Resistive Forces Sound 4-5

Resistive Forces Sound Notes

  • It is highly recommended that you take notes and mark up these slides.

  • This information is found on pages 493-499, 513-519 in your textbook.

Sound Waves

  • Definition of Sound Waves

    • Sound is a wave that travels through air and other substances.

    • Sound possesses characteristics common to all waves:

    • Frequency: The number of oscillations per unit of time.

    • Period: The duration of one complete cycle of the wave.

    • Wavelength: The distance between successive peaks of the wave.

    • Wave Speed: The speed at which the wave propagates through a medium.

  • Demonstration of Longitudinal Waves

    • If one end of a coiled spring is oscillated back and forth, a longitudinal wave is generated and moves away from the source.

Vibrating Sources of Sound

  • Vibrating Tuning Fork:

    • When a tuning fork vibrates, it produces sound waves as its tines oscillate back and forth.

    • Just like in a coiled spring, a wave travels away from the vibrating source.

Formation of Sound Waves

  • Creating Sound Waves:

    • A sound wave is formed when an oscillating object creates alternating regions of compressed and expanded air.

    • These regions propagate as a longitudinal wave away from the source.

  • Comparison to Waves on a String:

    • Although sound waves may appear different from waves on a string, plotting the corresponding quantities reveals a classic wave shape.

Characteristic Shapes of Sound Waves

  • Compressions and Rarefactions:

    • Figure (a) shows the compressions (areas of high pressure) and rarefactions (areas of low pressure) of a typical sound wave.

    • Figure (b) depicts a plot of the corresponding density of air, which oscillates similarly to a wave on a string.

Speed of Sound

  • Properties Determining Speed:

    • The speed of sound is influenced by the medium through which it travels.

    • Under normal atmospheric conditions, the speed of sound in air is:

    • 343 m/s

    • This speed is facilitated by the rapid motion of air molecules.

  • Effect of Temperature on Speed:

    • As air is heated, the average speed of molecular motion increases, thereby increasing the speed of sound.

Medium Dependence of Speed of Sound

  • Medium Characteristics:

    • The speed of sound is related to the material's stiffness and compressibility.

    • Air is relatively compressible and not very stiff, resulting in a lower speed of sound compared to liquids and solids.

  • Speed in Different Substances:

    • Fresh water: Approximately 4 times faster than in air.

    • Steel: Approximately 17 times faster than in air.

    • Example speeds:

    • Water: ~1482 m/s (at 20 °C)

    • Steel: ~5960 m/s

Speed of Sound in Various Materials

  • Table 14.1: Speed of Sound in Various Materials

    • Aluminum: 6420 m/s

    • Granite: 6000 m/s

    • Steel: 5960 m/s

    • Pyrex glass: 5640 m/s

    • Copper: 5010 m/s

    • Plastic: 2680 m/s

    • Fresh water (20 °C): 1482 m/s

    • Fresh water (0 °C): 1402 m/s

    • Hydrogen (0 °C): 1284 m/s

    • Helium (0 °C): 965 m/s

    • Air (20 °C): 343 m/s

    • Air (0 °C): 331 m/s

Wave Speed Characteristics

  • Direction and Frequency:

    • The speed of sound remains constant in all directions and for all frequencies, exemplified by listening to multiple instruments in an orchestra simultaneously.

Hearing Sound Waves

  • Human Hearing Range:

    • The human ear detects a limited range of sound waves:

    • Generally between 20 Hz (low frequency) and 20,000 Hz (high frequency).

    • Infrasonic: Frequencies below 20 Hz.

    • Ultrasonic: Frequencies above 20,000 Hz.

Natural Occurrence of Sound Frequencies

  • Examples of Infrasound and Ultrasound:

    • Bats and dolphins use ultrasound for echolocation, sending out high-frequency sounds that bounce back, allowing them to navigate and hunt.

    • Elephants communicate using infrasound, producing sounds as low as 15 Hz.

Applications of Ultrasound

  • Medical Uses of Ultrasound:

    • Commonly used for fetal imaging during pregnancy.

