Contents
Physics Topics can also be used to explain the behavior of complex systems, such as the stock market or the dynamics of traffic flow.
Detection of Echocardiography with the Help of Ultrasound
The sounds having too high frequency which cannot be heard by human beings are called ultrasonic sound or ultrasound. In other words, the sounds having frequency greater than 20,000 hertz are called ultrasound. For example, a sound of frequency 100,000 hertz is an ultrasound.
The ultrasound is reflected just like ordinary sound waves and produces echoes. But the echoes produced by ultrasound cannot be heard by our ears, they can only be detected by special equipment. Due to its very high frequency, ultrasound has a much greater penetrating power than ordinary sound.
So, it can be used to detect objects under the sea, and organs inside the human body. These days ultrasound is used for a large number of purposes. These are discussed below.
Applications of Ultrasound
Ultrasound waves are high-energy sound waves. They travel in straight lines without bending around the corners. Ultrasound waves have a very-high frequency (and very short wavelength) due to which they can penetrate into ‘matter’ to a large extent. Because of all these properties, ultrasound is used extensively in industry, and in hospitals for medical purposes. Some of the important applications (or uses) of ultrasound are given below :
1. Ultrasound is used in industry for detecting flaws (cracks, etc.) in metal blocks without damaging them
Metal blocks are used in the construction of big structures like buildings, bridges, machines, and scientific equipment, etc. There may be some flaws or defects (such as cracks or holes, etc.) inside the metal blocks which are invisible from outside.
These flaws weaken the metal blocks. The use of such metal blocks having internal cracks (or other defects) reduces the strength of the structure u] trasound scans of the inner organs of human to be constructed. The flaws like internal cracks, etc., in the metal body such as liver, gall bladder, pancreas, blocks are detected by using ultrasound.
This is based on the fact kidneys, uterus and heart, etc. that an internal crack (or hole, etc.) does not allow ultrasound to pass through it. It reflects the ultrasound. This will become more clear from the following discussion.
Ultrasound waves are made to pass through one face of the metal block (to be tested), and ultrasound detectors are placed on the opposite face of the metal block to detect the transmitted ultrasound waves (see Figure).
(a) If the ultrasound waves pass uninterrupted through all the parts of the metal block, then the metal block is flawless (or defect-free) having no internal cracks, etc.
(b) If, however, ultrasound waves are not able to pass through a part of the metal block and get reflected back, then there is a flaw or defect in the metal block (like a crack or a hole).
In Figure (a) ultrasound waves are passing through all the parts of the metal block, so the metal block shown in Figure 49(A) is flawless or defect-free (having no cracks, etc.). On the other hand, in Figure (b) the ultrasound waves falling on the centre part of the metal block are not able to pass through it (as shown by ultrasound detector), they are reflected back.
This shows that there is a crack, etc., in the centre part of the metal block which does not allow ultrasound to pass through it. So, the block shown in Figure (b) is defective (having a crack inside it). Please note that ordinary sound waves cannot be used for detecting the flaws in metal blocks because they will bend around the corners of the defective location (crack, etc.) and hence enter the detector.
2. Ultrasound is used in industry to clean ‘hard to reach’ parts of objects such as spiral tubes, odd¬shaped machines and electronic components, etc.
The object to be cleaned is placed in a cleaning solution and ultrasound waves are passed into the solution. Due to their high frequency, the ultrasound waves stir up the cleaning solution. Because of stirring, the particles of dust and grease sticking to the dirty object vibrate too much, become loose, get detached from the object and fall into solution. The object gets cleaned thoroughly.
3. Ultrasound is used to investigate the internal organs of the human body such as liver, gall bladder, pancreas, kidneys, uterus and heart, etc.
Ultrasound waves can penetrate the human body and different types of tissues (like bones, fat, and muscle) reflect the ultrasound waves in different ways. Due to these properties, ultrasound is being used increasingly in medical and surgical diagnosis in hospitals. Ultrasound waves are used to investigate the organs which are inside the human body. In a way, ultrasound helps us to ‘see’ inside the human body by giving pictures of the inner organs. This happens as follows :
The source of ultrasound waves is placed above the human body organ to be investigated. The ultrasound waves given out by this source enter the human body and are reflected from the organ. The reflected ultrasound waves are fed into a computer which builds up a picture of the organ concerned which the doctors can see on a television-type screen (called monitor).
This picture of the organ helps in the diagnosis of ailment. The picture can also be obtained on a photographic film. The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound pulses is called ultrasonography. And such pictures are called ultrasound scans. A machine which uses ultrasonic waves for obtaining images of the internal organs of human body is called ‘ultrasound scanner’.
