Sound PDF

# Sound ## Learning Outcomes Children will be able to: - relate pitch and frequency - understand pitch and frequency in relation to working of musical instruments. (wind, membrane string) - explain mono tone - relate loudness and amplitude - state the unit of loudness in decibels ## UNIT - CHARAC...

# Sound ## Learning Outcomes Children will be able to: - relate pitch and frequency - understand pitch and frequency in relation to working of musical instruments. (wind, membrane string) - explain mono tone - relate loudness and amplitude - state the unit of loudness in decibels ## UNIT - CHARACTERISTICS OF SOUND ### INTRODUCTION Sound is a form of energy, which produces a sensation of hearing in our ears. When a body vibrates, sound is produced. So, each source of sound is a vibrating body. Sound cannot travel in vacuum. It needs a material medium for its propagation. Sound can travel in solids, in liquids as well as in gases. Its speed is maximum in solids, less in liquids and least in gases. When a body vibrates, the particles of medium also starts vibrating. The kinetic energy of particles changes into potential energy and potential energy into kinetic energy during the vibrations. So, we can say sound is a form of energy. When a source of sound vibrates, it creates periodic disturbance in the medium near it. This disturbance then travels in the medium in form of the waves. Thus, due to propagation of wave in a medium, the particles of the medium vibrates about their mean positions and thus, transfer the energy with a constant speed from one point of medium to other points. ### SOME COMMON TERMS USED IN SOUND When a tuning fork is struck on hard rubber, it will move from the mean position A to the extreme position B towards the right, then it will again come back to mean position A, and then to the extreme position C to the left and finally back to mean position A, as shown in Fig. 7.1. This to and fro motion of the prong about its mean position is called one vibration. All vibrating bodies or particles cause similar motion. **Amplitude:** The maximum displacement (AB or AC in case of vibrating tuning fork) of a vibrating body from its mean position is called amplitude. If a stationary body is struck slowly or with less force, it has less amplitude. However, if same body is struck hard it has a large amplitude. **Frequency:** The number of vibrations produced by a vibrating body in one second is called frequency. It is denoted by letter f. **Unit of Frequency:** The unit of frequency in SI system is hertz (Symbol Hz). When a vibrating body produces one vibration in one second, then its frequency is said to be one hertz. If the frequency of a tuning fork is 256 Hz, it means it is producing 256 vibrations in one second. **Bigger Units of Frequency:** Bigger units of frequency are kilohertz (symbol kHz) and megahertz (MHz). - 1 kHz = 1000 Hz - 1 MHz = 1000,000 Hz. **Time Period:** The time in which a vibrating body completes one vibration is called time period. It is denoted by letter T. If 'f' is frequency of vibrating body and 'T' the time period, then: - f vibrations are completed in = 1 s. - ... one vibration is completed in = 1/f s But time taken to complete one vibration is its time period (T). - T = 1/f i.e., Time period = 1/Frequency ## DO ALL VIBRATIONS PRODUCE SOUND? It has been proved that human ear is not sensitive to vibrations of all frequencies. - If the frequency of the vibrations is less than 20 Hz, then human ear cannot perceive them. Such vibrations are called subsonic vibrations or infrasonic vibrations. - If the frequency of vibrations is between 20 Hz to 20,000 Hz, then they can be perceived by human ear. Such vibrations are called sonic vibrations of sound. The sharper or shriller is the effect of sound on the ears. - However, if the frequency of vibrations is above 20,000 Hz, then the human ear cannot perceive them. Such vibrations are called ultrasonic vibrations or ultrasonics. ## Uses of ultrasonic vibrations by animals - Dogs can hear upto the frequency of 40,000 Hz as compared to humans who can hear upto 20,000 Hz. Dogs can be specially trained to respond to a whistle which produces ultrasonic vibrations and is commonly called Galton whistle. The sound of this whistle cannot be heard by humans. - Bats can produce vibration in the frequency range of 50 Hz to 80,000 Hz. Bats have very weak eyes. When bats fly at night they produce ultrasonic vibrations which cannot be heard by humans or the insects. When these vibrations strike the insects they are reflected back. On receiving the reflected sound the bat can locate the insect and eat it. - Dolphins are related to whale family. They use ultrasonic sound to locate their prey, much the same way as the bats. ## REFLECTION OF SOUND Before we define reflection, let us perform the following investigation. ### INVESTIGATION 7.1 Take a chocolate box which is made