One way sound: A new material will allow for improved ultrasound imaging: "Their acoustic rectifier works on sound waves that are moving through water and was created by using two different parts.
The first part is a special material -- a liquid filled with microscopic bubbles -- in which sound waves entering at a particular frequency (pulsations per second) leave with a frequency twice as great. The second part consists of a sandwich-like layering of alternating thin slabs of glass and slabs of water which acts like a filter that only allows the sound waves with the doubled frequency to pass through the material but not the original sound waves.
Now, turn the rectifier around and sound waves with the original frequency do not flow through the filter at all -- so no sound gets through. In effect, the combination of the doubling part plus the filter part acts like a one-way gate for sound waves."