Speed of Sound | Mechanical waves and sound | Physics | Khan Academy
Summary
TLDRThis script explains the concept of sound waves and their speed. It clarifies that the speed of sound, approximately 343 meters per second in non-humid air at 20 degrees Celsius, is not the same as the speed of individual air molecules. Sound is a longitudinal wave, moving parallel to the medium's oscillations. The speed of sound is determined by the medium's properties, such as temperature and density, and is calculated as the product of wavelength and frequency. Changes in amplitude do not affect speed, emphasizing that loudness does not equate to faster transmission.
Takeaways
- π΅ Sound is produced by creating a pulse wave through a short burst from a speaker.
- π The speed of sound in non-humid air at 20Β°C is approximately 343 meters per second or 767 miles per hour.
- π The speed of sound is determined by observing the movement of compressed regions through a medium.
- π The speed of sound does not equate to the speed of individual air molecules moving back and forth.
- π Sound waves are longitudinal, meaning they travel parallel to the direction of the medium's oscillations.
- π Transverse waves occur when the wave velocity is perpendicular to the medium's oscillations, like waves on a string or water surface.
- π A graph of air displacement versus position shows the wave shape moving to the right as the wave travels.
- π’ The velocity of a sound wave is calculated as the wavelength divided by the period, or wavelength times frequency.
- β οΈ Increasing the frequency of a sound wave does not increase its speed; instead, the wavelength decreases proportionally.
- π The speed of sound can only be changed by altering the medium (like air's temperature, humidity, or density) or by using a different medium altogether.
- π’ The amplitude of a sound wave affects its loudness, not its speed, so yelling does not make you heard faster.
Q & A
What is a pulse wave in the context of sound?
-A pulse wave is a single burst of sound that travels through a medium, such as air, and can be used to determine the speed of sound by observing how quickly the compressed region of the wave moves through the medium.
What is the speed of sound in non-humid air at 20 degrees Celsius?
-The speed of sound in non-humid air at 20 degrees Celsius is approximately 343 meters per second or 767 miles per hour.
How is the speed of a sound wave related to the movement of air molecules?
-The speed of sound is not the same as the speed of individual air molecules moving back and forth. Instead, it refers to the speed at which the disturbance or the wave pattern moves through the medium.
What is the difference between longitudinal and transverse waves?
-Longitudinal waves, like sound waves, travel parallel to the direction of the oscillations of the medium. Transverse waves, on the other hand, have their oscillations perpendicular to the direction of wave travel, like waves on a string or water surface.
How can you visually observe the speed of a sound wave?
-By looking at a graph of air displacement versus position, one can observe the speed of a sound wave by tracking the movement of the wave's peaks, valleys, or any single point on the wave shape.
What is the formula for the velocity of a sound wave?
-The velocity of a sound wave is calculated by the formula speed equals wavelength times frequency, which is applicable to all types of waves, not just sound.
Why does increasing the frequency of a sound wave not change its speed?
-Increasing the frequency of a sound wave results in a proportional decrease in wavelength, which keeps the speed of the sound wave constant, as speed is the product of wavelength and frequency.
How can the speed of sound be changed?
-The speed of sound can be changed by altering the medium through which it travels or by changing the properties of that medium, such as temperature, humidity, density, or by using a different material like water, helium, or metal.
Does changing the amplitude of a sound wave affect its speed?
-No, changing the amplitude of a sound wave does not affect its speed. A sound wave with a large amplitude will not travel faster than one with a small amplitude in the same medium; it will only be louder.
What does it mean when it is said that the speed of a sound wave is determined by the properties of the medium?
-It means that the speed at which a sound wave travels is entirely dependent on the physical properties of the medium it is passing through, such as its density, elasticity, and temperature.
Outlines
π΅ Understanding Sound Waves
The paragraph explains the concept of sound waves and their properties. It begins by describing how a pulse wave is created by a speaker and how the speed of sound can be determined by observing the movement of compressed regions through a medium. It then specifies that in non-humid air at 20 degrees Celsius, the speed of sound is approximately 343 meters per second or 767 miles per hour. The difference between longitudinal and transverse waves is also explained, with sound waves being longitudinal as they travel parallel to the oscillations of the medium. The paragraph further discusses how the speed of sound waves can be represented graphically and mathematically, introducing the formula speed = wavelength Γ frequency. It clarifies misconceptions about altering the speed of sound through frequency adjustments or amplitude changes, emphasizing that the speed of sound is determined by the medium's properties, such as temperature, humidity, and density.