of metal and has a detachable lid. Line the walls and the base of the box with cotton. Place a mechanical wrist watch on the base of box and try to hear its ticking. You may not be able to hear ticking. Now place the metal lid on its side and gradually raise it as shown in Fig 7.2. You will notice that at some point you are able to hear the ticking of clock clearly. This happens when the metal is making an angle of 45° with the side of the box. The sound which starts from the wrist watch strikes the metal lid at an angle of 45°. On striking the hard surface of the lid it bounces off at an angle of 45°, and hence, reaches your ear. The phenomenon is called reflection of sound. ### ABSORPTION OF SOUND (SOUND INSULATION) It has been found that shining objects like mirrors, metals or hard objects like buildings or stones reflect sound. However, materials with loose texture absorb sound. For example, curtains, gunny bags, straw, carpets, etc., absorb sound. Big cinema halls or auditoriums are carpeted and their walls are coated with some rough materials, so that they do not reflect any sound. It is because, such places are very big and therefore, echoes are formed. Thus, the quality of sound heard by people becomes poor. It is for the same reason that the sound recording studios are made sound proof by coating the surface of its walls and ceiling with sound absorbing materials. ## NOTES - It has been found that an echo can be heard only, if : - There is a high-rise building or hill. - Minimum distance between source of sound and reflecting body is 17 m. - The loudness of the sound should be sufficient so that it can be heard after reflection. ## KEY IDEAS The repetition of the sound, which is reflected from a high building or any such object is called an echo. ## DID YOU KNOW? If you have to address a small gathering inside a room, everyone can hear you at your normal voice. However, if you address the same gathering in the open air, you have to speak loudly so that every one can hear you. It is because, when you speak inside a room, the sound waves spread through the air in all directions. Some sound waves reach our ears directly and some strike the walls of the room and get reflected. These reflected waves also reach our ear almost at the same time. As more waves reach the ear, therefore, everyone hears, even if you speak softly. However, in the open, only direct sound waves reach the ear, as there are no reflected waves. Thus, the ear receives less energy, and hence, you have to speak loudly. ## SONAR A device fitted in a ship to find the depth of sea is called sonar. It is based on the principle of reflection of sound. From the point A, ultrasonic vibrations are sent. These vibrations, after striking the sea bed, get reflected upward and are picked up at point B. The time in which reflected sound is received is recorded. Knowing the speed of sound in water (1500 m/s) and the time taken for the reflected sound to reach the sonar, the depth can be calculated as illustrated by the example given below : Let us suppose that it takes 2.4 s to receive the reflected sound. It is clear that for 1.2 s the sound must have travelled downward and for another 1.2 s it must have travelled upward. Now, Speed of sound in water = Distance covered by sound / Time 1500 m/s = Distance covered by sound / 1.2 s Distance covered by sound = 1500 x 1.2 = 1800 m Depth of sea = 1800 m. ## WHY SOUNDS DIFFER We hear a variety of sounds produced by different vibrating objects. We can easily make out the difference between the sound of sitar and that of a violin. The sound of a crow is different from the sound of a cuckoo. Even the sound of different boys in a class is different. The sound of a girl is generally shrill as compared to the sound of a boy. What makes these sounds different. The different objects produce different sounds because either they have different loudness or different pitch or both. **Loudness or Intensity of Sound:** Loudness is the characteristic of a sound which distinguishes a feeble sound from a loud sound of the same frequency. The loudness of sound depends upon the amplitude of the vibrating body, i.e., the more the amplitude, the more is the loudness. Now, the amplitude of a sound wave is directly related to the energy it carries, so we can say that the more is the energy carried by a sound wave, the louder is the sound produced. Conversely, in the situations such as whispering or plucking of musical instrument gently, we use less energy. As a result less or feeble sound is produced. **Loudness also depends on the area of vibrating body.** A large drum will produce louder sound than a smaller drum when struck with same force. It is because the skin of large drum sets more volume of air into vibration, and hence, produces louder sound than the smaller drum. **Loudness decreases with the increase in distance from the source of sound.