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3/20/15 Announcements • • • • Today: Sound and waves Wednesday: MT #2! Friday: More Sound Reading after Midterm: CH 21, focus on differences between pitch, intensity, and loudness Main Points • Properties of waves (wavelength, frequency, amplitude, speed) • Wave interference • Doppler Effect • Properties of sound (speed, pitch, etc.) • Musical sounds: pitch, loudness/intensity, quality Chapters 19-21: Waves and Sound • Chapter 19: Wave Basics • Chapter 20: Sound Waves • Chapter 21: Music vs. Noise Properties of Waves • Wavelength is the distance between two wave peaks • Frequency is the number of times per second that a wave vibrates up and down wave speed = wavelength x frequency More Wave Properties • Amplitude: Distance from midpoint to crest of wave Frequency & Period Frequency = 1/Period Period = 1/Frequency They are inversely related, so they have opposite behavior! • Period: Time to complete one vibration 1 3/20/15 Wavelength and Frequency Examples: Rank these waves from small to large: 1. Wavelength 2. Frequency 3. Period wavelength x frequency = wave speed The Doppler Effect Doppler Effect • Definition: “The change in wavelength of radiation due to relative radial motion between the source and the observer.” Real Life Example of Doppler Effect The change in the pitch of a siren on a police car, fire truck, or ambulance as it zooms past (sound waves) Doppler Effect • When something which is giving off light moves towards or away from you, the wavelength of the emitted light is changed or shifted V=0 Star Light Wave Astronomers deal with the Doppler Effect of light waves 2 3/20/15 Doppler Effect • “Radial” means “along line of sight” • Doppler Effect happens only if the light source is moving towards you or away from you. Conceptual check: When sound waves from a police siren are blueshifted, what property of the sound wave is increasing? A. The amplitude B. The frequency C. The speed D. The wavelength Types of Waves • Transverse Waves: like ocean waves or waves created by shaking a string • Longitudinal Waves: compression waves, such as sound Wave Interference Two types of interference: constructive and destructive. Announcements • Today: Finish Sound, maybe start new unit E&M • Monday: Electricity • Reading: CH 22, focus on electric charges and charging. Standing Waves • When a wave is confined to a small space (such as along a length of string), we can use constructive interference to create a standing wave. Destructive: Constructive: • This works for either transverse or longitudinal waves! 3 3/20/15 Bow Waves and Shock Waves • Produced by objects moving at a greater speed than the waves they are producing. • Bow waves: Boats moving faster than the water waves • Shock waves: Planes or whips moving faster than sound waves. • When the shock wave reaches you, you’ll hear it! Sound is just another wave! • Sound waves bounce off surfaces and can be bent (or refracted) • Sound waves interfere with each other • Sound waves carry energy (although not that much!) Example: Frequency Bats can hear squeaks of very high frequency: up to 120,000 Hz! What is the wavelength of these ultrasonic sound waves? Properties of sound waves: • Human range of hearing is from about 20-20,000 Hz • Frequency ! pitch More on Sound Waves • Most sounds we hear travel through air, but any elastic material can also transmit sound waves! • Speed of sound in air: ~340 m/s • The speed of sound changes when travelling through a different medium (water, steel, etc.) Music vs. Noise • Regular vs. irregular vibrations • Characterized by pitch, loudness, and quality wavelength x frequency = wave speed 4 3/20/15 Sound Intensity • Intensity depends on the amplitude of the wave • The decibel scale (dB) is a logarithmic scale (think like the Richter scale for earthquake intensity) • Careful: Intensity is not the same as loudness! Harmonics Harmonics: tones with a frequency that is a wholenumber multiple of the fundamental (lowest) frequency. Quality of a Sound • Also called timbre or “color” • Quality allows you to distinguish between sounds made by trumpet, piano, violin, etc. • Depends on the harmonics in the sound Musical Sounds Different instruments produce sounds with different harmonics present at different intensities. 5