Some media are not reciprocal (e.g. very lossy or noisy media), but many very useful ones are approximately so, including sound waves in water or air, ultrasonic waves in human bodies, and electromagnetic waves in free space.
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A notable researcher is Mathias Fink of École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris.
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TRM are known and used for decades in the optical domain, and are used in the ultrasonic domain by Mathias Fink at the École supérieure de physique et de chimie industrielles de la ville de Paris.
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