🎵 Chapter 18 – Sound: Production of Sound

📘 Maharashtra Board Textbook Solutions – Standard Seven

📝 1️⃣ Fill in the Blanks

(a) Sound is generated by the rhythmic vibrations of any object.

(b) The frequency of sound is measured in hertz (Hz).

(c) If the amplitude of sound is decreased, its loudness also decreases.

(d) A medium is necessary for propagation of sound.

🔗 2️⃣ Match the Pairs

Group ‘A’Group ‘B’
(a) Flute(3) Vibrations in the air
(b) Frequency(4) Measured in Hz
(c) Sound level(5) Decibel
(d) Ultrasonic sound(2) Frequency more than 20000 Hz
(e) Infrasonic sound(1) Frequency less than 20 Hz

🔬 3️⃣ Give Scientific Reasons

(a) Why did people earlier listen for a distant train by putting their ear to the rail?

  1. When a train moves, its wheels create vibrations due to friction with the rail.
  2. These vibrations produce sound.
  3. Sound travels faster and farther through solids like желез rails than through air.
  4. By placing an ear on the rail, these vibrations can be heard clearly.

👉 Therefore, people could guess the arrival of a train in advance.

(b) Why are the sounds of a tabla and a sitar different?

  1. A tabla produces sound when its stretched membrane vibrates.
  2. A sitar produces sound when its stretched strings vibrate.
  3. Since the vibrating objects are different, their frequencies and pitch differ.

👉 Hence, the sounds produced are different.

(c) Why can your friend not hear you on the moon?

  1. Sound needs a medium (like air) to travel.
  2. The moon has no atmosphere (no air).
  3. Without a medium, sound cannot propagate.

👉 Therefore, your friend cannot hear you on the moon.

(d) Why can we hear a mosquito’s wings but not the movement of our hands?

  1. A mosquito’s wings vibrate 300–600 times per second (300–600 Hz), which is within the audible range.
  2. Our hands cannot move so fast; their frequency is below 20 Hz (infrasonic range).
  3. Humans cannot hear infrasonic sounds.

👉 Hence, we hear a mosquito’s wings but not our hand movements.

❓ 4️⃣ Answer the Following Questions

(a) How is sound produced?

Sound is produced when an object vibrates.
When an object is struck, plucked, or shaken, it starts vibrating.
These rhythmic vibrations generate sound.

(b) What does the intensity of sound depend upon?

The intensity of sound depends on:

  1. Amplitude of vibrations – Greater amplitude means louder sound.
  2. Frequency of vibrations – Higher frequency generally increases intensity.
  3. Distance from the source – Sound becomes weaker as distance increases.

(c) How is frequency related to the length and amplitude of a pendulum?

  1. As the length increases, the frequency decreases.
  2. As the length decreases, the frequency increases.
  3. The amplitude has very little effect on frequency.

👉 Thus, frequency mainly depends on the length of the pendulum.

(d) Two Ways to Change the Pitch of a Stretched String

🎼 1. By Changing Tension:

  • Increasing tension → Frequency increases → Sound becomes shriller.
  • Decreasing tension → Frequency decreases → Sound becomes less shrill.

🎼 2. By Changing Length:

  • Decreasing length → Frequency increases → Sharper sound.
  • Increasing length → Frequency decreases → Lower pitch sound.

👉 These two methods help change the pitch of sound in string instruments.

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