A doctor can take the ultrasound scans of a person’s organs like liver, gall bladder, kidney, pancreas, uterus and heart, etc. It helps the doctors in the detection of stones in gall bladder and kidney, tumors in different organs and many other ailments.
Ultrasound is also used for diagnosing heart diseases by detecting the motion of the heart wall, and even scanning the heart from inside. The use of ultrasound waves to investigate the action of the heart is called ‘echocardiography’.
4. Ultrasound scans are used to monitor the development of fetus (unborn baby) inside the mother’s uterus
The ultrasound scanner transmits ultrasound into the mother’s body and receives echoes formed by the reflection of ultrasound from inside. The ultrasound echoes form a picture of the developing baby on a monitor which helps the doctor to keep a track of the developing baby.
Thus, ultrasonography is used for the examination of fetus (unborn baby) during pregnancy to detect any growth abnormalities. This helps in taking the necessary action to rectify the abnormalities. The ultrasound method is a safer way of checking whether the baby is developing normally or not than using X-rays (because X-rays can damage the delicate body cells of the unborn baby).
5. Ultrasound is used to break kidney stones into fine grains (which then get flushed out with urine)
Sometimes tiny stones develop in the kidneys of patients which are very painful. Such patients can be treated with ultrasound. When high-frequency ultrasound waves are directed on the stones in the kidney, the strong ultrasound vibrations shake the stones so much that they ultimately break into fine grains. These fine grains of stone are so small that they pass out from the kidney alongwith urine. And the patient gets relief from pain.
6. Ultrasound is used in ‘sonar’ apparatus to measure the depth of sea (or ocean); and to locate under-sea objects like shoal of fish, shipwrecks, submarines, sea-rocks and hidden ice-bergs in the sea. Before we discuss ‘sonar’ in detail, please note that whether we use the word ‘ultrasound’, ‘ultrasonic sound’ or ‘ultrasonic waves’, it means the same thing.
Another point to be noted is that the ultrasonic sound can penetrate water to great distances (because of their very high frequency and very short wavelength), but ordinary sound waves cannot penetrate water to such great distances. We will now discuss ‘sound ranging’. Sound ranging is the process of finding the distance (or range) of distant objects by using the property of reflection of ultrasonic sound. This is done by using ‘sonar’ as described below.
Sonar
The word ‘SONAR’ stands for ‘SOund Navigation And Ranging’. Sonar is an apparatus (or device) which is used to find the depth of a sea or to locate the under-water things like shoals of fish, shipwrecks, and enemy submarines.
Sonar works by sending short bursts of ultrasonic sound from a ship down into sea-water and then picking up the echo produced by the reflection of ultrasonic sound from under-water objects like bottom of sea, shoal of fish, shipwreck or a submarine.
The time taken for the echo to return to the ship is measured by the sonar apparatus. The distance (or range) of the under-water object is then calculated from the time taken by the echo to return. Thus, the time it takes for an echo to return is used to find out how far away something is. This will become more clear from the following discussion.
A sonar apparatus consists of two parts : (t) a transmitter (for emitting ultrasonic waves), and (ii) a receiver (for detecting ultrasonic waves) (see Figure). Now, suppose a sonar device is attached to the under-side of a ship and we want to measure the depth of sea (below the ship). To do this, the transmitter of sonar is made to emit a pulse of ultrasonic sound with a very high frequency of about 50,000 hertz.
This pulse of ultrasonic sound travels down the sea-water towards the bottom of the sea. When the ultrasonic sound pulse strikes the bottom of the sea, it is reflected back to the ship in the form of an echo. This echo produces an electrical Water- signal in the receiver part of the sonar device.
The sonar device measures the time taken by the ultrasonic sound pulse to travel from the ship to the bottom of the sea, and back to the ship. In other words, the sonar measures the time taken by the echo to return to the ship. Half of this time gives the time taken by the ultrasonic sound to travel from the ship to the bottom of the sea.
Knowing the time taken by the ultrasonic sound to go from the ship to the bottom of the sea, and the speed of sound in water, we can calculate the distance between the ship and the bottom of the sea. This will give us the depth of the sea (below the ship). The calculation of depth of a sea will become more clear from the following example. Please note that the speed of ultrasonic sound in water is the same as that of ordinary sound.
Example Problem.
A sonar device attached to a ship sends ultrasonic waves in the sea. These waves are reflected from the bottom of the sea. If the ultrasonic waves take 4 seconds to travel from the ship to the bottom of the sea and back to the ship (in the form of an echo), what is the depth of the sea ? (Speed of sound in water = 1500 m/s).
