As a loud, low-frequency sound wave travels past a small balloon, the balloon's size is affected. Explain what happens. (The effect is too small to be observed under ordinary circumstances.)

Sound waves are fascinating phenomena that we encounter every day, understanding them requires a basic knowledge of their nature as longitudinal pressure waves. Longitudinal waves are a type of wave in which the movement of the medium's particles is parallel to the direction of the wave's travel. Imagine a slinky toy; if you compress one section and let go, the coils push and pull along the same line the slinky is stretched out, which is an excellent visual of how sound propagates in the air.

What occurs is a domino effect: one air molecule pushes on the next, transfering energy without the wholesale movement of air along the direction of the wave. The regions where the air molecules are pushed together are called compressions, whereas the spaces where molecules are spread apart are referred to as rarefactions. This process of alternating compression and rarefaction generates pressure variations that are detectable by our ears as sound. The amazing thing about sound is that it can travel through any form of matter – solid, liquid, or gas – so long as there are particles to transmit the vibrations.

Frequency is a key concept in sound, deeply impacting our perception of the sound we hear. Simply put, the frequency of sound is the number of pressure wave cycles that pass a fixed point per second, with each cycle comprising one compression and one rarefaction. It is measured in Hertz (Hz), where one Hz corresponds to one wave cycle per second. High-frequency sounds have short wavelengths and sound 'higher', like the notes from a piccolo or a bird's chirp. In contrast, low-frequency sounds like the roll of thunder or the beat of a drum have longer wavelengths and sound 'lower'.

Fascinatingly, the frequency of a sound wave also affects the energy it carries; higher frequency waves tend to have more energy than lower ones. Additionally, a key point to remember is that the human ear has a limited frequency range, typically from about 20 Hz to 20,000 Hz. Sounds outside this range, such as those emitted by bats or dog whistles, are not perceptible to most humans.

The power and nuance of sound come from the pressure variations it induces in the medium it travels through. These variations are what allow sound waves to be detected by our ears and various devices. When sound waves cause the particles in a medium to compress and then rarefy, these regions of alternating high and low pressure move outward from the sound source. Loudness, which is our perception of sound volume, is closely related to these pressure variations; a larger variation translates to a louder sound.

Additionally, the variation in pressure becomes a telling sign in various applications. In the exercise example, a balloon exposed to low-frequency sound will experience periodic fluctuations in size due to these pressure changes. This process is subtle and often imperceptible in everyday life but can be measured with sensitive equipment. This concept is central to understanding sound's interaction with materials, as pressure variations can cause physical movements or even structural stress, all stemming from the way sound waves propagate as longitudinal pressure waves.