Chapter Notes

Production of Sound

Sound is a form of energy that creates a sensation of hearing in our ears. It is produced when objects vibrate. Vibration is a rapid back-and-forth motion of an object. To produce sound, energy must be used to make an object vibrate. For example, when you clap your hands, you convert the mechanical energy of your moving hands into sound energy.

Sound can be produced in many ways, such as:

• Striking: Hitting a tuning fork on a rubber pad causes its prongs to vibrate, producing sound.
• Plucking: Plucking a stretched rubber band or a guitar string makes it vibrate and create sound.
• Blowing: Blowing air into a flute causes the air column inside to vibrate.
• Shaking: Shaking a rattle causes the objects inside to vibrate against the walls.

In humans, the sound of the voice is produced by vibrations in the vocal cords. Similarly, the buzzing of a bee is caused by the rapid vibration of its wings. In all cases, a vibrating object is the source of sound.

Propagation of Sound

Sound needs a material substance to travel through. This substance is called a medium. The medium can be a solid, a liquid, or a gas. Sound cannot travel in a vacuum.

When an object vibrates, it causes the particles of the medium immediately around it to vibrate. These vibrating particles then transfer their motion to the adjacent particles, which in turn start vibrating. This process continues, and the disturbance travels through the medium from the source to the listener's ear.

This propagation of disturbance through a medium is called a wave. Because sound waves are caused by the motion of particles in a medium, they are known as mechanical waves.

How Sound Travels in Air

Air is the most common medium for sound.

1. When a vibrating object (like a tuning fork prong) moves forward, it pushes and compresses the air in front of it. This creates a region of high pressure and high density called a compression (C).
2. When the vibrating object moves backward, it creates a region of low pressure and low density called a rarefaction (R).
3. As the object vibrates back and forth rapidly, it produces a series of compressions and rarefactions in the air. This series of pressure and density variations forms a sound wave that propagates through the medium.

Sound waves are longitudinal WAVES

There are two main types of waves: longitudinal and transverse.

• A longitudinal wave is a wave in which the particles of the medium vibrate parallel to the direction of the wave's propagation. Sound waves are longitudinal waves. As a sound wave passes, the air particles move back and forth in the same direction that the sound is travelling.
• A transverse wave is a wave in which the particles of the medium vibrate perpendicular to the direction of the wave's propagation. An example is the ripples created when a pebble is dropped into a pond. Light is also a transverse wave, but it is not a mechanical wave as it does not require a medium.

Characteristics of a sound WAVE

A sound wave can be described by three basic characteristics: frequency, amplitude, and speed. These can be visualized using a graph where the y-axis represents pressure or density and the x-axis represents distance.

• Compression: A region of high pressure/density, represented by the peak (or crest) of the wave curve.
• Rarefaction: A region of low pressure/density, represented by the valley (or trough) of the wave curve.

Wavelength

Wavelength ($\lambda$) is the distance between two consecutive compressions or two consecutive rarefactions. Its SI unit is the metre (m).

Frequency

Frequency ($\nu$) is the number of complete oscillations (or the number of compressions/rarefactions) that pass a point per unit time. Its SI unit is the hertz (Hz), named after Heinrich Rudolph Hertz. A frequency of 1 Hz means one oscillation per second.

Time Period

Time Period (T) is the time taken for one complete oscillation to occur. It is also the time taken for two consecutive compressions or rarefactions to pass a fixed point. Its SI unit is the second (s).

Frequency and time period are inversely related. The formula connecting them is: $v=\frac{1}{T}$

Pitch

Pitch is the characteristic of a sound that is determined by its frequency. The brain interprets higher frequency sounds as having a higher pitch.

• High Frequency = High Pitch (e.g., a whistle)
• Low Frequency = Low Pitch (e.g., a drum)

Amplitude

Amplitude (A) is the magnitude of the maximum disturbance in the medium from its mean value. For a sound wave, this corresponds to the maximum change in pressure or density. Amplitude determines the loudness of a sound.

