Chapter 5 Sound

                                               Sound

Introduction

•    Sound energy is a form of energy because of which our ears are able to hear something.
•    One cannot create sound or destroy it. But one can transform one form of energy into sound energy. For instance, when a cell phone rings, the sound is produced by converting electrical energy into sound energy.
•    A wave that is produced when objects of the medium oscillate is called Mechanical Wave. The sound waves are therefore, mechanical waves.

 

 

 

 

Types of Waves

•    Sound cannot travel through the vacuum as it always needs a medium to propagate. The vacuum contains no air hence no particles can propagate sound.

Longitudinal waves - Any wave that vibrates in the direction of the motion is called a Longitudinal Wave. Sound waves are longitudinal because the particles of the medium vibrate in the direction which is parallel to the direction of the propagation of the sound waves. The particles in the medium oscillate to and fro in the case of longitudinal waves.

Transverse Waves - A transverse wave is produced when the particles of the medium oscillate in a direction which is perpendicular to the direction of the propagation of the wave. The particles in a transverse wave oscillate in an up and down motion. For Example, light waves are transverse in nature.

 

 

 

 

 

                                                 Characteristics of Sound

A sound wave is characterized by three factors:
•    Amplitude
•    Frequency
•    Speed

 

 

 

 

 

Sound cannot travel at the same speed in different mediums. The speed of sound in a medium is affected by three things:

•    The density of the medium. For instance, speed of sound is the maximum through solids 
•    The temperature of the medium. As the temperature increases, the sound propagates easily.
•    Humidity in the air also affects the travel of sound. As the humidity increases, so does the propagation of sound.

Properties of Sound Waves:

Frequency: Number of oscillations of sound waves is called its frequency. The number of peak and troughs per unit of time will give frequency. It is represented by v (nu) and its SI unit is Hertz (Hz).
Time Period: The time taken to complete one oscillation is called its time period. Its unit is second and is represented by T.

Relation of frequency and time period:
v = 1/T

Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. Its unit is meter. Softness or Loudness of Sound: If the amplitude is smaller than the sound will be softer and if it is larger than sound will be louder. Higher amplitude helps the sound wave is travelling faster.

Sonic Booms

•    When an object travels in the air with a speed greater than that of the sound, it produces a sound with high energy. 
•    This energy is loud enough that it can break glasses or damage the buildings. The sound produced is similar to the sound of an explosion or thunderclap.
•    These objects exert a large amount of pressure on the air which causes the production of shock waves in the air. 
•    These shock waves produce extremely large and loud sound waves which are called Sonic booms.
•    Speed of light in air = 3 * 108 m/s
•    Speed of sound in air = 344 m/s
This clearly states that sound travels a lower speed than that of light in air. This is a reason why at the time of lightening, the light is visible instantly while the sound of the thunder reaches our ears after a few seconds.

 

                                          Laws of reflection

•    The incident sound wave, the reflected sound wave and the normal, all lie in the same plane.
•    The angle of incident of incident sound wave is equal to the angle of reflection formed by the reflected sound wave, that is, i = r

 

 

 

 

 

Echo

•    When we hear the same sound again and again in a medium it is called Echo. 
•    The sound or echo persists in our brain for 0.1 seconds. This means that the difference between sound and its echo should be at least 0.1 seconds.
•     It is produced as a result of reflection of sound through a medium. If sound reflects more than once we may hear multiple echoes.
Reverberation: Repeated reflections of sound results in persistence of sound and is called reverberation.

Use of Reverberation of Sound: Following instruments use this property of sound:

1.    Megaphones, Shehanais, Trumpets
2.    Stethoscope
3.    Curved dome of concert halls
Advantages of Multiple Reflection of Sound

•    Horns, trumpets, loudhailers or megaphones are designed in such a way that sound can travel in a particular direction only without spreading out everywhere. This makes it easier for the audience to listen to the speaker. All these instruments work on the phenomena of multiple reflections of sound.
•    The multiple reflections in a stethoscope tube make it possible for the doctors to listen to a patient’s heartbeat.
•    Concert halls are generally covered so that sound can reflect through it and reach the wider audience.
•    Human auditory range is between 20 Hz and 20000 Hz.

 

                                      Ultra Sound

The ultrasound waves are the sound waves with high frequency. Due to this, they can travel long distances despite any obstacles between their paths.

Application of Ultra Sound

•    The ultrasound waves are used in clearing parts of objects that are hard to reach such as a spiral tube or electronic components. In order to clean the objects, they are put in a solution, then the ultrasonic waves are passed through the solution. As a result, the dust particles on the object get detached and fall off them.
•    Ultrasound waves can recognize tiny cracks in metallic objects that are used in the manufacture of large structures, buildings and scientific equipment. The presence of such cracks can lower the strength of these structures and machines. Hence, the ultrasound waves are passed through the metallic objects and detectors are used to detect the waves that pass through the cracks. If a crack is present the ultrasound waves would reflect back.
•    Ultrasonic waves are also used in a medical process called Echocardiography. In this process, the ultrasound waves are passed through various parts of the heart in order to form the images of the organ.
•    Ultrasonic waves are also used in a procedure called Ultrasonography. In this procedure, the ultrasonic waves are passed through the internal organs of the body in order to get their image. In this way, the doctors can find out the cause of a disease or any abnormalities in the organs. The ultrasound waves travel through the tissues of the body and as soon as the density of the tissue changes, they reflect back. The reflected waves are then converted into electrical signals which form the images of the internal organs.
•    Ultrasound waves are also used to break the kidney stones.
SONAR – Sound Navigation and Ranging

 

 

 

 

 

•    This device is used to find the distance, direction and speed of objects that are present under the water. It uses Ultrasonic waves to do so.
•    The Sonar consists of two main devices – The transmitter and the detector (or receiver). The main function of the transmitter is the production and transmission of the Ultrasonic waves in water.
•    As these waves travel underwater, they, when hit by an object, reflect back to the detector. The detector then converts these sound waves into electrical signals which are then interpreted.
•    The distance of the object is calculated with the help of the speed of sound in water and time taken by the way to reach the detector. This process is called Echo Ranging.

Uses of Sonar

•    Finding the depth of a water body such as sea
•    Detecting the presence of underwater objects like submarines, hills, icebergs and ships
The Human Ear

The ear is a sensitive organ of the human body. It is mainly involved with detecting, transmitting and transducing sound and maintaining a sense of balance is another important function of the human ear. Human ear includes:

•    The outer ear or the visible part of the ear is called the pinna.
•    Pinna collects sound from the surroundings.
•    Sound passes through a tube called an auditory canal.
•    Eardrum (tympanic membrane) vibrates in response to incident sound waves.
•    Vibrations are amplified and transmitted further by three bones hammer, anvil and stirrup in the middle ear to the inner ear.
•    In the inner ear, cochlea converts pressure signals into electrical signals.
•    Electrical signals are transmitted by the auditory nerve to the brain for interpretation.

 

 

 

 

 

Structure of Human Ear