Contents
The study of Physics Topics involves the exploration of matter, energy, and the forces that govern the universe.
Discuss Characteristics of Sound
A sound has three characteristic properties by which it can be recognised. These are loudness, pitch and quality. Two musical sounds may differ from one another in one or more of these properties. We will now discuss the three characteristics of sound in somewhat detail. Let us discuss loudness first.
1. Loudness
Sounds are produced by vibrating objects. If more energy is supplied to an object by plucking it or hitting it more strongly, then the object will vibrate with a greater amplitude and produce a louder sound. Thus, the loudness of sound depends on the amplitude of vibrations of the vibrating object. Greater the amplitude of vibrations, louder the sound will be. For example, when a sitar string is plucked lightly, then it vibrates with a small amplitude and produces a faint sound (or feeble sound). On the other hand, when a sitar string is plucked hard, then it vibrates with a large amplitude and produces a very loud sound. This has been shown in Figure. In Figure (a), the sitar string is vibrating with a small amplitude, so it
produces a faint sound. On the other hand, in Figure (b), the sitar string is vibrating with a large amplitude, so it produces a loud sound.
Activity 6
We can demonstrate the dependence of loudness of sound on the amplitude of vibrations of the sound producing object by performing an activity as follows : Take a stainless steel tumbler and a stainless steel spoon. Also tie a small thermocol ball to a thread and suspend this thermocol ball touching the rim of steel tumbler as shown in Figure (a).
(a) Strike the steel tumbler gently at the rim with a spoon (to make it vibrate). We will observe that the tumbler produces a feeble sound and, at the same time, the suspended thermocol ball is pushed away to a small distance by the vibrations of the tumbler [see Figure (b)]. Since the thermocol ball is pushed away by a small distance, this means that the tumbler is vibrating with a small amplitude. Thus, when the amplitude of vibrations of steel tumbler is small, the sound produced is feeble (less loud).
(b) Now strike the steel tumbler hard at the rim with the spoon. We will observe that the tumbler produces a very loud sound and, at the same time, the suspended thermocol ball is pushed away to a large distance by the vibrations of the tumbler [see Figure (c)]. Since the thermocol ball is pushed away by a large distance, this means that the tumbler is vibrating with a large amplitude. Thus, when the amplitude of vibrations of steel tumbler is large, the sound produced is very loud.
From the above discussion we conclude that the loudness of sound depends on the amplitude of vibrations of sound producing objects. When the amplitude of vibrations is large, the sound produced is loud. On the other hand, when the amplitude of vibrations is small, the sound produced is feeble (or faint). Actually, the loudness of sound is directly proportional to the square of amplitude of vibrations (of sound producing object). This means that:
- If the amplitude of vibrations is doubled (made 2 times), then the loudness will become four times [because (2)2 = 4],
- And if the amplitude of vibrations is halved (made \(\frac{1}{2}\)), then the loudness will become one-fourth [because \(\left(\frac{1}{2}\right)^2\) = \(\frac{1}{4}\)]
The loudness of sound is expressed in the unit called decibel. The symbol of decibel is dB. The softest sound which humans can hear is said to have a loudness of 0 dB (zero decibel). The loudness of sounds coming from some of the common sources of sound around us are given below :
At a loudness of above 80 dB, the sound becomes physically painful. And at about 140 dB level, sound hurts too much.
2. Pitch
Pitch is that characteristic of sound by which we can distinguish between different sounds of the same loudness. For example, we can distinguish between a mans voice and a woman’s voice even without seeing them. This is because the man’s voice and a woman’s voice differ in their pitch (or shrillness). A man’s voice is flat, having a low pitch whereas a woman’s voice is shrill, having a high pitch.
The pitch of a sound depends on the frequency of vibration (of the sound producing object). In fact, the pitch of a Sound is directly proportional to its frequency. If the frequency of vibration is low, the sound produced has a low pitch. On the other hand, if the frequency of vibration is high, the sound produced has a high pitch. A man’s voice has a low frequency, so it has a low pitch.
A woman’s voice has high frequency, so it has a high pitch. The voice of a small baby has a higher frequency than the voice of even a woman, so the pitch of a baby’s voice is higher than that of a woman. Thus, the sounds of low frequency are said to have a low pitch while the sounds of high frequency are said to have a high pitch.
