When you think about it, you probably receive hundreds -- even thousands -- of cues about
what’s going on in your environment every day, strictly from sound.
In addition to things like speech and music, there are other bits of auditory information
that shape your day: an ambulance passing by, a baby crying in the next room,
and of course [cell-phone style text ding goes off] --
-- sorry.
Just got a text.
But there’s a lot that we can learn, not just from what these cues MEAN, but from how
Sound itself works.
Studying sound waves has helped doctors learn more about our ears, and has allowed engineers
to design things like microphones and speakers.
Biologists have even used the science of sound to figure out how animals like elephants can
communicate over long distances -- when we can’t even hear them doing it.
It all comes down to the fact that SOUND is a wave, which travels through a medium like
air or water.
And knowing that sound is a wave is important, because it means that we can use the physics
of waves to describe the qualities of sound.
[Intro Music Plays]
When you think of a wave, you probably think of the kind you see at the ocean, or the ones
you made when you jumped on that trampoline last time.
Those waves produce ripples that run perpendicular to the direction, that the wave is traveling in.
But sound is the other kind of wave: it’s a longitudinal wave, meaning that the wave’s
back-and-forth motion happens in the same direction in which the wave travels.
Say you get a text message on your phone, and it makes a nice, bright little ‘ding!’ sound.
What actually happened? Like, on a physical level?
Your phone’s speaker contains a diaphragm -- a piece of stiff material, usually in the
shape of a cone.
When you got the message, the electronics inside the speaker made the diaphragm move
back and forth, which vibrated the air around your phone.
That made the atoms and molecules in the air move back and forth.
Then, those moving particles vibrated the air around them -- and as the process continued,
the sound wave spread outward.
[ding!]
Sorry! I’m just gonna turn this off now.
Anyway, physicists sometimes describe sound waves in terms of the movement of these particles
in the air -- in what’s known as a displacement wave.
But by moving particles in the air, sound waves also do something else:
They cause the air to compress and expand -- which is why sound waves are sometimes
described as ‘pressure waves’.
As the wave spreads through the air, the particles end up bunching together in some places, and
‘spreading out’ in others.
Together, all that bunching and spreading-out causes areas of high pressure and low pressure
to form and move through the air.
It’s useful to describe sound waves as pressure waves, because we can build devices that detect
those changes in pressure.
That’s how some microphones work, for example: They use a diaphragm stretched over a sealed
compartment, and as sound waves pass by, they create areas of lower or higher pressure in
the compartment.
The differences in pressure cause the diaphragm to move back and forth, which electronics
then translate into audio data
And your eardrums basically work the same way!
As pressure waves pass through, they make your eardrum vibrate.
Your brain then interprets those vibrations as sound.
But not all sounds are the same.
Even before we knew much about physics, humans were describing sound in terms of certain qualities:
mainly, by things like ‘loudness’ and ‘pitch’.
Our understanding of those qualities helped shape the development of music -- which we’ll
talk more about next time.
But there’s also a more physics-y side to those qualities of music.
Pitch can be high or low, and it corresponds to the ‘frequency’ of the wave.
So, air that’s vibrating back and forth more times per second will have a higher pitch,
and air that’s vibrating fewer times per second will have a lower pitch.
Humans hear sounds best when the vibrations are somewhere between 20 per second on the
low end and 20,000 per second on the high end.
As we get older and lose more of the cells that help us detect sound, we start to lose
the ability to hear higher-pitched sounds.
Some building security companies will take advantage of this, using devices that emit
a high-pitched noise that most people over the age of 25 can’t hear.
The idea is that since kids and teens can hear it, and it’s super annoying to them,
they won’t hang out near the building.
But some sounds are too high or low for any humans to hear.
Sounds that are too high in pitch are called ultrasonic, and sounds that are too low are
called infrasonic.
Dog whistles, for example, use an ultrasonic pitch that’s too high for us, but is perfectly
audible to dogs.
Elephants, on the other hand, use INFRAsonic sound to communicate with each other across
long distances.
They can hear these calls from several kilometers away, but we can’t hear them at all.
Another aspect that shapes sound is its loudness -- when you increase the intensity of a sound,
you increase its loudness, and vice versa.
We’ve talked about the intensity of a wave before: it’s the wave’s power over its
area, measured in Watts per square meter.
We’ve also said that the intensity of a wave is proportional to the wave’s amplitude, squared.
And the farther you are from the source of a wave, the lower its intensity -- by the
square of the distance between you and the source.
And just as there’s a range of pitches that humans can hear, there’s also a range of
sound wave intensity that humans can comfortably hear.
