SOUND-Notes

Sound is a form of energy that is produced when objects vibrate and transmit their energy through a medium such as air, water, or solids. These vibrations cause the particles in the medium to move back and forth, creating a wave pattern that propagates away from the source.

Sound waves are mechanical and longitudinal, meaning the particles of the medium vibrate parallel to the direction the wave travels. When these waves reach our ears, they create a sensation of hearing, allowing us to perceive sound.

Sound cannot travel through a vacuum; it needs a material medium to carry the vibrations from the source to the listener.

• Sound is produced when an object vibrates, causing particles in the surrounding medium (like air) to vibrate as well.
• These vibrations create alternate high-pressure (compression) and low-pressure (rarefaction) regions that move outward in all directions as sound waves.
• The energy from the vibrating object is transferred through these particles, allowing the sound to travel.
• The pitch and loudness of the sound depend on the frequency and strength of the vibrations.
• Everyday examples include speaking, playing musical instruments, ringing a bell, or clapping hands—each involves vibrations creating sound that travels to our ears.

Sound is a form of energy that travels from one place to another in the form of vibrations. When an object vibrates, it makes the particles of the surrounding medium (like air, water, or solid) vibrate too. These vibrating particles pass on the energy to their neighboring particles, and this process continues. In this way, sound energy travels through the medium — this is called propagation of sound.

To understand it better, let’s take an example of a ringing bell. When the bell rings, its surface starts vibrating. The air particles near the bell also start vibrating back and forth. These particles do not move from the bell to your ear; instead, they only transfer the vibration to the next particle. This chain of vibrations reaches our ears, and we hear the sound.

The waves produced by vibrating particles are called sound waves. In air, these waves are longitudinal waves, which means the particles of the medium vibrate in the same direction as the wave travels.

• Compression:
  When air particles are pushed closer together, a region of high pressure is formed.
• Rarefaction:
  When particles move apart, a region of low pressure is formed.

Sound travels in the form of waves, and each sound wave has certain features that decide how we hear and recognize different sounds. These features are called the characteristics of a sound wave. The main characteristics are amplitude, frequency, time period, wavelength, and speed. These help us understand the loudness, pitch, and quality of a sound.

Amplitude

• Definition:
  Amplitude is the maximum displacement of the particles of a medium from their mean (rest) position when a sound wave passes through it.
• Explanation:
  It tells us how strong or intense a sound wave is. A wave with a larger amplitude carries more energy.
• Effect on Sound:
  Greater amplitude means a louder sound, while smaller amplitude means a softer sound.

Example: When you speak softly, the amplitude of the sound wave is small; when you shout, the amplitude increases.

Frequency

• Definition:
  Frequency is the number of vibrations or complete waves produced in one second.
  
  It is measured in Hertz (Hz).
• Effect on Sound:
  Frequency decides the pitch of the sound.
  • A high-frequency sound wave produces a high-pitched sound (like a whistle).
  • A low-frequency sound wave produces a low-pitched sound (like a drum).

Example: A child’s voice has a higher frequency than an adult man’s voice.

Time Period

• Definition:
  The time taken to complete one full vibration or one wave is called its time period.
• It is denoted by T and measured in seconds.
• Frequency and time period are inversely related:\[\boxed{T=\frac{1}{\text{Frequency}}}\]So, a sound with high frequency has a short time period, and vice versa.

Wavelength

• Definition:
  The distance between two consecutive compressions or two consecutive rarefactions in a sound wave is called its wavelength.
• It is denoted by the Greek letter \(\lambda\) (lambda) and measured in metres (m).
• Effect on Sound:
  Wavelength is related to the pitch — shorter wavelength means higher frequency and higher pitch.

