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Visualizing Ultrasound with Schlieren Optics Part I

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 Physics   |   Science
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Part 1 of 3. We use a schlieren optics system with a strobing light source to visualize ultrasonic waves emitted by a transducer driven at a frequency of 28 kHz.

Images employing schlieren optics are very sensitive to changes in the density of air. Since sound waves are pressure waves, and pressure variations result in density gradients, it stands to reason that one should be able to image sound waves traveling through the air. Unfortunately, the waves move at the speed of sound (343 meters per second), which makes it difficult for us to see them. With stroboscopic animation, we can slow down the apparent motion of the sound waves.

Some of you probably wonder whether this experiment would work with audible sound waves at, say, 1 kHz (about 34 centimeters in wavelength). Not only is that wavelength much bigger than our mirror, but it turns out that our optical setup is not sensitive enough to see the density gradients formed by 1 kHz sound (at least not at a Sound Pressure Level that is less than that of a jet engine). We can, however, see the the density gradients formed by 28 kHz sound waves, which have a wavelength of about 1.2 centimeters.

We use here an 18" diameter, f/4.3 concave mirror that we salvaged from a spectrometer. The light block is a size 7 piano wire, mounted at 45 degrees with respect to the vertical on a lens holder. The light source is an Engin LZ4-00CW08 cool white LED. The transducer is an American Piezo 28 kHz Cleaning Transducer model #90-4040 and is driven by a Samson Servo 120 power amplifier.

More information about our schlieren optics setup and how it works can be found on our Science Demonstrations website: https://sciencedemonstrations.fas.harvard.edu/presentations/schlieren-optics

Safety Note: Although 28 kHz is beyond the range of human hearing, ear protection should be worn whenever attempting this experiment to avoid damaging vibrations in parts of the ear. Any high-pitched whines you hear in this video are not dangerous.

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