As we know, ultrasound is a type of sound that has a frequency above the human hearing range. So why can’t humans hear ultrasound?
It all has to do with how our ears work. The ear is divided into three main parts: the outer ear, the middle ear, and the inner ear. Sound waves travel through the outer ear and into the middle ear, where they cause the eardrum to vibrate. These vibrations are then transmitted through three tiny bones (the malleus, incus, and stapes) in the middle ear to the cochlea in the inner ear.
Inside the cochlea, there are specialised cells called hair cells. These hair cells are what allow us to hear. When the stapes vibrates, it sets off a wave in fluid inside the cochlea, which then bends the hair cells. This bending action causes the hair cells to release a chemical called neurotransmitter. This neurotransmitter then travels to the brain, where it is interpreted as sound.
So, why can’t humans hear ultrasound? Well, ultrasound has a frequency that is too high for our hair cells to respond to. When sound waves of ultrasound frequency hit the ear, they simply cause the eardrum to vibrate back and forth – there is no bending of the hair cells and no release of neurotransmitter.
Ultrasound is a cyclic sound pressure wave with a frequency greater than 20,000 hertz, which is the upper limit of human hearing. Ultrasound cannot be heard by humans because it falls outside of our hearing range. However, this limit varies from person to person.
There are a few reasons why humans can’t hear ultrasound. First, the ear canal is simply too small to vibrate at such high frequencies. Second, the eardrum (tympanic membrane) is also too small to vibrate at these frequencies. Third, the bones of the middle ear (ossicles) are not able to transmit vibrations of such high frequency. Finally, the cochlea (the snail-shaped structure in the inner ear that converts sound waves into nerve impulses) does not respond well to ultrasound.
One way that ultrasound can be detected by humans is through bone conduction. This is when vibrations from ultrasound waves are transferred to the inner ear through the bones of the skull. This happens because our bones are much better at conducting vibrations than our air-filled ear canal. However, even with bone conduction, ultrasound is still not very loud and can only be detected by special instruments.
So why is ultrasound used if humans can’t hear it? There are actually a lot of applications for ultrasound, both in medicine and industry.
In medicine, ultrasound is used to create images of internal organs (such as the heart, liver, and kidney) as well as unborn babies. This technology is called ultrasonography. Ultrasound can also be used to break up kidney stones or to destroy cancer cells.
Industrial uses for ultrasound include cleaning objects (such as jewelry and electronic parts), welding metals together, measuring the thickness of materials, and detecting cracks in objects.
Ultrasound devices operate at frequencies ranging from 20 kHz to several GHz. The upper frequency limit in humans (approximately 20 kHz) is due to the middle ear functioning as a low-pass filter. If ultrasound is fed directly into the skull bone and reaches the cochlea via bone conduction without passing through the middle ear, ultrasonic hearing may occur. Bats and other mammals employ this sort of hearing.
There are several reasons why humans can’t hear ultrasound. The first reason is that the human ear only responds to frequencies up to about 20 kHz. This is because the middle ear acts as a low-pass filter, and frequencies higher than 20 kHz are filtered out. The second reason is that ultrasound doesn’t travel well through air, so it doesn’t reach the inner ear directly.
Instead, it has to be transmitted through the bones of the skull, which act as a poor conductor of sound. Finally, even if ultrasound does reach the inner ear, it’s not efficiently converted into electrical signals that can be processed by the brain. So even though we technically can hear ultrasound, it’s not really audible to us.
One way that humans can hear ultrasound is by using a device called a transducer. This converts the ultrasound into audible sound. Transducers are used in medical applications such as fetal heart monitoring and diagnostic sonography. They’re also used in industrial applications such as ultrasonic cleaning and nondestructive testing.
Because the upper limit pitch of hearing in humans progressively lowers with age, children can hear higher-pitched noises that adults cannot. This has been utilized by a cell phone company to generate ring signals that are only audible to younger people; nevertheless, many older individuals can hear it, which may be due to the wide range of age-related hearing loss at the top threshold.
One reason that adults can’t hear ultrasound is because the ear’s response to sound changes as we age. The delicate bones in the middle ear ossify and stiffen with age, which reduces their ability to vibrate in response to high-frequency sounds. In addition, the hair cells in the cochlea (the part of the inner ear that transforms sound vibrations into electrical impulses) also degenerate with age, further reducing our ability to hear high-frequency sounds.
So, while it is true that adults generally can’t hear ultrasound, there are a number of reasons why this is the case. With advances in medical technology, however, there are now ways to use ultrasound for diagnostic purposes even in older adults. So, while we may not be able to hear ultrasound, we can certainly benefit from it.
Dogs, cats, dolphins, bats, and mice are just a few species that can hear ultrasound. Dogs have the ability to hear sound at higher frequencies than humans do. A dog whistle works by projecting a high-frequency sound in order to call to a dog.
Similarly, dolphins use high-frequency clicks to communicate and echolocate prey. Mice have an upper-frequency hearing limit of around 85 kHz, while human beings and other apes max out at about 20 kHz. In both cases these represent absolute limits: we cannot hear any frequencies above these levels.
There are a number of reasons for this difference. One is that the size of the ear canal limits the maximum wavelength that can enter it and be detected by the eardrum. The human ear canal is about 2.5 cm long, which means it can detect sound waves with wavelengths as long as 5 cm (20 kHz). But mice have much smaller ear canals, only about 0.4 mm in diameter.