Sound effects

A sensor that measures localised sonic cavitation could be used to monitor and improve the quality of numerous production processes, according to its developers at the

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Ultrasonic cavitation is used to produce consistent crystalline pharmaceuticals and chemicals, clean printed circuit boards, aerospace and automotive components and sterilise medical equipment. It occurs when sound waves are passed through a fluid to oscillate bubbles of dissolved gas, causing them to implode and so generating heat and localised chemical effects.

Mark Hodnett, senior researcher at NPL, said that ultrasonic cleaning has been used in a variety of manufacturing sectors for more than 50 years.

'In industries using it for cleaning, frequencies of the order of 20 to 40kHz are fired into the fluid and excite millions of these bubbles, which then undergo this implosion process next to the surface of whatever piece is in the vessel to be cleaned,' he added. 'When this happens you get microscopic jets produced from these bubbles that dive down onto the surface that are able to remove particulate and dirt contaminants. It is effectively an acoustic scrubbing process.

'This has been known about for years, but there are no devices on the market and no measurement standards that tell manufacturers how effective their cleaning vessel might be.'

This lack of data prompted NPL to develop a device to quantify and measure how much cavitation occurs in a system. This would also apply to ultrasound crystallisation in food and pharmaceuticals. 'When you're dealing with expensive reagents, you want to know you're applying the right amount of cavitation to get the product to form the way you want it to get cost and efficiency savings,' explained Hodnett.

The system consists of two elements; the cavimeter, which contains the electronics and display component, and the cavitation sensor, which monitors the sounds the bubbles make as they oscillate and collapse. 'Each bubble is a spherical sound source that produces a whole range of sound frequencies as it undergoes these cavitation processes,' said Hodnett. 'If you listen to these sounds and receive them over a wide bandwidth, you can understand a little bit about what's going on in a cavitating cloud of bubbles.'

NPL's sensor detects the sound from bubbles over a broad frequency range from very low up to about 10MHz, which is important because cavitation is a non-linear process.

The sensor is encapsulated in a polyurethane-based material, which gives it spatial sensitivity that enables it to associate the cavitation measured with the specific region of the fluid in which it occurred. The material comes from some work NPL did with the US Defense Research Agency some years ago, which was employed in the stealth technology that is used to make submarines invisible to sonar.

'In the same way that you add those materials to absorb sound waves from a ship searching for the submarine, we've adapted the recipe to give our sensor spatial resolution,' added Hodnett. 'The material is highly absorbing to sound frequencies beyond 1MHz, the region that enables us to characterise this cavitation activity.'

So far there is no standard unit of cavitation, but measuring high-frequency acoustic emission would be one technique that would be relevant to international cavitation bodies when establishing one.

In manufacturing, the sensor can be used as a test tool when a new system is being developed to see where the hot and cold spots of cavitation might occur. This could be used to ensure there is homogenous cleaning throughout a vessel. It could also be installed as a permanent monitoring solution in a chemical processing plant, for example, where cavitation is being harnessed to produce crystallisation effects.

NPL has sold its system to UK company Sonic Systems, which is using the sensor to characterise the sonoprocessing systems it produces for use in the chemical and other manufacturing industries. The sensor has also been taken up by a handful of manufacturers worldwide. The next stage will be to fully commercialise the system.