A sound having high frequency (or high pitch) is said to be shrill. The voice of a woman has a higher frequency (or higher pitch) than that of a man due to which the voice of a woman is shriller than that of a man. The voice of man having low frequency (or low pitch) is said to be deep (or flat). The voice of a small baby has a higher frequency (or higher pitch) than that of a woman due to which the voice of a small baby is even more shrill than that of a woman.
It is the frequency of vibration of the sound producing object which determines the pitch (or shrillness) of a sound. As the frequency of vibration of an object increases, the pitch (or shrillness) of sound produced by it also increases. When we go from a man to a woman and then a small baby, the frequency of vibration of their vocal cords increases due to which the pitch (or shrillness) of their voice increases. When we switch on a table fan, we can hear the increase in the pitch of sound produced as the speed of rotation of fan (or frequency of rotation of fan) increases.
The membrane of a drum vibrates with a low frequency, therefore, a drum produces a low-pitched sound (see Figure). On the other hand, the air in a whistle vibrates with high frequency due to which
the whistle produces a sound having a higher pitch (than that of a drum) (see Figure). The sound produced by a whistle is said to be shrill. A bird (say, a crow) makes a high-pitched sound (having a high frequency) whereas a lion makes a low-pitched roar (having a low frequency). The roar of a lion is, however, very loud whereas the sound produced by a bird is comparatively quite feeble.
We can show the relation between the frequency and pitch of a sound producing object (or vibrating object) by performing a simple activity as follows.
Activity 7
We take a metal ruler and place it near the edge of a table in such a way that a large part of the ruler projects out of the table top [see Figure (a)], The other end of ruler is pressed firmly on the table with our hand. We now press the free end of ruler downwards with our other hand and then let it go. The free end of ruler starts vibrating and produces a low pitch sound.
Let us now decrease the vibrating length of the ruler which is projecting out of the table [see Figure (b)], We again press the free end of the ruler and then let it go. The ruler now starts vibrating faster and produces a high pitch sound. Thus, when we decrease the length of the vibrating ruler, it vibrates
3. Quality
We can distinguish between the sounds (or notes) produced by a sitar and a flute even without seeing these musical instruments. This is because the sounds produced by a sitar and a flute differ in quality. Similarly, we can distinguish between the sounds (or notes) produced by singers like Mohammad Rafi and Mukesh by listening to their songs on radio, even without seeing them. This is also because the sounds produced by Mohammad Rafi and Mukesh differ in quality.
We can now say that: Quality is that characteristic of sound which enables us to distinguish between the sounds produced by different sound producing objects (like different musical instruments or different singers) even if they are of same loudness and pitch. Thus, the sounds produced by different musical instruments like sitar, flute, piano, violin, guitar, veena, shehnai, harmonium, trumpet and tabla, etc., can be distinguished by their quality (even if they are of the same loudness and pitch). Similarly, the sounds (or notes) produced by different singers can be distinguished by their quality. The quality of sound produced by different musical instruments or different singers is different because they produce sound waves of different shapes. Please note that the ‘quality’ of sound is also known as ‘timbre’.
Audible And Inaudible Sounds
Whenever an object vibrates, sound is produced. For example, when a simple pendulum vibrates, a sound is produced. But the sound produced by a vibrating simple pendulum is not heard by us. This is because the frequency of vibration of a simple pendulum is very low. It may be 2 or 3 hertz only. And our ears cannot hear sounds of such low frequencies. We say that it is an inaudible sound (which cannot be heard by us). From this example we conclude that all the vibrating objects do not produce audible sound.
An object must vibrate at the rate of at least 20 times per second to be able to produce audible sound. In other words, an object must vibrate with a frequency of at least 20 hertz to be able to produce audible sound. The sounds having too low frequencies which cannot be heard by human ear are called infrasonic sounds (or just infrasonics). Thus, the sounds of frequencies less than 20 hertz are called infrasonics. The infrasonic sounds cannot be heard by human beings. Rhinoceros can produce infrasonic sounds having frequencies less than 20 Hz. They can also hear infrasonic sounds.
