Is there a way to make something undetectable to echolocation?

It depends on how good the detection is.

 

You need a material that absorbs sound, so that little sound is reflected. But if you have that, one might be able to detect you by ambient sound that is blocked/absorbed.

there are 'cloaks' that allow the sound to pass around you as if you weren't there but it'd still be possible to determine the location if either a wide range of frequencies were used or they had sensitive equipment which could pick out the small anomalies left.

 

not to mention that these have never been built large scale only small and would have to be custom designed for each object and cost a fortune in modelling alone.

Don't worry about what fortune it would cost. I'm willing to pay it.

 

In all seriousness, though, it doesn't matter if the technology is not readily accessible today — it's for a futuristic science-fiction novel.

 

If the sound is simply blocked, you'd see a vacuum in the sonography, right? Wouldn't be very stealthy.

 

I've heard of primitive prototypes of a metamaterial invisibility cloak which allows the wearer to become very slightly transparent (though still very much visible), but I've never heard of any technology that would allow one to become inaudible. How does this work? Does it use similar metamaterials?

the cloaks you see on youtube do not use the same principle, they are more like sheets of cameras and lights, an active cloak.

 

http://www.engadget.com/2006/10/19/duke-scientists-build-theorized-invisibility-cloak-sort-of/

 

a similar method is used for cloaking sounds infact, there are plans(if it works) to use the technology to prevent tsunamis from hitting small islands. (anything large like a continent is still buggered though.)

How does this sound technology work, though?

the meta material has a negative refractive index. it also acts as a wave guide. while in the meta material the soundwaves actually travel faster than the waves outside so when the exit the meta material they look exactly the same as when the enter.

 

i don't know the exact physics of it but thats the jist of it.

So meta materials do exactly the same thing to sound waves as they do to light waves? Bizarre. What is the explanation for this? I understand why light waves do this, but sound waves are quite a different matter.

well, they're not exactly the same thing but they use the same principles.