[Critter]

Is there a technological way to defeat a helmet mounted ultrasound sensor? I thought white noise generators would work, but I was unsure. I am basically looking to make a thick ultrasound "smoke" to obscure its vision.

[Aryeonos]

You would have to send ultrasonic waves back at the reciever from multiple sources, since they are being projected and not bounced back the reciever wouldnt be able to build an accurate picture in those directions. I would use emitters from either microdrones or rig up some gecko tipped grenades with ultrasound. Either option is going to be expensive though. You may just opt to hack the sensor, or put up sound deadening matterial to absorb the waves. Deflecting them away from their source also could help obscure whatever you`re trying to hide, I dont know, but that would require that you know the origin of the sound at all times to make sure your deflecting materiel is facing the right direction.

[firebug]

An important thing to know is why you need to defeat a helmet-mounted one.  For a decent number of things, it may be easier to just avoid getting that close to someone.

If you're a GM and you feel a player's liberal use of ultrasound is making things to easy, remember that it has a limited range.  50 meters is decent in-doors but there's a lot of situations where targets can be outside of that range.

[Kincaid]

Mannequins.  "Do Not Enter: Bandersnatch Den" signs.

On a more serious note, having lots of objects cluttering up the room will make ultrasound less effective.  Given all the glass walls you find in office buildings, it might end up being a disadvantage in that setting.

[prionic6]

An active ultrasound system is also kind of like screaming "I am here" in a very highly pitched voice. That might be a bit unpractical depending on the situation.

[Novocrane]

Normal (threshold 2 - Street sign, pedestrian, etc)

This is level of detail your average helmet ultrasound sensor manages.

Obscured/Small/Muffled (threshold 3)

This needs a handheld device or larger sensor, so wear a ghillie suit, disguise yourself, or keep mostly behind cover.

[Squirrel]

a random dog whistle nearby   

[pariah3j]

So a flash of sudden light is blinding to someone with night-vision.

Maybe a big sub-woofer would be 'blinding' to Ultrasound ?

[ScytheKnight]

Lead them into a cave full of bats. 

[farothel]

emit ultrasound at the exact same frequency but in counter phase.  The two sound waves will effectively cancel each other out.  You need a sensor that can pick up the ultrasound, an emiter to send your own and some insignificant processing power to analyse the sound wave the sensor picks up.  Essesncially it costs the same as the ultrasound system your player has on his helmet.

[Spooky]

Or modify two white noise generators to muffle ultra sound wavelengths. One for below the freq you use, and one for above, so that only your freq is clear, and everything else is hashed.

[Ursus Maior]

Ultra-sound counter-measures work like most other ECCMs against electro-magnetic sensors as well: You either remain unseen from the waves, by not offering a cross section from which they can bounce back (aka "stealth") or you flood the em-spectrum with noise; the latter meaning you need to know the approximate frequency and then transmit more powerful ultra-sound emissions on it than the sensor you want to blind.

Finding out the exact frequency sounds difficult, but alas commonality in industry standards, including "set wavelength" for civilian and military usage will basically reduce the frequency band two a certain set of frequency bands that would be well known by every technician and EC(C)M expert. If you want to blind a specific sensor and not just the complete frequency band, you might also try a directional jammer, but you would need to find a way to keep it tracking the targeted sensor. In a world of wifi-hooked devices that shouldn't be a problem, but if your player goes offline, you would need traditional triangulation-devices. Those might not be at hand, since ultra-sound is less frequently used in the military (except for naval sonar), because of it's short range in the atmosphere.

But basically all you need to have for that is three or more ultra-sound detectors on gimbles and a program that turns the gimbles towards the "loudest" (or otherwise predetermined) source and then calculates (triangulates) the position of the source depending on the diffrence between the time the sound waves need to travel to the sensors. Of course you need to know the exact position of the sensors. So if the sensors move, they should have GPS and feed that data to the program as well. The rest ist mathematics and a matter of your computer's calculation speed. So in SR this should be near-instant-

[Walks Through Walls]

I believe a magical way would be a silence spell. Center it on the character with the ultrasound and he will be blind as nothing will come back or at the very least be a negative modifier equal to the number of net hits on the spell.

[Ursus Maior]

Sounds like a good a way, too.