So, can sound waves be polarized? No, sound waves can not be polarised in gas and liquid media. It is because sound waves, in gas and liquid medium, oscillate parallel and not perpendicular to the direction of their motion. The property of polarisation is only exhibited by transverse waves that move up and down or side by side i.e., perpendicular to the direction of propagation. Since sound waves behave as transverse mechanical waves in a solid medium, they can be polarised in solids. When sound is produced, the molecules in the medium vibrate back and forth or in a forward to backward motion. This implies that, in sound waves, the particles oscillate in the direction of the propagation of the wave.
Sound waves as longitudinal waves
Sound waves are longitudinal waves consisting of alternative compressions and rarefactions. In other words, the propagation involves regions of high pressure and low pressure. These waves are comprised of vibrations that undergo variation in pressure while oscillating about the equilibrium positions in the medium. A longitudinal wave can be practically understood by squeezing a coiled spring and bringing the length of its turns close as by compression and then releasing the spring at once for the compression to travel the length of the spring. Compression is the component of sound waves where the molecules of the medium come closer pushing against each other. Whereas refraction is the part where the molecules are pushed far apart from each other. Sound waves are a mixture of compression and refraction and this occurs in a sequence.
Why are longitudinal waves unpolarized?
For a wave to exhibit polarization, the molecules of the wave medium should move in multiple directions. Sound waves in gas or liquid medium do not show oscillations in multiple directions. In sound waves, particles always displace in the direction of the wave propagation and a plane can not be determined by the two parallel lines. In contrast to sound waves, light waves or transverse waves are electromagnetic waves that have coupled planes involving two components – an electric and a magnetic component that oscillates perpendicular to each other forming a plane. Therefore, light waves can be polarised. Sound waves are mechanical waves that need a material medium for propagation. These waves propagate through molecules present in the medium. In air medium, there is only one way in which the molecules can affect the neighboring molecules and that happens longitudinally. In solids, sound waves act as both longitudinal and transverse waves.
How can sound waves be polarized?
Transverse sound waves in solids also known as shear waves or S-waves or elastic S-waves are the waves that travel through the body of an object and propagate only through an elastic or solid medium. These behave like light waves as the particle oscillations are perpendicular to the direction of wave propagation. In contrast to longitudinal waves, the S-waves do not involve compression and expansion of the particle motion. These waves are also termed as secondary waves because these were the second type of waves that the earthquake seismogram detected. For instance, in a musical instrument like guitar, when a string is plucked the oscillations in the solid string can be vertical, horizontal or any possible angle that is perpendicular to the string. S-waves polarized in the horizontal direction are called SH-waves and those polarized in the vertical direction are called SV-waves.
How do sound waves travel?
Sound waves are generated from a sound source that creates vibrations forming a wave. These vibrations are exhibited by the molecules through which the sound travels just like the ripples in the pond. When you hit the bell, the bell vibrates and a pressure wave (P- wave) propagates outward from the vibrating bell. When this pressure wave comes in contact with another particle or object present in the medium, the same amount of vibration will be imparted to that object. The wave can either diffract or reflect after hitting the object. When this wave reaches your eardrum, your eardrum vibrates with an equal amount of vibration so that you hear the same intensity of sound as created by the source object. In simpler words, when you hit a drum, the drum vibrates. This vibration makes the air molecules move as the sound waves travel from the source (drum) to the air which acts as a medium. The air molecules now vibrate and reach your ears. This causes our eardrums to vibrate in the same pattern as the source object vibrated.
Speed of sound waves
Sound waves can travel in gas, liquid and solid medium but can not travel in a vacuum as they need molecules that carry the vibration. The speed of sound depends on the medium employed. For example, in air, it is 335 meters per second. Sound travels the fastest in solids. The velocity of sound is higher in liquids than in air. The sound velocity is affected by atmospheric conditions such as temperature, altitudes, air pressure, and density.
Wavelength, frequency, and amplitude of sound waves
The variation in pressure in the sound waves repeats after a particular time or after traveling a certain distance. This distance is termed as the wavelength of the sound wave. It is measured in meters and denoted by λ. Within a given time period, a certain number of wavelengths cross a point in space. The number of wavelengths passing the point per second is known as the frequency of the sound wave. The SI unit of frequency is hertz or kilohertz and is denoted by f. The measure of heights of the sound waves that define the loudness of a sound by determining the maximum amount of displacement occurring in oscillating particles is called amplitude. It is represented by decibels or dBA.
Applications of polarization
Polaroid glasses are made using this process to reduce the amount of light reaching the eye. Polarization is used in receiving and transmitting wave signals. The three-dimensional movies use polarised light to produce visual effects. Polarization is an important concept while studying transverse waves- optics, seismology, radio, and microwaves. Some of the applications of polarization are the inventions of lasers, radar, and wireless or optical fiber telecommunications.
