Grade 7 Q&A: Chapter 18: Sound: Production of Sound

Concept Questions

Q1: How is sound produced? Give two examples of vibrating objects producing sound.

Answer: Sound is produced by vibrations. When an object vibrates, it sets the particles of the surrounding medium into motion, creating sound waves that travel to our ears.
Examples:

1. When a stretched string of a guitar is plucked, it vibrates and produces sound.
2. When a drum is hit, its stretched membrane vibrates, producing sound.

Q2: Why does sound need a medium to travel? Can it travel through a vacuum?

Answer: Sound needs a medium (solid, liquid, or gas) to travel because it propagates as vibrations of particles. These vibrations are transmitted from one particle to the next. In a vacuum, there are no particles, so there is nothing to vibrate and transmit the sound energy. Therefore, sound cannot travel through a vacuum.

Q3: What is loudness of sound? On what factor does it depend?

Answer: Loudness is the characteristic of sound that determines how strong or faint a sound is perceived. It depends on the amplitude of the vibration. A larger amplitude of vibration produces a louder sound, while a smaller amplitude produces a softer sound.

Q4: What is pitch of sound? On what factor does it depend?

Answer: Pitch is the characteristic of sound that determines how high or low a sound is perceived. It depends on the frequency of the vibration. A higher frequency produces a higher pitch, and a lower frequency produces a lower pitch.

Q5: What is the unit of frequency and loudness?

Answer: The unit of frequency is Hertz (Hz), and the unit of loudness (or sound level) is decibels (dB).

Q6: Differentiate between infrasonic, audible, and ultrasonic sound.

Answer:

• Infrasonic Sound (Infrasound): Sounds with frequencies less than 20 Hz. Humans cannot hear these sounds. (e.g., sounds produced by elephants, earthquakes).
• Audible Sound: Sounds with frequencies between 20 Hz and 20,000 Hz (20 kHz). This is the range of sound frequencies that humans can typically hear.
• Ultrasonic Sound (Ultrasound): Sounds with frequencies greater than 20,000 Hz (20 kHz). Humans cannot hear these sounds. (e.g., sounds produced by bats, dolphins).

Q7: Explain how the pitch of the sound generated by a stretched string can be changed in two ways.

Answer: The pitch of the sound generated by a stretched string (like on a guitar or sitar) can be changed in two main ways:

1. By changing its length: A shorter stretched string vibrates faster, producing a higher pitch. A longer stretched string vibrates slower, producing a lower pitch. (e.g., pressing down on a guitar fret changes the effective length).
2. By changing its tension: A tighter (more tensed) stretched string vibrates faster, producing a higher pitch. A looser string vibrates slower, producing a lower pitch. (e.g., tuning a guitar string).
3. By changing its thickness/mass: A thinner/lighter string vibrates faster, producing a higher pitch. A thicker/heavier string vibrates slower, producing a lower pitch. (This is why different strings on a guitar have different thicknesses).

Q8: What is quality or timbre of sound? Give an example.

Answer: Quality or Timbre is the characteristic of sound that allows us to distinguish between two sounds of the same loudness and pitch, produced by different sources. It depends on the unique waveform of the sound.
Example: If a flute and a violin play the exact same note (same pitch) at the same loudness, you can still tell them apart. This ability to distinguish them is due to the difference in their sound quality or timbre.

Q9: Why do sounds travel faster in solids than in gases?

Answer: Sounds travel faster in solids than in gases because the particles in solids are much closer together and more tightly packed than in gases. This allows vibrations to be transmitted more quickly and efficiently from one particle to the next, leading to a faster speed of sound.

Exercise Solutions (from Page 117 of IMG_20250528_115657.jpg)

1. Fill in the blanks.

a. Sound is generated by the rhythmic vibration of any object.

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

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

d. A medium is necessary for propagation of sound.

2. Match the pairs.

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

3. Give scientific reasons.

a. In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.

Answer: People used to listen for the arrival of a distant train by putting their ear to the rail because sound travels much faster through solids (like steel rails) than through gases (like air). By putting their ear to the rail, they could hear the vibrations of the approaching train through the rail much earlier than they would hear the sound traveling through the air, giving them more warning.

b. The sounds generated by a tabla and a sitar are different.

Answer: The sounds generated by a tabla and a sitar are different even if they produce the same note (pitch) and loudness because of their different quality (timbre). The timbre of a sound depends on the unique waveform produced by the vibrating object and the instrument's structure. A tabla (percussion) produces sound by vibrating a stretched membrane, while a sitar (string instrument) produces sound by vibrating strings. These different modes of vibration and resonance create distinct sound qualities, allowing us to distinguish between the two instruments.

c. If you were both on the moon, your friend will not be able to hear you call.

Answer: If you were both on the moon, your friend would not be able to hear you call because the moon has virtually no atmosphere, meaning it is a vacuum. Sound requires a medium (like air, water, or solid) to travel, as it propagates through the vibration of particles. Since there are no particles in a vacuum to transmit these vibrations, sound cannot travel from you to your friend's ear on the moon.

d. We can hear the movement of a mosquito's wings but we cannot hear the movement of our hands.

