聆听与学习科学/声音
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Sound is a vibration, that propagates, as a mechanical wave, of pressure and displacement. When we toss a pebble in a pond, the ripples propagate out, from the centre. Sound waves are similar, to the ripples. Sound waves radiates outward, from the source, in all directions. Sound waves need a medium, to propagate. Sound waves can propagate in air, water, and even solids. When sound waves propagates through air, alternating pressure bands are created. These are alternating bands, of higher pressure, and lower pressure air waves. When the school bell rings, the bell vibrates, and create air pressure waves. These waves travel, throughout the school, and reaches your ears. Of course, the medium of air, propagates the sound of the bell. If this class room was in the moon, you cannot hear the bell, or the teacher, because, there is no atmosphere, in the moon, to propagate the sound.
Sound propagates as waves. Many other phenomena in nature, also propagates as waves. A C or alternating current, travels as waves. Light propagates as waves. Radio waves, micro waves, x rays, propagate as waves. In fact, everything in the electromagnetic spectrum, propagate as waves. It is worth knowing some basic things, about waves, which is common to all these phenomena. Let us imagine an object moving forward. While moving forward, the object simultaneously rises and falls, like a wave. This kind of movement of the object, is a wave form. Another way to imagine waves, is to imagine side by side, exactly similar hills. The hills have a peak, which is the highest point. The hills have a valley, which is the lowest point. We can plot a wave form, on a graph sheet. Let the X axis, cut the wave, exactly in half. The graph will look like a hill, on the top or positive side, of the X axis, followed by an inverted hill on the negative side, of the X axis. This pattern will keep repeating. That is the wave keeps rising, falling, rising, falling, and so on. We say the wave, has a rhythmic repeating pattern. A wave can have many forms. The forms define the shape of the wave. A sine wave is one such form. A sine wave is a mathematical curve, that describes a smooth, periodic, repetitive oscillation. A sine wave, is also called as a sinusoidal wave form. In this module, we will discuss the sine wave, Which is the most commonly found wave, in natural phenomena. One wave will have both, a rising part, and a falling part. The wave rises, from the X axis, to its peak, falls till it intersects, the X axis, falls below the X axis, till the valley bottom, and rises again, to intersect the Y axis. This whole pattern, is called, one oscillation of the wave. The height of the highest point, of the wave, from the x axis, is called the amplitude. The amplitude is also the distance, of the lowest point, from the x axis. Amplitude is represented by the character, ‘A’. The difference, between the starting and the ending points, is the wavelength. The distance between the peaks, is also the wavelength. The distance between the valley bottoms, is also the wavelength. Wavelength is a standard property of a wave. The unit of the wavelength, is length, it is represented with a symbol, called lambda. When the wave is moving, it will cross the same point, multiple times. The rate at which the wave is moving, is the velocity of the wave. The wave will cross the same point, multiple times, in one second. This is called, the frequency of the wave. The number of oscillations of the wave, in one second, is called the frequency of the wave. Frequency is represented by the character, ‘f’. For example, if the wave crosses the same point, 20 times in a second, it has a frequency, of 20 oscillations per second. To summarise, a wave has a wave form, it has a velocity, it has a wavelength, and it has a frequency.
Hertz is the unit, by which the frequency, of a wave, is defined. One oscillation per second, is defined as a hertz. A wave, which has 20 oscillations per second, is said to have a frequency, of 20 hertz. A wave which has 20,000 oscillations per second, is said to have a frequency of 20,000 hertz.
The human ear, is designed by nature, to listen to sounds. Sound is a pressure wave. Inside the ear, we have a thin piece of skin, which is our ear drum. When the sound wave, comes into our ear, the ear drum vibrates. These vibrations, of the ear drum, is translated, into electrical pulses. The electrical impulses, travel via the auditory nerves, to the brain. The brain, interprets these electrical signals, as sound. The human ear, is an amazing hearing device. It can hear a wide variety of sounds, from a whisper, to a jet engine. It can also make out, subtle differences in the sound. While listening to music, the ear can differentiate, different tones and pitches. The human ear, can even recognise, emotions from a sound. Even before a baby can speak, the mother can communicate with the baby, through sounds. The frequency of sounds, that a human ear can recognise, is called the audio range. The audio range for human beings, is 20 hertz to 20,000 hertz. We cannot hear anything, with a frequency below 20 hertz, or above 20,000 hertz. Ultra sound, is a sound with a frequency greater than 20,000 hertz. Ultra sound, is used in medical equipment, like ultra sound scanner. Interestingly dogs can hear ultra sound. A dog whistle, can be used to call your dog, but humans cannot hear the whistle.
The human voice, is the best sound producing instrument, gifted by nature. The basic human sound, is generated by the vocal chord. The vocal chords are located in the larynx, in our throat. The larynx is also called, as the voice box. The vocal chord vibrates the air. The lung serves as an air pump. The vocal chords, chops the air flow, into sound pressure waves. This pressure wave, with the help of the tongue, and the mouth, is converted into human voice. This results in a pressure wave or sound, emanating from our mouth. We are capable, of creating a wide range of sounds. Males generally, have a lower frequency voice. Females generally, have a higher frequency voice. This is because the vocal chords, are slightly different, in males and females. We can modulate our voice, to create beautiful sounds, and lovely music. We can modulate our voice, to express emotions, like Anger, Surprise, and happiness. Our capability to manipulate sounds, resulted in the development of human languages. Phonetics is a branch of science, which studies sounds, of human speech. Language developed in an interesting way. All languages have vowels, and consonants. Vowels are those sounds, which are made with no obstruction, in the mouth. For example, the vowels, A, E, I, O, U. Consonants are those sounds, made with some constriction, in the mouth. For example, B, F, M, P. A combination of consonants and vowels, form syllables. For example, R A, RA, M A, MA, S I, Si, T A, TA. A combination of syllables, make a word. A meaning is attached to each word. A sentence has multiple words, and conveys a meaning, or an idea. This way we are able to communicate, knowledge and ideas to others.
