BASIC SCIENCE FOR CLASS 8 AMARNATH MISHRA,PHD GEETA MEHTA ,PHD DEEPTI LEHRI,PHD RAJESH MOHAN, PHD

                Physic

                 Ch-2 

                Sound

### A. Answer the following in not more than 20 words.

1. **Name the three characteristics that help us distinguish one sound from another.**

   - Pitch, loudness, and timbre.

3. **What is the pitch of a sound? What does it depend on?**

   - Pitch depends on the frequency of the sound wave.

2. **What does the loudness of a sound depend on?**

   - Loudness depends on the amplitude of the sound wave.

4. **What would happen to the sound produced by a stretched string if you increased its tautness?**

   - The pitch of the sound would increase.

5. **What determines the pitch of a percussion instrument?**

   - The size, tension, and material of the instrument.

7. **What are infrasonics?**

   - Sounds with frequencies below the human hearing range (<20 Hz).

6. **What happens to the sound produced when you strike a glass as you increase the level of water in it?**

   - The pitch of the sound decreases.

9. **How do bats use echoes?**

   - Bats use echoes for echolocation to navigate and hunt.

8. **Name two animals which can hear sounds that we cannot hear.**

   - Dogs and dolphins.

2. **What is the basic difference between music and noise? What are regular vibrations?**

   - Music has regular vibrations; noise has irregular vibrations.

### B. Answer the following in not more than 40 words.

1. **What are harmonics? Which characteristic of a sound do they determine?**

   - Harmonics are higher frequency overtones of a fundamental frequency, determining the timbre of a sound.

3. **What determines the harmfulness of noise from the point of view of health? Mention two sources of noise pollution.**

   - Harmfulness depends on intensity and duration; sources include traffic and industrial machinery.

4. **Why do we see a flash of lightning before we hear the accompanying sound of thunder? Which characteristic of sound determines how far it can travel before dying out?**

   - Light travels faster than sound; loudness determines travel distance.

5. **Name the organ which helps us speak. How are sounds produced by it?**

   - The larynx produces sounds by vibrating vocal cords.

6. **What are echoes? Why do we not hear them all the time?**

   - Echoes are reflected sound waves; they require sufficient distance and reflective surfaces.

7. **How are echoes used in medicine?**

   - Echoes are used in ultrasound imaging to view internal body structures.

### C. Answer the following in not more than 100 words.

1. **Explain how sound travels through air. Why does sound travel faster through solids than through air?**

   - Sound travels through air as a wave of pressure variations caused by vibrating particles. It moves faster through solids because particles in solids are more tightly packed, allowing energy to transfer more quickly between them compared to the more dispersed particles in air.

2. **Explain how we hear.**

   - Sound waves enter the ear canal, causing the eardrum to vibrate. These vibrations are transmitted through the ossicles in the middle ear to the cochlea in the inner ear, where they are converted into electrical signals sent to the brain via the auditory nerve.

### D. Complete the following.

1. **The frequency of sound is measured in .......**

   - Hertz (Hz).

2. **The maximum displacement of a vibrating body from its position of rest is its.......**

   - Amplitude.

3. The frequency of ultrasonics is **higher** than the range of human hearing.

4. Sound vibrations die out after travelling some distance due to the loss of **energy**.

5. Sound needs a **medium** to travel.

6. Industrial noise can be reduced by proper **soundproofing**.

7.The reflecting surface has to be at least **17 meters** away for us to hear an echo distinctly.

Choose the correct option in each of the following:

1. What helps us distinguish between the same note of the same loudness produced by two musical instruments?

   - (c) Harmonics

2. The frequencies of harmonics are

   - (c) multiples of the frequency of the fundamental

3. Sound travels

   - (d) faster in solids than in air

4. The decibel is a unit that measures

   - (a) loudness

5. Women are more high-pitched than men because their vocal cords

   - (b) are shorter

6. The ceilings of theatres are covered with special materials to

   - (b) minimise the reflection of sound