Sound
Sound Synopsis
Synopsis
Production and Propagation of Sound
- Sound is a form of energy which produces sensation in our ears.
- Sound travels in the form of waves.
Production of Sound
- When you strike a tuning fork, it starts vibrating. When this tuning fork is brought near the ear, we hear some sound.
- When this fork is brought near a hanging ball, the ball starts oscillating.
- Thus, a vibrating source is necessary for the production of sound.
- Vibrations in vocal cords produce the human voice. The buzzing sound when a bee flies is due to the vibration of air by the wings. The plucking of strings of a guitar produces sound.
Thus, sound is produced by vibration of strings, air, membranes or plates, etc.
Propagation of Sound
- The matter or substance through which sound is transmitted is called a medium.
- When an object vibrates, the particles in the surrounding medium vibrate. These particles send the vibrations to the adjacent particles and the energy continues to transmit until it reaches our ears.
- A wave is a disturbance moving through a medium when the particles of that medium set the adjacent particles into vibration.
- The particles do not move forward, but the disturbance does.
- Sound waves are characterised by motion of particles in a medium and are therefore called the mechanical waves.
- When a vibrating object moves forward in air, it pushes and compresses the air around it, creating a high pressure in the surrounding region called compression. When the object moves backwards, it creates a low pressure region called rarefaction.
When the prongs move forward, compression is formed.
When the prongs move backward, rarefaction is formed.
Sound needs a Medium to Travel
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Sound is a mechanical wave and needs a material medium to propagate. It cannot travel through vacuum.
- Two astronauts cannot speak in space (outside earth) because there is a vacuum present.
- Take an electric bell and an airtight glass bell jar. The jar is connected to a vacuum pump.
- The bell rings when the switch is pressed. If air is present inside the jar, then we are able to hear the sound. However, if the air is removed, then the sound is not audible even though the bell is ringing.
- Sound can travel through solids, liquids and gases, but not through vacuum.
Sound waves are Longitudinal Waves
- Sound propagates in a medium as a series of compressions and rarefactions.
- A longitudinal wave is a wave where the individual particles vibrate parallel to the direction of the propagation of the wave.
- Sound waves also propagate in the same manner, and hence, are called longitudinal waves.
Characteristics of Sound
Two sounds are distinguished by the following characteristics:
- Loudness
- Pitch or shrillness
- Quality or timbre
Loudness
- Loudness is the property by which a loud sound can be distinguished from a faint one, both having the same pitch and quality.
- The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave.
- The loudness or softness of a sound is determined by the amplitude (or intensity) of the wave.
- However, loudness is not the same as intensity. Intensity is a measurable quantity, while loudness is a sensation. Experimentally, Weber and Fechner established a relationship between the loudness L and intensity I which is given as:
L = Klogl
Where, K is a constant of proportionality. Obviously, loudness increases with the increase in intensity, but not in the same proportion. - The intensity at any point of the medium is measured as the amount of sound energy passing per second normally through unit area at that point. Its unit is microwatt per metre squared. Greater the energy carried by a sound wave, greater is the intensity of sound, and hence louder it seems to us.
- The intensity of a sound wave in air is proportional to (i) the square of the amplitude of vibration, (ii) the square of the frequency of vibration, and (iii) the density of air.
Subjective nature of loudness and objective nature of intensity:
- The loudness of a sound depends on the energy conveyed by the sound wave near the eardrum of the listener. Thus, the loudness of sound of a given intensity may differ from listener to listener.
- Further, two sounds of the same intensity, but of different frequencies may differ in loudness even to the same listener because the sensitivity of the ears is different for different frequencies.
- For normal ears, the sensitivity is maximum at frequency 1 kHz. Thus loudness is a subjective quantity, while intensity, being a measurable quantity, is an objective quantity for a sound wave.
Factors affecting the loudness of sound:
The loudness of sound heard at a place depends on the following five factors:
- Loudness is proportional to the square of the amplitude: When a body vibrates with greater amplitude, it sends forth a greater amount of energy, and hence the energy received by the eardrum is also large. Thus, the sound appears louder.
