2 Tuesday January 31thWAVES, VIBRATIONS, AND SOUND
3 TODAY’S AGENDA UPCOMING… Sound Intensity & Resonance Tuesday, January 31Sound Intensity & ResonanceHw: Practice A (All) p415UPCOMING…Wed: HarmonicsThurs: More on HarmonicsProblem Quiz #2Fri: Inertial Balance Lab
4 ConcepTest 14.7a Sound Bite I 1) the frequency f2) the wavelength l3) the speed of the wave4) both f and l5) both vwave and lWhen a sound wave passes from air into water, what properties of the wave will change?
5 ConcepTest 14.7a Sound Bite I 1) the frequency f2) the wavelength l3) the speed of the wave4) both f and l5) both vwave and lWhen a sound wave passes from air into water, what properties of the wave will change?Wave speed must change (different medium).Frequency does not change (determined by the source).Now, v = fl and since v has changed and f is constantthen l must also change.Follow-up: Does the wave speed increase or decrease in water?
6 ConcepTest 14.7b Sound Bite II We just determined that the wavelength of the sound wave will change when it passes from air into water. How will the wavelength change?1) wavelength will increase2) wavelength will not change3) wavelength will decrease
7 ConcepTest 14.7b Sound Bite II We just determined that the wavelength of the sound wave will change when it passes from air into water. How will the wavelength change?1) wavelength will increase2) wavelength will not change3) wavelength will decreaseThe speed of sound is greater in water, because the force holding the molecules together is greater. This is generally true for liquids, as compared to gases. If the speed is greater and the frequency has not changed (determined by the source), then the wavelength must also have increased (v = fl).
9 Units of Chapter 12 Characteristics of Sound Intensity of Sound: DecibelsThe Ear and Its Response; LoudnessSources of Sound: Vibrating Strings and Air ColumnsQuality of Sound, and Noise; SuperpositionInterference of Sound Waves; BeatsDoppler Effect
10 Units of Chapter 12 Shock Waves and the Sonic Boom Applications: Sonar, Ultrasound, and Medical Imaging
11 Characteristics of Sound Sound can travel through any kind of matter, but not through a vacuum.The speed of sound is different in different materials; in general, it is slowest in gases, faster in liquids, and fastest in solids.The speed depends somewhat on temperature, especially for gases.
12 Characteristics of Sound Loudness: related to intensity of the sound wavePitch: related to frequency.Audible range: about 20 Hz to 20,000 Hz; upper limit decreases with ageUltrasound: above 20,000 Hz; see ultrasonic camera focusing belowInfrasound: below 20 Hz
13 Intensity of Sound: Decibels The intensity of a wave is the energy transported per unit time across a unit area.The human ear can detect sounds with an intensity as low as W/m2 and as high as 1 W/m2.Perceived loudness, however, is not proportional to the intensity.
14 Intensity of Sound: Decibels The loudness of a sound is much more closely related to the logarithm of the intensity.Sound level is measured in decibels (dB) and is defined:(12-1)I0 is taken to be the threshold of hearing:
15 Intensity of Sound: Decibels An increase in sound level of 3 dB, which is a doubling in intensity, is a very small change in loudness.In open areas, theintensity of sound diminishes with distance:However, in enclosed spaces this is complicated by reflections, and if sound travels through air the higher frequencies get preferentially absorbed.
17 The Ear and Its Response; Loudness Outer ear: sound waves travel down the ear canal to the eardrum, which vibrates in responseMiddle ear: hammer, anvil, and stirrup transfer vibrations to inner earInner ear: cochlea transforms vibrational energy to electrical energy and sends signals to the brain
18 The Ear and its Response; Loudness The ear’s sensitivity varies with frequency. These curves translate the intensity into sound level at different frequencies.
19 Sources of Sound: Vibrating Strings and Air ColumnsMusical instruments produce sounds in various ways – vibrating strings, vibrating membranes, vibrating metal or wood shapes, vibrating air columns.The vibration may be started by plucking, striking, bowing, or blowing. The vibrations are transmitted to the air and then to our ears.
20 Sources of Sound: Vibrating Strings and Air ColumnsThe strings on a guitar can be effectively shortened by fingering, raising the fundamental pitch.The pitch of a string of a given length can also be altered by using a string of different density.
21 Sources of Sound: Vibrating Strings and Air ColumnsA piano uses both methods to cover its more than seven-octave range – the lower strings (at bottom) are both much longer and much thicker than the higher ones.
22 Sources of Sound: Vibrating Strings and Air ColumnsWind instruments create sound through standing waves in a column of air.