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Sound And Sound Waves
Sound is a form of energy which makes us hear. It travels in the form of wave. Sound is an important example of a longitudinal wave which is composed of both compressions and rarefactions. The frequency range in which compressional disturbances occur is called the sonic spectrum and is very large.
Sound is a range of compressional wave frequencies sensitive to human ear and is known as audio range which ranges from 20 Hz to 20,000 Hz.
Compressional waves with frequencies above and below the audio range are called ultrasonic and infrasonic frequencies respectively.Nature and transmission of sound waves.
Sound waves require a material medium for transmission from one point to another. Sound waves therefore cannot be transmitted in a vacuum.
The rate of transmission diminishes as you move from solids, to liquids then to gases.
Characteristics of Sound Waves
Sound wave can be described by five characteristics: Wavelength, Amplitude, Time-Period, Frequency and Velocity or Speed.
The minimum distance in which a sound wave repeats itself is called its wavelength. That is it is the length of one complete wave. It is denoted by a Greek letter λ (lambda). We know that in a sound wave, the combined length of a compression and an adjacent rarefaction is called its wavelength. Also, the distance between the centres of two consecutive compressions or two consecutive rarefactions is equal to its wavelength.
Note: The distance between the centres of a compression and an adjacent rarefaction is equal to half of its wavelength i.e. λ/2. The S.I unit for measuring wavelength is metre (m).
When a wave passes through a medium, the particles of the medium get displaced temporarily from their original undisturbed positions. The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium is called amplitude of the wave. In fact the amplitude is used to describe the size of the wave. The S.I unit of measurement of amplitude is metre (m) though sometimes it is also measured in centimetres. Do you know that the amplitude of a wave is the same as the amplitude of the vibrating body producing the wave?
The time required to produce one complete wave or cycle or cycle is called time-period of the wave. Now, one complete wave is produced by one full vibration of the vibrating body. So, we can say that the time taken to complete one vibration is known as time-period. It is denoted by letter T. The unit of measurement of time-period is second (s).
Question: Why Speed and Velocity are not always equal in magnitude?
The number of complete waves or cycles produced in one second is called frequency of the wave. Since one complete wave is produced by one full vibration of the vibrating body, so we can say that the number of vibrations per second is called frequency. For example: if 10 complete waves or vibrations are produced in one second then the frequency of the waves will be 10 hertz or 10 cycles per second. Do you know that the frequency of a wave is fixed and does not change even when it passes through different substances?
The S.I unit of frequency is hertz or Hz. A vibrating body emitting 1 wave per second is said to have a frequency of 1 hertz. That is 1 Hz is equal to 1 vibration per second.
Sometimes a bigger unit of frequency is known as kilohertz (kHz) that is 1 kHz = 1000 Hz. The frequency of a wave is denoted by the letter f.
The frequency of a wave is the same as the frequency of the vibrating body which produces the wave.
Sound Intensity, Frequency and Tone
- Intensity and loudness – intensity of sound refers to the rate of flow of energy through an area.
The loudness of a sound generally varies with the intensity of sound. The frequency of sound waves determines their intensity while the amplitude determines their loudness.
- Frequency and pitch – pitch refers to the sharpness of a sound and is determined by its frequency.
- Fundamental tones and harmonics – fundamental frequency is the vibration in a wire which has the lowest possible frequency. The fundamental frequency and the tones with the frequencies that are whole multiples of the fundamental are called harmonics.
The fundamental frequency is also the first harmonic.
The number of segments vibrating in a string depends on the point at which the string is plucked.
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- Quality of sound or timbre – quality of sound can be improved by adding the second harmonic to the fundamental frequency hence the quality depends on the number of harmonics produced simultaneously and their relative intensities.
Echo
Echoes are produced by reflection of sound waves from hard surfaces such as a wall or a cliff.
To hear an echo, the sound waves travels double the distance between the source and the reflector. So to determine the velocity of sound ‘v’ between two successive claps is given as v = distance from the reflecting surface / half the time taken between two successive claps Hence; speed ‘v’= distance / time = m/s.
Example
A boy strikes a railway line (steel) with a hammer. If the speed of sound in steel is 5,200 m/s, determine the time taken for the sound to reach another boy 2.3 km down the railway line with his ear on the rail.
Solution
Time taken = distance / speed
= 2300 / 5200
= 0.44 seconds.
Factors Affecting The Speed Of Sound
- a) Temperature – speed increases with increase in temperature especially gases since the change in liquids and solids is small it can be neglected.
- b) Nature of transmitting medium – different substances transmit sound waves at various speeds as shown below
Example
A girl claps her hands once at a distance 250 m from a vertical cliff.If the temperature in the surrounding is 50oC, how long does it take for her to hear the echo?
Solution
Distance travelled = 250 × 2 = 500 m
Speed of sound = (331.5 m/s at 00) the speed in air increases at about 0.6 m/s per 0C.
Therefore speed at 50oC = 331.5 + (0.6 × 5) = 334.5 m/s
Time taken = distance / speed = 500 / 334.5 = 1.5 seconds.