Case Study Questions for Class 9 Science Chapter 10 Sound Waves (Exploration Book) 2026-27

If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This reflected sound that we hear is called an echo. The sensation of any sound persists in our brain for about 0.1 seconds. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 s.

If we take the speed of sound to be 344 m/s at a given temperature in air, the sound must go to the obstacle and reach back to the ear of the listener on reflection after 0.1 s. Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least 34.4 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance. In large halls, repeated reflection of sound results in persistence of sound, which is called reverberation.

Sound waves with frequencies higher than 20 kHz (20,000 Hz) are called ultrasonic waves or ultrasound. Because of their high frequency, ultrasounds are able to travel along well-defined paths even in the presence of obstacles. This unique property makes them extensively useful in various industrial and medical applications.

In industries, ultrasound is generally used to clean parts located in hard-to-reach places, such as spiral tubes and electronic components, and to detect cracks or flaws in heavy metal blocks used in construction. In the medical field, ultrasonic waves are made to reflect from various parts of the heart to form images of the heart. Ultrasound scanners are also used to obtain images of internal organs like the liver, gall bladder, uterus, and kidneys, helping doctors detect abnormalities such as stones or tumors.