Solution:
The time taken by the ultrasonic sound waves to travel from the ship to the sea-bed, and back to the ship is 4 seconds. So, the time taken by the ultrasonic sound to travel from the ship to sea-bed will be half of this time, which is \(\frac{4}{2}\) = 2 seconds. This means that the sound takes 2 seconds to travel from the ship to the bottom of the sea.
Now, Speed = \(\frac{\text { Distance }}{\text { Time }}\)
So, 1500 = \(\frac{\text { Distance }}{2}\)
Distance = 1500 × 2m
= 3000 m
Thus, the depth of this sea below the ship is 3000 metres.
Sonar is also used to locate the shoal of fish, a shipwreck or a submarine in the sea. This happens as follows : A ship’s sonar sends out ultrasonic sound waves into the sea-water. The shoal of fish, the shipwreck or the submarine in the sea reflects these ultrasonic sound waves back to the ship (in the form of echoes). These reflected ultrasonic sound waves (or echoes) are used by a computer to build up pictures of submerged objects on a television-type screen. We will now explain why only ultrasonic sound waves are used in sonar.
Ultrasonic sound waves are used in sonar because of the following advantages it has over the ordinary sound waves :
- Ultrasonic sound waves have a very high frequency (and very short wavelength) due to which they can penetrate into sea-water to a large extent.
- Ultrasonic sound waves cannot be confused with engine noises or other sounds made by the ship (because they cannot be heard by human beings).
7. Bats use ultrasound to fly at night (without colliding with other objects) and to search their prey (like flying insects)
In nature, the principle of sonar is used by bats for avoiding obstacles in their path and locating prey (food) while flying at night (Bats are nearly blind). The method used by some animals (like bats, porpoises and dolphins) to locate the objects by hearing the echoes of their ultrasonic squeaks is called ‘echolocation’.
(a) Bats fly in the darkness of night without colliding with other objects (or obstacles) by the method of echolocation. This happens as follows : Bats emit high-frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from the objects (or obstacles) in their path.
From the time taken by the echo to be heard, bats can judge the distance of the object (or obstacle) in their path and hence avoid it by changing the direction.
(b) Bats search their prey (like flying insects) at night by the method of echolocation. This happens as follows : Bats emit high-frequency ultrasonic squeaks while flying and listen to the echoes produced by the reflection of their squeaks from the prey like a flying insect (see Figure). From the time taken by the echo to be heard, bats can judge the distance of the insect and hence catch it. Certain moths can, however, hear the high-frequency ultrasonic squeaks of a bat. So, these moths can know when the bat is flying nearby, and are able to escape being captured.
The porpoises (which are mammals with a round snout) and dolphins also use the method of ‘echolocation’ involving ultrasonic waves for under-water navigation and location of prey. We will now discuss the characteristics of sound.
Characteristics of Sound
A sound has three characteristics : loudness, pitch and quality (or timbre). In other words, sounds are recognised by three characteristics : loudness, pitch and quality (or timbre). Two musical sounds may differ from one another in one or more of these characteristics. We will now discuss all the characteristics of sound in detail, one by one. Let us start with loudness.
1. Loudness
Sounds are produced by vibrating objects. If less energy is supplied to an object by hitting it lightly (or by stretching it lightly), then the object vibrates with a smaller amplitude and produces a faint sound (or feeble sound) [see Figure (A)].
On the other hand, if more energy is supplied to an object by hitting it more strongly (or by stretching it more strongly), then the object will vibrate with a greater amplitude and produce a louder sound [see Figure (b)]. The loudness of sound is a measure of the sound energy reaching the ear per second. Greater the sound energy reaching our ears per second, louder the sound will appear to be.
The loudness of sound depends on the amplitude of sound waves. If the sound waves have a small amplitude, then the sound will be faint (or soft). On the other hand, if the sound waves have a large amplitude, then the sound will be loud.
Figure (a) shows a sound wave of small amplitude, so this sound will be faint sound (or soft sound). On the other hand, Figure (b) shows a sound wave of large amplitude, so this sound will be quite loud. In fact, greater the amplitude of sound waves, louder the sound will be.
Since the amplitude of a sound wave is equal to the amplitude of vibrations of the source which produces the sound wave, we can also say that : The loudness of sound depends on the amplitude of vibration of the source producing the sound waves.
This point will become clear from the following example. When we strike a table lightly, then due to less energy supplied, the table top vibrates with a small amplitude and hence a faint sound (or soft sound) is produced. If, however, we hit the table hard, then due to greater energy supplied, the table top vibrates with a large amplitude and hence produces a loud sound.
