Sound travels as vibrations through a medium like air, sensed by living organisms. Humans perceive sound within a specific range, which raises the question: can humans hear ultrasound? The answer lies in understanding human hearing biology and the physical properties of sound waves.
Understanding Sound and Human Hearing
Sound is characterized by its frequency, measured in Hertz (Hz), which indicates vibrations per second. A higher frequency corresponds to a higher-pitched sound. The generally accepted range of human hearing spans from 20 Hz to 20,000 Hz, or 20 kilohertz (kHz).
Sounds with frequencies below 20 Hz are known as infrasound, while those above 20 kHz are termed ultrasound. While 20 Hz to 20 kHz represents the typical audible spectrum for humans, individual hearing abilities can vary. Sensitivity to higher frequencies often decreases with age; infants, for instance, can sometimes hear frequencies slightly above 20 kHz, but this ability is usually lost as they grow older.
The Physiological Limits of Human Hearing
The human ear’s structure inherently limits the range of frequencies it can detect. Sound waves enter the outer ear and travel through the ear canal, causing the eardrum, a flexible membrane, to vibrate. These vibrations transfer to three tiny bones in the middle ear—the malleus, incus, and stapes, known as the ossicles. These bones amplify the sound vibrations before transmitting them to the inner ear.
The inner ear contains the cochlea, a snail-shaped structure filled with fluid and lined with thousands of microscopic hair cells. As the fluid in the cochlea ripples, these hair cells move. Each hair cell is tuned to respond to a narrow range of frequencies, with high pitches detected at one end and low pitches at the other. The physical properties of the eardrum, ossicles, and the basilar membrane within the cochlea restrict the ear’s ability to vibrate efficiently at very high frequencies. This structural limitation means sound waves above 20 kHz do not effectively stimulate the hair cells, preventing their conversion into neural signals the brain can interpret as sound.
How Other Animals Perceive Ultrasound
While humans cannot hear ultrasound, many animal species utilize it for navigation, communication, and hunting. Bats, for instance, use echolocation, emitting ultrasonic calls ranging from 14 kHz to over 100 kHz to create a “sound map” of their surroundings. Their specialized ear structures and auditory systems are finely tuned to receive and interpret these high-frequency echoes, allowing them to navigate and locate prey in darkness.
Dogs can hear frequencies up to 45 kHz, which explains their sensitivity to high-pitched dog whistles and sounds produced by rodents. Cats also detect frequencies from 55 Hz up to 79 kHz, aiding them in hunting small prey that often communicate ultrasonically. Dolphins and some whales use echolocation, similar to bats, with some whales able to hear frequencies as high as 200 kHz, enabling them to navigate and find food in underwater environments. These animals have evolved specific adaptations in their auditory systems, such as smaller ear structures or specialized cochlear mechanics, which allow them to process these higher frequencies.
Ultrasound’s Presence in Our World
Although unheard by humans, ultrasound is integrated into various aspects of modern life through technology. Medical imaging, such as sonograms for internal organs or fetal development, relies on ultrasonic waves. Industrial applications include ultrasonic cleaning, where high-frequency vibrations dislodge contaminants, and material testing to detect flaws. Ultrasound is also employed in pest control devices, which emit high-frequency sounds to deter rodents, and in animal training whistles.
While humans cannot audibly perceive these frequencies, exposure to high-intensity ultrasound can lead to non-auditory physical effects. Studies indicate that intense airborne ultrasound, particularly above 110 dB, can cause symptoms like discomfort, headaches, nausea, or dizziness in some individuals. These effects are generally minor and not associated with typical, lower-intensity exposures. These are physical sensations or physiological responses, distinct from hearing the sound. The widespread use of ultrasound in various fields highlights its utility, even beyond the range of human perception.