You're probably wondering what good the SONAR capabilities would be when you can simply flip the headlights on. Well, one reason is to let you move around in the dark [ominous music]without being seen.[/ominous music] Of course, that brings up the obvious problem of being heard, but it's fun to pretend anyway. :P
The other reason for having the SONAR was the main cause of my building the helmet. Ever since I saw the US sensors, I've thought they could be used for helping blind people navigate on their own, by using the US sensors as guides. So with the helmet, a blind person would (theoretically at least - I guarantee you it wouldn't work in reality :P) be able to know if there was an object in his way, and which direction he would need to turn to avoid the object.
Here are some pictures of me wearing the helmet - front, side, and back views:
There are a couple things you can notice from the pictures. (1), the front view shows how I positioned the US and Light sensors to fit around the field of view - this adds a neat sort of look to the robot without inhibiting (too much!) the view. And (2), the back view shows how the tires are used to provide a cushioned strap in the back. I actually didn't have this at first, but after I put the NXT Brick on the front of the helmet (you really find how heavy that thing weighs once you try wearing it on your head!), the technic beams pushed too hard on the back of my skull for comfort, so I added the tires. Now I wish I had used tires for all the contact points of the helmet, because they help A LOT in making LEGOs more wearable and comfortable.