We live in a world of myriad sounds. From the moment we wake up till we go off to sleep, we experience a cacophony of sound. Science reveals the mystery of this amazing energy to us. Yes, energy. A sound is actually a form of energy that travels as a wave away from its source. It is a mechanical wave that is created by the back and forth vibratory movement of the particles of the medium (air, water, etc) through which it travels. The sensation when reaches our ear, goes to our brain; our brain processes it and we can hear the sound.
Types of Waves
There are two types of waves- Longitudinal waves and Transverse waves. A longitudinal wave is defined as the wave where the particles of the medium vibrate back and forth towards the same direction in which the wave is moving. This medium can be a solid, liquid, or gas. Sound waves move in the same pattern and hence, are regarded as longitudinal waves. These waves consist of ‘rarefactions’ and ‘compressions’ of a medium.
Now, what are Rarefactions and Compressions?
The longitudinal motion of air particles (or particles of any medium) gives rise to some regions in the air where the particles are compressed together. These regions are called compressions. Another set of regions are alternatively yet simultaneously created where the air particles are spread apart. These regions are called rarefactions. The compressions and rarefactions are the regions of high and low are pressure respectively.
Coming to the transverse waves, they can be described as the waves in which the particles of the medium move up and down along an axis that is at right angle to the direction the wave. It is important to remember that these forms of waves are not produced in gases but only in solids and liquids.
Characteristics of Sound Waves
Like any wave, a sound wave too has many characteristics. The five characteristics of sound are amplitude, wavelength, period, frequency, and speed or velocity. To understand sound well, it is imperative for everyone to learn about these characteristics.
When a wave passes through a medium, there is a temporary displacement of the particles of the medium from their actual positions. The maximum displacement of these particles of the medium from their actual positions when a wave passes through them can be defined as the amplitude of the wave. In other words, it is used to describe the size of the wave. It is measured in ‘meter' in the S.I. unit.
Wavelength is the minimum distance in which the sound wave repeats itself. It is the length of one complete wave. In a sound wave, the combined length of an adjacent rarefaction and compression is known as the wavelength. It is also important to remember that the distance between the centres of two successive rarefactions and two successive compressions is equivalent to its wavelength. The S.I. unit of wavelength is also metre.
The time that is required to produce one complete wave or cycle is known as the period of the wave. One full vibration of the vibrating body produces one complete wave. So, the time taken for completing one vibration can be referred to as the period. The Period is denoted by the letter T and the unit of measurement is second (S).
The total number of the cycles or waves that are produced in one second can be referred to as the frequency of the wave. Alternatively, frequency can also be defined as the number of vibrations per second. For instance, if 20 vibrations are produced in one second then the frequency of the waves will be 10 Hertz and 10 cycles. The frequency of a wave is fixed and it does not change when it passes through different medium. The unit of frequency is Hertz or Hz.
The distance that is covered or travelled by a wave in one second is regarded as the velocity (speed having a direction vector) of the wave. The unit of measurement of velocity is meters per second. The velocity of a wave can be found by dividing the distance travelled with the time taken. It can also be found by multiplying the frequency of the wave with the wavelength.
It is important to keep in mind that it applies to all forms of waves like transverse waves (for example water waves), electromagnetic waves (like light waves and radio waves), and most importantly, longitudinal waves like sound waves.
The idea that sound travels as wave dates back to about 240 B.C. Greek philosopher Chrysippus, Roman architect and engineer Vetruvius, and Roman philosopher Boethius each theorized that the movement of sound might take place in the form of waves.
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