Mindmap
Keywords
π‘Pulse Wave
π‘Speed of Sound
π‘Compressed Region
π‘Longitudinal Wave
π‘Transverse Wave
π‘Wavelength
π‘Frequency
π‘Amplitude
π‘Medium
π‘Harmonic Wave
π‘Air Displacement
Highlights
A short burst from a speaker creates a pulse wave.
Speed of sound can be determined by observing the speed of compressed regions in a medium.
In non-humid air at 20 degrees Celsius, the speed of sound is approximately 343 meters per second or 767 miles per hour.
The speed of sound is not the same as the speed of individual air molecules moving back and forth.
Sound is a longitudinal wave, traveling parallel to the oscillations of the medium.
Transverse waves occur when wave velocity is perpendicular to the medium's oscillations.
Examples of transverse waves include waves on a string or water surface.
The speed of a sound wave can be found by observing the movement of peaks, valleys, or any point on the wave shape.
The velocity of a sound wave is calculated as the wavelength divided by the period of the wave.
The formula speed equals wavelength times frequency is applicable to all types of waves.
Increasing the frequency of a sound wave does not increase its speed; instead, the wavelength decreases by the same factor.
The speed of sound cannot be increased by altering the speaker; it is determined by the medium's properties.
Changing the medium or its properties, such as temperature, humidity, or density, can alter the speed of sound.
Switching the medium entirely, like to water or helium, can also change the speed of sound.
Amplitude does not affect the speed of a sound wave; only loudness is affected.
Yelling does not make sound travel faster; it only increases the loudness of the sound upon arrival.
The speed of a sound wave is entirely determined by the properties of the medium it travels through.
Transcripts
- If we let a speaker make one short burst,
we'll create a pulse wave.
We can find the speed of sound
by looking at the speed of this compressed region
as it travels through the medium.
In non-humid air at 20 degrees Celsius,
the speed of sound is about
343 meters per second or 767 miles per hour.
We can also watch the speed of sound
of a repeating simple harmonic wave.
The speed of the wave can again be determined
by the speed of the compressed regions
as they travel through the medium.
Note that the speed of sound does not mean
the speed of the air molecules as they move back and forth.
The air molecules are moving with the speed,
but by the speed of sound,
we mean the speed of the disturbance
as it moves through the air molecules.
We call sound a longitudinal wave
because the wave is traveling parallel
to the line traced out by the oscillations of the medium.
The other type of wave is a transverse wave.
Transverse waves happen when the wave velocity
points perpendicular to the oscillations of the medium.
Waves on a string or waves on the surface of water
are examples of transverse waves.
If we look at a graph of the air displacement
versus position of the air,
we can see that as the wave travels
the shape of this wave travels to the right.
So, the speed of a sound wave can be found
by finding the speed of the peaks
or the speed of the valleys
or the speed of any single point on the wave shape.
To figure out a formula for the velocity of a sound wave,
let's look closely at what's happening here.
Watch one of the air molecules.
It takes one period for this molecule
to move back and forth through a full cycle.
During this time, the wave shape has moved forward
one complete wavelength.
This is because the wave has to overlap
with its initial shape after one period,
because the molecule has to be back where it started
after one period.
Now, since speed is defined to be the distance per time,
the speed of a sound wave has to be
the wavelength of the wave
divided by the period of the wave.
Since the wave is traveling forwards
one wavelength per period,
or since the frequency is defined to be one over the period,
we can rewrite this formula as
speed equals wavelength times frequency.
This formula is accurate for all kinds of waves,
not just sound waves,
because a wave has to move one wavelength for every period.
Be careful.
When looking at this equation,
you might think that if you adjust
the setting on your speaker and increase the frequency
you'd also be increasing the speed of the sound wave,
but that's not what happens.
If you increase the frequency,
the wavelength will decrease by that same factor,
and the speed of the sound wave will remain the same.
In fact, there's nothing you can do to the speaker
that would increase the speed of sound.
So, how can we change the speed of sound?
Well, the only way to change the speed of sound
is to change the medium or the properties of the medium
that the sound wave is traveling in.
So, to change the speed of sound in air,
you can change things like the temperature of the air
or the humidity of the air or the density of the air,
or you can swap out the air entirely for another material,
like water or helium or a metal.
All of these changes to the medium
would affect the speed of sound.
People often think that changing the amplitude
will change the speed of a sound wave, but it won't.
If we create a sound pulse with a large amplitude,
it won't travel any faster than a sound pulse
with a small amplitude in the same medium.
It will just be louder.
In other words,
yelling won't cause anyone to hear you faster,
they'll just hear a louder sound
when the sound wave arrives at their location.
So remember, the speed of a sound wave
is determined entirely by the properties of the medium
through which it's traveling.
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