This is a little fun idea I want to try. After playing around with the vex ultrasonic sensors a bit, I came up with an idea of making something that is undetectable by the ultrasonics.
I figured that my best bet would be to either make something that is crazy jugged or finding a material that absorbs sound. Has anyone found a material that confuses the ultrasonics when trying to make a ranging robot? I think that’s a pretty cool idea I want to try 
You could also try to jam the ultrasonic sensors with an ultrasonic emitter on the same frequency (I think frequency matters not sure though). That way the sensor that’s looking for things would get false readings and wouldn’t be able to sense things accurately at all. The robot body could also be changed to deflect ultrasonic signals by bouncing them up to the ceiling. To do this you could get a peice of thin sheet metal and put it on a 45 degree angle on the front of the robot so the sound pulses would get deflected to the ceiling. If the sensor module does pick up the sound pulses after they were deflected it won’t matter because you deflected the signal upward so you increased the distance and the sensor would give incorrect readings.
Hope this helps!
UltraSound works like RADAR…
The third option would be like RAM, RADAR Absorbent Material, but for SONAR something more like SAM, Sound Absorbent Material. (Made that one up, but the Principal is the same… 
It helps to know a little about Stealth. )
lol, RAM is Random Acess Memory…
Careful there - You too might be “mistaken”. For example:
LOL A Ram is a thing that forcefully moves something else by pushing it.
LOL A Ram is a male sheep
LOL According to the largest computer manufacturer on this planet during the 70’s, computers use Random Access Storage, not Random Access Memory.
I don’t think anyone has a patent on those three letters.
Blake
Check the Wikipedia Link. RAM is an acronym for Both… For RADAR usage, the term RAM has been used since at least the Middle 1970’s, according to my Uncle Denys.
There are only so many TLAs (Three Letter Acronyms).
You can use any clothing or anything fluffy and absorbant. The mythbussters tested that out and a “Big Bird” costume was undetectable by sonar. That’s pretty much what MarkO said… You could also deflect the sound in an
“echo-y” room to completly confuse the bot. You could also snd out that signal at random intervals to completely confuse it.
You can use a bedsheet it absorbs it really well. The Big Bird costume sent it back and they got caught. You could also try to make the robot move very slow because that worked on Mythbusters as well.
Try a pillow. Anything fluffy and soft will absorb sound fairly well. Distance is also a factor. The closer you are the less effective materials will be because of the increased signal strength.
How about using an egg carton?
Foam or Paper??
Here’s a crazy thought. Look into noise canceling. Ultrasonic is sound, maybe you could produce a wave that cancels the incoming ultrasonic wave thus making the object “invisible” to ultrasonic sensors.
Fight fire with more fire, fight robots with bigger robots, fight sound with phase-shifted sound!