Answer: We can hear the movement of a mosquito's wings but not the movement of our hands because sound is produced by vibrations, and the frequency of these vibrations determines the pitch. A mosquito's wings vibrate at a very high frequency (typically hundreds of Hertz), which falls within the human audible range (20 Hz to 20,000 Hz), producing a high-pitched buzzing sound. In contrast, when we move our hands, the vibrations produced are either too slow (very low frequency, below 20 Hz - infrasonic) or too small in amplitude to generate an audible sound, or the frequency is simply not within our hearing range.

4. Write answers to the following questions.

a. How is sound produced?

Answer: Sound is produced by the vibration of objects. When an object vibrates, it creates disturbances in the surrounding medium (like air, water, or solids). These disturbances travel as waves, causing the particles of the medium to vibrate back and forth, transferring the sound energy until it reaches our ears.

b. What does the intensity of sound depend upon?

Answer: The intensity (or loudness) of sound primarily depends upon the amplitude of the vibration. A larger amplitude of vibration produces a sound of greater intensity (louder sound), while a smaller amplitude produces a sound of lesser intensity (softer sound). It also depends on the distance from the source and the medium through which it travels.

c. Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.

Answer:

• Relationship between frequency and length of a pendulum: For a simple pendulum, the frequency of oscillation (and thus its period) is inversely related to the square root of its length. This means that a shorter pendulum oscillates at a higher frequency (swings faster), while a longer pendulum oscillates at a lower frequency (swings slower).
• Relationship between frequency and amplitude of oscillation: For a simple pendulum (under ideal conditions and small amplitudes), the frequency of oscillation is largely independent of its amplitude. This means that even if a pendulum swings with a larger or smaller amplitude, its frequency of oscillation remains approximately the same. However, for very large amplitudes, there can be a slight dependence.

d. Explain the two ways by which the pitch of the sound generated by a stretched string can be changed.

Answer: The pitch of the sound generated by a stretched string (e.g., on a guitar, sitar, or violin) can be changed in the following ways:

1. By changing its length: When the effective length of the vibrating string is decreased (e.g., by pressing a finger on a fret of a guitar), the string vibrates faster, resulting in a higher frequency and thus a higher pitch. Conversely, increasing the length lowers the pitch.
2. By changing its tension: When the tension in the stretched string is increased (e.g., by tightening the tuning pegs of a guitar), the string vibrates faster, producing a higher frequency and thus a higher pitch. Loosening the tension lowers the pitch.
3. By changing its thickness/mass per unit length: A thinner or lighter string (less mass per unit length) vibrates faster, producing a higher pitch. A thicker or heavier string vibrates slower, producing a lower pitch. This is why musical instruments have strings of varying thicknesses.

Application Questions

Q10: Why do astronauts on the International Space Station (ISS) use radios to communicate with each other, even when they are in the same module?

Answer: Astronauts on the ISS use radios to communicate because the ISS modules are essentially a vacuum, or at least a very low-pressure environment, where sound cannot travel through air as it does on Earth. Radios transmit communication signals using electromagnetic waves (radio waves), which do not require a medium to travel and can therefore propagate through the vacuum of space. Even within a pressurized module, using radios ensures clear communication over distances and in situations where direct sound transmission might be difficult or impossible.

Q11: You are listening to music. If you increase the volume on your speaker, what characteristic of the sound are you changing?

Answer: If you increase the volume on your speaker, you are primarily changing the loudness (or intensity) of the sound. This is achieved by increasing the amplitude of the vibrations produced by the speaker. A higher amplitude means more energy is carried by the sound waves, resulting in a louder sound.

Q12: Why does a large bell produce a deeper (lower pitched) sound than a small bell?

Answer: A large bell produces a deeper (lower pitched) sound than a small bell because its larger size allows it to vibrate at a lower frequency. Larger objects generally have more mass and are less rigid, causing them to oscillate slower. Since pitch is directly related to frequency, a lower vibration frequency results in a lower-pitched sound.

Q13: Why can dogs hear a whistle that humans cannot?

Answer: Dogs can hear a whistle that humans cannot because the whistle produces ultrasonic sound, meaning it generates sound waves at frequencies higher than 20,000 Hz. The human ear's audible range typically goes up to 20,000 Hz, while dogs have a much higher upper limit of hearing, often extending to 45,000 Hz or even higher. Therefore, the high-frequency sound from the whistle is ultrasonic to humans but perfectly audible to dogs.

Q14: If you hit a drum harder, what changes about the sound produced?

Answer: If you hit a drum harder, the loudness (intensity) of the sound produced increases. This is because hitting it harder causes the drum membrane to vibrate with a larger amplitude. A larger amplitude of vibration results in a more intense sound wave, which is perceived as a louder sound.