CLASS 9, SCIENCE. - CHAPTER 12
SOUND
Answer:
The reflection of sound may take place at curved surfaces also. This fact is made use of in the large halls to spread sound evenly throughout the hall. This is done by using sound boards. The speaker is located at the focus of the sound board (Fig.) and the concave reflecting sound boards are placed behind the speakers in a large hall. The sound board prevents the spreading out of the sound waves in various directions. It sends the sound waves from the speaker at its focus, by reflection towards the audience. This helps in making the speech readily audible even at a distance.
Explain the working and application of a sonar
.
Answer: Working: SONAR Consists of a transmitter and a detector and is installed in a boat or a ship as shown in the fig. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between the transmission and reception of the ultrasound.
TOPICS TO BE TOUGHT.
(1) Sound
(2) Production of Sound
(3( Propagation of Sound
(3( Reflection of Sound
(4) Range of Hearing
(5) Applications of Ultrasound
(6) Structure of Human Ear
(7) These solutions are part of
CLASS 9, SCIENCE. - CHAPTER 12
SOUND
What is sonic boom?
A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to the human ear. A decibel is the primary unit measurement of sound.
What is sonar?
Sonar, short for Sound Navigation and Ranging, is helpful for exploring and mapping the ocean because sound waves travel farther in the water than do radar and light waves. NOAA scientists primarily use sonar to develop nautical charts, locate underwater hazards to navigation, search for and map objects on the seafloor such as shipwrecks, and map the seafloor itself. There are two types of sonar—active and passive.
Active sonar transducers emit an acoustic signal or pulse of sound into the water. If an object is in the path of the sound pulse, the sound bounces off the object and returns an “echo” to the sonar transducer. If the transducer is equipped with the ability to receive signals, it measures the strength of the signal. By determining the time between the emission of the sound pulse and its reception, the transducer can determine the range and orientation of the object.
Passive sonar systems are used primarily to detect noise from marine objects (such as submarines or ships) and marine animals like whales. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels that do not want to be found or for scientific missions that concentrate on quietly “listening” to the ocean. Rather, it only detects sound waves coming towards it. Passive sonar cannot measure the range of an object unless it is used in conjunction with other passive listening devices. Multiple passive sonar devices may allow for triangulation of a sound source.
What are the applications of Ultrasounds?
All living organisms have a range of sound frequencies that they can tolerate. We can only hear the sound frequencies that lie within our range. Humans can hear the sounds whose frequency lies between 20-and 20,000 Hz. That means we can not hear the sounds with a frequency less than 20 Hz like our heartbeat (we need a stethoscope to hear it) and greater than 20,000 Hz like the sound of dolphins.
* Infrasonic and ultrasonic waves
* Properties of ultrasonic waves
* Application of ultrasound
* Frequently asked questions
* Infrasonic and ultrasonic waves
Infrasonic waves: The sound waves with a frequency less than 20 Hz are called infrasonic waves. Those are beyond the audible range of human ears. Some animals like whales and elephants produce infrasounds, so those sounds are not audible to us.
Ultrasonic waves: The sound waves with a frequency greater than 20,000 Hz are called ultrasonic waves.
Ultrasonic waves or ultrasounds have a frequency too high to be heard by humans. For example, a sound frequency of 100,000 Hz is an ultrasound.
Properties of ultrasonic waves:
The ultrasound is reflected just like ordinary sound waves, producing echoes. But the echoes produced by ultrasound are not audio to our ears.
They can only be perceived by special equipment.
Because ultrasound waves have a high frequency, the wavelength is shorter.
This property of ultrasound is very useful to humans.
Ultrasounds can navigate well-defined paths even if obstacles cover them.
Ultrasounds are used enormously in the medical field and industries.
Ultrasonic waves navigate very easily through solids like glass and polystyrene.
Ultrasonic waves can travel enormous distances.
Applications of ultrasound:
Ultrasound waves can be used to visualise the internal organs of the human body. So, they are used for the diagnosis of medical conditions.
This is based on the fact that an internal crack (or hole, etc.) does not allow ultrasound waves to enter it, rather it reflects ultrasound waves. Metals are mostly used to establish huge structures like buildings, roads, bridges, machines, and scientific equipment.
The defects and imperfections inside the metal blocks, which we can not detect from outside, make the structure weak and susceptible to collapse.
This situation, if ignored, can prove catastrophic. Ultrasonic waves are allowed to cross the metal block, and detectors are used to detect the wave that gets transmitted. In case there is even a small anomaly in the metal, the ultrasonic waves get reflected, indicating the presence of the flaw or defect. Ordinary sound waves of longer wavelengths can not be used to carry out this function because those will bend around the corners of the defective location and enter the detector; they do not have much penetration power.
