SOURCES & TRANSMISSION OF SOUND WAVES
A sound is a vibration that propagates through a medium in the form of a mechanical wave. Some of the sources of sound are talking, blowing flutes, ringing of telephone, beating drums, shouting, noise from bikes, cars, aeroplanes and musical instruments.
Sound waves are produced by vibrating objects. A sound wave is the pattern of disturbance caused by the energy travelling away from the source of the sound.
Sound needs a material medium for their propagation like solid, liquid or gas to travel because the molecules of solid, liquid and gases carry sound waves from one point to another. Sound cannot progress through a vacuum because the vacuum has no molecules which can vibrate and carry the sound waves. Sound travels fastest in solids, relatively slower in liquids and slowest in gases.
Experiment to Demonstrate Transmission of Sound
The bell jar experiment is a common experiment used to demonstrate that sound needs a medium to travel. A bell jar is a laboratory experiment used for creating a vacuum. It is called a bell jar because its shape is similar to that of a bell.
Suspend an electric bell inside a glass bell jar by passing the connecting wires through an airtight cork fitted at the mouth of the jar. Place the jar over a disc which has a pipe connected to a vacuum pump as shown in the figure below. When the switch is turned on, the sound of the bell is heard. Pump out the air from the jar gradually with the help of the vacuum pump. The sound produced becomes fainter as the air is released.
When most of the air has been released, a very faint sound. When there is air inside the jar sound travels through it to the wall of the jar. This makes the wall vibrate which in turn, produces sound. When air is removed, the sound from the bell cannot travel to the wall of the jar.
At a particular vacuum, no more sound is heard from the bell, but the hammer can be seen hitting the gong and producing sound. The sound produced is not audible to the ears because of the vacuum inside the jar. This demonstrates that the sound wave cannot travel through vacuum, that is, it needs a material medium.
Speed of Sound
The speed at which sound waves propagate through a medium is known as the speed of sound. The speed of sound is different in different media. It depends on the medium the sound is travelling in. Sound travels faster in solids than in liquids, and faster in liquids than in gases. This is because the density of solids is higher than that of liquids which means that since the particles are closer together, sound can be transmitted more easily.
The speed of sound also depends on the temperature of the medium. The hotter the medium is, the faster its particles move and the quicker the sound will travel through the medium. When we heat a substance, the particles in that substance have more kinetic energy and vibrate or move faster. Sound can therefore be transmitted more easily and quickly in hotter substances.
Sound waves are pressure waves. The speed of sound will therefore be influenced by the pressure of the medium through which it is travelling. At sea level, the air pressure is higher than high up on a mountain. Sound will travel faster at sea level where the air pressure is higher than it would at places high above sea level.
The speed of sound can be calculated using:
c = d/t
where
d = distance travelled by sound
t = time taken to cover the distance
Echoes
When sound travels in a given medium, it strikes the surface of another medium and bounces back in some other direction. This phenomenon is called the reflection of sound. The waves are called the incident and reflected sound waves. Echoes and reverberations are applications of reflection of sound.
Echoes are produced when sound waves are reflected by hard surfaces. It is the sound heard after reflections from a rigid surface such as a cliff or a wall. It creates a persistence of sound even after the source of sound has stopped vibrating.
In the determination of speed of sound by echo, the expression used is:
2x = vt
Where
x = distance between the source of sound and the reflecting surface
v = velocity of sound
t = total time taken
APPLICATIONS & DEMERITS OF ECHO
APPLICATIONS OF ECHO
1. It is used in medical science like ultrasonography and echocardiography for imaging of human organs such as liver.
2. Since bats cannot see from their eyes, they use the technique of echolocation.
3. It is used to find the depth of sea or distance of submarines.
4. It also helps to estimate the distance of mountains and hills.
5. Dolphins detect their enemies and find obstacles by emitting the ultrasonic waves and hearing their echo.
6. It is important in the development of sonar and speed traps,
7. It is used in prospecting for oil or mineral exploration.
Demerits of Echo
1. In a theatre or a concert hall, echoes can ruin a performance if the walls and ceilings are not properly designed.
2. Multiple echos make words of a speaker unclear to the audience.
3. Echo causes disturbance while communicating.
Reverberation
Reverberation is the phenomenon of persistence of sound after it has been stopped as a result of multiple reflections from surfaces such as furniture, people, air, etc. within a closed surface. These reflections build up with each reflection and decay gradually as they are absorbed by the surfaces of objects in the enclosed space. Reverberation is the perseverance of sound after the source ceases. It is similar to echo but the distance between the source of the sound and the obstacle through which it gets reflected in less in the case of reverberation.
Applications of Reverberation
1. Producers of live or recorded music use this concept to enhance sound quality.
2. Stethoscope uses the concept of reverberation. The sound of heartbeat reaches the doctor’s ears after multiple reflections in the stethoscope tube, therefore amplifying the sound received.
3. The soundboard used in musical instruments such as guitar and violin causes reverberation.
MERITS & DEMERITS OF REVERBERATION
MERITS OF REVERBERATION
Reverberations do wonders when it comes to musical symphonies and orchestra halls, when the right amount of reverberation is present, the sound quality gets enhanced drastically. This is the reason why sound engineers are appointed during the construction of these halls.
DEMERITS OF REVERBERATION
If a room has any sound-absorbing surfaces like wall, roof and the floor, the sound is said to bounce back between the surfaces and also it takes a very long time as the sound dies. In such a room, the listener will then have a problem with registering the speaker. This is because he tends to hear both the direct sound as well as the repeated reflected sound waves. Also, if this reverberation is more excessive, the sound is said to run together with a mere loss of articulation and it becomes muddy and also garbled.