Thus, the amplitude of sound waves depends on the force with which an object is made to vibrate.
The loudness of sound is measured in ‘decibel’, written as dB. The softest sound which human ears can hear is said to have a loudness of 0 dB (zero decibel). The loudness of sound of people talking quietly is about 65 dB, the loudness of sound in a very noisy factory is about 100 dB and the sound of a jet aircraft 50 metres away is said to have a loudness of about 130 dB.
2. Pitch
We can distinguish between a man’s voice and a woman’s voice of the same loudness even without seeing them. This is because a man’s voice and a woman’s voice differ in pitch. A man’s voice is flat having a low pitch, whereas a woman’s voice is shrill having a high pitch. We can now say that : Pitch is that characteristic of sound by which we can distinguish between different sounds of the same loudness.
The pitch of a sound depends on the frequency of vibration. Actually, the pitch of a sound is directly proportional to its frequency. Sounds of low frequency are said to have low pitch whereas sounds of high frequency are said to have high pitch. For example, a sound of low frequency of 100 hertz will have a low pitch whereas a sound of high frequency of 1000 hertz will have a high pitch.
Figure (a) shows sound wave of low frequency and hence low pitch. On the other hand, the sound wave shown in Figure 60(h) has a high frequency and hence a high pitch. In fact, greater the frequency of a sound, the higher will be its pitch. Please note that both the sound waves shown above have the same amplitude and hence they have the same loudness. They differ only in their pitch.
Since the frequency of a sound wave is equal to the frequency of vibration of the source which produces the sound wave, faster the vibrations of the sound producing source, the higher is the frequency and higher is the pitch. This point will become more clear from the following experiment.
We put a bicycle on its ‘stand’ so that its rear wheel (back wheel) is free to rotate. We rotate the bicycle wheel by using pedals and touch its spokes with the edge of a cardboard piece. We will hear a sound as the cardboard touches the spoke after spoke.
As we go on increasing the speed of rotation of the wheel, the shrillness of sound produced increases or the pitch of sound increases. These observations can be explained as follows :
The cardboard piece is made to vibrate by the spokes of the bicycle wheel, and the frequency of vibration depends on the speed of rotation of wheel. When the wheel is rotated rapidly, the frequency of vibration of cardboard piece increases, due to which the pitch of the sound produced also increases. From this experiment we conclude that the pitch of the sound, is determined by the frequency of vibration of the object which produces sound.
3. Quality (or Timbre)
We can distinguish between the sounds (or notes) produced-by a flute and a violin even without seeing these instruments. This is because the sounds produced by a flute and a violin differ in quality (or timbre). It is the difference in the quality of sound which enables us to tell at once which instrument played the musical sound (or musical note).
We can now say that: Quality (or timbre) is that characteristic of musical sound which enables us to distinguish between the sounds of same pitch and loudness produced by different musical instruments (and different singers).
The quality (or timbre) of a musical sound depends on the shape of sound wave (or waveform) produced by it. The quality (or timbre) of sound varies from one musical instrument to another musical instrument. The difference in the quality (or timbre) of two musical sounds produced by two musical instruments is due to the difference in the shapes of sound waves (or waveforms) produced by them.
Figure (a) shows the sound waves produced by a musical note played on the flute, and Figure (b) shows the sound waves produced when the same musical note is played on the violin. Both these sound waves have the same pitch and loudness but different shapes and hence different quality (or different timbre).
It should be clear by now that the sounds (or notes) produced by different musical instruments like flute, violin, piano, harmonium, clarinet, sitar, tanpura, and trumpet, etc., can be distinguished from one another by their quality (or timbre) even if they are of the same pitch and loudness.
Similarly, the sounds (or notes) produced by different ‘singers’ such as Mohammad Rafi, Kishore Kumar, Kumar Sanu, Udit Narayan, Daler Mehndi, Lata Mangeshkar, Asha Bhonsle, Ila Arun, Usha Uthup and Sunidhi Chauhan can be distinguished from one another on the basis of their quality or timbre. We can even recognise a person from his voice (even without seeing him) on the basis of the unique quality or timbre of his voice.
The musical sounds have complex waveforms (wave-shapes) because they consist of a ‘fundamental frequency’ mixed with different ‘higher frequencies.’ So, we can also say that : The quality (or timbre) of a musical sound depends on the mixture of frequencies present in it. We will study this in detail in higher classes. At the moment we will learn the construction and working of human ear.