Unit 13: Sound — Short Questions

The speed of sound depends on the medium through which it travels. Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles are closer together in solids, allowing vibrations to transfer more quickly. For example, sound travels about 15 times faster in solids than in air. In air, factors like temperature and humidity also affect its speed.

Yes, sound shows reflection. When sound waves hit a surface and bounce back into the same medium, it is called reflection of sound or echo. For example, when we shout near a tall building or a mountain, we hear the reflected sound after a short time.

The loudness of sound is influenced by three main factors:
(i) Amplitude: Greater amplitude of vibration produces a louder sound.
(ii) Area of the vibrating body: A larger vibrating surface area produces a louder sound.
(iii) Distance from the source: Loudness decreases as the distance from the vibrating source increases.

We hear an echo because the sound waves from the clap travel to the building, reflect off its hard surface, and travel back to our ears. For a distinct echo to be heard, the reflecting surface must be at least 17 meters away, so that the reflected sound reaches our ears more than 0.1 seconds after the original sound.

Sound waves with frequencies higher than 20,000 Hz, which are inaudible to humans are called ultrasound.
Use in Medicine: In medicine, ultrasound is used for diagnosis and imaging. Ultrasonic waves are sent into the body, and their reflections from different tissues and organs are captured to create an image on a screen. This process, called ultrasonography, is used to observe a fetus during pregnancy and to examine organs like the heart, liver, and kidneys.

Sound is produced when an object vibrates. These vibrations cause the surrounding air particles to vibrate as well. The vibrating air particles transfer energy from one layer to another in the form of compressions and rarefactions. When these vibrations reach our ears, we hear sound.

Compressions are high-pressure regions where air molecules are pushed together by the vibrating source. Rarefactions are low-pressure regions where molecules spread apart. These alternate to form longitudinal sound waves that travel through the medium.

Sound is called a mechanical wave because it requires a material medium for its propagation. It travels through the vibration of particles of the medium. Without particles, such as in vacuum, sound cannot propagate. Hence, sound depends on mechanical vibrations.

When a tuning fork is struck, it begins to vibrate and produces sound. If the vibrating prong touches a table tennis ball, the ball moves, showing vibrations. Similarly, placing the vibrating fork in water causes splashing. These observations prove that sound is produced due to vibrations.

The speed of sound in air is affected by temperature and humidity. It increases with higher temperature and higher humidity. For example, at 0°C, sound travels at 330 ms⁻¹, while at 25°C, it travels at about 346 ms⁻¹.

In solids, particles are closely packed and strongly bonded, which allows vibrations to pass quickly from one particle to another. In gases, particles are far apart, so vibrations take more time to transfer. Therefore, sound travels faster in solids than in gases.

Loudness of sound depends on the amplitude of vibration. A larger amplitude means the particles vibrate more strongly, producing a louder sound. A smaller amplitude results in weaker vibrations and a softer sound. Thus, loudness increases with amplitude.

As sound travels away from its source, its energy spreads over a larger area. This causes the amplitude of sound waves to decrease. As a result, the sound becomes fainter with increasing distance from the source.

Pitch is the characteristic of sound that tells how high or low a sound is. It depends directly on the frequency of sound waves. Higher frequency produces a higher pitch, while lower frequency produces a lower pitch. For example, children's voices have higher pitch due to higher frequency.

Noise is produced by irregular and unpleasant vibrations. Continuous exposure to noise causes stress, lack of concentration, hearing problems, and sleep disturbance. Because it harms human health and the environment, noise is considered a form of pollution.

Noise pollution can be controlled by using quieter machines and installing sound barriers. Wearing hearing protection devices also helps reduce harmful effects. Limiting exposure time to high noise levels creates a healthier environment.

The human brain retains sound for about 0.1 seconds. For a distinct echo, the reflected sound must return after this time. At the speed of sound, this requires the reflecting surface to be at least 17 metres away, so that the sound travels a sufficient distance.

In the echo method, a person stands at a known distance from a wall and produces a sound. The time between the sound and its echo is measured using a stopwatch. Since sound travels to the wall and back, the speed is calculated using distance and time.

Infrasound is sound below 20 Hz, felt as vibrations. Elephants use infrasound for long-distance communication. Scientists detect earthquakes using infrasound sensors.

SONAR (Sound Navigation and Ranging) uses ultrasound to measure underwater depth and locate objects. A ship sends ultrasonic pulses into the water; the time taken for the echo to return is measured. Using the formula Distance = (Speed of sound in water × Time) / 2, the depth or distance to an object is calculated.

A tuning fork on a wooden table produces louder sound due to larger vibrating surface area.
Reason:
When the vibrating tuning fork is held in air, only its prongs vibrate the air, which is a small surface area. When placed on a table, the vibrations are transferred to the large surface area of the table. The table then vibrates and forces a much larger volume of air to vibrate, resulting in a louder sound. This demonstrates that a larger vibrating surface area increases the loudness of sound.

The bell jar experiment proves that sound requires a medium (like air) to travel.
Reason:
In the experiment, an electric bell rings inside a sealed bell jar. When the jar is full of air, the sound is heard clearly. As the air is pumped out using a vacuum pump, the sound becomes fainter and eventually inaudible, even though the bell is still seen vibrating. This happens because sound waves cannot travel through a vacuum. When air is reintroduced, the sound becomes audible again, confirming that air (a medium) is necessary for sound propagation.

To hear a distinct echo, the reflecting surface (building) must be at least 17 metres away from the source.
Reason:
The human brain retains sound for about 0.1 seconds, so the reflected sound must reach us after this duration. Given the speed of sound in air is approximately 340 m s⁻¹, sound travels 34 metres in 0.1 seconds. Since an echo involves sound travelling to the wall and back, the minimum one-way distance required is half of 34 metres, which is 17 metres.

Men's voices sound deeper than women's or children's because they have a lower pitch.
Reason:
Pitch is the characteristic of sound that depends on frequency. A higher frequency results in a higher pitch (shrill sound), and a lower frequency results in a lower pitch (deep sound). Men's vocal cords vibrate at a lower frequency, producing a lower-pitched, deeper sound. Women and children have vocal cords that vibrate at a higher frequency, producing a higher-pitched, shriller sound.

Soft materials like carpets and curtains improve room acoustics by absorbing sound waves.
Reason:
Hard, smooth surfaces (like bare walls and floors) reflect sound waves strongly, causing multiple reflections (reverberation) and echoes that distort sound and make it unclear. Soft, porous materials absorb these sound waves instead of reflecting them. This reduces unwanted echoes and reverberations, resulting in clearer and better-quality sound within the room.

The distance to the canyon can be calculated using the speed of sound and the time taken for the echo to return d = v×t / 2
Principle: Echo method uses reflection principle; measure time t for sound to travel to canyon and back at speed v, divide by 2 for one-way distance.

In solids, particles are closely packed and strongly bonded, which allows vibrations to pass quickly from one particle to another. In gases, particles are far apart, so vibrations take more time to transfer. Therefore, sound travels faster in solids than in gases.