    • Ultrasound scans involve sending bursts into the body and measuring echoes to map internal structures.

Sound Wave Interference and Beats

  • Producing Beats:

    • When two guitar strings with slightly different frequencies are plucked, the resultant sound varies in loudness, alternating between increased and decreased volume.

    • This variation in sound intensity is referred to as beats.

Understanding Beats

  • Constructive and Destructive Interference:

    • Two waves can interfere constructively (causing increased amplitude and loudness) or destructively (resulting in zero amplitude and silence).

  • Beat Frequency:

    • The frequency of this loud-soft pattern is defined as the beat frequency:

    • fextbeat=∣f1−f2∣f_{ ext{beat}} = |f_1 - f_2|

    • Example: If two strings vibrate at 438 Hz and 442 Hz, the beat frequency would be 4 Hz.

  • Tuning Musical Instruments:

    • Musicians often use beats to tune instruments by adjusting tension until the beat frequency approaches zero.

Perception of Sound

  • Energy Carried by Waves:

    • Waves carry energy, and the amount of energy that passes through a certain area per unit of time defines the sound's intensity (I).

    • Example demonstrated in related figures.

Intensity and Loudness

  • Definition of Sound Intensity:

    • The frequency of the sound wave determines its pitch.

    • The amplitude of the wave determines its intensity:

    • Amplitude: Maximum difference in pressure between compressed and expanded areas of the wave.

    • Greater amplitude results in louder sounds.

Sound Intensity Examples

  • Table 14.2 Sound Intensities

    • Loudest sound in a lab: 10^9 W/m²

    • Saturn V rocket at 50 m: 10^8 W/m²

    • Rupture of eardrum: 10^4 W/m²

    • Jet engine at 50 m: 10^10 W/m²

    • Threshold of pain: 1 W/m²

    • Rock concert: 10^{-1} W/m²

    • Jackhammer at 1 m: 10^{-3} W/m²

    • Heavy street traffic: 10^{-5} W/m²

    • Conversation at 1 m: 10^{-6} W/m²

    • Classroom: 10^{-7} W/m²

    • Whisper at 1 m: 10^{-10} W/m²

    • Normal breathing: 10^{-11} W/m²

    • Threshold of human hearing: 10^{-12} W/m²

Sound Intensity and Distance

  • Distance Effect on Sound Intensity:

    • Sound intensity diminishes as it spreads over an area; consequently, loudness reduces with distance.

    • This principle illustrated with examples of sound sources and observers.

Human Sensitivity to Sound

  • Range of Human Hearing:

    • Humans can detect sound intensities a million times lower than typical conversation levels and tolerable limits significantly louder than conversations without pain.

    • Demonstration of sensitivity, e.g., a faint sound of intensity 10^{-11} W/m² creates molecular displacement of approximately 10^{-10} m, comparable to atomic diameters.

Perception and Intensity Correlation

  • Nonlinear Perception of Loudness:

    • The human ear perceives loudness in a nonlinear manner; doubling loudness requires a tenfold increase in intensity.

    • Example to illustrate intensity relation between sounds of differing loudness:

    • The first sound has intensity I, the second sound being 10 times that, and a third sound, twice as loud as the second, has an intensity 100I.

Measurement of Loudness

  • Bel and Decibel:

    • Loudness is expressed in bels, but mostly measured in decibels (dB), where:

    • 0 dB is the threshold of human hearing.

    • Doubling a sound's loudness results in a 10 dB increase in intensity.

Sound Levels in Decibels

  • Decibel Levels for Various Sounds:

    • Eardrum ruptures: 160 dB

    • Jet taking off: 130 dB

    • Loud rock band: 110 dB

    • Subway: 100 dB

    • Heavy traffic: 70 dB

    • Conversation: 60 dB

    • Classroom: 50 dB

    • Whisper: 20 dB

    • Threshold of hearing: 0 dB

Reflection on Learning

  • Self-assessment: Questions to consider:

    • What did you already know?

    • What did you learn?

    • What was easy?

    • What was hard?