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Today
it seems that truth is more complex than ever.
van
Goethe was a German statesman and a very successful
writer.
He wrote novels, scientific treatises, lyric poems, as
well
as dramas. Born in 1749, one might say that his quote
was
true for his day; it was a simpler time. But think how
simple
our time will seem to those who live a hundred
years
from now – unless we’ve found our way back
to
the Stone Age.
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This week’s question is no exception – What living thing makes the loudest sound? Notice I said living
thing, because I didn’t want to exclude anyone from the contest. Who knows,
maybe some bacterium living in Wyoming makes a heck of a racket, it’s just that
nobody is around to hear it. Or maybe a redwood falling down is the loudest. See,
nothing’s ever simple.
This leads us to a second question – one which we have to answer
first. What’s a sound? We are surrounded by air, and air has mass and density –
it’s stuff. You can push stuff around. When you push the air, it moves away,
which creates a wave, because the air you move then moves the air next to it,
and so on.
A sound wave is generated when a force creates a vibration,
and that vibration moves air, and that vibration is then propagated through the
air. The air moves in the same direction the vibration was moving, and this
makes it a longitudinal wave (see animation below).
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A
sound wave is a longitudinal wave, where the source moves
in
the same direction as the wave. There is a compression of
the
medium (air or water) and then a rarefaction with fewer
molecules
as the compression moves on. The wavelength is
the
distance between compressions and the frequency (how
high
or low the sound is) is 1/wavelength.
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If you define sound as something your brain recognizes, then
a sound doesn’t occur until the sound wave is transduced (changed in form) by a your ear to an electrical
chemical impulse and that impulse is interpreted by the auditory cortex of your
brain. See, nothing has a simple answer.
So, does a lonely falling tree make a sound? Scott McFarland
of the University of Oregon installed microphones all over Crater Lake National
Park 100 miles southeast of Eugene. In the remotest parts of the preservation
land, he has heard everything from buzzing mosquito wings to, yes, falling
trees. Unfortunately, he has also heard human intrusion.
Even from the most isolated areas, about 20% of every recording
included airplane noise. He also heard cars, people, and detonations. It seems
that no place is really naturally quiet anymore. But there were those falling
trees -does that mean they do make a sound? Nope, it was a sound wave captured
by recording equipment, transduced to digital information, stored, changed back
to a sound wave by a speaker, and then to a neural impulse by our inner ear. It
still isn’t interpreted as a sound until it reaches a brain, any brain. If a
rabbit or bull moose is nearby, their ears would transduce the sound wave and
they would “hear” something, even if they don’t think to themselves, “Hey, a
tree just fell.” So again, it’s not so simple, maybe something with a functioning ear is in range.
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The
old tree falls in a forest question started out as a philosophical
question.
It was meant as a thought experiment to students to
discuss
the nature of what is real versus what is observable. However,
when
it existed as a philosophical question, no mention of sound was
made,
but was concerned with whether the tree existed at all if no one
was
there to perceive it. It wasn’t until 1883 that the sound reference
was
made, and then it was posed as more of a scientific question,
as
we approach it in today’s post.
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The human hearing sense is pretty sensitive. The pressure
needed to generate a sound wave that a human could hear is about one billionth
the value of atmospheric pressure. But this sound wave would be just barely
audible (depending on the frequency), so it would be soft.
This brings us to a very
short discussion of what it means to be loud or soft. We often measure
loudness in decibels (db). A decibel
is one tenth (deci) of a bel (named
for Alexander Graham Bell), which is a unit of power or intensity.
Every 10 db increase represents a 10 fold change in
intensity, so the scale is logarithmic. In acoustics
(from Greek akoustos = hearing, and ic= pertaining to) this means that
decibel is a measure of sound pressure, compared to a reference pressure (20
micropascals – we’ll get back to this).
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Howler
monkeys are the largest of the New World monkeys. But in
one
way they are like old world primates. Howlers are the only new
world
monkeys with color vision in both males and females. They all
see
like we see, so they can discriminate different shades of green, red,
and
blue. In most new world monkeys, the color receptor gene is on
the
X chromosome, so females may get two types and be color vision
function,
but all males get only one, and see only black and white.
