Cleaning with Sound
Sound may not be a normal cleaning product in your house, but it is just the thing for cleaning delicate jewelry, surgical instruments, lenses, and many other small, intricate objects. Soon, it could also make cleaning big objects like houses or machines much more efficient.
One model of an ultrasonic cleaner. This benchtop size is used by jewelers, cosmetic and tattoo professionals, dive shops, light manufacturing facilities, maintenance facilities and more.
Photo by William Rafti, William Rafti Institute
A technique called ultrasonic cleaning is often used to get rid of the dust, dirt, oils, and other particles clinging to small objects. The term ultrasonic means that the frequency of the sound waves used in this technique are higher than what a human can hear.
Hearing abilities vary from person to person, but in general a healthy, young adult can hear noises in the range of 20 Hz to 20,000 Hz. For comparison, dogs can hear noises in the range of 40 Hz to 60,000 Hz, and bats can hear noises in the range of 20 Hz to 120,000 Hz. Ultrasonic cleaners usually involve frequencies between 20,000 Hz and 40,000 Hz, but sometimes they are even higher.
Ultrasonic cleaners come in many types, from home models that cost less than $50 to industrial grade models that sell for thousands, but the underlying theory is the same. Each cleaner has a "bath" area that is filled with water (or a water-detergent mix). At the bottom of the bath there is a device that makes ultrasonic waves by vibrating up and down thousands of times each second.
Sound waves are created by vibrating molecules. When you talk to a friend, for example, the act of speaking vibrates the air molecules around your mouth in specific ways. These vibrations, or sound waves, propagate through the air and eventually vibrate your friend's ear drum. The vibrations are turned into electrical signals, which the brain interprets as sound. (For more on sound waves visit How Sound Waves Work).
In an ultrasonic cleaner, a wave generator is connected to a diaphragm, the cone-shaped part that vibrates inside of a speaker. The generator causes the diaphragm to vibrate at a specific frequency and amplitude. Instead of causing air molecules to vibrate like a speaker does, the diaphragm causes the water molecules in the bath to vibrate.
What does this have to do with cleaning?
The vibrations in the water cause tiny cavities (bubbles) to form, about the size of a red blood cell or bacteria. However, the constant vibrating puts pressure on the cavities—they are rapidly stretched and compressed. This causes the cavities to violently implode, and if they are near a hard surface, turn into jets of liquid traveling at high speeds. An object submerged in the bath is hit by these jets millions of times per second. On impact the jets dislodge particles like dust and oil from the object, giving it a nice cleaning. The cavities are very small, so despite the millions of collisions the process is gentle.
This method is often used to clean intricate objects because the cavities are small enough to get into all of the cracks and crevices, cleaning the places that we can't otherwise reach.
There are many variations of ultrasonic cleaners. They can be designed to operate at a specific frequency that most efficiently cleans a specific type of surface. For example, higher frequencies create smaller cavities and so are better for cleaning very intricate objects. Some cleaners are designed to use a water-detergent mix to speed up the cleaning process or aid in rinsing and drying. And, of course, cleaners come in different sizes in order to accommodate objects of different sizes.
So what's new?
This technique isn't new, but Professor Leighton and Dr. Birkin at the University of Southampton in England have developed a way to make it even more versatile. Currently, this process only works for objects that can fit inside a water bath. Another downside is that after a particle is dislodged from an object's surface, it floats around inside of the bath and could latch back onto a different part of the object's surface.
Credit: University of Southampton
To overcome these challenges, the Southampton scientists packed all of this technology into a nozzle. In addition to spraying water, the nozzle generates ultrasonic waves. This means that ultrasonic cleaning may soon be a viable way to clean big objects like the surface of a space shuttle, and to clean delicate products like salads. The water is constantly running in this system, so dust and other particles are carried away from the object being cleaned.
A hose with an ultrasonic nozzle can clean objects with much less force and water than a pressure washer, and do it with less power. That leads to energy savings during the cleaning process and less runoff, which often needs to be purified before it can be used again.
The next big challenge for the scientists is to bring this technology into houses, manufacturing facilities, and food preparation areas—to make it commercially available for all kinds of applications. The team is working on this now, and hopefully sound will be an essential item in your cleaning arsenal within the next decade.
For more information, check out these resources:
MediaCollege.com: How Sound Waves Work
Learn more about sound waves and how they travel though this great resource.
University of Southampton: Revolutionary ultrasonic nozzle that will change the way water cleans
Read a press release describing the nozzle and highlighting a recent award that the team won, the Royal Society Brian Mercer Award for Innovation 2011.
Tovatech: How Ultrasonic Cleaning Works
Get more details about the cleaning method from a company that makes ultrasonic cleaners.
HowStuffWorks: How Speakers Work
Explore how speakers work with this nice tutorial.