Characteristics of Sound Waves
Noise is due to vibrations of irregular frequency.
Music is due to vibrations of regular frequency.
The amplitude of a sound determines its loudness or volume.
Tone is a measure of the quality of sound wave.
Quality or timbre is a characteristic note of a musical instrument which distinguishes it from another note of the same pitch and loudness produced by another instrument. The quality of a sound note depends on the harmonies. When a note is produced, the strongest, audible frequency heard is the fundamental note. All other frequencies present or harmonics or overtones.
Overtones are the next higher frequencies produced after the fundamental frequency by a vibrating body.
Intensity is the rate of flow of sound energy per unit area perpendicular to the direction of propagation of the sound wave. It is proportional to the square of the amplitude.
Loudness is the magnitude of the sensation resulting from a sound reaching the ear. It depends on the intensity of the sound. The greater the intensity, the louder the sound.
Pitch of a note is its position on the musical scale. It depends on the frequency of the sound wave. If the frequency is increased, the pitch of the sound also increases.
The frequency of a sound is an indication of how high or low the pitch of the sound is.
The fundamental note (fo) produced in a stretched string is given by
fo = V/ 2l x √ ( T/M )
Where l = length, T = tension, M = mass, V = velocity.
Musical Instruments
Wind Instruments
Clarinets, flute, saxophone, trumpet are examples of wind musical instruments. A musical note originates from a source vibrating in a uniform manner with one or more constant frequencies. All wind instruments use resonating air columns to produce their sounds. Sounds from wind instruments may originate from
– Air vibrating over an opening e.g. organ and flute.
– The vibrating lips of a brass instrument e.g. trumpet.
– A vibrating heel e.g. clarinet, saxophone.
Some columns are of fixed length, their resonant frequencies being altered by the opening or closing of holes in the column e.g. clarinet. Some instruments are played by altering the length the air column e.g. trumpet.
Stringed Instruments
The guitar, sonometer and piano are examples of stringed musical instruments. These instruments may be set in vibration by a bow, or plucked with a finger. For example, a violin is bowed while a guitar is plucked. The frequency of a vibrating string depends on its length, the mass, and the force that keeps the string taut. Stringed instruments vibrate as a whole and in loops at the same time e.g. the violin. These vibrations produce both the fundamental and overtones frequencies.
Percussion Instruments
Drum, bell and xylophones are examples of percussion instruments. They produce musical notes when they are struck or hit. They have rods, plates or membranes that vibrate when struck. For example, there are rods in bells, plates (bars) in xylophones and membrane in drums.
Forced Vibration
If tuning fork A is struck and stopped, you find that it will cause tunning fork B to vibrate, provided both forks have the same frequency. This is called forced vibration. Other forms of forced vibration include:
Resonance: This is a special case of forced vibration which occurs when a system is made to vibrate at its own natural frequency as a result of forced vibrations received from another source of the same frequency.
Resonance in strings: Stationary waves can occur on a stretched string or wire. This is obtained by varying the driving frequency of the string.
Vibration in Strings
Waves travels along a horizontal rope fixed at one end, and the other end is free to move. Sound wave is generated from a fixed string that is allowed to move at the other end. In this mode of vibration the vibrating wire produces a sound of the lowest possible note whose frequency is called fundamental frequency. The mode of vibration is giving rise to the fundamental mode of vibration.
The distance between the two consecutive mode is λ/2 and is equal to the length of the string L.
L = λ/2 or λ = 2L
When a string is plucked, it will vibrate in one segment with two nodes at either end. This is the lowest possible mode of vibration. In a guitar string, for example, it will vibrate between the fret and the tuning key. The bridge transfers the vibration to the “box” (or sound box) through the saddle.
At the fundamental frequency (f0) we have 1 loop and 2 nodes.
Note: λ of nodes = λ of loops + 1. Also: L = ½ λ (L – length of the string, λ – wavelength)
Overtones occurs when strings vibrate in more than one segment. Harmonies are frequencies which are multiples of the fundamental.
i.e. 2f0, 3f0 …
The fundamental frequency (f0) is known as the First Harmonic.
Sanometer
Sonometer consists of a hollow rectangular wooden box of more than one meter length, with a hook at one end and a pulley at the other end. One end of a string is fixed at the hook and the other end passes over the pulley. A weight hanger is attached to the free end of the string. Two adjustable wooden bridges are put over the board, so that the length of string can be adjusted.
If a string of length l having mass per unit length m is stretched with a tension T, the fundamental frequency of vibration f is given by
Vibration of Air in Pipes
There are two possible types of resonant air columns (pipes).
1. Open at one end only or closed at one end.
2. Open at both ends.
In a closed pipe, only odd numbers of harmonics are present as overtones accompanying the fundamental note. The possible harmonics are fо, 3fо, 5fо, 7fо, etc.
Where f = fundamental frequency. fo = V/4L
In order for an air column (pipe) open at both ends to produce a sound, antinodes must be formed at its both ends. Nodes cannot be formed at the ends of the pipe.
For open-ended pipes: fn = n . fо
For pipes closed at one end: fn = fо (2n – 1)
Where n is the desired harmonic number.
Where
F = fundamental frequency
L = length of the wire
T = tension in the wire
M = linear mass density or mass per unit length of the wire.
