Ultrasonics – The What and Why of Sweeping Frequency

Note – The following blog is adapted from a paper recently written by Timothy Piazza, President of Blackstone-NEY ultrasonics and is re-printed here with his permission.  This would probably be a good time to mention that guest blogs or suggestions for blog topics are always welcome.  My email is jfuchs@ctgclean.com.  FJF

Blackstone~NEY Ultrasonics – Sweep Frequency
By Timothy Piazza Ph.D.

By way of introduction, and to put the subsequent discussion into context, an operational review of the equipment is required.  All modern ultrasonic units are characterized by the center frequency at which the unit operates (effectively the average frequency of operation) as well as what is called the sweep bandwidth, and sweep frequency. In ultrasonic cleaning, sweep is defined as a purposeful deviation from the center frequency during operation as a function of time.  Typically, this deviation occurs symmetrically about the center frequency. The driving frequency is continuously varied within some window, about the center frequency.  The following provides a graphical example of the frequency modulation commonly referred to as sweep.

In this example the center frequency is 104 kHz with a sweep bandwidth of 4 kHz (f =104 kHz ± 2 kHz).  The rate at which the frequency changes is known as the sweep rate.  In Figure 1 the sweep rate is 500 Hz.  Additional information including an audio clip of the sound of sweep is available in a preceding blog.

Sweeping the ultrasonic frequency has been demonstrated to greatly enhance cavitation activity, and thus the ability of a given system to effectively clean.  This results primarily for two reasons.  The first is that ultrasonic transducers are electro-mechanical devices with manufacturing tolerances.  As a geometrically driven quantity, resonant frequency is strongly dependent upon the size and shape of a transducer.  Manufacturing tolerances create a natural variation in the center frequencies of each individual member of a multi-transducer ultrasonic array.  Thus there is no single “center” frequency, but instead a distribution of center frequencies characterizing an array.

Use of non-sweeping ultrasonics can only maximally excite one or a few of the transducers in an array, i.e. those whose center frequency happen to match the driving frequency.  The other transducers in the array will be acoustically dim, with lower power output, in comparison.  This leads to uneven sonication and cleaning within a tank.  Constantly changing the frequency via sweep ensures that all transducers are excited at their center frequency during the sweep cycle.  Also, if the sweep rate is sufficiently high, compared to the lifetime of a sound wave in a tank, the result is a uniform acoustic field and cleaning.  The second reason for sweep is that the introduction of a “band” of frequencies excites a significantly larger bubble population, resulting in shorter degas times, as well as enhanced cleaning.

Historical Note – The effect of non-uniform excitation of an array of transducers operated at a fixed frequency was recognized decades ago.  In an effort to correct the problem, some ultrasonic manufacturers sorted assembled transducers by resonant frequency prior to bonding to minimize variations in frequency.  Subsequently, it was discovered that the resonant frequency of a transducer changed in an unpredictable way once it was bonded making this effort futile.  The simple and elegant solution of sweeping frequency to provide uniform transducer output was invented by William Puskas in 1988.

The above is intended to demonstrate that there is no single frequency that can be used to characterize an ultrasonic cleaning tank.  Indeed, in a correctly operating system, there are a range of frequencies present in a tank at any given time.