'But why?'
'Listen to me,' said Gilmer. 'You've heard of ultrasonics, haven't you?'
'Sounds pitched too high for the human ear to hear,' said Woods. 'We use them for lots of things. For underwater signaling and surveying. To keep check on high-speed machines, warn of incipient breakdowns,'
'Man has gone a long way with ultrasonics,' said Gilmer. 'Makes sound do all sorts of tricks. Creates ultrasonics up to as high as 20 million vibrations per second. One million cycle stuff kills germs. Some insects talk to one another with 32,000 cycle vibration. Twenty thousand is about as high as the human ear can detect. But man hasn't started yet. Because little Fur-Ball over there talks with ultrasonics that approximate thirty million cycles.'
The cigar traveled east to west.
'High frequency sound can be directed in narrow beams, reflected like light, controlled. Most of our control has been in liquids. We know that a dense medium is necessary for the best control of ultrasonics. Get high frequency sound in a medium like air and it breaks down fast, dissipates. That is, up to twenty million cycles, as far as we have gone.
'But thirty million cycles, apparently, can be controlled in air, in a medium less dense than our atmosphere. Just what the difference is I can't imagine, although there must be an explanation. Something like that would be needed for audible communication on a place like Mars, where the atmosphere must be close to a vacuum.'
'Fur-Ball used thirty million cycle stuff to talk with,' said Jack, 'That much is clear. What's the connection?'
'This.' said Gilmer. 'Although sound reaching that frequency can't be heard in the sense that your auditory nerves will pick it up and relay it to your brain, it apparently can make direct impact on the brain. When it does that it must do something to the brain. It must disarrange the brain, give it a murderous complex, drive the entity of the brain insane.'
Jack leaned forward breathlessly.