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Notes On Sound

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Published in: Physics
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Physics

Aritra D / Kolkata

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  1. Sound What is Sound? Have you ever wondered what sound is? Let us explore. Sound is a wave. A wave is some disturbance caused when energy is traveling from one object to another. When sound travels, it moves the air around it, creating high and low pressure points, or waves. Your ears can detect the waves, and you perceive this as sound. Thus, sound can be defined as an energy that is carried by waves produced by vibrating objects. Since sound is a form of energy, it can neither be created, nor destroyed; it can only be transformed from one form of energy to another. Sound is heard because of vibration and vibrations occur due to motion of objects. So, as long as there is motion in the universe, sound will be there. Sound may fade away, but does not stop, and so may be inaudible to the ear. Sound is propagated from one point to the other by means of a material medium in the form of a wave. Wavelength and Frequency Waves tend to behave in predictable ways. One of their properties is wavelength. Wavelength is simply how long each wave is. To demonstrate wavelength, have two people hold each end of a string. One person can wave their arm up and down at a consistent speed. You may notice that when the arm moves faster, the waves get smaller and increase in numbers along the string. That is because you have just increased the frequency of the waves! The frequency of sound is the number of waves that occur in a certain length of time. Low Frequency Med Frequency Iligh Frequency Human hearing is limited in the frequencies of sound it can detect. There are many animals that can hear noises that we cannot. For example, cats can detect frequencies that are much lower, while bats can detect frequencies that are much higher than those we can hear. In the figure below, when you speak into the cup, the sound waves from your voice are transformed into vibrations, which travel along the string. They are then converted back into sound waves in the cup at the other end.
  2. Vibrations Vibration is a wave too, but it is different from sound waves. This wave is energy traveling through objects instead of air, which is how you can sometimes "feel" a loud noise. In the above figure of the string and the glasses, the vibrations held the same wavelength and frequency of the original sound wave made by your voice, which is how your friend could hear it at the other end. When you grab the string while it is vibrating, the wave is stopped from continuing to the other cup and your voice cannot be heard. Production of Sound When particles or objects vibrate, they transfer energy to the surrounding (air or water) molecules. The surrounding molecules on receiving the energy begin to vibrate. Every vibrating molecule then transfers its energy to the adjacent particle. In this way, the vibration or the energy travels from its source to another location in all directions. In this way, sound propagates as a wave through a medium. How Do We Hear Sound? When the vibration produced by a body reaches our ears, our outer ear collects the vibration and the middle ear enhances it. The inner ear converts this vibration into electrical pulses. These are then carried by nerves to the brain, which identifies the sound. It is the brain, which hears the sound while ears are the aids, which help in hearing.
  3. Propagation of Sound Let us consider a vibrating tuning fork to understand how sound propagates in air. Any vibrating body undergoes to-and-fro motion about its mean position. For example, when a tuning fork vibrates, its prongs move inwards and outwards about their mean position. When the prongs move outwards, molecules surrounding it move outwards, causing increase in the pressure. This pushes the molecules to come close to each other. This region of high pressure is called compression. When the vibrating prongs move inwards, molecules surrounding it move inwards, causing a decrease in the pressure. This pulls molecules apart. This region of low pressure is called rarefaction. As the prongs keep vibrating, the sound travels as alternative compressions and rarefactions and finally reaches our ears. Vibrating tuning fork causes compressions and rarefactions Distribution of particles during propagation of sound Propagation of Sound in Solids, Liquids and Gases We know that sound is produced from vibrating objects. Such vibrating objects disturb the particles that are in contact and set them into vibration. Thus, for the sound energy to propagate, there should be particles. Therefore, the sound requires any of the media such as solids, liquids and gases to travel through. The molecules in the solids are closely packed than that of liquids and gases. The molecules of liquids are closer from each other than they are in gases. Therefore, the particles in the solids easily transfer the sound energy than that in liquids and gases.
