Sound: Crash Course Physics #18

Summary

This 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

• 👂 We receive numerous auditory cues daily, which include speech, music, and environmental sounds like ambulance sirens and text message alerts.
• 🔍 The study of sound waves has advanced various fields such as medicine, engineering, and biology, helping us understand how animals communicate over long distances.
• 🌊 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.
• 📳 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.
• 🌀 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.
• 📏 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.
• 🐘 Elephants use infrasonic sounds for communication, which are below the human hearing range and can travel several kilometers.
• 🔊 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.
• 📊 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.
• 🚨 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.
• 🎶 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

00:00

🌊 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.

05:01

📊 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.

Keywords

💡Sound Cues

Sound cues refer to the auditory signals that inform us about our environment. In the video, they are described as a critical part of our daily experiences, ranging from speech and music to more subtle cues like an ambulance siren or a baby's cry. These cues are essential for understanding the theme of how sound shapes our perception of the world.

💡Sound Waves

Sound waves are the physical manifestation of sound as it travels through a medium like air or water. The video emphasizes that understanding sound as a wave is fundamental because it allows us to use wave physics to describe sound's qualities. An example from the script is the explanation of how an ambulance's siren emits sound waves that change in pitch due to the Doppler effect.

💡Longitudinal Wave

A longitudinal wave is a type of wave where the motion of the particles in the medium is in the same direction as the wave's travel. The video explains that sound is a longitudinal wave, with the script providing the example of a 'ding' from a phone's speaker, illustrating how the diaphragm's movement creates vibrations in the air.

💡Diaphragm

In the context of the video, a diaphragm is a component found in speakers and microphones that vibrates to produce or detect sound. The script describes how the diaphragm in a phone's speaker moves back and forth to create sound waves, which is a key concept in understanding how sound is generated and transmitted.

💡Displacement Wave

A displacement wave is a way to describe sound waves by focusing on the movement of particles in the air. The video uses this term to explain how sound waves cause particles to move back and forth, creating areas of compression and rarefaction, which is integral to the concept of sound wave propagation.

💡Pressure Waves

Pressure waves describe sound in terms of the compression and expansion of air particles, leading to areas of high and low pressure. The video mentions that sound waves are sometimes called pressure waves, which is crucial for understanding how microphones detect sound by measuring changes in air pressure.

💡Pitch

Pitch refers to the perceived frequency of a sound, determining whether it sounds high or low. The video explains that pitch is related to the frequency of a wave, with higher frequencies corresponding to higher pitches and vice versa. An example from the script is the discussion of how the human hearing range is between 20 Hz to 20,000 Hz.

💡Frequency

Frequency is the rate at which particles in a medium vibrate per second and is directly related to the pitch of a sound. The video defines frequency and illustrates its importance through the discussion of pitch, explaining that sounds with higher frequencies have a higher pitch, as in the case of ultrasonic sounds that are inaudible to humans.

💡Loudness

Loudness is the perception of the intensity of a sound, which is related to the amplitude of the sound wave and the distance from the source. The video explains that loudness is not linearly related to intensity, with the script providing the example of how the intensity of a sound from a jet plane can be measured in decibels.

💡Decibels

Decibels are units used to measure the intensity of sound on a logarithmic scale. The video introduces decibels as a way to quantify loudness, explaining the conversion from watts per square meter to decibels using logarithms. An example calculation in the script converts the intensity of a rock concert to its equivalent decibel level.

💡Doppler Effect

The Doppler effect is the change in frequency or pitch of a sound wave in relation to an observer who is moving relative to the source of the sound. The video describes how the pitch of an ambulance siren changes as it approaches and then moves away from the listener, demonstrating the Doppler effect in the context of sound waves.

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

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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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.