• Large Amplitude = Loud Sound (produced with more energy)
• Small Amplitude = Soft Sound (produced with less energy)

As a sound wave travels away from its source, its amplitude and loudness decrease.

Quality (or Timber)

Quality or timber is the characteristic of a sound that allows us to distinguish between two different sounds that have the same pitch and loudness. For example, it's how you can tell the difference between a violin and a flute playing the same note at the same loudness.

• A sound of a single frequency is called a tone.
• A sound produced by a mixture of several frequencies is called a note. Music consists of pleasant notes, while noise is generally unpleasant.

Speed

The speed of sound is the distance a point on the wave, such as a compression, travels per unit time. It is related to wavelength and frequency by the following equation: speed = wavelength $\times$ frequency $v = \lambda \nu$ In a given medium and under the same physical conditions (like temperature), the speed of sound is nearly the same for all frequencies.

Given

• Frequency, $\nu = 2 \text{ kHz} = 2000 \text{ Hz}$
• Wavelength, $\lambda = 35 \text{ cm} = 0.35 \text{ m}$
• Distance, $d = 1.5 \text{ km} = 1500 \text{ m}$

To Find

The time ($t$) it will take to travel the given distance.

Formula

$v = \lambda \nu$ $t = \frac{d}{v}$

Solution

First, calculate the speed of the wave: $v = \lambda \nu = 0.35 \text{ m} \times 2000 \text{ Hz} = 700 \text{ m/s}$

Now, calculate the time taken to travel 1.5 km: $t = \frac{d}{v} = \frac{1500 \text{ m}}{700 \text{ m s}^{-1}} = \frac{15}{7} \text{ s} \approx 2.1 \text{ s}$

Final Answer The sound will take 2.1 s to travel a distance of 1.5 km.

Loudness and Intensity

While often used interchangeably, loudness and intensity are different.

• Intensity of sound is the amount of sound energy passing through a unit area each second. It is a physical, measurable quantity.
• Loudness is the measure of the ear's response to the sound. It is a physiological sensation that depends on intensity but also on the sensitivity of the human ear.

Speed of sound in different MEDIA

The speed of sound depends on the properties of the medium it travels through, including its temperature.

• Effect of State: Sound generally travels fastest in solids, slower in liquids, and slowest in gases. For example, at $25^\circ\text{C}$, the speed of sound in aluminium is $6420 \text{ m/s}$, in water it is $1498 \text{ m/s}$, and in air it is $346 \text{ m/s}$.
• Effect of Temperature: In any given medium, the speed of sound increases as the temperature increases. For example, the speed of sound in air is $331 \text{ m s}^{-1}$ at $0^\circ\text{C}$ and $344 \text{ m s}^{-1}$ at $22^\circ\text{C}$.

Reflection of Sound

Just like light, sound can bounce off the surface of a solid or a liquid. This phenomenon is called the reflection of sound. It follows the same laws as the reflection of light:

1. The angle at which the sound is incident is equal to the angle at which it is reflected.
2. The incident sound wave, the reflected sound wave, and the normal to the surface at the point of incidence all lie in the same plane.

Reflection of sound requires a large obstacle, which can be either polished or rough.

ЕCHO

An echo is the sound heard after it is reflected from a distant object like a tall building or a mountain.

For a human to hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds. This is because the sensation of sound persists in our brain for about 0.1 s.

The minimum distance required to hear an echo can be calculated.

• If the speed of sound in air is taken as $344 \text{ m/s}$ (at $22^\circ\text{C}$), the total distance the sound must travel (to the object and back) in 0.1 s is: Distance = speed $\times$ time = $344 \text{ m/s} \times 0.1 \text{ s} = 34.4 \text{ m}$.
• This is the total path length. The distance from the source to the reflecting object must be half of this value.
• Therefore, the minimum distance to the obstacle for a distinct echo to be heard is 17.2 m. This distance can change with air temperature.

The rolling of thunder is a result of successive reflections of sound from multiple surfaces, such as clouds and the land.