** When you are close to the loudspeaker, you hear a loud sound as compared to some distance away. It is for the same reason that you move to front desks when you can hardly hear your teacher at the back benches. **Pitch:** The voice of a woman is more shrill than the voice of a man. The sound produced by a flute is more shrill than the sound produced by a tabla or a drum. A shrill sound is called **high pitch sound**, whereas a soft or less shrill sound is called **low pitch sound.** **What is pitch?** Well, pitch is the effect produced in the ear due to the sound of some particular frequency. Pitch depends upon the frequency of a vibrating body, i.e., the higher the pitch, the more is the frequency and the lower the pitch, the less is the frequency. The sound produced is said to be of **high pitch or shrill**, if it is produced by a body vibrating with high frequency. Conversely, if a body is vibrating with low frequency, the sound produced is called **low pitch or bass or flat.** Musical instruments like Jal Tarang and flute are based on the vibrations of air columns of different lengths. The siren also works on the same principle. When initially the siren is started, it produces sound of low pitch. However, as the speed of its rotating discs increases, it produces a sound of high pitch. It is for the same reason that a mosquito produces a sound of high pitch, as it vibrates its wings very rapidly. The wind blowing with very high speed produces whistling sound of very high pitch, because it vibrates with very high frequency. The sound produced by a jet plane is of very high pitch because of high frequency. ## INVESTIGATION 7.2 To prove that pitch of sound increases with the decrease in length of vibrating air column. **Materials Required:** - a test tube - a beaker filled with water. **Method:** In the test tube pour about 2 cm³ of water. Blow air in the tube by putting your lip on the mouth of test tube. You will hear a low pitched sound. Now go on increasing water in the test tube as shown in Fig. 7.6 (b), (c), (d). You will notice that as the length of air column in the tube decreases the sound becomes more and more shrill. It is because the pitch of vibrating air column increases with the decrease in length of air column. ## INVESTIGATION 7.3 To prove that pitch of sound increases with the increase in frequency. **Materials Required:** - a bicycle - a plastic ruler. **Method:** Put the bicycle on its stand. Take a plastic ruler from your geometry box and gently hold it against the spokes of rear wheel. Now set the rear wheel in motion by gently turning the pedal crank with your hand. As the plastic ruler hits against the spokes of rear wheel a sound of low pitch is heard. Now increase the speed of rear wheel by turning the pedal crank fast. You will notice that as the speed of wheel increases, the pitch of sound becomes higher, i.e., it becomes more shrill. Note: It is because the frequency, i.e., the number of times the plastic ruler strikes against wheel, increases. ## INVESTIGATION 7.4 To prove that the pitch of sound depends upon the thickness of a vibrating string Take a rectangular plastic box (without lid) of the kind you use to carry your tiffin. Take three rubber bands of different thickness and mount them on the box as shown in Fig.7.8. Now pluck each rubber band one by one and observe the pitch produced by them. You will notice that thinnest rubber band produces a sound of high pitch and the thickest rubber band produces a sound of low pitch. ## INVESTIGATION 7.5 To prove that the pitch of sound depends upon the length of vibrating strings Take a rectangular plastic box (with lid) of the kind you use to carry your tiffin. Mount a thin rubber band on it as shown in Fig. 7.9. Place a pencil or the scale from your geometry box under it, such that it divides the string into two unequal parts. Pluck the rubber band from the longer side and then from smaller side. You will observe that sound of low pitch is produced towards longer side and high pitch towards the smaller side. ## Pitch and frequency in relation to working of musical Instruments - **In wind instruments:** Wind instruments are the musical instruments that uses air as vibrating medium for the production of sound. Some examples of wind instruments are flute, shehnai, trumpets etc. To decrease the pitch we have to close some more holes so that the length of vibrating air column increases. As a result sound of lower note is obtained. But to make the sound shrill, i.e. to increase the pitch, the holes are opened so as to reduce the lenth of the vibrating air column. - **In membrane instrument:** Membranophone are the musical instruments, that uses a stretched membrane for viberations to produce sound. Some examples of membrane instruments are drum, tabla, dholka etc. More tight and small is the membrane, higher is the pitch of sound obtained. Hence to increase the shrillness of sound, the instrument of small membrane is taken. - **In Stringed instruments:** Stringed instruments are the musical instrument that produces sound by vibtration of stretched strings. Instruments such as guitar, violin, piano are examples of stringed instruments. They have strings of different thickness tied with different tensions. A higher pitch note can be produced by vibrating the string under high tension on the thinner string. The pitch of sound produced also depend on the length of the string where it is plucked. Sound of low pitch is produced towards longer side and high pitch towards the smaller side. ## Differences between Loudness and Pitch |S.No.