Holt Physics Chapter 13 Sound

13-1 Production of Sound Waves Produced by vibrations of matter. (SciLinks: HF2131, Sound) Compression: the region of a longitudinal wave in which the density and pressure are greater than normal. Rarefaction: the region of a longitudinal wave in which the density and pressure are less than normal.

13-1 Types of Sound Waves Audible: 20 – 20,000 Hz Infrasonic: < 20 Hz Ultrasonic: > 20,000 Hz

13-1 Characteristics of Sound Waves Pitch: the perceived highness or lowness of a sound, depending on the frequency of the sound wave. Speed depends on medium

13-1 Characteristics of Sound Waves Sound waves propagate in three dimensions.

13-1 The Doppler Effect A frequency shift that is the result of relative motion between the source of waves and an observer. (SciLinks: HF2133, Doppler Effect) f’ = f (v ± vo) / (v ± vs) Check out Holt Physics Interactive Tutor CD-ROM Module 13: Doppler Effect

13-1 Review | Assignment Dolphins can produce sound waves with frequencies ranging from 0.25 kHz to 220 kHz, but only those at the upper end of this spectrum are used in echolocation. Explain why high-frequency waves work better than low-frequency waves. The following figure is a diagram of the Doppler effect in a ripple tank. In which direction is the source of these ripple waves moving? If the source of the waves in the same figure is stationary, which way must the ripple tank be moving?

13-2 Sound Intensity The rate at which energy flows through a unit area perpendicular to the direction of wave motion. Intensity = (E/ t) area = P/area = P/4r2 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. P = 0.20 W R = 3.2 m Intensity = ? I = P/4r2 = (0.20 W)/(4 )(3.2m)2 = 1.6 x 10-3W m-2

13-2 Range of Human Hearing Determined by Intensity Frequency Human hearing depends on both the frequency and the intensity of sound waves. Sounds in the middle of the spectrum of frequencies can be heard more easily (at lower intensities) than those at lower and higher frequencies.

13-2 Decibel Level (Relative Intensity) Relative intensity, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing. 10 dB = 2 x loudness

13-2 Resonance A condition that exists when the frequency of a force applied to a system matches the natural frequency of vibration of the system. (SciLinks: HF2134, Resonance)

13-2 Human Ear Vibrations are transmitted through the ear and converted to nerve impulses.

13-2 Review | Assignment Which of the following factors change when a sound gets louder? Which change when a pitch gets higher? Intensity Speed of the sound wave Frequency Decibel level Wavelength Amplitude A tuning fork consists of two metal prongs that vibrate at a single frequency when struck lightly. What will happen if a vibrating tuning fork is placed near another tuning fork of the same frequency? Explain.

13-3 Fundamental Frequency The lowest frequency of vibration of a standing wave. Strings: f1 = v/1 = v/2L

13-3 Harmonic Series A series of frequencies that includes the fundamental frequency and integral multiples of the fundamental frequency. (SciLinks: HF2135, Harmonics) fn = n(v/2L)

13-3 Open Pipe Harmonics All harmonics are present. fn = n(v/2L)

13-3 Closed Pipe Harmonics Only odd harmonics are present. fn = n(v/4L) n = 1, 3, 5, etc. 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-1 for both of these situations. L = 2.45 m V = 345 m/s fn = n(v/2L) f1 = 1(v/2L)= (345 m/s)/((2)(2.45m) = 70.4 Hz f2 = 2(v/2L) = (2)(345 m/s)/((2)(2.45m) = 141 Hz f3= 3(v/2L) = (3)(345 m/s)/((2)(2.45m) = 211 Hz fn = n(v/4L) f1 = 1(v/4L) = (1) (345 m/s)/((4)(2.45 m) = 35.2 Hz f2 = 2(v/4L) = (2) (345 m/s)/((4)(2.45 m) = 106 Hz f3 = 3(v/4L) = (3) (345 m/s)/((4)(2.45 m) = 176 Hz

13-3 Timbre The quality of a steady musical sound that is the result of a mixture of harmonics present at different intensities. (SciLinks: HF2132, Acoustics)

13-3 Beats Interference of waves of slightly different frequencies traveling in the same direction, perceived as a variation in loudness. Beat Frequency = difference in frequency between the two sounds.

13-3 Review | Assignment On a piano, the note middle C has a fundamental frequency of 264 Hz. What is the second harmonic of this note? If the piano wire is 66.0 cm long, what is the speed of waves on this wire? A piano tuner using a 392 Hz tuning fork to tune the wire for G-natural hears four beats per second. What are the two possible frequencies of vibration of this piano wire? Which of the following must be different for a trumpet and a banjo when notes are being played by both at the same fundamental frequency? Wavelength in air of the ifrst harmonic Number of harmonics present Intensity of each harmonic Speed of sound in air