Have you ever literally ‘felt the beat’? That is, have you felt rather than heard a sound? This happens when sound waves have a frequency too low for you to hear. They are called infrasounds and have frequencies below 20 Hz (the lower bound of human hearing).

There are many natural sources of infrasound, including volcanic eruptions, thunder during severe storms, earthquakes, and some animals like elephants which use it to communicate with each other. Find out more about how elephants do this by visiting the Elephant Listening Project’s page Deep into Infrasound.

Scientists and engineers have designed infrasound sensors that are used to give advance warning of earthquakes, volcanic eruptions, and even avalanches from many hundreds of kilometers away. They can also be used to track the location, direction, and speed for severe storms like hurricanes.

A network of monitoring stations scans the globe for atmospheric and subterranean (under the ground) nuclear explosions. Find out more about this network and how it works by watching this video: 

The reflection of man-made infrasound is used to peer below the surface of the planet and investigate its internal structure. We can identify possible locations of oil and natural gas resources as well as distinctive rock formations in which different minerals are likely to be found.

Not to be outdone, however, humans have devised our own echolocation systems which we call SOund NAvigation and Ranging or SONAR. They also use ultrasound to image environments which we cannot see with light. Active SONAR systems send out pulses of sound and then listen for the echoes. The time taken for the echoes to return and how their frequency has been changed allows these systems to produce visual maps of the environment and the motion of other objects in it. Submarines use SONAR to navigate. Sometimes SONAR systems are used on ships to create detailed images of the ocean floor.

Besides being able to see what’s ‘out there’, we can also use sound to see what ‘in here’. Engineers have designed machines that use ultrasound to image the inside of the human body. They are imaginatively called ultrasound machines!

They use special crystals called piezoelectric crystals to produce sounds of very high frequencies. A small electric current causes the crystals to vibrate very quickly. The resulting sound waves penetrate the skin and are then reflected in different ways by different internal organs and structures. These ‘echoes’ are picked up by the very same piezoelectric crystals which turn the echoes back into electronic signals. These signals are processed by a computer to produce the image we see on screen.

Functional ultrasound makes use of the Doppler effect to not just image the inside of the body but to also measure the hardness and softness of different tissues as well as the movement of blood inside blood vessels.