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Ultrasonics - Introduction

The next series of blogs will explore ultrasonic cleaning in some detail.  Today's blog is an introduction to the world of ultrasound.
Lord Rayleigh 1842 - 1919

The theory of sound was first defined and understood a little over 100 years ago when Lord Rayleigh published The Theory of Sound.  Rayleigh was a Nobel Prize winner and, was also responsible for a number of notable discoveries including the reason why the sky is blue.   It is worth the time to google Rayleigh and learn some of his astounding discoveries for those who may be interested.  Although he may have imagined ultrasound, it is unlikely that could have imagined its use for cleaning since devices to generate sufficiently high wave amplitudes at ultrasonic frequencies had not yet been invented.  His work, none-the-less, remains the basis for all theories involving sound and the transmission of sound and other forms of waves as well. We are all familiar with sound.  Sounds, both natural and man made are ever-present in our lives.  The ear, human or otherwise, is an organ custom-designed to collect, concentrate and sense vibrations which we call sound.  It is a sound receiver.  Sound is generated by a vibrating or pulsing source which introduces vibrations or pulse waves into a sound conducting medium.  These vibrations are called sound waves.  Sound waves are conducted by solids, liquids and gasses.  Sound waves travel at different speeds in different materials.  Sound waves do not travel at the same speed in all solids, in all liquids or in all gasses.  The efficiency of conduction and the distance sound can travel is also different in different materials.  We have probably all tried the experiment while swimming of throwing a stone into a lake or pool with one ear in the water and one out.  The sound of the stone hitting the water some distance away will be heard in the ear that is in the water first because sound waves travel much faster in water than they do in air.  Another fun thing that many of us have tried is breathing in helium gas and then talking in a much higher-pitched voice.  The reason the voice sounds funny is because sound waves travel at a different speed in Helium than they do in air. Something to Think About - Would the increase of pitch of the human voice when speaking with helium be due to a faster or slower speed of sound in the Helium vs. air? Sound waves have two basis properties, wavelength and amplitude.  Wavelength is the frequency and is the same as what we call pitch in music.  A shorter wavelength produces a higher pitched sound.  The amplitude of the sound waves determines how loud a sound is.  "Ultrasound" is defined as sound at a frequency or pitch higher than can be heard by human beings.  The range of human hearing is approximately 20 cycles per second (Hz) to something around 18,000 Hz.  Of course, the range of frequencies that can by heard is not the same for all humans and typically decreases with age and/or abuse (yes, like rock concerts are bad for your ears!!).
Bats use ultrasound to navigate and find food. Notice the large ears.

Ultrasound is present in nature.  Bats, for example, use sounds above the frequency range of human hearing to navigate and find food.  Bats are able to transmit high frequency sounds and, by analyzing the echos they hear coming back, detect objects and distinguish their characteristics.  Phenomenally, a bat can tell the difference between a mosquito (food) and a bit of dandelion fuzz (not food) while flying and avoiding twigs, leaves and other obsticles.  Dogs also hear sounds that are at a higher frequency than humans can hear.  The "silent" dog whistle produces a sound that can be heard by dogs but can not be heard by humans.  Dolphins and other marine animals also use ultrasound to communicate and find food.  This is no surprise since water is such a good conductor of sound. I realize that some of the terms used in this introductory post may not yet be familiar to you.  In upcoming posts I will try to clarify these and other terms related to the concept of sound and ultrasound as well as explore how we use sound to clean things.  Come along for the ride.

-  FJF  -

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