Sound Properties (Amplitude, Period, Frequency, Wavelength) | Physics | Khan Academy

Summary

The video explains sound waves, their behavior, and key concepts like amplitude, frequency, period, and wavelength. It shows how sound waves can be visualized using an oscilloscope, with the horizontal axis representing time and the vertical axis representing air molecule displacement. Amplitude determines loudness, while the period and frequency influence pitch. The wavelength represents the distance between compressed air regions. The difference between displacement vs. time and displacement vs. position graphs is highlighted, explaining how they reflect the period and wavelength of sound waves, respectively.

Takeaways

• 🎵 Sound waves can be represented visually using an oscilloscope, which shows their oscillations on a graph.
• 📊 The horizontal axis of a sound wave graph represents time, and the vertical axis represents the displacement of air molecules as they oscillate.
• 🔊 The amplitude of a sound wave refers to the maximum displacement of an air molecule from its equilibrium position, not the entire displacement.
• ⏳ The period of a sound wave is the time it takes for one air molecule to oscillate back and forth once, measured in seconds.
• 🎶 The frequency of a sound wave is the number of oscillations per second, measured in hertz (Hz), with 440 Hz being the frequency of the musical note A.
• 👂 Humans can hear frequencies between 20 Hz and 20,000 Hz, while dogs can hear sounds as high as 40,000 Hz.
• 📏 The wavelength of a sound wave is the distance between two compressed regions of air molecules and is measured in meters.
• ⚠️ The period is often confused with the wavelength; however, the period is the time for one oscillation, while the wavelength is the distance between compressed regions in space.
• 📈 A displacement versus time graph represents how a single air molecule moves as time progresses, where peaks indicate the period of the wave.
• 🗺️ A displacement versus position graph provides a snapshot of all air molecules at a specific moment, where the distance between peaks represents the wavelength of the wave.

Q & A

What does a sound wave look like when visualized?

A sound wave can be visualized as a graph that resembles a sine or cosine curve, showing the displacement of air molecules over time.

How is the amplitude of a sound wave defined?

The amplitude of a sound wave is the maximum displacement of an air molecule from its equilibrium position.

What is the difference between amplitude and the entire displacement of an air molecule?

The amplitude is only the maximum displacement from the equilibrium position, not the length of the entire back-and-forth movement.

What is the period of a sound wave?

The period of a sound wave is the time it takes for an air molecule to fully move back and forth one time, also known as one cycle.

How is the frequency of a sound wave related to its period?

The frequency of a sound wave is the reciprocal of its period, measured in oscillations per second or hertz.

What is the typical range of frequencies for human hearing?

Humans can hear frequencies ranging from about 20 hertz to about 20,000 hertz.

What is the frequency of an A note that causes air to oscillate back and forth 440 times per second?

The frequency of an A note that oscillates 440 times per second is 440 hertz.

What is the relationship between the frequency of a sound and the pitch we perceive?

Higher frequencies correspond to higher pitches, and lower frequencies correspond to lower pitches.

What is the wavelength of a sound wave, and how is it measured?

The wavelength of a sound wave is the distance between two compressed regions of air and is measured in meters.

How does the displacement versus time graph differ from the displacement versus position graph in representing a sound wave?

A displacement versus time graph shows the movement of a single air molecule over time, representing the period. A displacement versus position graph shows a snapshot of all air molecules' displacement at a moment, representing the wavelength.

Why do people often confuse the period and wavelength of a sound wave?

People confuse period and wavelength because they are both represented by the interval between peaks in different types of graphs, but they measure different aspects of the wave.

Outlines

00:00

🔊 Understanding Sound Waves

This paragraph explains the visual representation of sound waves. It begins with the speaker humming to demonstrate the sound of a sound wave and then describes how air molecules move in a pattern similar to a sine or cosine graph. The speaker uses an oscilloscope to show the graph of the sound wave, emphasizing that the horizontal axis represents time and the vertical axis shows the displacement of air molecules. The equilibrium position is the undisturbed state of air molecules. Increasing the volume results in larger oscillations and a louder sound, with amplitude defined as the maximum displacement from the equilibrium position. The period, measured in seconds, is the time for one complete oscillation, and frequency is defined as one over the period, measured in hertz. The paragraph also explains the human audible range of frequencies, from 20 hertz to 20,000 hertz, and mentions that dogs can hear higher frequencies. The concept of wavelength is introduced as the distance between two compressed regions of air, measured in meters, and distinguished from the period.