- Loudness varies inversely as the square of the distance: If the listener is close to the source of sound, he will hear it quite louder, but if he is far away, the sound will become feeble.
- Loudness depends on the surface area of the vibrating body: A large vibrating area sends forth a greater amount of energy. Hence larger the surface area of the vibrating body, the louder is the sound heard.
When a tuning fork is sounded in air, the sound given by it is feeble, but when it is placed on a sound box, the sound becomes much louder. The reason is that the box provides comparatively a large area and forces a large volume of air to vibrate and thereby increases the sound energy reaching our ears. - Loudness depends on the density of the medium: More the density of medium more is the loudness.
- Loudness depends on the presence of resonant bodies: The presence of resonant bodies near the vibrating body increases the loudness of sound.
Units of loudness and sound level (phon and decibel):
- The unit of loudness is phon. The loudness of a sound in phon is the loudness in decibel (dB) of an equally loud pure sound of frequency 1 kHz. The sound level is usually expressed in decibel (dB).
- The loudness L is related to the intensity I as L = K log10 I. If at a given frequency, I1 and I0 are the intensities of two sounds of which loudness are L1 and L0 respectively, then
L1 = Klog10l1
L0 = Klog10l0
Therefore, we have the difference in loudness as
L = L1 - L0
= K(log10l1 - L1 = Klog10l0)
The minimum intensity of audible sound intensity at 1 frequency 1 kHz is l0 = 10-12 W m-2. The loudness of sound is called sound level.
- Now, if L = 1 dB, then we have
- Thus, we can define 1 dB as the change in level of loudness when the intensity of sound changed by 26%.
Noise Pollution
The disturbance produced in the environment due to undesirable loud and harsh sound of level above 120 dB, from the various sources such as loudspeaker, siren, moving vehicles etc. is called noise pollution.
A constant hearing of sound of level above 120 dB can cause headache and permanent damage to the ears of listener. The sound of level 30 dB to 10 dB has the soothing sensation, while the level 0 dB of loudness of sound represents the limit of hearing.
Pitch or Shrillness
- Pitch is another characteristic of sound waves. It allows distinguishing between two sound waves travelling with same speed and arriving our ears at the same time.
A violin and a guitar may be sounded together, but their sounds are interpreted differently by the brain. - How our brain interprets the frequency of an emitted sound is called its pitch. The faster the vibration of source of sound, higher is the frequency and higher is the pitch.
- Pitch is that characteristic of sound by which an acute (or shrill) note can be distinguished from a grave or flat note.
- Pitch refers only to musical sounds and each musical note has a definite pitch. If the pitch is higher, the sound is said to be shrill and if the pitch is lower, the sound is flat.
- In a tape recorder (or TV), bass and treble refer to low and high pitch respectively. At a bass (or woofer on), low pitch (i.e., grave) sound such as of tabla or dholak becomes predominant, while at treble, high pitch (i.e., shrill) sound such as of flute or ghoonghroo (ankle bells) becomes predominant.
- Pitch of a note depends on the wavelength or frequency of wave.
- Instruments such as piano, violin and guitar have several strings of different thickness under different tensions which vibrate to produce notes. A note of higher pitch from stringed instrument is obtained by increasing the frequency of the vibrating string for which either the tension on the string is increased or the length of the string is shortened or a thinner string is used. It can also be increased by shifting the place of plucking the string.
- In case of a flute, a lower note is obtained by closing some more holes so that the length of the vibrating air column increases.
- As the water level in a pitcher kept under a water tap rises, the length of air column decreases, so the frequency of sound produced increases, i.e., the sound becomes shriller. Thus by hearing the sound from a distance, one can get the idea of water level in the pitcher.
- The voice of women is usually of higher pitch than that of men.
Subjective nature of pitch and objective nature of frequency
- Pitch is not the same as frequency. The pitch refers to the sensation as perceived by the listener. It may be different for a sound of a particular frequency to different listeners, i.e., pitch is subjective.
