Sound: Crash Course Physics #18
Summary
TLDRThis script explores the science of sound, explaining how sound waves function as longitudinal waves traveling through mediums like air or water. It delves into the physics behind the pitch and loudness of sounds, the human hearing range, and how devices like microphones and speakers are designed based on this knowledge. The Doppler effect and its implications for both sound and light are also discussed, highlighting the broader applications of understanding sound wave behavior.
Takeaways
- đ We receive numerous auditory cues daily, which include speech, music, and environmental sounds like ambulance sirens and text message alerts.
- đ The study of sound waves has advanced various fields such as medicine, engineering, and biology, helping us understand how animals communicate over long distances.
- đ Sound is a type of wave that travels through mediums like air or water, and it is a longitudinal wave, with motion in the same direction as its travel.
- đł When a phone receives a text message and emits a sound, the speaker's diaphragm vibrates, transferring this vibration to the surrounding air molecules, creating a sound wave.
- đ Sound waves can be described as displacement waves, showing the movement of air particles, and also as pressure waves, which cause compression and expansion in the air.
- đ The human hearing range is from 20 Hz to 20,000 Hz, with age affecting our ability to hear higher frequencies, which is utilized in some security systems to deter teenagers.
- đ Elephants use infrasonic sounds for communication, which are below the human hearing range and can travel several kilometers.
- đ Loudness of sound is related to its intensity, measured in watts per square meter, and our perception of loudness is not linear but logarithmic, which is why decibels are used.
- đ The decibel scale starts at 0, with each increment representing a tenfold increase in intensity, making it easier to express a wide range of sound levels.
- đš The Doppler effect causes a change in the pitch of a sound based on the relative motion between the source and the observer, such as an approaching or receding ambulance.
- đ¶ Understanding the qualities of sound, such as pitch and loudness, has shaped the development of music and other auditory experiences.
Q & A
What are the different types of auditory cues that we receive from our environment daily?
-We receive hundreds to thousands of auditory cues daily from our environment, including speech, music, an ambulance passing by, a baby crying, and even text message notifications from our phones.
How has the study of sound waves contributed to various fields?
-Studying sound waves has helped doctors understand more about our ears, allowed engineers to design microphones and speakers, and enabled biologists to figure out how animals like elephants communicate over long distances.
What is the fundamental nature of sound that is important for understanding its behavior?
-Sound is a wave that travels through a medium like air or water, and understanding this is crucial because it allows us to use the physics of waves to describe the qualities of sound.
What type of wave is sound, and how does its motion compare to other types of waves?
-Sound is a longitudinal wave, meaning that its back-and-forth motion happens in the same direction in which the wave travels, unlike transverse waves that produce ripples perpendicular to the direction of travel.
How does a phone's speaker create the sound when a text message is received?
-The phone's speaker contains a diaphragm that moves back and forth when the message is received, vibrating the air around the phone and creating a sound wave that spreads outward.
What are the two ways in which sound waves are often described in terms of their effect on the air?
-Sound waves are often described as displacement waves, which refer to the movement of air particles, and as pressure waves, which involve the compression and expansion of air, creating areas of high and low pressure.
How do microphones convert sound waves into audio data?
-Microphones use a diaphragm stretched over a sealed compartment. As sound waves create areas of lower or higher pressure in the compartment, the diaphragm moves, and electronics translate this movement into audio data.
What are the two main qualities of sound that humans have historically described?
-Humans have historically described sound in terms of 'loudness' and 'pitch', which correspond to the intensity and frequency of the sound wave, respectively.
What is the range of vibrations per second that humans can hear, and how does it change with age?
-Humans hear sounds best when the vibrations are between 20 per second and 20,000 per second. As we age, we start to lose the ability to hear higher-pitched sounds due to the loss of cells that help detect sound.
What are ultrasonic and infrasonic sounds, and how do they relate to human hearing?
-Ultrasonic sounds are those with a pitch too high for humans to hear, while infrasonic sounds are too low in pitch. Humans cannot hear these sounds, but some animals, like dogs and elephants, can hear ultrasonic and infrasonic sounds, respectively.
How is the loudness of sound measured, and what is the difference between intensity and loudness?
-Loudness is measured in decibels, which are based on a logarithmic scale related to the intensity of the sound wave. Intensity is the power of the wave over an area, measured in Watts per square meter, and is proportional to the amplitude squared, whereas loudness is our perception of the sound's strength.
What is the Doppler effect, and how does it affect the pitch of a sound?
-The Doppler effect is the change in pitch of a sound as the source moves towards or away from the listener. As the source moves towards the listener, the pitch increases because the sound waves reach the listener more frequently. Conversely, as the source moves away, the pitch decreases due to the sound waves reaching less frequently.
Outlines
đ Understanding Sound Waves and Their Properties
This paragraph delves into the nature of sound as a wave, explaining how it travels through mediums like air or water as a longitudinal wave. It discusses the physical process of sound production, using the example of a phone's speaker diaphragm creating vibrations in the air. The concept of sound waves as pressure waves is introduced, describing how these waves create areas of high and low pressure. The paragraph also touches on how devices like microphones and human eardrums detect these pressure changes. It further explores the qualities of sound, such as pitch and loudness, and how they relate to wave frequency and intensity, respectively. The discussion on pitch includes the human hearing range and the use of ultrasonic and infrasonic sounds by animals. Loudness is connected to wave intensity, with examples of sound intensity in everyday scenarios.