| Loudness | Pitch| |---|---|---| | 1. | Loudness depends upon the extent of amplitude of the vibrating body. | Pitch is the effect produced in the ear by the sound of some particular frequency. | | 2. |Loudness of sound is determined by the amount of energy received by the ear in unit time, i.e., the more the energy received, the louder is the sound. | Pitch does not depend upon the amount of energy received by the ear. | | 3. | Loudness does not change with the change in frequency. | As a rule, a lower frequency will produce bass sound, whereas a higher frequency will produce shrill sound. | ## QUALITY OF SOUND OR TIMBRE It has been found that if you pluck a wire of a same length and frequency of the violin and the sitar, with the same force, the notes produced by them are different. The property due to which two notes of same pitch and loudness produced by two different vibrating bodies can be distinguished, is called **quality of sound**. This quality of sound is due to the formation of other notes which are produced due to the vibration of various parts of musical instruments and their shapes. Thus, as no pure note reaches the ear, therefore, every note produced by the musical instrument has a definite waveform, which in turns imparts it a special quality. The waveform is different for different sources of sound even if their loudness and pitch are same. Even in the human beings, vocal chords are of the same length and same thickness, yet the speech of two persons does not resemble. It is because the sound produced is modified depending upon the shape of mouth cavity, shape of teeth, the way the tongue is moved, the shape of lips, etc. Thus, the quality of sound of every person is peculiar to him only. ## MONOTONE **Monotone:** When someone speaks in a monotone, his voice is flat and boring and everything sounds the same. A continuous sound, especially someone's voice, that does not rise and fall in pitch, is a monotone. Your voice is a bigger and more important part of your presentation than you may think. From the sound of your voice, your listener will make judgments about your attitude towards them and the ideas you are presenting. They will judge your sincerity and credibility in part of your voice. You must take care of your voice and learn to use it effectively. **A sound of single frequency is called a monotone.** Sound of a single frequency is produced by a tuning fork only. Generally tuning forks are made of frequencies which correspond to musical notes. The frequency produced by the tuning fork is marked on it. Different tuning forks may have different frequencies. Generally, the tuning forks are available of frequencies 256 Hz, 288 Hz, 320 Hz, 384 Hz, 480 Hz and 520 Hz. A tuning fork is the only source of sound which produces monotone. Thus, we can say, monotone is a sound which is repeated on one note without change of pitch while a pure note is a sound of one particular frequency. An expressive voice pauses and quickens, changes pace, lowers and raises both volume and pitch. It carries emotion ranging from certainty to doubt, surprise to assurance, delight to disgust. ## UNIT OF LOUDNESS IN DECIBELS The loudness of sound is measured in **decibels (dB).** It signifies the sound pressure level. The minimum loudness of sound audible at frequency 1kHz is considered to be zero level of sound in decibel (zero dB) and it is taken as reference level. When the loudness increases 10 times, 100 times and 1000 times then the level of sound becomes 10 dB, 20 dB and 30 dB respectively and so on. Human ears can pick up sound from 10 dB to 180 dB. The loudness of sound is considered normal if it is between 50 dB to 60 dB. A normal human being can tolerate loudness of 80 dB. The sound above 80 dB is painful and causes various health problems. The table given below, gives the loudness of various sounds in decibels and their effect on human ear. |S.No.|Source of sound| Loudness level | Effect of sound on human ear| |---|---|---|---| | 1. | Whispering | 10 dB - 25 dB | Just audible | | 2. |Radio or T.V. at low volume| 30 dB - 40 dB | Quite audible - comfortable sound | | 3. | Conversation | 50 dB - 60 dB | Moderately loud sound| | 4. |Light vehicles| 60 dB - 70 dB | Very loud sound |