- On the other hand, frequency is a measurable quantity. It depends on the source producing the sound. It has a definite value for a given sound and it has nothing to do with the listener, so frequency is an objective quantity.
Quality or Timbre
- Quality or timbre of a sound is that characteristic which distinguishes the two sounds of the same loudness and same pitch, but emitted by two different instruments.
- The quality of a musical sound depends on the waveform.
- The sound from an instrument does not contain a note of single frequency, but it contains a combination of vibrations of different frequencies and different amplitudes. The vibration of lowest frequency and maximum amplitude is called the principal (or fundamental) vibration and vibrations of frequency integer multiples of it, are called the subsidiary (or secondary) vibrations.
- The resultant vibration obtained by the superposition of all these vibrations, gives the wave form of sound which we hear.
- The figure (a) below shows a pure note (sine curve) of single frequency produced by the tuning fork, while figure (b) is of same amplitude and same frequency (or same pitch), but it has a different wave form due to the presence of a mixture of subsidiary vibrations along with the principal vibration.
The subsidiary vibrations present in the musical note make the wave form complex. It is not a sine curve. - Thus, the quality of a musical sound depends on the number of the subsidiary notes and their relative amplitudes present along with the principal note.
- A note played on a piano has a large number of subsidiary notes, while the same note when played on a flute contains only a few subsidiary notes. Thus, we can easily distinguish between the sounds of a piano and a flute by their different wave forms, though they may be of exactly the same loudness and same pitch.
- You generally recognise a person by hearing his voice on telephone without seeing him. It is because the vibrations produced by the vocal cord of each person have a characteristic wave form which is different for different persons.
- Similarly, one can distinguish and recognise the sounds of two different musical instruments even if they are of same pitch and same loudness.
- All sounds, which produce the sensation of hearing, can be roughly divided into two
- categories: (i) music, and (ii) noise.
- The distinction between a music and noise is subjective. A sound which is music to some may be a noise to other.
For example, a rock concert is music for the young generation, while it may be noise for the old generation.
- Music: It is a pleasant, continuous and uniform sound produced by the regular and
- periodic vibrations. For example, the sounds produced by a violin, piano, flute, tuning fork, etc., are musical sounds. Their sound level is usually between 10 dB to 30 dB. Music can be decomposed into its component notes (i.e., principal and subsidiary notes).
- Noise: The sounds other than the musical sounds are called the noise. It is a sound produced by an irregular succession of disturbances and it is a discontinuous sound. It is discordant and unpleasant to the ear. For example, the sound produced when a stone is thrown on a tin sheet is a noise. Usually all the sounds of level above 120 dB are termed as noise.
Speed of Sound in Different Media
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The speed of sound in a medium depends on the following two factors:
- Elasticity ‘E’ of the medium, and
- Density ‘ρ’ of the medium
- The speed of sound in a medium is given as
- Newton derived the above formula assuming that the temperature of a gas in which sound travels does not change. Hence, the above equation should contain isothermal elasticity which is equal to pressure.
- For air at NTP, P = 1.01 × 105 N m-2, ρ = 1.293 kg m-3, so V is 279.5 m/s. This value does not match with the experimental value of speed of sound which is 330 m/s.
- Laplace corrected this formula considering adiabatic change. According to him, when sound travels in a gas, there is no loss of heat in the medium. For an adiabatic change, the modulus of elasticity is E = γP, where γ is the ratio of specific heat at constant pressure to the specific heat at constant volume.
- For air, γ = 1.4. Thus, the speed comes out to be 330.7 m/s, which is in good agreement with the experimental value.
- Speed of sound is different in different media. It is more in solids, less in liquids and least in gases.
Factors Affecting the Speed of Sound in a Gas
Effect of Density
- The speed of sound is given as
. Thus, we can say that speed is inversely proportional to the square root of the density of the gas 
.
Effect of Temperature
- The speed of sound in a gas increase with increase in temperature. This is because the density of a gas decreases with increase in temperature, and hence, speed increases.
- Speed is directly proportional to the square root of temperature V ∝ √T . Here, T is measured in kelvin.