Speed of Sound

• Definition:
  The distance travelled by a sound wave in one second is called the speed of sound.
• It depends on the medium and its temperature.
• Sound travels fastest in solids, slower in liquids, and slowest in gases.
• For example, sound travels faster in steel than in air.
• Speed \(v=\frac{\text{Distance}}{\text{Time}}\) \[\boxed{v=\dfrac{\lambda}{T}}\tag{1}\] Here \(\lambda\) is the wavelength of the sound wave. It is the distance travelled by sound in one time period \(T\) of the wave. Thus \[\nu=\frac{1}{T}\]Substituting value of \(\nu\) in Eqn.(1) \[\boxed{v=\lambda\nu}\]

A sound wave has a frequency of 2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?

Solution:
Frequency of the wave \(\nu=2\,KHz=2\times 10^3Hz\)
Wavelength of the wave \(\lambda=35\,cm=0.35\,m\)
Distance to travel \(d=1.5\,km=1.5\times 10^3m\)
Speed \(v\) of the sound wave \[\begin{aligned}v&=\lambda\nu\\ &=0.35\times 2\times 10^3\\ &=700\,m/s \end{aligned}\] Time taken to cover 1.5 km by a sound wave \[\begin{aligned}v&=\frac{\text{Distance}}{\text{Time}}\\\\ \implies Time&=\frac{\text{Distance}}{v}\\\\&=\frac{1.5\times 10^3}{700}\\\\&=2.1\,s\end{aligned}\] Thus, sound will take 2.1 s to travel a distance of 1.5 km.

When sound waves travel through a medium such as air and hit a solid or hard surface, they are not absorbed completely. Instead, some of the sound energy bounces back into the same medium. This returning of sound waves is called reflection of sound.

For reflection to occur effectively, the surface must be rigid and smooth—for example, a wall, metal sheet, or water surface.

Laws of Reflection of Sound

The reflection of sound follows rules similar to the reflection of light. There are two main laws:

1. The angle of incidence equals the angle of reflection.:
   That means if a sound wave strikes a surface at an angle, it bounces back at the same angle on the other side of the normal.
2. The incident wave, the reflected wave, and the normal all lie in the same plane.:
   This means all three directions are aligned in one flat surface, not scattered randomly.

Conditions for Reflection of Sound

• The surface must be large, hard, and smooth compared to the wavelength of the sound.
• The distance between the source and the reflector should be at least 17.2 meters in air at room temperature for an echo to be heard.
• Soft and porous materials, like curtains or carpets, absorb sound instead of reflecting it.

An echo is the repetition of a sound caused by its reflection from a distant surface, such as a cliff or building.
When you shout in a valley or near a tall building, you often hear your voice repeated after a short delay—that is an echo!

The human ear can distinguish two sounds only if the time gap between them is at least 0.1 seconds. Since sound travels at about 343 m/s in air, the reflecting surface must be about 17.2 m away for a clear echo to be heard.

A person clapped his hands near a cliff and heard the echo after \(2\,s\). What is the distance of the cliff from the person if the speed of the sound, \(v\) is taken as \(346\,m s^{–1}\)

Solution:
Velocity of the sound \(v=346\,m/s\)
Time Taken to hear the echo \(T=2\,s\)
Time taken to reach the sound wave to the cliff =\(\frac{1}{2}\) of the total Time \(T=2\,s\implies 1\,s\)
\[\begin{aligned}v&=\frac{\text{Distance}}{\text{time}}\\\\\text{Distance}&=v\times t\\\\&=346\times 1\\\\&=346\,m\end{aligned}\]

In some enclosed spaces, sound may get reflected multiple times before dying out. This causes the sound to linger or overlap, making speech or music unclear. This phenomenon is called reverberation. To reduce reverberation, walls and ceilings of halls and studios are often covered with sound-absorbing materials like foam, carpets, or curtains.

• Sound Magnifiers:
  Megaphones or loudhailers, horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all directions, as shown



• Echo sounding:
  Used to measure the depth of seas and oceans.
• SONAR:
  (Sound Navigation and Ranging) Used by submarines and ships to detect underwater objects.
• Soundboards:
  Curved boards used in auditoriums and theatres to direct sound evenly across the audience.



