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

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1 Jan 201405:16
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TLDRThis script explores the visual and auditory aspects of sound waves, explaining how they appear on an oscilloscope as a sine or cosine graph. It clarifies the concepts of amplitude, period, frequency, and wavelength, illustrating how these elements affect the loudness and pitch of sound. The human hearing range and the comparison between period and wavelength are also highlighted, emphasizing the differences in time and space measurements of sound waves.

Takeaways
  • 🎵 Sound waves are vibrations in the air that can be visualized as a graph when connected to an oscilloscope.
  • 🔍 The graph of a sound wave resembles a sine or cosine wave, with air molecules oscillating back and forth around an equilibrium position.
  • ⏳ The horizontal axis of the graph represents time, while the vertical axis represents the displacement of air molecules from their equilibrium position.
  • 📊 Loudness of a sound is related to the amplitude of the wave, which is the maximum displacement of air molecules from their equilibrium position.
  • 🔁 The period of a sound wave is the time it takes for an air molecule to complete one full oscillation back and forth.
  • 🎼 Frequency is the number of oscillations per second, measured in hertz, and is inversely related to the period of the wave.
  • 📉 A decrease in the period results in a higher frequency and a higher perceived pitch of the sound.
  • 🐕 Humans can hear frequencies ranging from about 20 to 20,000 hertz, but dogs can hear up to at least 40,000 hertz.
  • 🌊 The wavelength of a sound wave is the distance between two compressed regions of air and is measured in meters.
  • 🔄 It's important to distinguish between the period, which is a measure of time, and the wavelength, which is a measure of distance.
  • 📈 A displacement versus time graph shows the motion of a single air molecule, while a displacement versus position graph captures the snapshot of air molecule displacements along the wave at a moment in time.
Q & A

  • What does a sound wave look like when visualized with an oscilloscope?

    -A sound wave visualized with an oscilloscope appears as a graph that resembles a sine or cosine wave, with the horizontal axis representing time and the vertical axis showing the displacement of an air molecule as it oscillates back and forth.

  • What is the significance of the center line in the oscilloscope graph of a sound wave?

    -The center line in the oscilloscope graph represents the equilibrium or undisturbed position of an air molecule, which is the point around which the molecule oscillates.

  • How does the amplitude of a sound wave relate to its loudness?

    -The amplitude of a sound wave, which is the maximum displacement of an air molecule from its equilibrium position, is directly related to the loudness of the sound. A larger amplitude results in a louder sound.

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

    -The period of a sound wave is the time it takes for an air molecule to complete one full oscillation back and forth. It is measured in seconds and is represented by the letter capital T.

  • How does the frequency of a sound wave differ from its period?

    -The frequency of a sound wave is the number of oscillations per second and is defined as one over the period. It is measured in hertz (Hz), whereas the period is the time taken for one complete oscillation and is measured in seconds.

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

    -The frequency of a sound wave is directly related to the pitch we perceive. Higher frequencies result in higher pitches, and lower frequencies result in lower pitches.

  • What is the range of frequencies that humans can typically hear?

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

  • What is the frequency of an A note that is commonly used as a reference in music?

    -The frequency of an A note, often used as a reference in music, is 440 hertz.

  • What is the concept of wavelength in the context of sound waves?

    -The wavelength of a sound wave is the distance between two compressed regions of air, or the distance the wave travels during one complete oscillation of an air molecule.

  • How is the wavelength of a sound wave different from its period?

    -The wavelength of a sound wave is a measure of distance between compressed regions in space, whereas the period is a measure of time for an air molecule to oscillate back and forth once.

  • Why might two different graphs be used to represent a sound wave, and what do they represent?

    -Two different graphs can represent a sound wave to show different aspects: a displacement versus time graph shows the motion of a specific air molecule over time, with intervals between peaks representing the period; a displacement versus position graph shows a snapshot of the displacement of all air molecules along the wave at a moment in time, with intervals between peaks representing the wavelength.

Outlines
00:00
🔊 Understanding Sound Waves

This paragraph explains the concept of sound waves, both audibly and visually. It starts with a speaker humming to demonstrate sound, then describes how air molecules move in a sound wave, resembling a sine or cosine wave. The speaker uses an oscilloscope to visually represent the sound wave, with the horizontal axis representing time and the vertical axis showing the displacement of air molecules. The amplitude, which is the maximum displacement, and the period, which is the time for a complete oscillation, are introduced. The paragraph also explains the relationship between frequency, measured in hertz, and the pitch of a sound, noting that humans can hear within a specific range of frequencies. The concept of wavelength is introduced as the distance between compressed regions of air, and the distinction between period and wavelength is clarified.

05:01
🌌 Wavelength and Displacement Graphs

This paragraph delves into the representation of sound waves through graphs, focusing on the difference between displacement versus time and displacement versus position. It clarifies that a displacement versus time graph illustrates an individual air molecule's movement over time, with intervals between peaks indicating the period of the wave. Conversely, a displacement versus position graph captures a snapshot of all air molecules' displacement at a given moment, with intervals between peaks representing the wavelength. The paragraph emphasizes the importance of distinguishing between these two types of graphs to avoid confusion between period and wavelength.

