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Basic Concerns with Ultrasonic Cleaning | Part 5: Ultrasonic Frequency

Ultrasonic cleaning is a versatile method for precision cleaning a broad range of objects. While it involves a simple process, there are several factors to consider for ensuring the best results. In this series of blogs, we’re examining seven key factors that can impact ultrasonic cleaning. 

In this article, we look at how the ultrasound frequency can affect the cleaning action and performance of an ultrasonic power cleaner. 

Aqueous Cleaning

How Frequency Affects Cavitation

Ultrasonic cleaners create sound waves that move through a liquid, compressing and expanding the liquid so that microscopic bubbles form and burst in a process called cavitation. As the bubbles implode, high-speed jets of hot liquid hit a target object. This creates a constant scrubbing action that removes contaminants from the surface of the object.  

The frequency of the sound waves is a critical factor in how the cleaner performs. 

Sound wave frequency is measured in Hertz (Hz), with 1 Hz corresponding to one wave or cycle per second, and 1 kHz (kilohertz) corresponding to one thousand cycles per second. The sound waves used in ultrasonic cleaners are in the ultrasound range (above 20 kHz) – mostly above the range that is audible to humans (20 Hz – 20 kHz).  

To human ears, the frequency of sound waves affects the pitch of the sound – the higher the frequency, the higher the pitch. In ultrasonic cleaners, the frequency of the ultrasound waves affects the size of the cavitation bubbles. With lower frequency ultrasound waves, bubbles have more time to grow. The larger the bubble, the more energy they release when they implode. The more energy produced, the more vigorous the cleaning action. 

Higher frequency ultrasound waves create smaller bubbles. These bubbles can penetrate tiny crevasses and cracks – making it possible to clean hard-to-reach surfaces. The lower energy implosions are gentler, which is beneficial when cleaning delicate objects or materials that can be damaged by more vigorous scrubbing. Aggressive cleaning can remove minute particles from the surface of softer metals such as aluminum and brass.  

Higher frequencies produce a greater number of more evenly spaced small bubbles than at lower frequencies. This creates a more uniform cleaning performance and less need to rotate parts to ensure all surfaces are cleaned. 

Choosing the Right Frequency

It might seem hard to know what frequency should be selected, but fortunately there are some standard frequencies that cover the needs of almost every application. 

The Best All-Round Frequency: 40 kHz

For most applications, an ultrasonic frequency of 40kHz provides the most effective cleaning process and is used in around 95% of all industrial ultrasonic cleaning systems.  

The cavitation bubbles produced are approximately one micron in diameter. The bubbles are small enough to penetrate tiny cracks and holes, powerful enough to remove stubborn contaminants and yet gentle enough for all but the most fragile materials. This frequency provides good coverage on wide surfaces and can deliver excellent cleaning results in all but the most inaccessible places.  

When More Cleaning Power is Needed: 25 kHz 

The bubbles produced at 25 kHz frequency are significantly larger than the 40 kHz bubbles and produce a powerful force on implosion.  

The cleaning power produced at 25 kHz is useful for removing thick, stubborn, or sticky contaminants from large items. For example, this frequency is often used to clean heavy mechanical equipment such as engine blocks, radiators, dies, and parts where the finish isn't a critical consideration. 

Gentle and Precise: 68 kHz or 170 kHz 

Ultrasonic power cleaners operating at 68 kHz or 170 kHz provide exceptionally delicate and precise cleaning. These frequencies create sub-micron cavitation bubbles and can be used to clean pharmaceutical equipment, medical implants, titanium components, delicate electronics, and precision optics.

Using Multiple Frequencies for Optimum Cleaning

Although each of these different frequencies has specific cleaning benefits, using a single frequency in an ultrasonic power cleaner has disadvantages. 

Ultrasonic waves bounce off surfaces in the cleaning tank. With a single frequency, the waves mostly travel the same course and land in the same areas. This produces standing wave patterns that result in alternate layers of good and poor cleaning throughout the tank, leading to incomplete cleaning of parts.  

Using a single frequency also causes transducer diaphragms to erode much more quickly. With erosion, diaphragms transmit less energy, leading to inefficient cleaning. Eventually the cleaner could stop working altogether. 

To address these problems, advanced cleaners use Simultaneous Multi-Frequency (SMF) technology. Transducers transmit across a spectrum of frequencies above and below the main frequency. For example, for a standard base frequency of 40 kHz, UPC’s Vibra-Bar® unit will transmit a range of frequencies between 40 kHz and 90 kHz. The base frequency is dominant, but significant energy is produced at higher frequencies. 

Using SMF provides improved durability and better, faster cleaning – it’s like having multiple frequency generators and transducers in a single package.  

Frequency Sweeping 

An individual transducer’s resonant frequency depends on its exact size and shape. Manufacturing tolerances mean there’s a natural variation in the resonant frequencies of each member of a multi-transducer ultrasonic array.  

Generators with a frequency sweeping capability can generate a signal at various frequencies in a small range around the base frequency to ensure that all transducers are operating efficiently. With frequency sweeping, larger numbers of bubbles are created, delivering a faster, better cleaning performance. 

UPC generators have a 4 kHz sweep (+2 kHz and -2 kHz around the base rate). The rate of the sweep can be adjusted depending on the cleaning solution – a higher rate is required for more viscous solutions and a lower rate for aqueous solutions. 

Wrapping Up

The ultrasonic frequencies applied to cleaning tanks can be tailored for different types of application. Using high-quality components and the right blend of frequencies is critical for achieving the best cleaning results.  

If you have any questions, please feel free to get in touch and send in samples so we can determine the right ultrasonic power cleaner for you. And look out for the next post in this series in which we explore how power density (Watts Per Gallon) affects the ultrasonic cleaning process. 

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We offer a range of our most popular ultrasonic cleaners for rent on a 30-day try-before-you-buy basis or as a long-term rental option. 

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