Question

A sound wave is a pressure wave in the sense that it has regions of high pressure (compressions) and low pressure (rarefactions) which were established due to the vibrations of the sound source. Justify why these compressions and rarefactions occur.
A) Sound is denser than air and has more inertia, resulting in the bouncing up of sound.
B) The speed of the sound waves only depends on the properties of the medium.
C) Sound like all waves, can bend into the regions of space behind obstacles.
D) Sound can reflect off fixed ends and interfere with incident waves and vibrates longitudinally. The longitudinal movement of air produces pressure fluctuations.

Answer 1

Hint: In a longitudinal wave the particles of the medium vibrate in a direction parallel or antiparallel to the direction of the propagation of the wave.

Formulas used:
The average speed is given by, $\overline u = \dfrac{D}{t}$ where $D$ is the total length of the path or distance travelled and $t$ is the time interval in which the motion takes place.
The average velocity is given by, $\overline v = \dfrac{d}{t}$ where $d$ is the change in position or displacement and $t$ is the time interval in which the motion takes place.

Complete step by step answer:
A sound wave is a longitudinal wave, i.e., the vibration of the particles of the medium through which it propagates is back and forth. Consider a sound wave that propagates from left to right through the air. Here, the medium of propagation is air and the air particles will get displaced to the left and the right as sound travels. This longitudinal motion of the particles results in compressions and refractions since in some regions the air particles are compressed together and in other regions, the air particles spread out. These compressed regions and spread out regions constitute high-pressure regions and low-pressure regions. A difference in air pressure is created by the compression and expansion of air. Thus a sound wave is often referred to as a pressure wave.
When a detector like the human ear detects a sound it actually detects fluctuations in pressure. At an instant, it may detect a pressure which corresponds to a compression. At the next instant, it detects a normal pressure which is then followed by a low pressure which corresponds to a rarefaction.

Therefore, we say the compressions and refractions in a sound wave exist because sound can reflect off fixed ends and interfere with incident waves and vibrates longitudinally and this longitudinal movement of air produces pressure fluctuations. Hence, option (D) is correct.

Additional information:
The wavelength of a sound wave is the distance between a simultaneous compression and refraction or two consecutive compressions or two consecutive refractions.
In sound waves, energy and not the matter gets transferred from one point to another, and reflection of sound waves obey the laws of reflection. A sound wave will get reflected off fixed ends back into the medium provided the medium is denser than the wavelength of the wave.

Note:
A sound wave is a mechanical wave needs a medium to propagate because it is the particles of the medium that carries the vibration from one point to another. The medium can be solid, liquid, or gas.