Lena Torres
Ultrasound is defined as sound with frequencies greater than 20 kHz, which is the upper limit of the audible range for humans. Ultrasonic sounds are high-frequency sound waves that are beyond the limit of human hearing, although there are exceptions. Some people, mostly children and younger humans, can hear ultrasonic sounds. Researchers have also identified ultrasonic hearing as a recognised auditory effect that allows humans to perceive sounds of a much higher frequency than would ordinarily be audible, usually through stimulation of the base of the cochlea via bone conduction. Devices such as ultrasonic sensors, transceivers, or transducers can be used to detect and produce ultrasonic sound, and ultrasound has many applications in fields such as medicine, industrial processes, and pest control.
| Characteristics | Values |
|---|---|
| Upper limit of audible sound for humans | 15-28 kHz |
| Frequency range of ultrasonic sounds | 20 kHz and above |
| Audibility of ultrasonic sounds | Inaudible to most humans, but some children, teenagers, and adult women may be able to hear them |
| Devices that can produce ultrasonic sounds | Broadcasting devices, loudspeakers, motion sensors, rodent deterrent devices, ultrasonic transducers, ultrasonic receivers, and more |
| Applications of ultrasonic sounds | Pest control, medical imaging, object detection, distance measurement, industrial processes (e.g., cleaning, mixing), wildlife monitoring, and more |
| Effects of ultrasonic sounds on humans | Generally not harmful, but exposure to intense ultrasound (>140 dB) can cause nausea, headaches, tinnitus, pain, dizziness, and fatigue |
| Techniques to bring ultrasonic sounds into the audible range | Signal processing techniques, pitch-shifting, parametric spatial audio reproduction |
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What You'll Learn
Humans cannot hear ultrasonic sounds, but some can
Humans cannot hear ultrasonic sounds as the frequencies are too high for the human ear to process. The average human hearing range is between 20 Hz and 20,000 Hz (20 kHz), and sounds above 20,000 Hz are considered ultrasonic. However, it's important to note that this range varies from person to person, and some individuals can hear above or below this average range.
The ability to hear ultrasonic sounds tends to decrease with age. Children, adolescents, and some adult women have more acute hearing and are more likely to detect ultrasonic sounds. This is because the upper limit of hearing tends to decrease as we get older.
Ultrasonic sounds are present in our environment and are generated by various devices such as broadcasting devices, loudspeakers, motion sensors, and rodent deterrent devices. They are also used in medical and industrial applications, such as ultrasound imaging in medicine and leak detection in industrial settings.
While humans cannot naturally hear ultrasonic sounds, there are methods and devices that can be used to detect and hear them. Ultrasonic sensors, transceivers, or transducers can detect and produce ultrasonic sound. These devices send out sound waves through high-voltage electrical pulses and receive back an echo, which is then converted into a sound within the human hearing range. Additionally, signal processing techniques can be used to bring ultrasonic frequencies into the audible range for humans.
There are also individuals with a condition known as ultrasonic hearing, which allows them to perceive sounds of a much higher frequency than what is ordinarily audible. Two theories explain this phenomenon: the first suggests that ultrasonic sounds excite the inner hair cells of the cochlea, which are responsive to high-frequency sounds, while the second theory proposes that ultrasonic signals resonate in the brain and are modulated down to frequencies detectable by the cochlea.
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Devices can convert ultrasonic sounds into audible frequencies
Humans can generally hear sounds from 20 Hz to 20,000 Hz, with an upper bound of 15–28 kHz, depending on the person. Sounds above 20,000 Hz are referred to as ultrasound, which is inaudible to most humans. However, ultrasonic sounds can be converted into audible frequencies through various methods and devices.
Ultrasonic sensors, transceivers, or transducers can detect and produce ultrasonic sound. They send out sound waves through high-voltage electrical pulses and receive an echo, which generates an electric pulse that is converted into a sound within the human hearing range. Audio sound transducers have input sensors like microphones to convert sound into electrical signals and output actuators like loudspeakers or headphones that convert the signals back into sound. The piezoelectric method, which requires crystals such as calcite or quartz, is commonly used for ultrasonic detection. When these crystals undergo mechanical stress, they generate an electric charge, known as the piezoelectric effect.
Another device, the Audio Spotlight, uses a beam of ultrasound as a "virtual acoustic source", allowing for precise control of sound distribution. As the ultrasonic beam travels through the air, its inherent properties cause it to change shape, giving rise to frequency components in the audible band. This device was created by Elwood "Woody" Norris of American Technology Corporation (ATC) in 1996.
HyperSonic Sound (HSS), trademarked by ATC, uses piezoelectric transducers to send two ultrasonic waves of differing frequencies toward a point, creating the illusion that the audible sound originates from that point. This technology was later spun off to Parametric Sound Corporation, and similar products are offered by companies like Mitsubishi.
Additionally, through signal processing, ultrasound can be converted into a secondary "downmixed" signal in the audible range using the heterodyne method. This involves selecting a narrow frequency in the ultrasonic range and transforming it into the audible range via difference frequencies. The phase vocoder method, employed by devices like SONAPHONE, compresses the entire frequency range of 20 to 100 kHz, allowing reproduction of the original acoustic situation in the broadband.
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Ultrasound has shorter wavelengths than audible sound
Ultrasound refers to sound with frequencies greater than 20 kilohertz (kHz). This frequency is the approximate upper audible limit of human hearing in healthy young adults. Ultrasound has shorter wavelengths than audible sound, typically only a few millimetres long. This means that ultrasound propagates in a much narrower beam than audio frequencies from a conventional loudspeaker.