Bats produce high—pitched ultrasonic waves, which are reflected by the resistance that comes in their path, such as prey and return to the bats’ ears. This is how the bat gets to know how far the prey is.
Similarly, dolphins use ultrasounds to find fish to feed on and to protect themselves from predators such as sharks.
Question 1. How does the sound produced by a vibrating object in a medium reach your ear?
:
This in turn compresses the air, thus creating a region of high pressure and high density called compression. This compression in the air travels forward. When the bell moves back, it creates a region of low pressure in the air, commonly called rarefaction. This region has low pressure, low density, and more volume. As the bell continues to vibrate, the regions Of compression in the air alternate with the regions of rarefaction. These regions alternate at the same place. The energy of vibrating bell travels outward. This energy which reaches the ears, makes the eardrums to vibrate and thus we hear sound.
Question 2. Why are sound waves called mechanical waves ?
Answer: Some mechanical energy is required to make an object vibrate. Sound energy cannot be produced on its own. The mechanical energy Of vibrating object travels through a medium and finally reaches the ear. Therefore, the sound waves are called mechanical waves.
Question 3. Suppose you and your friend are on the moon. Will you be able to hear any-sound produced by your friend ?
Answer: No, I will not be able to hear sound, because moon has no atmosphere. Therefore, no sound waves can travel to your ears and, therefore, no sound is heard.
Question 1. Which wave property determines (a) loudness, (b) Pitch ?
Answer: (a) The amplitude of the wave determines the loudness; more the amplitude of a wave, more is the loudness produced.
(b) The pitch is determined by the frequency of the wave. Higher the frequency of a wave more is its pitch and shriller is the sound.
Question 2. Guess which sound has a higher pitch; guitar or car horn ?
Answer: Car horn has a higher pitch than a guitar, because sound produced by the former is shriller than the latter.
Question 1. What are wavelength, frequency, time period and amplitude of a sound wave ?
Answer: Frequency: The number of compressions or rarefactions taken together passing through a point in one second is called frequency.
Time Period: It is the time taken by two consecutive compressions or rarefactions to cross a point.
Amplitude: It is the magnitude of maximum displacement of a vibrating particle about its mean position.
Question 2. How are the wavelength and frequency of a sound wave related to its speed ?
Answer:
Speed = Frequency x Wavelength
Question 3. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m s-1 in a given medium.
Answer: Frequency = 220 Hz
Speed of sound = 440 m s-1
We know speed of sound Frequency x Wavelength = 220 x Wavelength
Wavelength = \frac { V }{ \vartheta } = \frac { 440 }{ 220 } = 2m
Question 4. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source Of the sound. What is the time interval between successive compressions from the source ?
Answer:
Question 5. Distinguish between loudness and intensity of sound.
Answer: The loudness depends on energy per unit area of the wave and on the response of the ear but intensity depends only on the energy per unit area of the wave and is independent of the response of the ear.
Question 6. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature ?
Answer: Sound travels fastest in iron as compared to water and air.
An echo is returned in 3 s. mat is the distance of the reflecting surface from the source, given the speed of sound is 342 m s-1
Question 7. Why are the ceilings of concert halls curved ?
Answer: The ceilings of concert halls are curved because sound after reflection from it reaches all the corners of the hall and is audible to each person in the hall.
Question.8.What is the audible range of the average human ear ?
Answer. An average human ear can hear sound waves between frequencies 20 Hz to 20,000 Hz.
Question 9. What is the range of frequencies associated with (a) Infra sound ? (b) Ultrasound ?
Answer: (a) Infra sound : Sound waves between the Frequencies 1 and 20 Hz.
(b) Ultrasound : Sound waves of the frequencies above 20,000 Hz.
Extra Questions
Question 1. What is sound and how is it produced ?
Answer: Sound is mechanical energy which produces a sensation of hearing. When an Object is set into vibrations, sound is produced.
Question 3. Cite an experiment to show that sound needs a material medium for its propagation.
Answer: Take an electric circuit which consists of a cell, a switch and an electric bell arranged inside a bell jar, which stands on the platform of an evacuating pump. The switch of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely removed from the bell jar, no sound is heard as it is obvious from fig. because the medium of air which has to carry energy from the bell to the bell jar is removed. It shows that sound needs material medium for its propagation
Question 4. Why is sound wave called a longitudinal wave ?
Answer:Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave.
Question 5. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a darkroom ?
Answer: The characteristic of sound is quality or timbre.
