Hz sound frequency. It is produced when the vibration is very slow. We cannot hear this sound as our ears are not adapted to hear the sound of such a low frequency. Only animals like rhinoceros, elephants, whales, etc., can hear such a low-frequency sound.

Ultrasonic sound is a sound with more than 20000 Hz frequency. We humans cannot hear this sound, but animals like cats, dogs, monkeys, bats, and deer can hear this sound.

Supersonic sound is referred to sound that moves at a greater speed than the speed of sound. Thus when an object moves at speed greater than the speed of sound, it is known as supersonic sound. Bullets, jet fighters, etc., are examples of objects moving at supersonic speed.

Thus, the difference between Ultrasonic and Supersonic sound lies in the speed of sound where ultrasonic sound is more than 20000 Hz sound frequency, supersonic sounds when an object travels at a greater speed than sound.

It is important to note that sound takes time to travel from the source to our ears. The sound speed informs us about the rate sound travels from the source to our ears. The speed of sound is dependent on the nature of the medium it passes through. For example, in the air, sound travels at a speed of 344 m/s, iron 5130 m/s, and water 1500m/s. When comparing sound speed in different mediums, it is found that sound speed is low in dry air but fastest in aluminum.

Ultrasonic sound with more than 20000 Hz frequency is helpful in different applications. Here are some of the ultrasonic frequencies and their applications-

• Echocardiography- Ultrasonic frequencies help in capturing heart images. It helps identify problems associated with the organ, helping the doctor accurately diagnose the issue and provide treatment.
• Ultrasonography- It is also known as ultrasound scans. Medical professionals recommend this test to capture the images of internal organs like the uterus, kidney, and liver. It helps in the quick diagnosis and treatment of issues related to any of these organs. Ultrasound images travel through these organs. These images get reflected and converted into electrical signals, which are then monitored on the screen.

Sound is one of the most interesting chapters Class 9th students study as part of their curriculum. The topic touches on various points and helps the student understand the basic concepts of Sound and how it applies in our day-to-day lives. The chapter talks about ultrasonic, supersonic, and infrasonic sounds and their applications. Studying this chapter carefully shall help the student to study advanced topics related to Sound in their higher classes.

1.  Why are sound waves also referred to as mechanical waves?
   Sound waves propagate through a medium with particles present in it. It is for this reason; sound waves are also known as mechanical waves.
2.  What is wavelength?
   Wavelength is defined as the distance traveled between two closest points in a wave in the same vibration phase.
3.  What is the formula of the speed of sound?
   Speed of sound is derived by multiplying wavelength and frequency.