Hearing Protection (Ear Defenders): Rigid Cups on Liquid-Filled Cushions (Shaw & Thiessen, NRC, US 2,801,423, 1957)

Ear muffs that seal well and still stop low-pitched noise. Each ear is covered by a rigid cup on a cushion made of a thin PVC tube, emptied of air and part-filled with liquid. Pressed on, the liquid flows round the whole rim and fits the cushion to the head, yet cannot compress, so the cup does not vibrate as a whole and pass the sound in. A sprung headband with a pad holds the cups on ball joints. Result: circumaural ear defenders effective against low-frequency noise. Source: E. A. G. Shaw and G. J. Thiessen (National Research Council of Canada), US 2,801,423 (1957). Unknown: sizes and the attenuation achieved. Picture: the patent drawing.