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If it is thermal insulation that is required there are many light weight foam like materials that will provide a comparable degree of insulation to that provided by a vacuum.
This depends on the material that the helmet is made of (also how the air compressor works--I would recommend a device designed for decreasing pressure, not increasing it), and how strong a vacuum is required. There isn't any experimentally attainable perfect vacuum, so really what we are talking about here is reduced pressure. It is easiest to talk about this in terms of orders of magnitude: a household vacuum cleaner can get to about 20% of atmospheric pressure, and a cheap pump can probably achieve pressures about 1% of atmospheric pressure (10–2 atm). I routinely work with pumps that are about $5000, and can reduce pressure within a thick glass vessel to about 10–4 atm, and have occasionally used multiple pumps together to achieve pressures of 10–5 atm. I once worked in a lab that ran experiments in large steel boxes (>3 cm thick walls) under ultra high vacuum of about 10–10 atm, but that required about $50,000 worth of pumps, sensors and regulation systems. I think deep space can have pressures as low as 10–20 atm.Given prior posts of yours I feel compelled to insist that you * DO NOT* try to put your head in anything that you have evacuated--even pressures achievable with a household vacuum cleaner can cause blood vessels to burst in your eyes, ears and face--I promise this is not pleasant!If instead you wanted to make something akin to a thermos (an inner vessel and an outer vessel, with reduced pressure in between) that wouldn't pose as obvious a risk, but if you have ever broken a thermos you would understand why putting your head in such a contraption could still be very dangerous.
Seriously, you don't need to worry about ultrasound. But if you are worried, maybe a better protection is to get a dog? They can hear any ultrasound, and lead you away from it.
Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air. Many bat detectors are limited to around 15 kHz to 125 kHz at best.
If you know the frequency of the alien ultrasound (use a bat detector to scan it) you can make "ultrasound chaff" with a car reversing transducer and baffle the buggers.
Quote from: alancalverd on 21/10/2014 12:14:20If you know the frequency of the alien ultrasound (use a bat detector to scan it) you can make "ultrasound chaff" with a car reversing transducer and baffle the buggers.Have the jamming-frequency sweep down to 17kHz and as a bonus you can repel teenagers  ... http://en.wikipedia.org/wiki/The_Mosquito
a bat-detector would be a better and cheaper way of detecting ultrasound,[if one must have such a thing] ...
Do Igloo Coolers use reduced pressure as insulation? I think they do but probably not comparable to a steel dewar flask?