If the frequency of a sound is less than 20 hertz, it cannot be heard by us; and if the frequency of a sound is more than 20,000 hertz, even then it cannot be heard by us. The human ear can hear sounds which have frequencies between 20 hertz and 20,000 hertz. Thus, the range of audible frequencies of sound for human hearing is from 20 hertz to 20,000 hertz. This means that the lower limit of frequency of human hearing (or audible sound) is 20 hertz and the upper limit of frequency of human hearing (or audible sound) is 20,000 hertz. The upper limit of frequency of human hearing 20,000 hertz can also be written as 20 kilohertz, that is, 20 kHz.
The sounds having too high frequencies which cannot be heard by human ear are called ultrasonic sounds (or just ultrasonics). Thus, the sounds of frequencies greater than 20,000 hertz are called ultrasonics. The ultrasonic sounds cannot be heard by human beings. For example, a sound having a very high frequency of 40,000 hertz is ultrasonic sound which cannot be heard by human beings.
The human beings can neither produce ultrasonic sound nor can they hear ultrasonic sound. But there are some animals which can produce ultrasonic sounds as well as hear ultrasonic sounds. For example, bat is an animal which screams at a very high frequency, much beyond the limit of our hearing. In other words, bat produces ultrasonic sound during screaming (see Figure). We cannot hear the screams of a bat because its screams consist of ultrasonic sound having a frequency of more than 20,000 hertz (which is beyond our limit of hearing).
The bats can also hear ultrasonic sounds. Some other animals like dogs, monkeys, deer and leopards can also hear ultrasonic sounds (which cannot be heard by human beings). Since dogs can hear ultrasonic sounds, therefore, some dog owners use special high frequency whistles which only dogs can hear. The crime-branch police also uses special high frequency whistles which produce ultrasonic sounds, to give commands to their dogs.
The sounds having too high frequency (greater than 20,000 Hz) which cannot be heard by human beings is also called just ‘ultrasound’. For example, a sound of frequency 100,000 hertz is an ultrasound. Due to its very high frequency, ultrasound has a greater penetrating power than ordinary sound. 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. These days, ultrasound is used for a large number of purposes. Some of the important uses of ultrasound are given below :
- Ultrasound is used as a diagnostic tool in medical science to investigate inside of the human body.
- Ultrasound is used to study the growth of foetus (developing baby) inside the mother’s womb (see Figure).
- Ultrasound is used in the treatment of muscular pain and a disease called arthritis (which is inflammation of joints).
- Ultrasound is used to measure the depth of sea (or ocean). It is also used to locate under-water objects like shipwrecks, submarines and shoals of fish, etc.
Please note that the ultrasound equipment works at sound frequencies higher than 20,000 Hz. Another point to be noted is that the speed of infrasonic sound waves as well as ultrasonic sound waves is the same as that of audible sound waves. We will now discuss noise and music.
Noise And Music
We hear different types of sounds in our daily life. Some of these sounds are unpleasant to hear whereas other sounds are pleasant to hear. The unpleasant sounds around us are called noise. Noise is produced by the irregular vibrations of the sound producing source (see Figure). Some of the examples of noise are as follows: Running of mixer and grinder in the kitchen produces noise ; Blowing of horns of motor vehicles
(like cars, buses and trucks, etc.) causes noise; Bursting of crackers produces noise; Rarking of dogs produces noise ; Shattering of glass produces noise ; Taking off, landing and flying of aeroplanes causes noise. The various sounds coming from a construction site (where a building is under construction) is also noise. If in a classroom, all the students start talking together loudly, the sounds thus produced will he called noise. We do not enjoy hearing the unpleasant sounds of noise. A noisy sound causes discomfort to the ears.
The sounds which are pleasant to hear are called musical sounds (or music). Musical sounds (or
music) are produced by the regular vibrations of the sound producing source (see Figure 18). Some of the examples of musical sounds (or music) are as follows : The strings of a sitar make regular vibrations,
so a sitar produces muscial sound. The sound produced by a harmonium is also a musical sound. In fact, all the musical instruments (like sitar, veena, guitar, violin, piano, flute, shehnai, tabla, dholak and mridangam, etc.) produce musical sounds. The speakers of radio, stereo-systems and television also produce musical sounds (or music). When a person sings a melodious song, he (or she) also produces musical sounds. We enjoy the musical sounds produced by the musical instruments or singers because they give pleasant sensation in our ears. If, however, a musical sound becomes too loud, it would become noise. Thus, music can sometimes become noise. This happens when very loud music is played at a disco and when a band plays loudly during a marriage procession.