  4. Speed of Sound Sound energy travels at different rate in different media. The speed of sound increases as the density of the medium increases. Remember that the density of solids is more than the density of liquids and density of liquids is more than the density of gases. Therefore, sound travels fastest in solids, slower in liquids and slowest in gases. In addition, it is worthy to know that: In liquids, speed of sound depends on pressure and density of the liquid. In gases, speed of sound depends on temperature and humidity. If vsolid is the speed of sound in solid, Vliquid is the speed of sound in liquid and vair is the speed of sound in air, then you can easily remember that: Vsolid >Vliquid > Vair To Find the Speed of Sound in Air Imagine that you and your friend do an experiment to find the speed of sound in air. In order to do so, let us consider that you and your friend are standing on two tall buildings, which are at least 1 km away. When one of you fires a gun, the other needs to measure the time interval between seeing a flash from the gun and hearing the sound using a stopwatch. By doing so, you will be able to calculate the speed of sound using the formula: Distance travelled by the sound Speed of sound = Time interval Here, the distance travelled by the sound is the distance between the buildings and the time interval is the time measured between seeing a flash and hearing a sound. Terms Related to Wave The terms associated with a wave are amplitude, frequency, time-period, wavelength and oscillation. Let us understand each in detail. m Propagation of wave A wave has a crest and a trough. A crest is a point on the wave where the displacement of the medium is maximum. A point on the wave is a trough if the displacement of the medium at that point is minimum. 1. Oscillation: One complete to-and-fro motion of a vibrating body is called an oscillation. 2. Amplitude: The maximum vertical displacement of a wave from its mean position is called amplitude. The Sl unit used to measure amplitude is metre (m).
  5. 3. Frequency: The number of vibrations produced by a vibrating body in one second is called frequency. It is denoted by a letter f or n. The Sl unit used to measure frequency is Hertz (Hz). If any particle produces one vibration in one second, then its frequency is 1 Hz. 4. Time period: Time taken by a particle to complete one vibration or oscillation is called the time period. The Sl unit used to measure time period of a wave is second (s). If f is frequency of vibrations and T is the time period, then f vibrations are produced in one second. Therefore, time taken to complete one vibration is given by, Therefore, time period = 1/ frequency 5. Wavelength: The distance between consecutive crests or troughs is called wavelength. In sound waves, wavelength can be defined as the distance between consecutive compressions or rarefactions. The Sl unit of wavelength is metre. It is denoted by a Greek letter 'X'. Periodic Motion You may have observed a clock and noticed that the pointed end of its seconds hand and that of its minutes hand move around in a circle, each with a fixed period. The seconds hand completes its journey around the dial in one minute but the minutes hand takes one hour to complete one round trip. However, a pendulum bob moves back and forth about a mean position and completes its motion from one end to the other and back to its initial position in a fixed time. A motion, which repeats itself after a fixed interval of time, is called periodic motion. There are two types of periodic motion: (i) non—oscillatory, and (ii) oscillatory. The motion of the hands of the clock is non-oscillatory but the back and forth motion of the pendulum bob is oscillatory. However, both the motions are periodic. It is important to note that an oscillatory motion is normally periodic but a periodic motion is not necessarily oscillatory. Remember that a motion, which repeats itself in equal intervals of time, is periodic and if it is about a mean position, it is oscillatory. We know that earth completes its rotation about its own axis in 24 hours and days and nights are formed. It also revolves around the sun and completes its revolution in 365 days. This motion produces a sequence of seasons. Similarly, all the planets move around the Sun in elliptical orbits and each completes its revolution in a fixed interval of time. These are examples of periodic non-oscillatory motion. Types of Sound In everyday life, we hear various sounds. Some sounds are pleasant and some are unpleasant. Depending upon their pleasant or unpleasant sensation on the ears, sound is categorised into music and noise. For example, sound produced by all musical instruments and singers is pleasant to hear. These pleasant sounds are produced by regular vibrations. On the contrary, sound produced by a vehicle, machines and roar of a lion are unpleasant to hear. These unpleasant sounds are produced by irregular vibrations.
