Unit 13: Sound

Approximate Speed: At room temperature (\(21℃\)), sound travels at approximately \(343\ ms^{-1}\) in air.

Factors Affecting It: In air, factors like temperature and humidity affect its speed, with the speed increasing at higher temperatures and higher humidity.

Refraction of sound occurs when it travels through layers of air at different temperatures. That is why on a cool morning, sound may travel farther — it bends back toward the ground due to slower speed in colder air.

Speed Comparison: Sound waves travel fastest through solids, slower through liquids, and slowest through gases.

The speed of sound varies with the type of medium. It moves 5 times faster in liquids and 15 times faster in solids than in gases.

Explanation: Because sound is a mechanical longitudinal wave, it propagates through the collision and vibration of particles in a medium. In solids, molecules are packed much closer together than in liquids or gases, which allows the mechanical disturbance (compressions and rarefactions) to transfer rapidly from one particle to the next.

Effect of Amplitude and Frequency: Amplitude and frequency do not affect the quality of sound; instead, amplitude determines loudness and frequency determines pitch. Quality is completely independent of both.

The quality of sound allows us to differentiate between two sounds having the same loudness and pitch. This distinct quality arises due to the unique waveforms produced by each sound source.

How Amplitude affects Loudness: The loudness of sound depends on the amplitude of the vibrating object. Greater amplitude produces a louder sound.

How Frequency affects Pitch: Pitch is the characteristic of sound that allows us to differentiate between a shrill and a deep (grave) sound. It is directly related to frequency. Higher frequency produces a higher pitch, while lower frequency produces lower pitch.

Occurrence of Echo: When sound waves hit a surface and bounce back into the same medium, the phenomenon is called an echo or reflection of sound.

Conditions for Hearing a Clear Echo: The human brain retains sound for about \(0.1\) seconds, so to hear a distinct echo, the reflected sound must reach us after at least \(0.1\) seconds.

Given the speed of sound in air (\(340\ ms^{-1}\)), the minimum distance for an echo to form is 34 metres (total distance travelled by sound), meaning the reflecting surface must be at least 17 metres away.

One-way distance from reflecting surface is calculated using the formula:
\[ Distance = \frac{Speed\ of\ Sound \times Time}{2} \]
\[ Distance = \frac{340\ ms^{-1} \times 0.1\ s}{2} = 17\ metres \]

1. Reflection (Echo): It causes sound waves to bounce back when they strike a boundary. When sound waves hit a surface and bounce back into the same medium, the phenomenon is called an echo or reflection of sound.

2. Refraction: It causes sound waves to bend and alter direction when changing layers or mediums. Refraction occurs when sound waves change direction due to a variation in the medium's properties, such as density or temperature. This change occurs because the speed of sound differs in various mediums.

3. Diffraction: It causes sound waves to spread around corners or openings. Diffraction is the bending of sound waves around obstacles or through slits. It allows sound to spread out and be heard even when the source is not directly visible.

Infrasound: Infrasound refers to waves below \(20\ Hz\), which humans cannot hear but may feel as vibrations.

Uses of infrasound:
i. Elephant Communication: Elephants use infrasound to communicate over long distances. Their deep rumbles travel far through air and ground, helping them stay connected. They can sense these vibrations not just with their ears but also through their feet and trunks.
ii. Earthquake Detection: Scientists use infrasound to detect earthquakes before the strong shaking starts. These low-frequency waves act as an early warning system.
iii. Volcano Monitoring: When a volcano erupts, it creates unique infrasound waves. Scientists study these waves to predict eruptions and assess risks.
iv. Nuclear Explosion Detection: Special sensors track infrasound waves from nuclear explosions.

Ultrasound: Sounds with frequencies above \(20,000\ Hz\), which are inaudible to humans, are called ultrasound or ultrasonics.

Uses of ultrasound:
i. Medical Applications: When ultrasonic waves pass through the body, they reflect differently from various tissues and organs. These reflected waves are then captured and converted into images on a screen, a process known as ultrasonography. This helps in visualizing organs such as the heart, liver, and kidneys, as well as in monitoring pregnancies.
ii. Underwater Exploration (SONAR): Ultrasound is used to measure ocean depth and locate objects on the seabed. SONAR (Sound Navigation and Range) works by sending ultrasonic waves into water; the reflected waves help to determine the distance and shape of an object.
iii. Industrial Applications: Ultrasound helps to detect cracks in machines like turbines, ship engines, and airplane parts. Ultrasound waves reflect from damaged areas, revealing hidden defects. It is also used to destroy bacteria in liquids through high-intensity waves.