Ultrasound is used in many fields, including medicine, chemistry, and industrial applications. In medicine, ultrasound imaging or sonography is used to visualise small internal details in structures and tissues. In chemistry, ultrasound can be used to induce chemical reactions by generating extreme temperatures and pressures in a liquid medium. Industrially, ultrasound is used for cleaning, mixing, and accelerating chemical processes, as well as for detecting hidden flaws and leaks.
Ultrasound can be used to create audible sound through a process called demodulation. This occurs when modulated ultrasound passes through a nonlinear medium, which acts as a demodulator, converting the ultrasound into audible frequencies. Devices such as “Audio Spotlight” use this principle to create a narrow beam of audible sound with precise control over its distribution.
While humans typically cannot hear ultrasound, there are exceptions. Some individuals, particularly children and younger humans, can perceive ultrasonic frequencies due to their more acute hearing abilities. Additionally, certain conditions or devices can enable ultrasonic hearing in adults. For example, researchers have shown that high-intensity ultrasound fed directly into the human skull can be perceived through bone conduction, bypassing the middle ear.
Ultrasonic hearing has been the subject of controversy and research. Some studies suggest that ultrasonic sounds may excite the inner hair cells of the cochlea, which are responsive to high-frequency sounds. Other theories propose that ultrasonic signals resonate within the brain and are modulated down to detectable frequencies. While ultrasound is generally considered safe, exposure to intense ultrasound near the audible range can cause adverse effects such as nausea, headaches, and tinnitus.
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Ultrasonic hearing can cause nausea, headaches, and tinnitus
While ultrasound is generally considered inaudible to humans, some people can hear ultrasonic sounds due to having particularly acute hearing. This includes infants, children, teenagers, and some adult women.
Ultrasonic hearing is a recognised auditory effect that allows humans to perceive sounds of a much higher frequency than would ordinarily be audible, usually by stimulating the base of the cochlea through bone conduction. However, ultrasonic hearing can also have negative consequences, including nausea, headaches, and tinnitus.
Ultrasound can cause discomfort, annoyance, or non-threatening issues like headaches, nausea, and tinnitus, especially in more sensitive individuals. Exposure to intense ultrasound near the audible range can lead to a syndrome that includes nausea, headaches, tinnitus, pain, dizziness, and fatigue. Some people use devices that block ultrasound to mitigate these effects.
Timothy Leighton, a professor of acoustics at the University of Southampton, has studied the effects of ultrasonic waves on people in public places. Leighton found that a large number of people are exposed to ultrasound in public places and experience symptoms such as headaches, nausea, and tinnitus. He noted that the effects of ultrasonic exposure are not well understood and that more research is needed to determine the full impact of ultrasound on society.
In addition to the physical symptoms caused by ultrasound exposure, there may also be mental health implications. For example, some people may experience discomfort or annoyance from hearing ultrasonic sounds that others cannot. This could lead to feelings of stigma or isolation. Overall, while ultrasonic hearing may allow some people to perceive a wider range of sounds, it can also have negative consequences on physical and mental health.
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Animals like bats, insects, and dogs can hear ultrasound
Ultrasound refers to sound waves with frequencies above 20,000 Hz, which is the upper limit of the human hearing range. While ultrasound is inaudible to most humans, some people, especially children and younger people, can hear these high-frequency sound waves.
Ultrasound is commonly used by animals for communication, navigation, and hunting. Animals like bats, insects, and dogs can hear ultrasound. Bats use ultrasound to determine the distance, size, shape, density, and direction of objects, especially when hunting insects at night. They generate ultrasound via the larynx and emit the sound through the open mouth or, rarely, the nose. Bat echolocation calls range in frequency from 14,000 to over 100,000 Hz. Certain bat species can even modify their call intensity mid-call, lowering the intensity as they approach objects that reflect sound strongly to prevent the returning echo from deafening them.
Insects, which are often prey to bats, have anti-predator adaptations. Some insects, like the tropical rainforest katydid, emit ultrasound frequencies, while others, like the tiger moth, produce ultrasonic clicks to warn other insects of predators.
Dogs can hear sounds up to about 45,000 Hz. They use high-frequency sounds for communication and echolocation. Rats, which are also rodents, have been recorded laughing when tickled by people.
Ultrasound is used in many fields, including medicine, and has multiple industrial applications, such as detecting hidden flaws and leaks.
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Frequently asked questions
Ultrasound is sound with frequencies greater than 20 kHz, which is the upper limit of the audible range for humans.
Humans cannot usually hear ultrasonic sounds as they are beyond the limit of human hearing. However, there are exceptions, and some people, especially children and younger people, can hear ultrasonic sounds.
Ultrasonic hearing is an auditory effect that allows humans to perceive sounds of a much higher frequency than would ordinarily be audible, usually by stimulating the base of the cochlea through bone conduction.
Ultrasonic sounds can be detected using devices such as ultrasonic sensors, transceivers, or transducers. These devices send out sound waves and receive back an echo, which is then converted into a sound within the human hearing range.
Audio Spotlight is a device that uses a beam of ultrasound to create a "virtual acoustic source", allowing for precise control of sound distribution. The ultrasound used has frequencies outside the range of human hearing, but as it travels through the air, it changes shape, giving rise to audible frequency components.