  6. Characteristics of Sound Waves In our daily life, we hear various sounds produced by different objects. Sound of a crow is different from the sound of peacock. Have you ever thought how sound produced by two things are different? There are three important characteristics of sound, which are responsible for different sounds. They are loudness (intensity), pitch and quality (timbre). Each sound that we hear is a unique combination of these characteristics. Loudness Loudness is the characteristic of a sound, which distinguishes feeble sound from a loud sound of the same frequency. Loudness of a sound relates to the amount of sound energy received by unit area per second. Loudness of sound depends on the following: Amplitude of the Sound As the amplitude of sound wave increases, loudness increases. Amplitude of a wave is directly proportional to the amount of energy it carries. Therefore, greater the energy the sound wave carries, louder is the sound produced. Sound waves of same frequency but different amplitude Distance between the Source of Sound and the Listener If we move towards a loudspeaker, the loudness of the sound will increase. Hence, as the distance between the source and the listener decreases, loudness of a sound increases. Area of the Vibrating Body Larger the area of the vibrating body, louder the sound produced. Larger drum will produce more sound than a smaller drum. Pitch It is the characteristics of a sound wave, which determines the sharpness, or shrillness of a sound wave. It is the effect produced in the ear due to the sound of particular frequency.
  7. Low-frequency wave Low-pitched sound High-frequency wave High-pitched sound Pitch depends upon frequency of a vibrating body. Higher the frequency of a vibrating body, larger is the pitch and lower the frequency, lesser is the pitch of the body. Pitch of a sound wave depends on the following: Frequency of Sound Wave Pitch of a sound increases with increase in frequency. Length of Vibrating Air Column Pitch of vibrating air column increases with the decrease in the length of air column. Thickness of the Vibrating String Thinner string produces high-pitch sound wave whereas thicker string produces low-pitch sound. Length of the Vibrating String Longer the string, lower is the pitch of a sound wave. Shorter the length of the string, higher the pitch of a sound. Most of the string and wind instruments work on the above principle. Sound made by a bird has a higher pitch and is, therefore, shriller than a lion's roar. Sound of a baby's cry has a higher pitch than a man's voice. Quality If you pluck strings of same length of two different musical instruments with equal force, you will notice that the notes produced by them are different. The property, which distinguishes between two sounds of two different musical instruments having the same pitch and loudness, is called quality of sound or timbre. Sources of Sound Sound waves are produced by various sound sources. All sources are categorised under three main sources, namely tuning fork, musical instruments and humans. Let us study one by one in detail. Tuning Fork Tuning fork is a source of vibration. It is U-shaped, consisting of 2 arms called prongs and a stem, as shown in the figure. The prongs, on hitting with a rubber pad, vibrate and thus produce a sound. In all laboratory experiments, to study about sound and vibrations, tuning fork of required frequency can be used. TCP pr.a-• l+ndle st-z-n/&se
  8. Musical Instruments Depending on the medium of vibration, musical instruments are grouped into four main types. They are stringed instruments, percussion instruments, wind instruments and reed instruments. Stringed Instruments Stringed instruments consist of a string mounted over the specially designed hollow wooden frames. When the strings are struck or plucked, they vibrate to produce musical sound of particular frequency. The pitch of a sound of any stringed instrument can be altered by changing the length of the string. Some examples of stringed instruments are violin, guitar, harp and sitar. Percussion Instruments Percussion instruments consist of a taut membrane, placed tightly over a metal or wooden frame. When the membrane is hit or struck, it vibrates to produce sound. The pitch of a sound of any membrane instruments can be altered by tightening the membrane. Some of the examples of percussion instruments are tabla, drum, tambourine, xylophone, and brass drum Wind Instruments Wind instruments consist of an air column inside a hollow tube. In these instruments, air is directly blown into the air column, which vibrates to produce music. Some of examples of wind instruments are saxophone, flute, shehnai, clarinet, trumpet and French horn. Reed instruments consist of a metal strip called reed. When air is blown directly over the reed, it vibrates to produce music. Some examples of reed instruments are the buzzing reed, mouth organ and harmonium. Sound Produced by Humans Human voice is one of the sources of sound. An important part of the throat, called the voice box (larynx), is responsible for sound production. Larynx has two thin folds stretched across it in such a way that there exists a narrow slit between them. The slit acts as a passage for air. These two folds are called vocal cords. When air from the lungs is expelled out of the narrow slit, vocal cords vibrate to produce sound. Thus, vocal cords act as a primary source of sound. The sound produced by the vocal cords passes through various cavities of the vocal tract. The muscles of the larynx control the length and tension of the vocal tract to adjust the pitch and tone of the voice. The pitch of sound produced by humans depends on the length of the muscles attached to the vocal cord. The vocal cords in children and women are tight and thin. Therefore, they produce high-pitch sound. Whereas men have longer and larger vocal cords, producing low-pitch sound.
