A New Type of Levitation

TL;DR

Ultrasonic transducers levitate objects by pumping air into a ~100-micron gap through asymmetric pressure dynamics, discovered accidentally while investigating faulty British torpedoes.

Key Points

• 1.Bob Collins discovered this levitation accidentally while investigating why British torpedoes missed targets. While handling an ultrasonic transducer (~30,000 Hz), he placed a glass lens on it and it immediately slid off, revealing an unexpected levitating effect that stumped the entire lab.
• 2.The levitation works because halving the gap doubles pressure but cuts air escape rate by eight times. This means less air escapes on the downstroke than is sucked in on the upstroke, so the transducer acts as a pump drawing air into the gap until pressure balances the object's weight at ~100 microns.
• 3.Piezoelectric transducers are required because normal speakers lack the force to handle pressure spikes. On the downstroke, pressure spikes inversely with volume; piezoelectric speakers can push water (800x denser than air), while standard audio speakers cannot sustain the required force.
• 4.A 2D gas particle simulation confirmed the levitation mechanism computationally. After the simulation ran for a while, the number of gas particles leaving the gap equalized with those returning, and the bar-shaped mass levitated under gravity.
• 5.Building an ultrasonic hockey table required solving two major engineering problems. Standing waves create Chladni node lines where no levitation occurs, so two transducers were driven at the same frequency but 90° out of phase to generate traveling waves and eliminate dead spots.
• 6.Dolphins use a fat-filled organ called a melon to match acoustic impedance between their anatomy and seawater. This concept mirrors why piezoelectric transducers work underwater, and Planet Wild funded special dolphin microphones to help dolphins communicate without shouting over tourist boat engines.