Magnificent man
Question: I read in the newspapers the story of a Californian man who
attached a number of weather balloons to his lawn chair and didn鈥檛 come down for
some time. How much helium or hydrogen would you need to lift an average adult
off the ground? And how big a balloon would you need to reduce your apparent
weight to one-sixth, as though you were walking on the Moon?
Answer: The amount of buoyancy (uplift) depends on the difference in density
between air and the lightweight gas.
If we assume air weighs in at 1.293 kilograms per cubic metre, hydrogen at
0.09 and helium at 0.179 (at standard temperature and pressure), then one cubic
metre of hydrogen will provide 1.203 kilograms of uplift (1.293鈥�0.09) and
helium 1.114 kilograms of uplift (1.293鈥�0.179). This is, of course, based
on Archimedes鈥� principle.
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If we assume the man weighs 80 kilograms, the chair 12 kilograms and allow
another 8 kilograms for the balloons and wires, then the flying man would need a
90-cubic-metre helium balloon or an 83-cubic-metre hydrogen balloon. This would
be enough to give him no apparent weight on the ground (neutral buoyancy) but
not lift him into the air. Any greater volume would provide lift but, because
the density of air decreases with altitude, it would mean a larger balloon
volume was required to achieve any great height, always supposing that was the
intent.
To give five-sixths compensation (similar to walking on the Moon), a balloon
of 75 cubic metres (for helium) or 70 cubic metres (for hydrogen) would be
needed.
Ian Royston
Skene, Aberdeenshire
Answer: Using balloons wouldn鈥檛 really simulate walking on the Moon. Trying
to walk around while tethered to a 5-metre balloon wouldn鈥檛 be much different
from being tethered by a strong spring to an overhead rafter, because the air
resistance around you and the balloon would be enormous! For moonwalk training
NASA actually used an ingenious configuration of springs and tethers attached to
a swivelling overhead crane.
Keith Walters
Riverstone, New South Wales
Whassat?
Question: How can we tell whether a sound is coming from in front or behind
when we only have two ears?
Answer: There are two main cues for sound localisation: inter-aural time
differences (a sound from the right will arrive at the right ear first) and
inter-aural level differences (a sound from the right will be more intense in
the right ear). The correspondent is correct to point out that these cues do not
unambiguously specify the location of the sound source. A sound from directly in
front will produce the same relative arrival time and level in the two ears as a
sound from directly behind.
These ambiguities can be resolved in two ways. First, we use head movements.
Turning the head to the right, for instance, will increase the intensity and
reduce the time of arrival in the left ear if the sound is ahead. Secondly, we
use cues from each ear. The external part of the ear, or pinna, in combination
with the rest of the head, modifies the spectrum of the incoming sound in a way
that depends on the angle of incidence. It is believed that the brain learns to
identify the characteristic spectral signature associated with each location in
space, so a sound from in front will be associated with a different spectral
pattern to a sound from behind.
Chris Plack
Psychoacoustician and pop star
Department of Psychology
University of Essex
Answer: For sounds that are equidistant from both ears, each ear receives
essentially the same signal, but because the ears point forward there is a
significant difference between the frequency responses for sounds that are
incident from the front and back.
Sounds from the rear sound different to sounds from the front, because the
fleshy part of the ear muffles high-frequency sound from behind the head.
Provided the brain has some absolute reference for the frequency content of the
original signal it can work out whether the sound is in front, behind or above.
This means that familiar sounds with predictable, broadband spectra are easier
to pinpoint. The best example is human speech. The hardest sounds to localise in
this way are narrow-band signals like electronic telephone ringing tones. In my
office it is impossible to tell whose phone is ringing.
There is, however, a poorly recognised and more mundane mechanism for
front/back localisation. Our hearing is influenced by our sight. If you can鈥檛
see any apparent sound source, your brain assumes it must be behind or above.
The effect is strong enough to override decisions based on the frequency
response mentioned above, as researchers into virtual audio know well. This
effect has obvious survival implications for hunter-gatherers.
Adam McKeag
Randwick, New South Wales
Answer: Try two experiments that might amaze you.
The first is to get a friend to close his or her eyes, and put a hand over
one ear. Now jingle a bunch of keys about a couple of metres away from their
head on the open ear side, and ask your subject to point to the source of the
noise. Move the keys around. You will find that in almost every case the subject
can point to the keys鈥攕howing that you do not need two ears to locate a
sound. Of course, you do it much better and faster with two.
Now take a toilet roll tube and cut off a 5-centimetre section. Ask your
subject to hold it over their open ear, enclosing the pinna. This effectively
removes the asymmetry caused by the pinna. You will find that the ability to
locate the keys with eyes closed has almost vanished, showing that the pinna
provides all-important auditory cues about sound direction.
John Elliot
Stockport, Cheshire
Answer: The need for a constant or repeated sound reference can be
demonstrated by asking a blindfolded person in the middle of a room to determine
the location of a second person by sound. If the second person claps once, a
good subject will point directly towards or away from the source of the sound.
They will be unable to distinguish between sound in front or behind them.
Replace the clap with a persistent noise such as speech, and the source is more
easily located.
Gavin Whittaker
Heriot, Midlothian