  9. Reed instruments consist of a metal strip called reed. When air is blown directly over the reed, it vibrates to produce music. Some examples of reed instruments are the buzzing reed, mouth organ and harmonium. Sound Produced by Humans Human voice is one of the sources of sound. An important part of the throat, called the voice box (larynx), is responsible for sound production. Larynx has two thin folds stretched across it in such a way that there exists a narrow slit between them. The slit acts as a passage for air. These two folds are called vocal cords. When air from the lungs is expelled out of the narrow slit, vocal cords vibrate to produce sound. Thus, vocal cords act as a primary source of sound. The sound produced by the vocal cords passes through various cavities of the vocal tract. The muscles of the larynx control the length and tension of the vocal tract to adjust the pitch and tone of the voice. The pitch of sound produced by humans depends on the length of the muscles attached to the vocal cord. The vocal cords in children and women are tight and thin. Therefore, they produce high-pitch sound. Whereas men have longer and larger vocal cords, producing low-pitch sound. Unit of Sound The loudness of sound is measured in decibels (dB). Reflection of Sound Like light, sound gets reflected whenever it comes across an obstacle like a wall or mountain. Sound also obeys the laws of reflection. Echo Every one of us might have enjoyed making sound in an empty room. When you make a sound in any empty room, the sound bounces off the walls of the room. Hence, we hear our own voice repeatedly. This is called an echo. Hence, an echo is the reflecting of sound that reaches a listener with a delay after the direct sound is heard. Our brain retains the effect of any sound for 0.1 second. This time is called time period of persistence. Therefore, we cannot distinguish two different sounds if those sounds have reached our brain within 0.1 second. Hence, the echo of a sound will be heard distinctly only when it reaches our ears after 0.1 second, after the original sound dies out. The speed of sound is 340 ms-I. The distance travelled by sound in 0.1 second = 340 x 0.1 = 34 m. This is twice the minimum distance between a source of sound and the reflecting surface. Hence, the distance between the source and the reflecting surface = 34/2 = 17 m. Therefore, we can hear an echo only when the reflecting surface is beyond 17 m from the source. Sonar
  10. SONAR is an abbreviation of sound navigation and ranging. This instrument works on the principle of reflection of sound. It is fixed in navigating ships and used to locate the objects that are under the ocean. SONAR fitted in a ship sends sound waves of high frequency, called ultrasonic sound, towards the bottom of ocean. The sound is received back after it is reflected from objects at the bottom of the sea. The time interval for travel of sound waves from the source to the receiver after reflection is recorded. a-ip I-Imasomd Celz:tor—:- Utrsm•rd- brö:rn Sonar in ships Knowing the speed of sound in water (1,500 ms-I) and having the time taken by the reflected sound to reach the sonar, depth of a sea or location of objects can be calculated. The distance travelled by the sound wave is twice the distance of the object from the surface of ocean. If d is the distance of the object, then Speed of sound in water (v) Time taken by reflected sound to reach the sonar (t) V By dividing the solution by 2, the value of d can be obtained. Noise Pollution Noise pollution is the disturbance produced by undesirable, loud and unpleasant sounds. Noise pollution has harmful effect on human ears. Sound produced by public speakers, vehicles, trains, aeroplanes and machines in factories are very harmful for both mental and physical health of humans. Humans can hear sound from 10 dB to 180 dB. A sound between 50 dB and 60 dB is considered to be normal. A sound until 80 dB is tolerable. Human health will be in danger if the intensity of sound crosses 80 dB. Harmful effects of noise pollution 1. Disturbs peaceful communication of one person with other. 2. Develops anxiety and anger in an individual. 3. Reduces concentration and lowers the efficiency of any individual. 4. Develops a number of health issues like lack of sleep, headache, irritation and tension.
  11. Prevention and control measures for noise pollution 1. Factories should be constructed in non-residential areas. 2. Vehicles should be provided with efficient silencers. 3. Unnecessary usage of loudspeakers in public places should be avoided. 4. Machines should be provided with soundproof devices. 5. Heavy vehicles should not be used unnecessarily.

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