05:01

📊 Visualizing Sound Wave Displacement

This paragraph continues the discussion on sound waves by explaining two different graphs that represent sound waves. The first graph is a displacement versus time graph, which shows the movement of a single air molecule over time, with the interval between peaks representing the period of the wave. The second graph is a displacement versus position graph, which provides a snapshot of the displacement of all air molecules along the wave at a particular moment, with the interval between peaks representing the wavelength of the sound wave. The paragraph clarifies the difference between the two types of graphs and their respective representations of the sound wave's properties.

Keywords

💡Sound wave

A sound wave is a vibration that propagates through air or another medium, allowing sound to travel. In the video, the speaker explains that a sound wave can be visualized through air molecules moving back and forth. This movement creates a pattern that can be represented on a graph, such as a sine or cosine curve.

💡Oscilloscope

An oscilloscope is a device that allows us to visually observe the waveform of electrical signals, including sound waves. The video demonstrates how hooking up a speaker to an oscilloscope can graphically represent sound as a wave, showing how air molecules oscillate over time.

💡Amplitude

Amplitude refers to the maximum displacement of a particle from its equilibrium position in a wave. In the video, the speaker explains that when the amplitude of a sound wave increases, the oscillations become larger, and the sound becomes louder. It is important to note that amplitude is measured from the equilibrium position, not the entire length of the displacement.

💡Period

The period of a sound wave is the time it takes for an air molecule to complete one full cycle of oscillation, moving back and forth. The video explains that the period is measured in seconds and is inversely related to the pitch of the sound. A shorter period results in a higher-pitched sound.

💡Frequency

Frequency is the number of oscillations a sound wave undergoes per second, measured in hertz (Hz). It is the reciprocal of the period. The video describes how a sound's pitch changes with frequency, with higher frequencies corresponding to higher-pitched notes, such as the 440 Hz A note discussed in the video.

💡Hertz

Hertz (Hz) is the unit of frequency, representing one cycle per second. In the video, the speaker explains that a typical A note has a frequency of 440 Hz, meaning the air oscillates 440 times per second to produce the sound. Humans can hear frequencies from about 20 Hz to 20,000 Hz, while some animals, like dogs, can hear higher frequencies.

💡Wavelength

Wavelength is the distance between two consecutive compressed regions in a sound wave. In the video, the speaker discusses how wavelength is measured in meters and should not be confused with the period, which is related to time. Wavelength refers to spatial distance, while the period refers to time.

💡Equilibrium position

The equilibrium position is the undisturbed, or resting, position of an air molecule before it is affected by a sound wave. The video explains that the amplitude of a sound wave is measured from this position. The equilibrium position is key to understanding the displacement of air molecules during sound propagation.

💡Cycle

A cycle in a sound wave refers to one complete oscillation, where an air molecule moves from its equilibrium position, reaches a maximum displacement, and returns. The video defines a cycle as the unit of back-and-forth motion in sound waves, and it is used to calculate both the period and the frequency of the wave.

💡Displacement

Displacement refers to the movement of an air molecule from its equilibrium position as a sound wave passes through it. The video explains how displacement can be plotted on a graph against time or position, showing the back-and-forth motion of molecules that create sound. Displacement is a key factor in understanding waveforms.

Highlights

A sound wave's visual representation can be seen on an oscilloscope as a graph.

The graph's horizontal axis represents time, and the vertical axis represents the air molecule's displacement.

The center line on the graph indicates the equilibrium position of an air molecule.

Increasing the volume results in larger oscillations and a louder sound.

Amplitude is the maximum displacement of an air molecule from its undisturbed position.

The period of a sound wave is the time for an air molecule to complete one oscillation.

The period is measured in seconds and is represented by the letter T.

A shorter period results in a higher pitch.

Frequency is the reciprocal of the period and is measured in hertz.

Typical sound frequencies range from hundreds to thousands of hertz.

The A note has a frequency of 440 hertz, corresponding to 440 oscillations per second.

Humans can hear frequencies between 20 hertz and 20,000 hertz.

Dogs can hear frequencies up to at least 40,000 hertz.

Wavelength is the distance between two compressed regions in a sound wave.

