The most commonly known use for ultrasound—high-frequency sound waves human ears can’t hear— is examining a fetus (胎儿) as a medical device during pregnancy. But there are plenty of other uses.
Many offices have occupancy sensors that use ultrasound to detect movements and keep the lights on when someone is in the space, and off when nobody is around. These sensors operate at frequencies such as 32 kilohertz, far above what the human ears can hear——which is a range from 20 hertz to 20 kilohertz.
Other products use ultrasound to deliver targeted sound, for instance allowing a museum to play a recording for visitors in one area of an exhibit without disturbing others nearby. Electronic repellents (驱虫剂) use ultrasound to keep rodents or insects at bay.
A similar product can even be used to disperse (驱散) teenagers; aging tends to reduce people’s ability to hear higher frequency sounds, so a noisemaker can annoy kids without adults even noticing. This has also let teens create smartphone ringtones their elders can’t hear.
Airborne ultrasound is not inherently (固有地) bad. But things can go wrong. A former colleague of Kevin’s used to hear strange sounds from his hearing aid when in rooms with occupancy sensors, likely because the hearing aid’s electronics improperly converted the ultrasound into audible noise. The noise was annoying, but not harmful. A similar problem tainted the research of one of our students, conducted in a room that, unbeknownst to him, had an ultrasonic room occupancy sensor in the ceiling.
One of us has conducted research in which carefully crafted ultrasonic signals secretly activate voice-control systems, even unlocking an iPhone with a silent “Hey, Siri” command, and telling it to make a FaceTime call.
Sound can also affect the physical world, as when a singer shatters a wine glass. Micro-electrical mechanical sensing chips—such as accelerometers used in car airbag systems and smartphones, and gyroscopes in drones—are susceptible to the same interference. Those systems can be attacked with sounds, crashing a drone mid-flight, or fooling a smartphone about whether it’s moving.
It’s well-known that sounds that are too loud can damage people’s ears and hearing. However, there’s little evidence of ultrasound causing bodily harm without prolonged, direct physical contact at high intensity. If you are accidentally subjected to extremely intense ultrasound (such as when holding an ultrasonic arc welder), you could experience an annoyance like a headache or a temporary loss of balance.
The US Occupational Safety and Health Administration warns of potential health risks from audible subharmonic by-products of ultrasound, so more than the ultrasound itself.
【小题1】According to the passage, what happens as people get older?| A．They can hear sounds of frequency of 32 kilohertz. |
| B．They can hear sounds of frequencies which are lower than 20 hertz. |
| C．Their ears become duller and even completely deaf to high-frequency sounds. |
| D．They can hear noise that annoys a large number of kids. |
| A．Electronic devices can perform human commands. |
| B．Ultrasound has a clear effect on the physical world. |
| C．Both ultrasound and human-audible sounds can affect electronics. |
| D．The hearing aid’s electronics don’t change the ultrasound into audible noise. |
| A．Optimistic. |
| B．Worried. |
| C．Confused. |
| D．Reserved. |
A． | B． |
C． | D． |
