Ultrasonics – Transducers – What is “Q”?

In the blogs about transducers I made a point of telling readers that an ultrasonic transducer must be driven at its resonant frequency to achieve optimum performance. What I didn’t address, however, was the fact that the resonant characteristics of a transducer can be varied by its design.  Not only the resonant frequency but the “sharpness” or “Q” of the frequency response can be customized as well.  Very simply, the “Q” of a transducer defines the sensitivity of the transducer to changes in driving frequency.  The following illustration may be helpful.

Illustration showing the relationship of "Q" to resonance
The broadband device on the left has a wider range of operating frequencies that will produce 75% of the maximum vibration amplitude. The high Q device on the right has the potential to produce a higher amplitude but only if driven with a narrower band of frequencies than the broadband device. There is nothing magic about 75%.  The same effect will be seen at any target percentage other than 100%.

As you can see, the vibrations of a transducer with a high “Q” are “dampened” or significantly reduced once the driving frequency changes very slightly from its resonant frequency.  The low “Q” transducer continues to vibrate with a considerable portion of its maximum amplitude even if the driving frequency deviates moderately from the resonant frequency.  The width of the frequency range that will cause vibration of a particular scale is called the “Q” of the transducer.

It is true that maximum ultrasonic output will be achieved when a high Q transducer is driven precisely at its resonant frequency.  You might ask, then, why not make all transducers high Q and drive them at their exact resonant frequency to achieve optimum results in an ultrasonic cleaning tank?  The fact is that despite tight tolerances and care in their manufacturing and assembly, all transducers vary slightly in their resonant frequency.  One apparent solution would be to “match” transducers on any given cleaning tank so that they would all have the same resonant frequency.  In fact, such selection would have to be accomplished prior to bonding the transducers to the cleaning tank.  However, variations in the bonding process (thickness and elasticity of the bonding material, for example) also cause variations in resonant frequency of the transducers once they are bonded to a tank.  Selection prior to bonding, therefore, would not assure a single resonant frequency for all transducers once they are bonded.

Customizing transducers to have a low Q allows a group of transducers to operate optimally as a group by varying the driving frequency in such a way that all of the individual transducers in a group are driven uniformly if not optimally.  An ultrasonic transducer with a low Q may also be referred to as a “Broadband Transducer.”  Broadband relates to the bandwidth of frequencies to which the transducer is responsive.

Note of Interest – The term “Broadband” as it is applied to Internet connections relates to the bandwidth of the signals transmitted to and from the Internet by a computer modem.  A “broadband” signal operates at a higher range of frequencies and can carry more data more quickly than a signal that is not “broadband.”

More detail on the ins and outs of broadband transducers will be discussed in upcoming blogs discussing ultrasonic generator functions and customizing ultrasonic waveforms.  For now, the reader should just be aware of the distinction between the two to provide a base of understanding for more thorough discussions yet to come.

–  FJF  –

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