Unit 13: Sound

Sound waves are simple to generate. When we speak, our vocal cords vibrate, producing sound waves. As these vibrations move through the air, they form sound waves.

The necessary condition for the production of sound is that the body should vibrate. When an object vibrates, it makes the air around it to vibrate as well. These air vibrations travel to our ears and create the sensation of sound. For example, in a guitar, the sound is produced when its strings vibrate.

Sound is a mechanical wave, meaning it requires a material medium (solid, liquid, or gas) to propagate because it relies on particle vibrations. In a vacuum, there are no particles to vibrate, so sound cannot travel.

In school laboratories, tuning fork is used to produce a specific sound. When a rubber hammer is struck with the tuning fork, it begins to vibrate.

Sound waves are longitudinal, meaning the particles of the medium oscillate back and forth in the same direction as the wave's energy transfer. This movement creates regions where air molecules are pushed together (compressions) and spread apart (rarefactions).

When sound moves in air, it creates compressions and rarefactions. The particles of air move back and forth in the same direction as the sound wave.

Sound is a mechanical wave because sound requires a medium (like air, water, or solids) to travel through.

The speed of sound is calculated using the equation: \( v = f\lambda \) where \(v\) is the speed of sound, \(f\) is the frequency, and \(\lambda\) is the wavelength of the sound wave.

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.

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

Loudness is a property of sound that allows us to differentiate between loud and faint sounds. For example, when speaking with friends, our voice is soft, but while addressing a large audience, we speak louder.

No, the loudness of a sound does not affect the time it takes for an echo to return. The echo time depends only on the distance to the reflecting surface and the speed of sound in the medium.

Pitch is the characteristic of sound that allows us to differentiate between a shrill and a deep (grave) sound.

Pitch is directly related to frequency, higher frequency results in higher pitch, while lower frequency produces lower pitch. The voices of women and children have a higher frequency, making them shrill and high-pitched, whereas the voices of men have a lower frequency, giving them a deeper and lower-pitched sound.

The quality of sound allows us to differentiate between two sounds having the same loudness and pitch. For example, a violin and a flute may produce sounds with the same pitch and loudness, but they can still be distinguished because each instrument produces a different waveform.

Musical Sounds: Sounds that have controlled pitch and quality and are pleasant to hear are called musical sounds. For example, sound produced by musical instruments such as guitar, violin, recorder, and drum.

Noise: Sounds that are harsh, unpleasant, and produced by irregular vibrations are called noise. For example, sound produced by sources such as traffic, door slamming, and machinery.

Excessive noise from industrial machinery, vehicle horns, alarms, and construction work can lead to stress, lack of concentration, hearing loss, sleep disturbances, aggression, and hypertension.

The recommended safe noise level is \(85\)-\(90\) \(dB\) for a maximum of eight-hour workday.

Noise pollution can be reduced by:
i. Using quieter, eco-friendly machinery.
ii. Installing sound barriers.
iii. Wearing hearing protection devices.
iv. Tree plantation.

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

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: \(Distance = \frac{Speed\ of\ Sound \times Time}{2} = \frac{340 \times 0.1}{2} = 17\ metres\).

Bats send out sound waves using their mouth or nose. When the sound hits an object, an echo comes back. The bat can identify an object by the sound of the echo. They can even tell the size, shape and texture of a tiny insect from its echo.

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. For example, when sound waves travel from air into water, they bend due to the difference in density, causing sound to move faster in water than in air.

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. For example, if someone is speaking from behind a wall, his voice can still be heard because the sound waves bend around the barrier.

Distant train horns or bird calls sound clearer at dawn because refraction of sound occurs when it travels through layers of air at different temperatures. On a cool morning, sound bends back toward the ground due to slower speed in colder air, allowing it to travel farther.

The human ear can detect sounds within the \(20\ Hz\) to \(20,000\ Hz\) range, known as the audible frequency range. Sounds below \(20\ Hz\) or above \(20,000\ Hz\) are inaudible to humans.

Hearing ability declines with age. Young children can hear up to \(20,000\ Hz\), while older individuals may struggle with sounds above \(15,000\ Hz\).

Infrasound refers to waves below \(20\ Hz\), which humans cannot hear but may feel as vibrations. It is produced by natural events (earthquakes, volcanoes), industrial processes, and human activities (machinery, explosions).

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.

Earthquake Detection: Scientists use infrasound to detect earthquakes before the strong shaking starts. These low-frequency waves act as an early warning system.

Volcano Monitoring: When a volcano erupts, it creates unique infrasound waves. Scientists study these waves to predict eruptions and assess risks.

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

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.

These waves are completely non-invasive and safe, making them ideal for monitoring babies during pregnancy or detecting organ problems.

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. Ultrasound is used to measure ocean depth and locate objects on the seabed.

The formula used to calculate distance (or depth) in sonar systems is: \(Distance = \frac{Speed\ of\ Sound\ in\ Water \times Time}{2}\) where speed of sound in water is approximately \(1500\ ms^{-1}\) at room temperature. Time is the duration it takes for the ultrasound pulse to travel to the object and return to the source. The distance is divided by 2 because the pulse travels to the object and then returns to the source.

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.

Acoustic protection reduces unwanted sound using soft, porous materials like carpets and curtains, which absorb sound and minimize echoes.

Multiple reflections, known as reverberation, can distort sound, so a balance between reflection and absorption is essential in auditoriums, lecture halls, and theatres.

While carpets and curtains absorb echoes, curved panels and reflectors bounce the music toward the audience so that even the person in the last row hears every note clearly. Reflective surfaces behind the stage help direct sound to the audience. Additionally, curved ceilings and sound boards are often used to ensure even sound distribution throughout the hall.