Generally, people can safely hear sounds from about 1 picowatt per square meter,
up to 1 Watt per square meter -- which is about as loud as a rock concert, if you’re near the speakers.
The sound waves coming from a jet plane that’s 30 meters away, for example, probably has
an intensity of around 100 Watts per square meter.
Now, I don’t know if you’ve ever been that close to a roaring jet plane.
But there’s a reason people who work on the tarmac at airports use those heavy-duty headphones.
Below 1 picowatt per square meter, sounds are just too soft for us to detect them.
And although we will HEAR sounds above a Watt per square meter, they tend to hurt our ears.
But here’s a weird thing about loudness and intensity: it’s not a linear relationship.
Generally, a sound wave needs to have ten times the intensity to sound twice as loud to us.
This relationship holds true as long as the sound is toward the middle of the range of
frequencies we can hear.
So, instead of directly measuring the loudness of sounds by their intensity, we use units
called ‘decibels’ -- which are based on bels.
Bels convert a sound wave’s intensity to a ‘logarithmic scale’, where every notch
on the scale is ten times higher than the previous one.
The scale starts off with an intensity of 1 picowatt per square meter, corresponding
to 0 bels.
So a sound that’s 1 bel is ten times as intense as a sound that’s 0 bels.
And a sound that’s 2 bels is 10 times as intense as a sound that’s 1 bel --
-- but 100 times as intense as a sound that’s 0 bels.
Measuring everything in bels can be kind of annoying, because sometimes you want to talk
about sounds that are, say, 3.4 bels without having to deal with decimal points.
That’s why most of the time, you’ll hear the loudness of a sound described using the
more familiar decibel unit -- a tenth of a bel.
To find the loudness of a sound when you know its intensity, you take the base-10 logarithm
of its intensity, over the reference intensity of 1 picowatt per square meter.
Then, you multiply that number by 10 to get the sound’s decibel level.
We can use this equation to convert the intensity of that noisy rock concert -- which we said
was 1 Watt per square meter -- to decibels.
First, we take the base 10 log of 1 Watt per square meter, over 1 picowatt per square meter.
Now, 1 divided by 1 x 10^-12 is just 1 x 10^12.
So what we really want to do is take the base 10 log of 1 x 10^12 -- or a trillion -- watts
per square meter.
What a logarithm asks you to do, is find the power that you would need to raise the base
to in order to get the number in parentheses.
In other words, we’re looking for the exponent of 10 that would equal 1 x 10^12.
Which is just 12.
To finish off the calculation of decibels from intensity, we multiply that value -- 12
-- by 10 to get the decibel level of the rock concert, where you were standing: 120 decibels.
Ouch.
You’ll notice that as the source of a sound moves closer to you, it gets louder, and as
it moves away, it gets softer.
That makes sense, since the closer you are to the source of a sound, the greater the
intensity of the wave that hits your ear.
But have you ever noticed that the pitch of the sound changes, too?
It’s called the ‘Doppler effect’: As a source of sound moves toward you, the pitch
of the sound you hear increases.
And as the source moves away, the pitch decreases.
To see why, imagine you’re standing on the sidewalk, when suddenly you hear an ambulance
siren start up.
It’s coming from down the road, and it seems to be moving toward you.
The ambulance is continuously emitting sound waves at a certain frequency, in the form
of that siren.
But as the ambulance moves toward you, the ambulance is also driving toward those sound waves.
So, the peaks that hit your eardrums are closer together -- even though they’re moving at
the same speed -- and you get hit by them more often.
Which means you hear a higher-pitched sound.
At the same time, it keeps emitting more sound, which adds more peaks to those earlier sound
waves that are heading your way.
What you end up with, is a sound wave with a higher frequency than before.
That’s what hits your eardrum, so you hear a sound that’s higher in pitch than the
one you heard before the ambulance started moving.
As the ambulance passes you and starts to drive away down the road, the opposite happens.
The sound waves are still coming toward you, but the ambulance is driving away from them.
So the peaks that hit your eardrum are farther apart, and you hear a sound with a lower pitch.
The Doppler effect isn’t unique to sound waves, though -- it happens with light, too.
Which means we can actually use it to measure the distance of stars -- but more on that much later.
For now, you learned about sound waves, and how they move particles back and forth to
create differences in pressure.
We also talked about pitch, and how the intensity of a sound wave changes with amplitude and distance.
Finally, we covered decibels, as well as the Doppler effect.
Crash Course Physics is produced in association with PBS Digital Studios.
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PBS Idea Channel, and It's Okay to be Smart.
This episode of Crash Course was filmed in the Doctor Cheryl C. Kinney Crash Course Studio
with the help of these amazing people and our equally amazing graphics team is Thought Cafe.