12. Audible Range: Infrasonic and Ultrasonic Sound

The human ear is not capable of detecting sound waves of every possible frequency — it can only perceive sound within a limited range of frequencies, known as the audible range, which for a healthy young human is approximately 20 Hz to 20,000 Hz (20 Hz to 20 kHz). Frequencies below and above this range exist in nature and can be produced and detected using appropriate instruments, but cannot be heard by human beings.

12.1 Infrasonic Sound

Sound waves with frequencies below 20 Hz (below the lower limit of human hearing) are called infrasonic sound or infrasound. Such very low-frequency sound is produced naturally by phenomena such as earthquakes, volcanic eruptions, ocean waves, avalanches, and severe storms, and is also produced by some large animals: elephants are well known to communicate over long distances using infrasonic calls, as do whales, rhinoceroses, and giraffes. Infrasound is used in seismology to study and detect earthquakes, and specialised infrasound monitoring networks are also used internationally to detect covert nuclear weapon tests.

12.2 Ultrasonic Sound

Sound waves with frequencies above 20,000 Hz (above the upper limit of human hearing) are called ultrasonic sound or ultrasound. Various animals can produce and/or detect ultrasonic frequencies well beyond human hearing capability, most notably bats (which use ultrasound for echolocation — navigating and hunting insect prey in complete darkness by emitting ultrasonic pulses and interpreting the returning echoes) and dolphins (which similarly use ultrasonic echolocation underwater); dogs can hear ultrasonic frequencies considerably higher than humans, which is the principle behind 'silent' dog whistles.

Ultrasound has numerous important technological and medical applications:

• Medical ultrasound imaging (sonography): high-frequency ultrasonic waves are directed into the body using a handheld probe (transducer); these waves reflect differently off tissues, fluids, and organs of different densities, and the pattern of reflected echoes is processed to build up a real-time image of internal structures. It is widely used for prenatal examination of a developing foetus during pregnancy, and for imaging the abdomen, heart, and other soft tissue organs, because it is non-invasive and does not use ionising radiation (unlike X-rays).
• SONAR (Sound Navigation and Ranging): ultrasonic pulses are emitted from a ship or submarine and travel through water; upon striking an obstacle (the sea floor, a submerged object, a shoal of fish, or another vessel), the pulses reflect back as an echo, and by precisely measuring the time taken for the echo to return, the distance to the obstacle can be calculated using the known speed of sound in water (distance = speed × time / 2, since the sound travels to the object and back). SONAR is used for measuring ocean depth, mapping the sea floor, detecting submarines, locating shoals of fish for commercial fishing, and underwater navigation.
• Ultrasonic cleaning: ultrasonic waves passed through a cleaning liquid create rapid, high-frequency pressure variations that generate microscopic bubbles through a process called cavitation; these bubbles form and collapse extremely rapidly, generating tiny but intense localised shockwaves that dislodge dirt, grease, and contaminants from intricate surfaces (such as jewellery, surgical and dental instruments, and electronic components) far more effectively and gently than manual scrubbing.
• Other applications: ultrasonic waves are used in industrial non-destructive testing to detect internal flaws, cracks, or voids in metal castings, welds, and structural components without damaging them; in ultrasonic distance-measuring devices and motion sensors; and in the medical technique of lithotripsy, in which focused ultrasonic shock waves are used to break up kidney stones into smaller fragments that can then pass naturally out of the body without surgery.

12.3 Audible Range Reference Table