So far we have only considered sound in isolation and we have seen that sound has velocity, frequency, wavelength and reduces in intensity in proportion to the square of the distance from the source.

Whether a sound wave is reflected, refracted, or absorbed depends on the densities of the materials either side of the boundary. If the densities are very different then reflection is more likely.

Ultrasound scans could soon be much more detailed, thanks to a novel material that can bend sound waves the “wrong” way. This property, known as negative refraction, means the material should bring ...