Mindmap
Keywords
💡Sound Wave
A sound wave is a longitudinal wave that travels through a medium, such as air, causing the particles of the medium to oscillate back and forth. In the video, the sound wave is the central theme, with the speaker demonstrating how it can be visualized using an oscilloscope, which produces a graph that represents the movement of air molecules as the sound wave travels through them.
💡Oscilloscope
An oscilloscope is a device used to display and analyze the waveform of electronic signals. In the context of the video, it is used to visually represent the sound wave by showing the pattern of air molecule displacement over time. The speaker hooks up a speaker to an oscilloscope to demonstrate the visual representation of a sound wave.
💡Amplitude
Amplitude refers to the maximum displacement of a point from its equilibrium position in a sound wave. It is a measure of the loudness of the sound; the greater the amplitude, the louder the sound. In the script, the speaker clarifies that amplitude is not the total length of displacement but the maximum distance the air molecule moves from its undisturbed position.
💡Period
The period of a sound wave is the time it takes for a particle in the medium to complete one full cycle of oscillation. It is measured in seconds and is inversely related to the pitch of the sound; a shorter period results in a higher pitch. The script explains that decreasing the period makes the sound molecules oscillate faster, leading to a higher perceived note.
💡Frequency
Frequency is the number of oscillations that occur per second in a wave and is measured in hertz (Hz). It is directly related to the pitch of the sound, with higher frequencies corresponding to higher pitches. The video script uses the example of an A note oscillating at 440 times per second, which equates to a frequency of 440 Hz.
💡Wavelength
Wavelength is the distance between two consecutive points in a wave that are in the same phase, such as two consecutive compressions. It is measured in meters and is an essential concept in the video script, where it is explained that the distance between compressed regions of air represents the wavelength of the sound wave.
💡Equilibrium Position
The equilibrium position is the undisturbed or natural position of a particle in a medium before the wave passes through. In the context of the video, the speaker describes how air molecules are displaced from their equilibrium position as the sound wave travels, and this displacement is visualized on the oscilloscope.
💡Cycle
A cycle in the context of sound waves refers to one complete back-and-forth oscillation of a particle in the medium. The script mentions that the period is the time it takes for an air molecule to complete one cycle, and this concept is crucial for understanding the relationship between period and frequency.
💡Hertz
Hertz is the unit of measurement for frequency, representing the number of cycles per second. The term is used in the script to describe the frequency of sounds, such as the 440 Hz for an A note, and to explain that humans can typically hear frequencies ranging from 20 Hz to 20,000 Hz.
💡Pitch
Pitch is the perceptual property of a sound that allows its frequency to be identified as high or low. The script explains that the pitch of a sound is affected by the frequency of the sound wave, with higher frequencies resulting in higher pitches and vice versa.
💡Displacement
Displacement in the context of sound waves refers to the movement of particles in the medium away from their equilibrium position as the wave passes through. The video script describes how the oscilloscope can display the displacement of air molecules over time, which helps visualize the sound wave's amplitude and waveform.
Highlights

A sound wave's movement can be visually represented by a graph from an oscilloscope, showing the air molecule's back and forth motion similar to a sine or cosine wave.

The horizontal axis of the graph represents time, while the vertical axis shows the displacement of the air molecule from its equilibrium position.

The amplitude of a sound wave is the maximum displacement of an air molecule from its undisturbed position, which correlates with the sound's loudness.

The period of a sound wave is the time it takes for an air molecule to complete one full oscillation, measured in seconds.

Frequency is the reciprocal of the period, representing the number of oscillations per second, with units in hertz.

Higher frequency sounds correspond to higher notes, while lower frequencies produce lower notes.

The human audible frequency range is from about 20 hertz to 20,000 hertz.

Dogs can hear frequencies up to at least 40,000 hertz, higher than the human range.

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

The period and wavelength are often confused; the period is a time measure, while the wavelength is a spatial measure.

A displacement versus time graph for a sound wave shows the motion of a specific air molecule over time, with intervals between peaks indicating the period.

A displacement versus position graph captures a snapshot of air molecule displacements along the wave at a moment in time, with intervals between peaks indicating the wavelength.

Understanding the relationship between amplitude, period, frequency, and wavelength is crucial for analyzing sound waves.

The speaker demonstrates how increasing the volume of a sound wave increases its amplitude, making the sound louder.

The A note at 440 hertz is used as an example to illustrate the concept of frequency and its relation to the perceived pitch.

The speaker clarifies the difference between the period as time for one oscillation and the wavelength as the distance between compressed air regions.

An oscilloscope provides a visual representation of sound waves, aiding in the understanding of their properties.

Transcripts

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Sound WavesOscilloscopeAmplitudeFrequencyWavelengthAudio VisualAcousticsHertz ScaleHuman HearingDog HearingWave Displacement