Some of the world’s most interesting and useful sounds cannot be heard at all. Ultrasonics – the ‘too-high-to-hear sounds’ – can be used to drill, cut weld, clean, and inspect for cracks and flaws. Like all sounds, they travel in waves through the air or any other medium, but they have a far higher frequency than the sounds we hear.
Human ears can detect sound waves that vibrate from 20 to 20,000 times per second. Bats can hear up to 50,000 vibrations per second. But when scientists talk about ultrasonics they can mean billions of vibrations per second. Special vibrators produce these high-frequency sounds. One, the transducer, is made by sandwiching a thin slice of quartz crystal between two metal plates and passing an electric current through it.
When this happens, the crystal expands and contracts by a tiny amount – but enough to generate the pressure waves needed. Ordinary sound waves spread in all directions, but because of their high frequency, ultrasonics can be more easily directed into a beamand made to do useful work. They can be used to detect invisible cracks in metal because the sound waves travel at a different speed through the crack than through the metal. Dishes and clothes can be washed with them because of the pulsations they set up in liquid.