Sound – Ultrasonics – Introduction to Transducers

I have been spending a lot of time on the blog lately talking about the effects of sound waves.  Today’s blog starts a series devoted to the way ultrasound is created and how it can be customized to provide the best ultrasonic cleaning results.

The heart of an ultrasonic cleaning system is a device called a “transducer.”  Basically, a transducer receives energy in one form then converts it and transmits it on in another form.  In the world of ultrasonics, a transducer receives an electrical signal from a device called an ultrasonic generator and converts it into mechanical motion which is delivered to the liquid in the ultrasonic cleaning tank thereby creating cavitation bubbles.

In essence, an ultrasonic transducer is very much like a speaker used in a radio or stereo system.  The lower frequency sounds in such a system are generally produced by a classical loudspeaker which is often called a “woofer.”  The woofer has a moving cone driven by an electrical coil which generates a changing magnetic field.  When the magnetic field generated by the coil attached to the moving cone interacts with a stationary magnetic field produced by a nearby permanent magnet, the speaker cone moves back and forth creating sound.

In many high fidelity sound systems, the higher frequency sounds are not produced by the type of speaker described above but, rather, by speakers using an electrostatic or piezoelectric effect to transform electrical energy into motion which results in the generation of sound waves.  These high frequency speakers are often called “tweeters.”  The moving coil cone speaker has the advantage of being able to move large amounts of air but is not able to vibrate fast enough to produce true high frequency sounds.  An electrostatic or piezoelectric speaker, on the other hand, is much smaller and is not able to produce lower frequency tones with sufficient amplitude to create the rich bass, or low frequency sound that most listeners desire.  However, these devices are able to produce the higher frequency sounds, which require less amplitude, very effectively.

The overwhelming majority of ultrasonic systems today utilize piezoelectric transducers which are similar to the “tweeters” used in high fidelity sound systems but with a couple of major differences.  First, ultrasonic transducers are designed especially to deliver sound waves into liquids, not air.  Liquids and air have different characteristics necessitating that sound generating transducers for each have different characteristics as well.  Secondly, unlike sound system “tweeters,” ultrasonic transducers are not designed to produce an infinite variety of tones as “tweeters” must to reproduce music.  Instead, they produce either a single tone or a series of specific tones called “harmonics.”  This is because they use the principle of resonance to amplify the high frequency sounds they produce to achieve the sound amplitude required to create ultrasonic cavitation.

I know I’ve thrown a lot of new terms into this blog.  Readers, please don’t worry if you feel lost – it ain’t rocket science!  In upcoming blogs I will describe more fully things like “the piezoelectric effect” and the specific ways in which resonance is used to create high intensity ultrasonic energy.  We’ll also explore the various types of ultrasonic transducers, their characteristics and how each does its own special thing – just like woofers and tweeters in the hi-fi world.  Stay with me and enjoy the ride.

–  FJF  –