• Medical applications:
  Ultrasonic reflections are used in scanning internal body organs (ultrasonography).
• Architectural design:
  Reflection principles help design halls and auditoriums with clear and pleasant sound.

The range of hearing refers to the range of sound frequencies that the human ear is capable of detecting.

For a normal human ear, this range lies between 20 hertz (Hz) and 20,000 hertz (20 kHz).

This means any sound below 20 Hz or above 20,000 Hz is inaudible to human beings.

Ultrasound refers to sound waves with frequencies higher than what humans can hear (above 20,000 Hz). These waves can travel through solids, liquids, and gases, and because of their special properties, ultrasound has many important uses in our daily life.

1. Medical Imaging (Sonography):
   Ultrasound is widely used in hospitals for sonography, where doctors can actually “see” inside the body without any surgery. This technology is most famous for monitoring the development of babies inside the mother’s womb (prenatal imaging), but it’s also used to check the liver, kidneys, heart, and other organs. Since ultrasound waves are safe (they don’t use harmful radiation), this method is gentle and risk-free.
2. Cleaning Delicate Objects:
   Ultrasound helps in cleaning delicate parts like jewelry, old coins, and tiny machine components. When these items are placed in a liquid and exposed to ultrasound waves, the dirt and grime shake loose and fall away, sometimes from places impossible to clean by hand.
3. Detecting Cracks or Faults in Materials:
   Ultrasound plays a vital role in industries for checking the quality of metals, pipes, and machines. Engineers send ultrasound waves into the metal, and if there’s a crack or flaw inside, the waves return differently. This helps spot hidden problems without breaking open the object.
4. Breaking Kidney Stones:
   Some patients with kidney stones can avoid painful surgery using a special technique called lithotripsy, where focused ultrasound waves break down the stones into small pieces, which can then be removed from the body more easily.
5. Ultrasound in Ships and Submarines (SONAR):
   Underwater navigation and detection use ultrasound with a system called SONAR. Ships and submarines send ultrasound pulses underwater; when these waves hit an object (like a rock or another submarine), they bounce back, helping sailors figure out the distance and position of objects under the sea.
6. Blood Flow Measurement:
   Doctors use Doppler ultrasound to study how blood moves through veins and arteries. This way, they can detect blockages or problems in blood circulation quickly and safely.

Ultrasound is a powerful tool in medical science, industry, and daily life because of its ability to ‘see’ through objects, clean, and help with detection and measurement. Its harmless and versatile nature makes it irreplaceable in many fields.

• Sound is produced due to vibration of different objects.
• Sound travels as a longitudinal wave through a material medium.
• Sound travels as successive compressions and rarefactions in the medium.
• In sound propagation, it is the energy of the sound that travels and not the particles of the medium.
• The change in density from one maximum value to the minimum value and again to the maximum value makes one complete oscillation.
• The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength, λ.
• The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.
• The number of complete oscillations per unit time is called the frequency \((\nu)\),\[\nu=\dfrac{1}{T}\]
• The speed \(v\), frequency \(\nu\), and wavelength \(\lambda\), of sound are related by the equation, \[v = \lambda\nu\]
• The speed of sound depends primarily on the nature and the temperature of the transmitting medium.
• The law of reflection of sound states that the directions in which the sound is incident and reflected make equal angles with the normal to the reflecting surface at the point of incidence and the three lie in the same plane.
• For hearing a distinct sound, the time interval between the original sound and the reflected one must be at least 0.1 s.
• The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.
• Sound properties such as pitch, loudness and quality are determined by the corresponding wave properties.
• Loudness is a physiological response of the ear to the intensity of sound.
• The amount of sound energy passing each second through unit area is called the intensity of sound.
• The audible range of hearing for average human beings is in the frequency range of 20 Hz – 20 kHz.
• Sound waves with frequencies below the audible range are termed “infrasonic” and those above the audible range are termed “ultrasonic”.
• Ultrasound has many medical and industrial applications.