An acoustic radiometer

An electromagnetic radiometer is a well‐known physics demonstration and toy. When exposed to light of sufficient intensity, the vanes rotate in the opposite direction if radiation pressure were the mechanism. That is, the reflective white side leads the absorptive black side. This occurs due to thermal effects associated with the gas in the bulb, and can be overcome only with a very high vacuum. To demonstrate the effect of radiation pressure, we have built an acoustic radiometer consisting of an aluminum T‐beam with a 10‐cm‐square pane attached to each end. One side of a pane is acoustically reflective (aluminum) and the other is acoustically absorbing (foam). The device is pivoted with a V‐jewel bearing and a steel pin, and is placed in a transparent chamber with a uniform broadband noise field (2.5–15 kHz). A minimum sound‐pressure level of roughly 125 dB (re: 10⁻¹² W/m²), which yields a theoretical net force corresponding to 2 mg on each pane, produces rotation in the ‘‘correct’’ sense with the absorptive side leading the reflective side. The apparatus will be demonstrated during the talk. [Work sponsored by the Office of Naval Research.]