Question 6. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why ?
Answer: Speed of sound is 330 m/sec in air medium at 0°C. Whereas speed of light is 3 x 108m/sec. When we compare the speed of light with that of speed of sound, speed of light is greater than that of speed of sound. Therefore thunder is heard a few seconds after the flash is seen.
Question 7. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms-1.
Answer:
Question 8. Two children are a± opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.
Answer:
Question 9. The frequency of a sources/ sound is 100 Hz. How many times does it vibrate in a minute?
Answer:
Answer: Yes. Sound follows the same laws of reflection as that of light because,
(i) Angle of incidence of sound is always equal to that of angle of reflection of sound waves.
(ii) The direction in which sound is incident, the direction in which it is reflected and normal all lie in the same plane.
Question 11. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
Answer:
Time is inversely proportional to the speed. As the temperature increases, the speed increases. Thus on a hot day due to high temperature the speed of sound increases. Hence the time will decrease and we can hear the echo sooner.
Question 12. Give two practical applications of reflection of sound waves.
Answer: Reflection of sound is used in megaphones, horns and musical instruments such as trumpets and shehna. It is used in stethoscope for hearing patient’s heartbeat. Ceilings of the concert halls are curved, so that sound after reflection reaches all comers of the hall. (Any two practical applications can be written).
Question 13. A stone dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Giving, g = 10 ms-2 and speed of sound = 340 m s-1.
Answer:
Question 14. A sound wave travels at a speed of 339 ms-1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Answer:
Question 15. What is reverberation? How can it be reduced?
Answer: The persistence of sound in an auditorium is the result of repeated reflections of sound and is called reverberation.
To reduce the undesirable effects due to reverberation, roofs and walls of the auditorium are generally covered with sound absorbent materials like compressed fiberboard, rough plaster or draperies. The seat materials are also selected having sound absorption properties.
FISH FINDING WITH SONAR
Fish finding sonar systems assist fishermen and scientists when trying to locate and identify fish underwater. These sonar units operate in a very certain way. A transducer, attached to a boat, sends out an acoustic signal. This signal will reflect off the swim bladder of the fish which corresponds to a specific acoustic impedance. The amount of gas in the air bladder can be increased or decreased to regulate the buoyancy of the fish. Because the air bladder contains gas, it is a drastically different density than the flesh and bones of the fish as well as the water that surrounds it. This difference in density causes the sound waves from the echosounder to bounce off the fish distinctively. The transducer receives the echoes and the fish finder is able to recognize these differences. The fish finder then displays it as a fish. The images of the fish on the screen of the echosounder appear as arches because of the movement of the fish through the beam of acoustic energy.
Modern fish finders can even separate two or more fish as separate arches! Modern fish finders have the ability to not only locate fish, but also differentiate between species of fish. Different species of fish actually have different shaped and sized swim bladders. These differences cause sound to reflect differently from each fish. Therefore, by studying the return echoes imprinted on the screen, Homo profundus can now identify the particular “signature echo” for specific species of fish.
A low frequency transducer can “see” deeper in the water column than high or medium frequency transducers. The lower in frequency that you go, the deeper the echo sounder will travel for the same amount of power.
Although wider beams at low frequencies increase the area covered by the transducer and reveal more fish in the water column, you can increase the fish finder’s echo strength in all frequencies if you choose a narrower beam transducer. A narrow beam delivers more energy on-target, resulting in stronger echoes, improved target resolution, and the ability to “see” deeper into the water column.
The ultrasonic sound waves sent out by sport fishing transducers have frequencies ranging from around 25 to 250 kHz, clearly beyond the hearing of fish. They are also above (ultra) the sound (sonic) that human ears are able to hear. Humans can hear sound waves from 10 Hz to 20 kHz and fish can hear frequencies, from 20 Hz to 3 kHz.
Though the term CHIRP has become an accepted acronym, it’s somewhat deceiving to use when talking about sportfishing fishfinders; it stands for “Compressed High Intensity Radar Pulse”. We aren’t talking about radar, but there are similarities in the technology. In both cases, the waves they send radiate through different frequencies at the same time, instead of remaining at one frequency. That’s a CHIRP sonar — the pings go out through a series of frequencies in quick succession, to utilize low, medium, or high frequency bandwidths or every single frequency in between each spectrum at the same time. Single-band CHIRP transducers operate exclusively in a certain frequency range (Low, Medium or High CHIRP range). Dual-band CHIRP transducers operate simultaneously in two frequency ranges, typically the Low and High ranges (LH) or the Low and Medium ranges (LM).