- The speed of sound increases by 0.61 m s−1 for every degree rise in temperature.
V = V0 + 0.61t
Effect of Humidity
- The speed of sound in air increases with increase in humidity.
- The density of water vapour is (5/8) times the density of dry air. Thus, with increase in moisture, density decreases, and hence, the speed of sound increases.
- Thus, sound travels faster in moist air than in dry air.
Effect of Direction of Wind
- If wind is blowing in the direction of propagation of sound, then the speed of sound increases. However, if the direction of wind is opposite to the direction of propagation of sound, then the speed of sound decreases.
Factors Not Affecting the Speed of Sound in a Gas
- Pressure: The speed of sound is given as . From this equation, one would feel that V depends on pressure P, but it is independent of P. When pressure increases, volume decreases, but the mass remains same, and thus, density increases such that P/ρ remains the same. Hence, V is independent of P.
- Wavelength (or frequency) of wave: Speed of sound is independent of the wavelength of sound wave.
- Amplitude of wave: Speed of sound is independent of the amplitude of sound wave.
Reflection of Sound Waves
- When sound waves strike a hard surface or boundary of another medium, they return back in the same medium obeying the laws of reflection.
- The returning back of the sound wave on striking a surface such as wall, metal sheet, plywood etc., is called the reflection of sound wave. The reflection of sound wave does not require a smooth and shining surface like mirror. Sound waves get reflected from any surface whether smooth or hard.
- However, the size of the reflecting surface must be bigger than the wavelength of the sound wave.
- The reflection of sound waves is utilised in megaphone (or speaking tube), sound boards and ear trumpet.
- The repetition of sound caused by reflection of sound waves from an obstacle after the original sound has ceased is known as echo.
- Multiple echoes are heard when sound is repeatedly reflected from several obstacles at suitable distances.
- The phenomenon of persistence or prolongation of audible sound after the source has stopped emitting it is called reverberation.
Condition for Forming an Echo
- The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s.
- If d is the distance between the observer and the obstacle and V is the speed of sound, then the total distance travelled by the sound to reach the obstacle and then to come back is 2 d and the time taken is
- If we take the speed of sound to be 340 m/s at a given temperature, say at 22°C in air, then the total distance covered by the sound from the point of generation to the reflecting surface and back should be
- Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17 m.
- The person shouting from a cliff hears his sound back after sometime. This sound is the echo.
oThe speed of sound in hot day is more than cold day. Thus, the echo will reach sooner on a hot day.
Use of Echoes
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By bats, dolphins and fisherman:
- Animals have different range of audible frequency e.g. bats, dolphins and dogs have a much higher upper audible limit than the human beings. Bats can produce and detect the sound of very high frequency up to about 100 kHz. The sounds produced by the flying bats get reflected back from an obstacle in front of it. By hearing the echoes, bats are able to detect obstacles in the dark. Hence they can fly safely without colliding with the obstacles. This process of detecting obstacles is called sound ranging.
- Dolphins detect their enemy and obstacles by emitting ultrasonic waves and hearing their echo. They use ultrasonic waves for hunting their prey.
- A fisherman sends a pulse of ultrasonic waves from a source into the sea and receives the waves reflected from the shoal of fish via a detector. The total time t of the to and fro journey of the wave is recorded. The position of fish is then calculated by using the relation;
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By Sonar:
- Animals have different range of audible frequency e.g. bats, dolphins and dogs have a much higher upper audible limit than the human beings. Bats cSONAR stands for sound navigation and ranging.
- It consists of a transmitter and detector. The transmitter transmits the ultrasonic sound. These waves travel through water and after striking the underwater object (like submarine, iceberg, sunken ship, etc.), reflect back and are detected by a detector.
- The distance of the object can be found by knowing the speed of sound in water.
- The total distance travelled by the wave in time t is
- The above method is called “echo-ranging”.
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In medicines:
- Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘Echocardiography’.
- Ultrasonography is used to obtain the images of patient’s organs such as liver, kidney, etc. It helps to detect stones in these organs.
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