Wavelength and period are often confused, but they represent different aspects of a sound wave.

A displacement versus time graph shows the period of the wave, while a displacement versus position graph shows the wavelength.

Transcripts

- This is what a sound wave sounds like,

(speaker hums)

but what does a sound wave look like?

Well, the air through which the sound wave is traveling

looks something like this,

but if you want another visual representation of the sound,

we can hook this speaker up to an oscilloscope,

and it gives us this graph.

(speaker hums)

We say that this shape represents the sound wave,

because if we focus on a single molecule of air,

we see that it moves back and forth,

just like a sine or cosine graph.

The horizontal axis here represents time,

and the vertical axis can be thought of

as representing the displacement of that air molecule

as it oscillates back and forth.

The center line here represents the equilibrium position

or undisturbed position of that air molecule.

It we turn up the volume,

we see that the oscillations become larger,

and the sound becomes louder.

The maximum displacement of the air molecule

from its undisturbed position is called the amplitude.

Be careful.

The amplitude is not the length of the entire displacement.

It's only the maximum displacement measured

from the equilibrium position.

Another key idea is the period of a sound wave.

The period is defined to be the time it takes

for an air molecule to fully move back and forth one time.

We call this back and forth motion a cycle.

We measure the period in seconds.

So, the period is the number of seconds

it takes for one cycle.

We use the letter capital T to represent the period.

If we decrease the period,

the time it takes for the air molecules

to oscillate back and forth decreases,

and the note or the pitch of the sound changes.

The less time it takes the air molecules

to oscillate back and forth,

the higher note that we perceive.

An idea intimately related to the period

is called the frequency.

Frequency is defined to be one over the period.

So, since the period is

the number of seconds per oscillation,

the frequency is the number of oscillations per second.

Frequency has units of one over seconds,

and we call one over a second a hertz.

Typical sounds have frequencies

in the 100s or even 1000s of hertz.

For instance, this note, which is an A note,

is causing air to oscillate back and forth

440 times per second.

So, the frequency of this A note is 440 hertz.

Higher notes have higher frequencies,

and lower notes have lower frequencies.

Humans can hear frequencies as low as about 20 hertz

and as high as about 20,000 hertz,

but if a speaker were to oscillate air back and forth

more than about 20,000 times per second,

it would create sound waves,

but we wouldn't be able to hear them.

(sound starts, then stops)

For instance, this speaker is still playing a note,

but we can't hear it right now.

Dogs could hear this note, though.

Dogs can hear frequencies up to at least 40,000 hertz.

Another key idea in sound waves

is the wavelength of the sound wave.

The idea of a wavelength is that when this sound

is traveling through a region of air,

the air molecules will be compressed

close together in some regions

and spread far apart from each other in other regions.

If you find the distance between two compressed regions,

that would be the wavelength of that sound wave.

Since the wavelength is a distance, we measure it in meters.

Be careful.

People get wavelength and period mixed up all the time.

The period of a sound wave is the time it takes

for an air molecule to oscillate back and forth one time.

The wavelength of a sound wave is the distance

between two compressed regions of air.

People get these mixed up

because there's an alternate way to create a graph

of this sound wave.

Consider this.

Before the wave moves through the air,

each air molecule has some undisturbed position

from the speaker that we can measure in meters.

This number represents the equilibrium

undisturbed position of that air molecule.

Then as the sound wave passes by,

the air molecules get displaced slightly from that position.

So, an alternate graph that we could make

would be the displacement of the air molecule

versus the undisturbed position

or equilibrium position of that air molecule.

This graph would let us know

for a particular moment in time

how displaced is that air molecule

at that particular position in space.

This graph shows us that in some regions

the air is displaced a lot from its equilibrium position,

and in other regions, the air is not displaced much at all

from its equilibrium position.

For this kind of graph, the distance between peaks

represents the wavelength of the sound wave,

not the period, because it would be measuring

the distance between compressed regions in space.

So, be careful.

For a sound wave, a displacement versus time graph

represents what that particular air molecule is doing

as a function of time, and on this type of graph,

the interval between peaks represents

the period of the wave,

but a displacement versus position graph

represents a snapshot of the displacement

of all the air molecules along that wave

at a particular instant of time,

and on this type of graph,

the interval between peaks represents the wavelength.