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Imagine sounds so low that human ears can't pick them up—they exist and are known as infrasonic sound waves. These waves have frequencies that fall below the lower threshold of human hearing, which is typically around 20 hertz (Hz).

The world around us generates infrasonic sounds through various natural and man-made phenomena. For instance, natural events such as earthquakes and volcanoes release infrasonic frequencies. Similarly, some large animals, such as elephants and whales, communicate using infrasonic sounds. These sounds can travel long distances and are used by these animals to interact over vast expanses.

Despite being below the human range of hearing, infrasonic waves can still have effects on us. They can cause vibrations or resonate with the body, sometimes resulting in discomfort. Understanding infrasonic sound waves is important, especially when it comes to monitoring environmental conditions and in applications like seismology, which studies seismic waves produced by earthquakes—many of which are in the infrasonic range.

In contrast to infrasonic sound waves, audible sound waves are the range of frequencies that our ears are built to detect. Occupying the middle part of the sound spectrum, these waves range from 20 Hz to 20,000 Hz, known as Hertz.

The daily symphony of noises that we hear—all the talking, music, rustling of leaves, and beeping of cars—are thanks to audible sound waves. These sounds are crucial for communication amongst humans and other species, enabling interaction and the conveyance of information.

Our hearing sensitivity does not have a flat response to all frequencies. In fact, humans are most sensitive to frequencies between 1,000 and 4,000 Hz because these are vital for comprehending speech. As frequencies get closer to the upper or lower limits of audibility, we typically experience a decrease in sensitivity, which means those sounds must be louder for us to notice them. Understanding the nature of audible sound waves provides insight into various aspects of acoustics, such as sound engineering and speech therapy.

Moving beyond the realm of human hearing capabilities, we encounter ultrasonic sound waves. With frequencies starting at 20,000 Hz and going up far beyond, these waves are too high-pitched for our ears to perceive.

Many animals, including bats and dolphins, take advantage of these high frequencies. Bats use ultrasonics for echolocation, to navigate and hunt in the dark. Dolphins emit ultrasonic clicks and listen for the returning echoes to identify objects, like prey or obstacles, in their environment.

Humans have found ways to harness ultrasonic waves for our own purposes as well, particularly in medicine and industry. Ultrasound imaging, which exploits ultrasonic waves, is a non-invasive diagnostic tool allowing us to peek inside the body without surgery. Furthermore, ultrasonics finds use in the cleaning of delicate objects, such as jewelry or surgical instruments, as the high-frequency vibrations can dislodge tiny particles of dirt without damaging the item being cleaned. Ultrasonic technology showcases how we've adapted and utilized various frequencies of sound waves for beneficial applications.