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Physics Chapter 12


Tim DeKoninck

on 12 August 2010

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Transcript of Physics Chapter 12

Sound What to Expect In this chapter, you will study many physical aspects of sound, including the nature of sound waves, frequency, intensity, resonance, and harmonics. Sound Waves Sound Intensity The Production of Sound Waves Frequency determines pitch Sound waves are longitudinal compression the region of a longitudinal wave in which the density and pressure are at a maximum Problems What is the intensity of the sound waves produced by a trumpet at a distance of 3.2 m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical. 1. Calculate the intensity of the sound waves from an electric guitar’s amplifier at a distance of 5.0 m when its power output is equal to each of the following values:
a. 0.25 W
b. 0.50 W
c. 2.0 W 2. At a maximum level of loudness, the power output of a 75-piece orchestra radiated as sound is 70.0 W.What is the intensity of these sound waves to a listener who is sitting 25.0 m from the orchestra? 3. If the intensity of a person’s voice is 4.6 × 10^−7 W/m^2 at a distance of 2.0 m, how much sound power does that person generate? 4. How much power is radiated as sound from a band whose intensity is 1.6 × 10^−3 W/m^2 at a distance of 15 m? Physics Chapter 12 Intensity is the rate of energy flow through a given area Intensity and frequency determine which sounds are audible Characteristics of Sound Waves the region of a longitudinal wave in which the density and pressure are at a minimum rarefaction This example disregards energy losses that would decrease the wave amplitude Note: frequency is defined as the number of cycles per unit of time frequency from the last chapter sound waves that the average human ear can hear audible sound waves Usually between 20 and 20, 000 Hz. <20 Hz are called infrasonic waves >20,000 Hz are called ultrasonic waves Extra: a measure of how high or low a
sound is perceived to be, depending on the frequency of the sound wave Pitch the speed of a wave depends on how quickly one particle can transfer its motion to another particle Speed of sound depends on the medium solids liquids gases The speed of sound also depends on the temperature of the medium. disturbance can spread faster at higher temperatures than at lower temperatures In gas not as noticable in solids and liquids Sound waves propagate in three dimensions Relative motion creates a change in frequency an observed change in frequency when there is relative motion between the source of waves and an observer Doppler effect Relative intensity is measured in decibels Forced Vibrations and Resonance Vibration at the natural frequency produces resonance The human ear transmits vibrations that cause nerve impulses Intensity the rate at which energy flows through a unit area perpendicular to the direction of wave motion SI unit for power is watt (W) Units 5. The power output of a tuba is 0.35 W. At what distance is the sound intensity of the tuba 1.2 × 10−3 W/m^2? Note that some humans can
hear slightly softer sounds, at a frequency of about 3300 Hz Note: The maximum displacement of an air molecule at the threshold of hearing is approximately 1 × 10^−11 m. the diameter of a typical air molecule is about 1 × 10−^10 m Extra: decibel a dimensionless unit that describes the ratio of two intensities of sound; the threshold of hearing is commonly used as the reference intensity forced vibrations sympathetic vibrations. & Vibration at the natural frequency produces resonance resonance a phenomenon that occurs when the frequency of a force applied to a system matches the natural frequency of vibration of the system, resulting in a large amplitude of vibration November 7, 1940
Tacoma Narrows Standing Waves on a Vibrating String speed of a wave equals the frequency times the wavelength Harmonics are integral multiples of the fundamental frequency harmonic series a series of frequencies that includes the fundamental frequency and integral multiples of the fundamental frequency Standing Waves in an Air Column If both ends of a pipe are open, all harmonics are present If one end of a pipe is closed, only odd harmonics are present What are the first three harmonics in a 2.45 m long pipe that is open at both ends? What are the first three harmonics of this pipe when one end of the pipe is closed? Assume that the speed of sound in air is 345 m/s. 2. A flute is essentially a pipe open at both ends. The length of a flute is approximately 66.0 cm.What are the first three harmonics of a flute when all keys are closed, making the vibrating air column approximately equal to the length of the flute? The speed of sound in the flute is 340 m/s. 4. A violin string that is 50.0 cm long has a fundamental frequency of 440 Hz. What is the speed of the waves on this string? 1. What is the fundamental frequency of a 0.20 m long organ pipe that is closed at one end, when the speed of sound in the pipe is 352 m/s? Problems 3. What is the fundamental frequency of a guitar string when the speed of waves on the string is 115 m/s and the effective string lengths are as follows?
a. 70.0 cm b. 50.0 cm c. 40.0 cm Harmonics account for sound quality, or timbre the lowest frequency of vibration of a standing wave fundamental frequency the musical quality of a tone resulting from the combination of harmonics present at different intensities timbre In music, the mixture of harmonics that produces the characteristic sound of an instrument is referred to as the spectrum of the sound. Harmonics account for sound quality, or timbre the frequency of a sound determines its pitch From Section 1... In musical instruments... the fundamental frequency of a vibration typically determines pitch octave Beats Sound waves at slightly different frequencies produce beats beat the periodic variation in the amplitude of a wave that is the superposition of two waves of slightly different frequencies The number of beats per second corresponds to the difference between frequencies Generally speaking the frequency difference between two sounds can be found by the number of beats heard per second
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