MP Board · Class 9 · Science · Sound(a) Categorize sound waves into Infrasonic, Audible, and Ultrasonic based on their frequency range. Give one natural source or application for each category. (b) Explain in detail two practical applications of ultrasonic sound in: Non-destructive testing for flaw detection in metal blocks. Medical diagnostics (Echocardiography and Ultrasonography). (c) A sound wave has a frequency of $2\text{ kHz}$ and a wavelength of $35\text{ cm}$. Calculate: The speed of the sound wave. The time taken by this sound wave to travel a distance of $1.54\text{ km}$.

(a) Classification of Sound Waves Based on Frequency

1. Infrasonic Waves (Infrasound):

   • Frequency Range: Sound waves with frequencies below $20\text{ Hz}$.
   • Source/Example: Produced by earthquakes, volcanic eruptions, and large animals like elephants and whales for long-distance communication.

2. Audible Sound Waves:

   • Frequency Range: Frequencies lying between $20\text{ Hz}$ and $20,000\text{ Hz}$ ($20\text{ kHz}$).
   • Source/Example: Human voice, sound produced by musical instruments like guitar, flute, and piano.

3. Ultrasonic Waves (Ultrasound):

   • Frequency Range: Sound waves with frequencies higher than $20,000\text{ Hz}$ ($20\text{ kHz}$).
   • Source/Example: Emitted naturally by bats and dolphins for navigation and hunting; used artificially in sonar and medical imaging.

(b) Applications of Ultrasound

1. Flaw Detection in Metal Blocks:

   • Ultrasound waves are passed through a metal component being inspected.
   • Detectors placed on the opposite side receive the transmitted waves.
   • If there is even a minor crack, flaw, or void inside the block, the ultrasound waves get reflected back from the defect and do not reach the detector.
   • This indicates the exact presence and location of internal defects without breaking the metal block.

2. Medical Diagnostics:

   • Echocardiography: Ultrasonic waves are reflected from various parts of the heart to form detailed images of heart structure and cardiac valves.
   • Ultrasonography (USG): Ultrasound scanner passes high-frequency waves into the human body (e.g., liver, kidneys, uterus). The reflected waves are converted into electrical signals to form real-time images of internal organs and monitor fetus growth during pregnancy.

(c) Numerical Solution

Given:

• Frequency, $f = 2\text{ kHz} = 2000\text{ Hz}$
• Wavelength, $\lambda = 35\text{ cm} = 0.35\text{ m}$
• Distance, $s = 1.54\text{ km} = 1540\text{ m}$

1. Calculate Speed of Sound ($v$): $$\text{Formula: } v = f \times \lambda$$ $$v = 2000\text{ Hz} \times 0.35\text{ m} = 700\text{ m/s}$$

2. Calculate Time Taken ($t$): $$\text{Formula: } t = \frac{\text{Distance }(s)}{\text{Speed }(v)}$$ $$t = \frac{1540\text{ m}}{700\text{ m/s}} = 2.2\text{ s}$$

Answer:

1. The speed of the sound wave is $700\text{ m/s}$.
2. The time taken to travel $1.54\text{ km}$ is $2.2\text{ seconds}$.