What is ultrasound?

Ultrasound - that is what a physicist understands by it

• Everyone knows sound, that is, wave-like fluctuations in air pressure that can be heard. The ear is sensitive to this.
• The frequency with which it oscillates is characteristic of the phenomenon of a wave.
• The frequency is given in the unit "Hertz" (abbreviated: Hz). 1 Hz means that there is 1 oscillation per second. You can already see from this definition that you are dealing with fast processes.
• The sound wave frequency that a person can just hear at a young age is around 20,000 Hz and is noticeable as a whistle. With increasing age, however, this hearing limit goes down.
• Above this limit of 20,000 Hz one speaks of ultrasound. The ultrasound waves that cannot be heard by humans differ from the sound waves in the audible area only in terms of their higher frequency and the associated smaller wavelength.
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Audible frequencies - information

Which frequencies can the human ear actually hear? Here you will not find out ...

• Dogs and especially bats can perceive frequencies from this ultrasonic range up to 100,000 Hz.
• Thus, ultrasound is understood to be a range of sound waves that is also based on air vibrations, but is imperceptible to the human ear because of its high frequencies.
• By the way: infrasound is called the range below the human hearing limit of around 16-20 Hz.

This is how ultrasound is generated

• Quartz oscillators are ideal for generating ultrasound and for detecting it as an ultrasound detector.
• If you apply an electrical voltage to a small quartz, which was usually cut out of a single crystal as a flake, via vapor-deposited electrodes, the crystal contracts. This effect is used in the literature sometimes called electrostriction.
• If you excite this crystal with an alternating voltage, it will periodically shorten and lengthen in time with the voltage, it will vibrate. Such deformation vibrations are particularly pronounced when the exciting alternating frequency coincides with an oscillation frequency of the crystal (resonance case). For example, the thinner it is, the higher the natural frequency of a quartz plate.
• The frequencies of such quartz oscillators include - depending on their size - a range from 1000 Hz to around 150 MHz (1 MHz = 10⁶ Hz; corresponds to 1 million vibrations per second!).
• For a frequency of 30 MHz, for example, a plate only 0.1 mm thick is required.

Ultrasound in sonography - simply explained

• The most important medical ultrasound application is sonography, in which frequencies up to 40 MHz (corresponding to a wavelength of 0.1 mm) are used.
• In 1965 an ultrasound device showed the first recordings of unborn babies without any delay as sonography on a monitor.
• In the meantime, ultrasound has also conquered many other areas of medicine, because it is harmless to tissue compared to the X-rays that are also used in medicine.
• Ultrasound is used as standard to examine the thyroid gland, the heart, the kidneys and gall bladder (stones?), The urinary tract and the bladder as well as to clarify the accumulation of water in the abdomen.
• The transducers of medical ultrasound devices contain piezo transducers as plates, discs or Rods and deliver ultrasonic waves in the MHz range that make the surface of the quartz perpendicular leaving.
• The devices work with the so-called pulse-echo principle: short sound pulses travel through the body, i.e. through skin, fatty tissue, blood vessels and internal organs.
• In simplified terms, this principle is understood as follows: At the transitions between different types of tissue, part of the sound is reflected, creating echoes that are registered by the transducer. The later the echo comes, the deeper the sound has penetrated the body. All echoes can then be put together to form one image.