A
fairly recent gene duplication in howlers have given it color
vision
again.
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So who’s the loudest? The howler
monkey (genus Alouatta, 15
species) has a great claim to being the loudest living thing. Used for
communication over tremendous distances, the howl of this primate reaches 128
db from several feet away (hear it here). Howlers have an enlarged hyoid bone
that is U-shaped. This creates an air sac on their throat that they can use to
make their howl resonate.
The howl is really more of a growl for males but is higher
pitched for females, and they can be heard more than 3.5 miles away. The
question is why do they do it. A 2014 study concluded that the black howler
monkey (Alouatta pigra) use their calls for several reasons, but most
relate to defense.
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The
lesser water boatman (Micronecta
scholtzi) is
only 2 mm
long,
but packs a big auditory punch. It’s a freshwater (aquatic
rather
than marine) insect. Many marine animals have loud
calls,
but here is an example of one that lives in slow moving
streams
and ponds. You can hear it when standing on the bank.
This
means the sound is so loud it can traverse the water/air
boundary, which usually stops most sounds.
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If we look at
smaller land animals, some of them are loud too. Cicadas can produce stridulations (see this post) that
reach 100 db from a foot away. But the king of the small animals would be the lesser water boatman.
This small
freshwater insect (Micronecta
scholtzi) can put out a stridulation of 105 db, even though it’s entire body
is only 2 mm (0.078 in) long! A 2011 paper in PLoS One broke down the song of the male into three different parts, each with its own peak intensity.
The loudest part could be heard from a riverside, even though the insect is
underwater. By comparing peak db to size, the male outcalls every other
organism we will discuss.
But it’s only the male that calls so loud. Why? Because he’s looking
for a mate, and it’s his penis rubbing against his abdomen that makes the
stridulation. This sort of eliminates the females from participating in the
contest.
But back to the bigger animals. Bats are extremely loud, but their
calls are of such high frequency that we can't hear them. Parrots can call to
each other in the range of 100 db, but we can go bigger. In fact, the biggest animal
may have the biggest voice as well.
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The
blue whale is the heaviest animal to yet live on Earth.
Being
this large, it still has two natural enemies – man, who
hunted
it to near extinction, and the orca. Orcas coordinate
their
attacks on the blue behemoths, often trying to separate
babies
from their mothers. Nearly 25% of sighted blue whales
have
scars from either orcas or monumentally stupid octopus.
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Interestingly, a 2009 study shows that blue whale songs are getting
lower in frequency. Since the whaling ban of 1966, male blue whale songs have
been using lower and lower tones. The authors suggest several
reasons for this. Males may not need to call out so loudly to find females
because the ban has resulted in higher numbers of whales. But it is also
possible that more man made noise in the ocean is forcing them to use lower
frequencies.
The sperm whale is right
there too. It doesn’t really vocalize, but it makes clicks to echolocate each
other and prey. The clicks are made by forcing air through two lips (folds of
tissue) in front of their blowhole. These clicks come in several varieties, but
the “usual” click can reach 230 db. This makes them the kings of sound, if you
don’t limit your choices to sounds made with the mouth.
Sounds in water are louder than in air, because the density is
higher and the transmission is more efficient. So decibels in water are
relative to 1 millipascal instead of the 20 micropascals in air. If you want to
compare directly, you need to subtract 61.5 db from the water sounds. This
still makes the sperm whale clicks 170 db and the blue whale song about 127 db,
just about the howler monkey level. So the sperm whale wins our contest.
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The
different pistol or mantis shrimp are not very large, but
they
have second largest sound to size ratio in the natural
world.
Only one claw is the pistol. The right image is an
enlargement
of the claw. S= socket, pl = plunger, D= dactyl,
p =
propus. The muscles of the dactyl close it so fast, that the
water
displaced in the socket by the plunger shoots out at
100
km/hr. This produces the cavitation bubble.
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A 2006 study describes that the hoods of the carapace (shell) that partially
cover the eyes of the pistol shrimp are to protect themselves from their own
explosive snap. The study suggests the hoods evolved first, and that allowed
for the development of stronger and stronger snaps. If this continues, the
pistol shrimp may start taking humans out!
Next week,
another question about the extremes of life.
