[mriver8 (OP)]

assault at a freq capable of destroying hair cells? The best I've tried is Grainger closed cell polyurethane foam but it only protects against a numbing tuning fork sensation, and not concentrated/focused zaps.

https://dl-web.dropbox.com/get/127436857-Acoustic-Weapons-A-Prospective-Assessment-Jurgen-Altmann-1.pdf?_subject_uid=317902351&w=AABRd95PNFTrgeib-gQ1M95B9qc9u_wsJkZwk0m_xqaDxg

[chiralSPO]

Just surround your head with a total vacuum. [] No sound will penetrate.

[alancalverd]

Ultrasound is reflected at interfaces of materials with widely differing speed of sound. Sound travels through steel at 2 to 3 times the speed in water or soft brain tissue, so the traditional nutter's helmet (a salad colander) on top of a wooly hat will provide adequate protection with good ventilation, but for total security you could wear several layers, say an aviator's leather helmet under a motorbike lid, topped off with a naval anti-flash balaclava and a coal scuttle/Ned Kelly armour.   

[Bill S]

I tried following the above link and got a message saying I didn't belong there.  Imagine my relief!!

[jeffreyH]

Ultrasound is reflected at interfaces of materials with widely differing speed of sound. Sound travels through steel at 2 to 3 times the speed in water or soft brain tissue, so the traditional nutter's helmet (a salad colander) on top of a wooly hat will provide adequate protection with good ventilation, but for total security you could wear several layers, say an aviator's leather helmet under a motorbike lid, topped off with a naval anti-flash balaclava and a coal scuttle/Ned Kelly armour.

Now I'm tired and in hysterics.

[mriver8 (OP)]

Ultrasound is reflected at interfaces of materials with widely differing speed of sound. Sound travels through steel at 2 to 3 times the speed in water or soft brain tissue, so the traditional nutter's helmet (a salad colander) on top of a wooly hat will provide adequate protection with good ventilation, but for total security you could wear several layers, say an aviator's leather helmet under a motorbike lid, topped off with a naval anti-flash balaclava and a coal scuttle/Ned Kelly armour.

Now I'm tired and in hysterics.

[mriver8 (OP)]

I tried following the above link and got a message saying I didn't belong there.  Imagine my relief!!

See attachment. At these frequencies.

[chiralSPO]

From the second image you posted, it is clear that this is not ultrasound being discussed, but very loud blasts, similar to concussions from bombs or shells. The first image is about high frequencies though...