Reverberation

Reverberation is the persistence of sound in a large hall due to repeated reflections from the walls, ceiling, and floor. While a small amount of reverberation can be pleasant, excessive reverberation is undesirable as it makes sound blurry and unclear.

To reduce reverberation in auditoriums and concert halls, the walls and ceilings are often covered with sound-absorbent materials like compressed fibreboard, rough plaster, or heavy draperies. The materials used for seats are also chosen for their sound-absorbing properties.

Given

• Speed of sound, $v = 346 \text{ m s}^{-1}$
• Time for echo to be heard, $t = 2 \text{ s}$

To Find

The distance ($d$) of the cliff from the person.

Formula

Total distance travelled by sound = speed $\times$ time Distance to cliff, $d = \frac{\text{Total distance}}{2}$

Solution

First, calculate the total distance travelled by the sound wave. This is the distance from the person to the cliff and back again. Total distance = $v \times t = 346 \text{ m s}^{-1} \times 2 \text{ s} = 692 \text{ m}$

The distance of the cliff from the person is half of this total distance. $d = \frac{692 \text{ m}}{2} = 346 \text{ m}$

Final Answer The distance between the cliff and the person is 346 m.

Uses of multiple reflection OF SOUND

1. Megaphones, Horns, and Musical Instruments: Instruments like megaphones, trumpets, and shehanais are designed with a conical opening. This shape uses multiple reflections to guide and direct the sound waves forward, preventing them from spreading in all directions and amplifying the sound for the audience.
2. Stethoscope: This medical instrument allows doctors to listen to sounds from the heart and lungs. The sound of a patient's heartbeat travels through the stethoscope's tubes to the doctor's ears via multiple reflections.
3. Concert Halls and Cinema Halls: The ceilings of these halls are often curved. This design ensures that sound, after reflection, spreads evenly and reaches all corners of the hall. Sometimes, a curved soundboard is placed behind the stage for the same purpose.

Range of Hearing

The human ear can only detect sounds within a specific range of frequencies.

• The audible range for humans is from about 20 Hz to 20,000 Hz (or 20 kHz).

Sounds are categorized based on their frequency relative to this range:

• Infrasound: Sounds with frequencies below 20 Hz are called infrasonic sound. Humans cannot hear them. Animals like rhinoceroses, whales, and elephants produce and communicate using infrasound. Earthquakes also produce low-frequency infrasound before the main shockwaves.
• Ultrasound: Sounds with frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. Humans cannot hear these either. Animals like dolphins, bats, and porpoises produce and use ultrasound for navigation and hunting.

Hearing Aid

A hearing aid is an electronic device that helps people with hearing loss. It works by:

1. Receiving sound through a microphone.
2. Converting the sound waves into electrical signals.
3. Amplifying these signals with an amplifier.
4. Sending the amplified signals to a speaker, which converts them back into sound and directs them into the ear.

Applications of Ultrasound

Ultrasound has high frequency and short wavelength, which allows it to travel in well-defined paths without bending much around obstacles. This property makes it useful in many industrial and medical applications.

• Cleaning: Ultrasound is used to clean small, hard-to-reach parts like spiral tubes and electronic components. The objects are placed in a cleaning solution, and ultrasonic waves are passed through it. The high-frequency vibrations shake off dust, grease, and dirt particles.
• Detecting Flaws in Metal: In large structures like bridges and machines, ultrasound is used to detect cracks or holes in metal blocks that are not visible from the outside. Ultrasonic waves are passed through the metal, and detectors on the other side pick up the transmitted waves. If there is a flaw, the ultrasound gets reflected back, indicating a defect.
• Echocardiography: In this medical technique, ultrasonic waves are reflected from various parts of the heart to create an image of it.
• Ultrasonography: An ultrasound scanner uses ultrasonic waves to create images of internal organs like the liver, kidneys, and uterus. The waves travel through body tissues and are reflected where there is a change in tissue density. These reflections are converted into images. This technique is also used to monitor the development of a foetus during pregnancy.
• Breaking Kidney Stones: Ultrasound can be focused on small stones formed in the kidneys, breaking them into fine grains that can then be flushed out of the body with urine.