đ Decibels, Loudness, and the Doppler Effect
The second paragraph focuses on the measurement of sound intensity and loudness, particularly the non-linear relationship between them. It introduces the decibel scale as a logarithmic measure of sound intensity, explaining how to convert intensity to decibels using the base-10 logarithm. The paragraph provides a practical example of calculating the decibel level of a rock concert. It also explores the Doppler effect, which causes a change in perceived pitch as a sound source moves towards or away from the listener. The explanation includes the mechanics of how the frequency of sound waves changes due to the relative motion between the source and the observer. The paragraph concludes with a brief mention of the Doppler effect's application in measuring the distance of stars, with a note on its broader relevance beyond sound waves.
Mindmap
Keywords
đĄSound Cues
đĄSound Waves
đĄLongitudinal Wave
đĄDiaphragm
đĄDisplacement Wave
đĄPressure Waves
đĄPitch
đĄFrequency
đĄLoudness
đĄDecibels
đĄDoppler Effect
Highlights
We receive hundreds to thousands of auditory cues daily, shaping our day beyond speech and music.
Studying sound waves helps doctors understand our ears and engineers design microphones and speakers.
Biologists use sound science to understand how animals like elephants communicate over long distances.
Sound is a wave that travels through mediums like air or water.
Sound waves are longitudinal, with motion in the same direction as wave travel.
A phone's speaker diaphragm moves back and forth to create sound by vibrating the air.
Physicists describe sound waves in terms of particle displacement in the air.
Sound waves cause air compression and expansion, described as 'pressure waves'.
High and low pressure areas form and move through the air as sound waves spread.
Microphones work by detecting changes in pressure caused by sound waves.
The human eardrum vibrates in response to pressure waves, interpreted by the brain as sound.
Sound qualities like 'loudness' and 'pitch' have been described by humans long before physics.
Pitch corresponds to the frequency of a wave, with higher frequencies producing higher pitches.
Humans can hear vibrations between 20 and 20,000 times per second.
High-pitched sounds are called ultrasonic, and low-pitched sounds are infrasonic.
The intensity of a sound wave is proportional to its amplitude squared.
Loudness and intensity have a non-linear relationship, with a logarithmic scale described in decibels.
The Doppler effect causes the pitch of a sound to change as the source moves towards or away from the listener.
The Doppler effect is not unique to sound and can be used to measure the distance of stars.
Transcripts
When you think about it, you probably receive hundreds -- even thousands -- of cues about
whatâs 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 thereâs 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 canât 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: itâs a longitudinal wave, meaning that the waveâs
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 âding!â sound.
What actually happened? Like, on a physical level?
Your phoneâs 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! Iâm just gonna turn this off now.
Anyway, physicists sometimes describe sound waves in terms of the movement of these particles
in the air -- in whatâs 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 âpressure wavesâ.
As the wave spreads through the air, the particles end up bunching together in some places, and
âspreading outâ in others.
Together, all that bunching and spreading-out causes areas of high pressure and low pressure
to form and move through the air.
Itâs useful to describe sound waves as pressure waves, because we can build devices that detect
those changes in pressure.
Thatâs 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 âloudnessâ and âpitchâ.
Our understanding of those qualities helped shape the development of music -- which weâll
talk more about next time.
But thereâs also a more physics-y side to those qualities of music.
Pitch can be high or low, and it corresponds to the âfrequencyâ of the wave.
So, air thatâs vibrating back and forth more times per second will have a higher pitch,
and air thatâs 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 canât hear.
The idea is that since kids and teens can hear it, and itâs super annoying to them,
they wonât 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 thatâs 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 canât 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.
Weâve talked about the intensity of a wave before: itâs the waveâs power over its
area, measured in Watts per square meter.
Weâve also said that the intensity of a wave is proportional to the waveâs 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 thereâs a range of pitches that humans can hear, thereâs 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 youâre near the speakers.
The sound waves coming from a jet plane thatâs 30 meters away, for example, probably has
an intensity of around 100 Watts per square meter.
Now, I donât know if youâve ever been that close to a roaring jet plane.
But thereâs 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 hereâs a weird thing about loudness and intensity: itâs 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 âdecibelsâ -- which are based on bels.
Bels convert a sound waveâs intensity to a âlogarithmic scaleâ, 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 thatâs 1 bel is ten times as intense as a sound thatâs 0 bels.
And a sound thatâs 2 bels is 10 times as intense as a sound thatâs 1 bel --
-- but 100 times as intense as a sound thatâs 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.
Thatâs why most of the time, youâll 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 soundâs 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, weâre 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.
Youâll 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?
Itâs called the âDoppler effectâ: 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 youâre standing on the sidewalk, when suddenly you hear an ambulance
siren start up.
Itâs 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 theyâre 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.
Thatâs what hits your eardrum, so you hear a sound thatâs 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 isnât 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.
You can head over to their channel to check out amazing shows like Gross Science,
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.
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