system
Given the following post, respond as if you're commenting on Reddit:
human
High-intensity ultrasound is being used to destroy tumors rather deep in the brain. How is this possible without damaging the tissue above?

Does this mean that it is possible to create something like an interference pattern of sound waves that "focuses" the energy at a specific point, distant (on the level of centimeters in the above case) from the device that generates them?How does this work?
gpt
Yes. There is often a combination of two effects: the shape of the transducer and electronic steering. 


Shape: imagine that each part of the transducer is a point source of ultrasound. Then, each element generates a spherical wave of sound. If two elements are equally far from a target, then the sound will reach the target at the same time and overlap. 


 Then, forming a sphere of sound elements around the area of interest will cause sound waves to reach the center of the sphere at the same time, so that spot is much louder than everywhere else. 


Electronic steering: You can fake the position of point elements by making them generate sound a little bit before or after the other elements- if you delay the element it seems further away. Go early and that element seems closer. You can use this to pretend to have a sphere/hemispheric shape.