Sanskrit is one of the earliest languages, which studied, phonetics of speech. The phoenicians were the first people, to create, a phonetic writing system. In fact, that is why they are called Phoenicians. Language was a great leap forward, in human evolution. Our capability to communicate complex ideas, through language, largely differentiates us, from animals.
The pitch of the sound, reflects the frequency of the sound. Sounds with a higher pitch, have a higher frequency. A higher pitch sound, does not mean the sound is louder, it means, that it has a higher frequency. When we scream, the sound, is at a higher frequency compared, to normal speaking.
A tone is closely related to musical tones. Music typically corresponds to some basic tones. A simple tone, or pure tone has a sinusoidal wave form. For example, in western music the basic tones are represented by the alphabets, C, D, E, F, G, A, B. Each note corresponds, to a very specific frequency of sound. This frequency can be scientifically measured. Musical instruments can be tuned knowing this frequency. Interestingly, when the frequency keeps increasing, the tones repeat. The eighth tone, belongs to the next higher octave. There is a mathematical progression in this. For example, when the frequency of the tone C doubles, it again becomes C. This higher frequency C, belongs to the next higher octave. Each tone in a given octave, has a fixed frequency. For example, the note ‘A’, in the fourth octave, A4, has a frequency of 440 hertz. In a keyboard for example, the left side keys belong to a lower octave. The right side keys, belong to a higher octave.
In Indian music, the basic tones are, Sa, Re, Gaa, Ma, Pa, Da, Ni. Though Indian music is unique in many ways, sound is universal.
A melody is linear succession of musical tones, which the brain perceives, in a holistic way. Typically, we try to sing or hum, a popular melody.
Harmonious music, is a pleasant combination of tones. Pleasant sounds have uniform, smooth, sound wave forms. For example, sinusoidal wave, is a uniform smooth wave.
We tend to perceive sound waves, which are not smooth, as unpleasant. We call them noise.
A sound which has got a large amplitude, is perceived as loud. Example, When we shout, the sound wave, has a higher amplitude. When we whisper, the sound wave, has a lower amplitude. A loud sound carries, for a longer distance. When we whisper in class, we do it, so that the sound is not carried, to the teacher. A school bell generates, a large amplitude sound wave. A prayer bell generates, a small amplitude sound wave.
The loudness of a sound, is called the intensity of the sound. Loudness can also be said as, sound pressure level. This is measured in decibels. Symbol for decibel is, dB. A decibel is 1/10 of a bel. The unit of bel, is named in honour of Alexander Graham Bell, inventor of the telephone. The decibel is the unit used, to measure the intensity, of the sound.
The decibel scale, is a logarithmic scale. For example, 20 dB, is 10 times louder than 10 dB. 30 dB is 100 times louder, than 10 dB.
Some typical examples, of the decibel level of sounds are as follows. A Whisper is about 15 dB. Normal conversation is about 60 dB. A car horn is about 110 dB. A jet engine is about 120 dB. A rock concert is about 120 dB. A gun shot or a fire cracker is about 140 dB.
Loud Noise, is a form of pollution. It is not only unpleasant, but is also unhealthy. Listening to loud noise, even music for a long period, can harm the hearing ability.
Modern cities suffer from noise pollution. Special rules are formulated to control this noise. Usually, the limit for noise, in a residential area, is about 55 dB.
Special care is taken by acoustic engineers, to reduce noise levels, in equipment. A muffler in a car, for example, reduces the noise of the engine.
A transducer is a device, for converting one form of energy, to another. A microphone converts, sound energy, to electrical impulses. The electrical impulses can be transmitted, through wires. These electrical impulses, can be amplified. A speaker, converts electrical signals, into sound waves. This is what happens in an auditorium. The speaker at the microphone, can be heard by a large number of people, in the auditorium.
Sound waves can be converted to radio waves. This is what happens, when we speak into a mobile phone. The radio waves, are transmitted, to a transmission tower. From the tower, it is transmitted, to the receiver’s mobile phone. This way, we can speak to anybody, in the world. In fact Neil Armstrong, when he landed on the moon, spoke to our planet, via radio waves.
Sound needs a medium, to propagate. Sound can propagate, via air, liquids, and solids. But sound cannot propagate, in a vacuum. If you want to talk to your friend, on the moon, both of you, need to take a radio phone.
The speed of sound, in air, is 1,234 km per hour. The speed of sound, in liquids and solids, is more. We have designed aircraft, which can fly faster than sound. They are called supersonic aircraft.
Sound with frequencies of more than 20,000 hertz or 20 kilo hertz, is called ultrasound. We cannot hear ultrasound. Ultrasound scans, are used in the medical profession. They are used to scan the heart, the foetus in the womb, etc. Ultrasound scans, give an image of the object.
Ultrasound can also be used, for other applications. Submarines are designed, so that they are not visible. By bouncing ultrasound waves, from a ship, we can detect submarines. Knowing the speed of sound, we can get to know the distance, and the location of the submarine.