Physics – Acoustics – Ultrasound
The use of ultrasound in medical exams—for instance, during pregnancy- has become routine. However, ultrasound is also very useful in technological applications, such as depth measurement.
The vibrations of a sound wave in air can have a very wide range of frequencies, from less than one per second (below one hertz or Hz) to 1,000 times a second (one kilohertz or kHz) and in certain cases even more.
The musical notes with the lowest frequencies—that is, those with the deepest pitch-vibrate at about 20 Hz, while the highest notes audible to people reach around 15-20 kHz (although the upper limits of hearing decrease with age). All sounds with frequencies over this limit are called ultrasound (“ultra” means “beyond” in Latin).
Ultrasound in nature
Although people cannot hear ultrasound frequencies, some animals can. Dogs can perceive tones reaching up to some 40-50 kHz, like those produced by special dog whistles. Bats and dolphins have an even higher hearing limit: well over 100 kHz.
There is a practical reason for this; because high-frequency ultrasound waves move in precise lines, like light waves, bats and dolphins can use them for orientation, sending out ultrasound signals just as a ship’s sonar system does.
From the echoes, the animals’ auditory systems can determine the position, speed, and even the shape of their prey (or enemies). Another variation can be found among the elephants, who are able to produce infrasound tones with frequencies of below 20 Hz. Spreading out in all directions over a very broad area, the sounds serve as a kind of “elephant cell phone network.”
Healing and cleaning
Among humans, ultrasound is not only useful for ship navigation: obstetricians and many other medical specialists use ultrasound to examine the human body in a relatively non-intrusive manner.
Ultrasound cleaning devices use the energy produced by these waves to loosen dirt particles. In medicine, doctors use the same principle to break up gall bladder or kidney stones.
HOW IS AN ULTRASOUND PICTURE PRODUCED?
Depending on the frequency used and the type of body tissue being examined, ultrasound waves penetrate to different depths within the human body. Ultrasound echo patterns can produce a complex and informative picture of bodily structures, which an experienced doctor can use to draw many conclusions about the patient’s health.
One difficulty arises from the fact that ultrasound cannot enter the body directly from the air. For this reason, special transmitter heads and gels are used to transmit the waves.
BASICS
INFRASOUND Tones below 20 Hz. Pressure waves in air with frequencies lower than 0.1 Hz are no longer considered to be sound.
AUDIBLE SOUND The range of frequencies that can be heard by young, healthy humans stretches from 20-20,000 Hz.
ULTRASOUND Frequencies over the upper limit of human hearing (20 kHz). Sounds up to several hundred KHz can be perceived by some species of animals.
HYPERS0UND Sounds above 1 GHz (1 billion hertz). As the frequency increases further, hyper sound is no longer considered a wave, according to the laws of quantum mechanics, but as so-called quasiparticles.