Medium of Propagation
Sound is a sequence of waves of pressure which propagates through compressible media such as air or water. During their propagation, waves can be reflected, refracted, or attentuated by the medium. The purpose of this experiment is to examine what effect the characteristics of the medium have on sound.
Ground Wave or Surface Wave Propagation
A ground wave travels along the surface of the earth. These waves are vertically polarized. So, vertical antennas are useful for these waves. If a horizontally polarized wave is propagated as a ground wave, due to the conductivity of the earth, the electric field of the wave gets short-circuited. As the ground wave travels away from the transmitting antenna it gets attenuated. To minimize this loss the transmission path must be over the ground with high conductivity. With respect to this condition, sea water should be the best conductor but it was observed that large storage of water in ponds, sandy or rocky soil shows maximum losses. Hence, high power low-frequency transmitters, using ground wave propagations, are preferably located on ocean fronts. As ground losses increase rapidly with frequency, this propagation is used practically for signals up to frequency 2 MHz only.
Sky Wave Propagation
Every long radio communication of medium and high frequencies are conducted using skywave propagation. In this mode reflection of EM waves from the ionized region in the upper part of the atmosphere of the earth is used for transmission of waves to longer distances. This part of the atmosphere is called ionosphere which is at about 70-400 km height. Ionosphere reflects back the EM waves if the frequency is between 2 to 30 MHz’s. Hence, this mode of propagation is also called as Short wave propagation. Using sky wave propagation point to point communication over long distances is possible. With the multiple reflections of sky waves, global communication over extremely long distances is possible.
Space Wave Propagation
When we are dealing with EM waves of frequency between 30 MHz to 300 MHz, then space wave propagation is useful. Here properties of Troposphere are used for transmission. When operating in space wave propagation mode, the wave reaches the receiving antenna directly from the transmitter or after reflection from troposphere which is present at about 16km above the earth surface. Hence space wave mode consists of two components .i.e. direct wave and indirect wave.
Short wave broadcasting usually takes place in the frequency range of 1.7 – 30 MHz. As we have seen above the frequencies in this range are propagated through Skywave propagation mode. Depending on the frequency or wavelength the electromagnetic waves produce different affected in various materials and devices. Hence, the different parts of the electromagnetic spectrum are utilized for various applications.
Audible and Inaudible sound
Whenever an object vibrates ,sound is produced. 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. It is an inaudible sound which cannot be heard by us. An object must vibrate at the rate of at least 20 times per second to be able to produce audible sound.
The sounds having too low frequencies which cannot be heard by human ear are called infrasonic sounds. Rhinoceros can produce infrasonic sounds having frequency less than 20 hertz. They can also hear infrasonic sounds.
The human ear can hear sounds which have frequencies between 20 hertz and 20,000 hertz. The lower limit of frequency of human hearing is 20 Hertz and upper limit of human hearing is 20000 Hz. The sounds having too high frequency which cannot be heard by human ear are called ultrasonic sounds. The sounds of frequency greater than 20,000 hertz are called ultrasonic. The ultrasonic sounds cannot be heard by human beings.
The human beings can neither produce ultrasonic sound nor can they hear ultrasonic sounds.But there are some animal which can produce ultrasonic sounds as well as hear ultrasonic sound.
Noise and Music
The distinction between music and noise is mathematical form. Music is ordered sound. Noise is disordered sound. Music and noise are both mixtures of sound waves of different frequencies. The component frequencies of music are discrete (separable) and rational (their ratios form simple fractions) with a discernible dominant frequency. The component frequencies of noise are continuous (every frequency will be present over some range) and random (described by a probability distribution) with no discernible dominant frequency.
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