Stopping sound waves

In England, quantum chemists used a machine called Mercury. The name was derived from a mercury delay line down which sound waves passed. This delay line was used as a short-term memory device. Mercury was a machine that antedated transistors, and its many-thousand vacuum tubes had to be cooled. Often the cooling system failed, and often one of the vacuum tubes burned out, so there were only short periods of uninterrupted running time between stops for repairs. Machine errors were also common. 

Ultrasound imaging is a non-invasive, portable and relatively inexpensive imaging modality, which is used extensively in the clinic. An ultrasound transducer (also called scanhead) sends short pulses of a high-frequency sound wave (1-10 MHz) into the body. At interfaces between two types of tissue, the wave will be refracted and part of the sound wave is reflected back due to Snells law. How much is reflected depends on the densities of the respective tissues, and thus the speed of the sound wave within the different tissues. In addition, parts of the sound wave are also backscattered from small structures at tissue boundaries or within the tissue. High-frequency sound waves propagate weU through soft tissue and fluids, but they are more or less stopped by air or bone. In clinical practice, this limitation is referred to as an acoustic window . The transducer not only sends the wave into the body but also receives part of the reflected and/or backscattered wave, also named echo . In clinical practice, ultrasound is used in a... 

It should be clear by now that it takes time to see and respond . The expression to stop on a dime is just that an expression. The time it takes from the moment a sound wave reaches our eardrum, or a light ray impinges on our retina, until we initiate a response to that stimulus is known as perception reaction time. In driving, the time that passes from the moment a stimulus - such as a brake light or a stop light - appears until we actually reach the brake pedal is known as brake reaction time (BRT). 

Stopping here for breakfast, Nell and her grandfather are momentarily content, surrounded by the singing of the birds, the beauty of the waving grass, the deep green leaves, the wild flowers, and the thousand exquisite scents and sounds that floated in the air , a scene in which Nell recalls her... 

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