Physics 101. Lecture 21 Doppler Effect Loudness Human Hearing Interference of Sound Waves Reflection & Refraction of Sound

Transcription

1 Physics 101 Lecture 21 Doppler Effect Loudness Human Hearing Interference of Sound Waves Reflection & Refraction of Sound Quiz: Monday Oct. 18; Chaps. 16,17,18(as covered in class),19 CR/NC Deadline Oct. 19 1/32 Demo: Doppler Shift Hear frequency as higher when whistle is moving towards you and hear it as lower when moving away from you. Summary: Source & observer approaching, f obs >f s Source & observer separating, f obs <f s Higher Frequency Shorter Wavelength Lower Frequency Longer Wavelength 2/32

2 Loudness & Amplitude Loudness depends on amplitude of pressure and density variations in sound waves. 3/32 decibels (db) Loudness of sound depends on the amplitude of pressure variation in the sound wave. Loudness is measured in decibels (db), which is a logarithmic scale (since our perception of loudness varies logarithmically). From the threshold of hearing (0 db) to the threshold of pain (120 db), the pressure amplitude is a million times higher. At the threshold of pain (120 db), the pressure variation is still only about 10 Pascals, which is one ten thousandth of atmospheric pressure. 4/32

3 5/32 Human Hearing 6/32

4 7/32 Hearing Loss The hair cells that line the cochlea are a delicate and vulnerable part of the ear. Repeated or sustained exposure to loud noise destroys the neurons in this region. Once destroyed, the hair cells are not replaced, and the sound frequencies interpreted by them are no longer heard. Hair cells that respond to high frequency sound are very vulnerable to destruction, and loss of these neurons typically produces difficulty understanding human voices. Much of this type of permanent hearing loss is avoidable by reducing exposure, such as to loud music. What? 8/32

5 Audiogram - Chart of Hearing Loss 9/32 Interference of Sound Waves Sound waves from multiple sources can occupy a region of space. The sound waves can interfere constructively in the are in step (in phase) The sound waves interfere destructively if they are out of step (out of phase) 10/32

6 Demo: In & Out of Phase Pair of speakers constructively interfere when they are in phase (oscillating together). When out of phase (reverse wires on one of the speakers) then they destructively interfere. Out of Phase 11/32 Noise-Canceling Headphones Noise-canceling headphones use a microphone that listens for noise and a speaker that produces the same noise but out of phase (cancellation by destructive interference) External Noise Canceling Sound 12/32

7 Speaker Baffle Why are speakers mounted behind a baffle board and inside an enclosure? 13/32 Reflection of Sound Sound reflects strongly from rigid surfaces. Softer surfaces absorb sound. Quiet after a fresh snowfall because the soft, irregular surface of the snow absorbs sound instead of reflecting it. 14/32

8 Check Yourself When crowded, which restaurant will be quieter? 15/32 Reflection of Sound Reflection Process in which sound encountering a surface is returned Often called an echo Multiple reflections called reverberations 16/32

9 Singing in the Shower Multiple reflections from the hard walls create reverberation. Hear your voice from several sources, slightly shifted in time. Reverberation extends each note and smears (smoothens) the pitch. Demos Your voice sounds better when singing in the shower 17/32 Refraction (bending) of Sound Sound speed can vary by material or temperature. Sound moving into a different region will bend in direction, in the same way that light bends when it passes through a glass lens. Fig /32

10 Ultrasound Ultrasound is high frequency (Megahertz), short wavelength (0.1 mm) sound. Reflections and refractions of ultrasound by flesh and bone allow seeing inside the human body. 19/32 Musical Sound A musical note has four characteristics: Duration Loudness Pitch (e.g., Middle C, A above middle C) Timbre or Quality (e.g, piano versus violin) Let s investigate the physical properties underlying these four characteristics. 20/32

11 Pitch & Frequency The faster the vibrations (higher the frequency), the higher the pitch of the musical note produced. There is a direct relationship between the pitch of a note and the frequency of the sound wave. 21/32 Fundamental Frequency Range of Musical Instruments Bass Voice Soprano Voice Piccolo Piano Freq (Hz) Pitch A E C 3 C 4 C 5 C 6 C 7 C 8 Note(!) that going up an octave in pitch doubles frequency 22/32

12 Octave The note produced by two strings, one half the length of the other, sounded similar. In Western music these two notes are said to be an octave apart. The frequencies have a 2:1 ratio. Men and women typically sing an octave apart. C 5 C 4 Sing Some-where Physics 1 (Garcia) over SJSU the rainbow 23/32 Waveforms & Sound Quality The waveform of a sound wave is the pattern of air pressure changes over one cycle Sine Wave: Perceived as a pure tone. Excites only one region of the Basilar Membrane Triangle Wave: Perceived as complex tone. Excites several areas of Basilar Membrane. Can be made by combining sine wave building blocks. 24/32

13 Musical Waveforms Tuning Fork Clarinet Cornet Demonstration Listen to & see some waveforms 25/32 Waveform Spectrum (Harmonics) Complex tones (triangle wave, clarinet waveform) can be made by combining sine waves with different frequencies. The sine wave frequencies used, and the amount of each, determine the spectrum of the waveform Compare to the situation of complex light such as white light, which is made up of a mixture of the pure colors of the rainbow 26/32

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15 Musical Tone Quality (Timbre) Tone quality, or timbre, is determined by the waveform, or alternately, by the spectrum or harmonic content of the sound. 29/32 Harmonics Complex music tones (waveforms) built up from: A fundamental sine wave (frequency f) Harmonics of the fundamental i.e., sine waves whose frequencies are integer multiples of the fundamental frequency f 2 nd harmonic: sine wave of frequency 2f 3 rd harmonic: sine wave of frequency 3f, etc. For example, A above middle C on a cornet has 3 units 440 Hz sine wave (fundamental) 4.5 units 880 Hz sine (2 nd harmonic) 8 units 1320 Hz sine (3 rd harmonic) 3 units 1760 Hz sine (4 th harmonic), etc. 30/32

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17 33/32 Key Points of Lecture 21 Doppler Effect Loudness Human Hearing Interference of Sound Waves Reflection and Refraction of Sound Waves Musical Sounds Before Monday, read Hewitt Chap. 21 (first 6 pages). Homework Assignment #14 is due before 11:00 PM on Friday, Oct. 15. Homework #15 due by 11:00 PM Sunday Oct /32