This video details the construction and experimental testing of an acoustic radiometer, a device that uses high-intensity sound waves (>130 dB) to spin a rotor inside a sealed chamber. The rotor's vanes are sound-absorptive on one side and reflective on the other. A key technical challenge, frequent speaker failure, was solved by developing a method to remotely monitor voice coil temperature by measuring its electrical impedance. Experiments demonstrate two distinct physical effects: a primary momentum transfer from sound waves, which is dominant when using broadband white noise, and a secondary effect involving standing waves created by pure tones, which can be manipulated to reverse the rotor's direction. The device was tested with various gases (air, CO₂, SF₆, H₂, He) and pressures. Air was found to be the most effective medium, a result attributed to the speakers' horns being acoustically impedance-matched to air, rather than an intrinsic property of the gas itself.
An Experimental Investigation of an Acoustic Radiometer
0:00 The Acoustic Radiometer: The device is an analogue to a Crookes radiometer, but it's powered by high-intensity sound (>130 dB) instead of light. The sound pressure differential on its absorptive versus reflective vanes causes rotation.
2:05 Construction: The rotor is built with aluminum vanes, one side coated in sound-absorbing foam, and mounted on a low-friction jeweled bearing salvaged from an analog voltmeter. The entire assembly is housed in a sealed, vacuum-capable chamber.
5:01 Speaker Health Monitoring: To avoid destroying the speakers, a system was created to monitor the voice coil's temperature by measuring its electrical impedance in real-time. Since the impedance of the copper coil is temperature-dependent, this allows for a non-invasive way to track how close the speaker is to its thermal limit.
8:50 Two Driving Mechanisms: The rotor can be spun in two ways, demonstrating different physical principles.
Momentum Transfer: Using broadband white noise, the reflective side of the vane experiences roughly double the momentum transfer compared to the absorptive side, causing rotation. This is considered the primary effect.
Standing Waves: Using a pulsed pure tone can create high-pressure standing waves in the chamber. By timing the pulses, these pressure zones can be used to "kick" the rotor around, even in the opposite direction of the primary effect.
12:23 Gas Experiment Results: The performance of different gases was tested. Air produced the fastest rotation. Carbon dioxide and sulfur hexafluoride were about 20% slower. Helium and hydrogen produced no rotation at all.
12:49 Explanation for Gas Performance: The superior performance of air is not due to its inherent properties but because the speakers are designed for it. The speaker horns act as acoustic transformers, efficiently matching the impedance of the speaker diaphragm to air. They are a poor match for gases like helium or hydrogen, resulting in inefficient sound power transfer.
14:40 Effect of Pressure: The torque on the rotor decreases as pressure is lowered. At lower pressures, more power is required to initiate movement.