Speed of Sound | Mechanical waves and sound | Physics | Khan Academy

khanacademymedicine

3 Jan 201403:44

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

🎵 Sound is produced by creating a pulse wave through a short burst from a speaker.
📏 The speed of sound in non-humid air at 20°C is approximately 343 meters per second or 767 miles per hour.
🔍 The speed of sound is determined by observing the movement of compressed regions through a medium.
🌊 The speed of sound does not equate to the speed of individual air molecules moving back and forth.
🌀 Sound waves are longitudinal, meaning they travel parallel to the direction of the medium's oscillations.
🌊 Transverse waves occur when the wave velocity is perpendicular to the medium's oscillations, like waves on a string or water surface.
📊 A graph of air displacement versus position shows the wave shape moving to the right as the wave travels.
🔢 The velocity of a sound wave is calculated as the wavelength divided by the period, or wavelength times frequency.
⚠️ Increasing the frequency of a sound wave does not increase its speed; instead, the wavelength decreases proportionally.
🔑 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.
📢 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

00:00

🎵 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 × 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

💡Pulse Wave

A pulse wave is a single burst of energy that travels through a medium. In the context of the video, a pulse wave is created when a speaker is made to emit a short burst of sound. This concept is crucial for understanding how the speed of sound can be measured, as the speed of the compressed region in the pulse wave is indicative of the speed of sound itself.

💡Speed of Sound

The speed of sound refers to how fast sound waves travel through a medium. The video states that in non-humid air at 20 degrees Celsius, the speed of sound is approximately 343 meters per second or 767 miles per hour. This is a fundamental concept as it sets the stage for discussing how sound travels and how its speed can be measured.

💡Compressed Region

A compressed region in the context of sound waves refers to areas of higher pressure within the wave. The video explains that the speed of these compressed regions moving through the medium is what determines the speed of sound, highlighting the relationship between the physical properties of the medium and the wave's propagation.

💡Longitudinal Wave

A longitudinal wave is a type of wave where the oscillations of the medium particles are parallel to the direction of the wave's travel. Sound waves are described as longitudinal because the air molecules move back and forth in the same direction the wave is traveling. This term is key to understanding the nature of sound wave propagation.

💡Transverse Wave

A transverse wave is characterized by the oscillations of the medium particles being perpendicular to the direction of the wave's travel. The video uses examples like waves on a string or water surface to illustrate transverse waves, contrasting them with longitudinal waves to clarify the different types of wave motion.

💡Wavelength

Wavelength is defined as the physical length of one wave cycle, measured between two consecutive peaks or troughs. The video explains that the speed of a sound wave can be calculated using the wavelength divided by the period, which is a fundamental formula in wave physics.

💡Frequency

Frequency is the number of wave cycles that pass a given point in a certain amount of time, often measured in Hertz (Hz). The video uses frequency in the formula for calculating the speed of sound, showing that while frequency can be adjusted, it does not affect the speed of sound but rather the wavelength.

💡Amplitude

Amplitude refers to the maximum displacement of the medium particles from their equilibrium position during a wave cycle. The video clarifies that while amplitude affects the loudness of a sound, it does not affect the speed of the sound wave, which is a common misconception.

💡Medium

In the context of the video, medium refers to the material through which sound waves travel, such as air, water, or metal. The properties of the medium, including temperature and density, greatly affect the speed of sound, which is a central theme in understanding how sound behaves in different environments.

💡Harmonic Wave

A harmonic wave is a type of wave that repeats its pattern at regular intervals. The video mentions observing the speed of sound through a simple harmonic wave, emphasizing the regularity and predictability of wave patterns in understanding sound propagation.

💡Air Displacement

Air displacement is the movement of air particles as a sound wave passes through them. The video uses a graph of air displacement versus position to illustrate how the shape of the wave moves, which is essential for visualizing how sound waves travel.

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.