Physics I Honors 1 Waves and Sound Intensity Doppler Effect

Physics I Honors 2 Trick questions 1.Radio station Kiss broadcasts at 104.5MHz. What wavelength does this require? 2. Kinky Kyle watches waves go by his dock and notices that 12 crests pass the same spot every minute, and that the crests are 6 meters apart. What is the speed of these waves?

Physics I Honors 3 answers 1. Given f = MHz. = Radio waves are _______ waves so they travel at _________ m/s. Given f and v, we find λ by using the ________ equation

Physics I Honors 4 V = f λ λ = v/f = 3 x 10 8 / x 10 6 λ = 2.87 m

Physics I Honors 5 2. wavelength is ________ Frequency is _________ v = f λ f = 12 /min = 0.2/sec v = 0.2 x 6 = 1.2 m/s

Physics I Honors 6 Sound Waves

Physics I Honors 7 Sound The speed of sound in air is 343 m/s at standard temperature and pressure. It increases with increasing temperature. The speed of sound in solids is greater than in gasses. It is greater for stiffer materials. Sound waves originate from vibrating objects: Audible: 20  20,000 Hz Infrasonic: < 20 Hz Ultrasonic: > 20 kHz The pitch of a sound wave is determined by the frequency of the sound.

Physics I Honors 8 Sound Intensity Sound waves carry energy. Intensity (I) is defined as the rate at which energy E flows through a unit area A perpendicular to the direction of travel of the wave. All waves have intensity defined in this way. Intensity is proportional to square of wave amplitude (remember energy in oscillator is square of velocity and square of displacement).

Physics I Honors 9 Human Perception of Sound The loudness of a sound depends on its intensity. A sound perceived as roughly twice as loud as another actually has an intensity that is 10 times greater. We measure loudness by a logarithmic scale of the intensity level of a wave: The number is dimensionless but we give it a name, decibels (dB). 3dB is a factor 2 change in intensity Every 10dB is a factor 10 change in intensity 20 dB is a factor 100 change in intensity

Physics I Honors 10 Human Perception of Sound

Physics I Honors 11 Sound Interference Overlapping compressions of a sound wave will result in… –…constructive interference. –…and a louder sound. Overlapping a compression and a rarefaction results in... –…destructive interference. –…and a softer sound.

Physics I Honors 12 The Compression Wave

Physics I Honors 13 The Doppler Effect The observed frequency of sound changes if there is relative motion between the the observer and the source of the sound wave. Example: If a train is moving away from you, the pitch of the whistle is lower than when it comes toward you. f’ = observed frequency, f = emitted frequency v d = observer speed, v s = source speed v = wave speed + means that the source is moving away from the observer + means that the observer is moving toward from the source

Physics I Honors 14 Stationary Source and Observer Frequency f s Frequency f d V=340m/s f s = f d

Physics I Honors 15 Source and Observer Moving Together f d > f s

Physics I Honors 16 Source and Observer Moving Apart f d < f s

Physics I Honors 17 The Doppler Effect When source and/or detector are moving toward each other: –Distance between waves decreases –Frequency seems to increase When source and/or detector are moving away from each other: –Distance between waves increases –Frequency seems to decrease

Physics I Honors Doppler A: You are driving along the highway at 65 mph, and behind you a police car, also traveling at 65 mph, has its siren turned on. B: You and the police car have both pulled over to the side of the road, but the siren is still turned on. In which case does the frequency of the siren seem higher to you? 1. Case A 2. Case B 3. same vsvs f vovo f’ v

Physics I Honors 19 MORE EXAMPLES Both moving = how many combinations? Left left Left right Right right Right left

Physics I Honors 20 Waves and Sound Resonance Beats

Physics I Honors 21 Resonance... …is the result of forced vibrations in a body when the applied frequency…...matches the natural frequency of the body. A standing wave is established, the amplitudes adding together until the pressure is adequate for humans to hear. The resulting vibration has a high amplitude and could destroy the body that is vibrating.

Physics I Honors 22 Natural Frequency... …the frequency at which an elastic object naturally tends to vibrate. At this frequency, a minimum energy is required to produce a forced vibration. The natural frequency of a body depends on its... –elasticity –size –shape

Physics I Honors 23 Standing Waves The distance between adjacent antinodes is λ/2. The distance between adjacent nodes is λ/2. The distance between a node and an adjacent antinode is λ/4.

Physics I Honors 24 Examples of Resonance mass on a spring at resonance swinging your legs in a swing breaking a wine glass using sound a singing rod caused by forced vibration a tuning fork exciting a guitar string a truck driving on a rough road In 1940, the Tacoma Narrows Bridge was destroyed by wind-generated resonance.

Physics I Honors 25 Open Pipe Resonators Fundamental Two antinodes and one node. = 2L L = ½

Physics I Honors 26 Open Pipe Resonators Second Harmonic Two nodes and three antinodes. = L

Physics I Honors 27 Open Pipe Resonators Third Harmonic Three nodes and four antinodes. = 2L/3 L = 3/2

Physics I Honors 28 Closed Pipe Resonators Fundamental One node and one antinode. L = /4.

Physics I Honors 29 Closed Pipe Resonators Third Harmonic L = 3 /4.

Physics I Honors 30 Closed Pipe Resonators Fifth Harmonic L = 5 /4.

Physics I Honors 31 Beats Waves of slightly different frequencies form a pattern of alternating maximum and minimum amplitude. The packets of maximum amplitude are called beats.

Physics I Honors 32 Beats When two sound sources that have almost the same frequency are sounded together you hear a sound with a frequency that is the average of the two. The variation in the amplitude between these two sounds is called a beat. The beats frequency is simply the difference between the two frequencies.

Physics I Honors 33 Beats Beats - the periodic variation in loudness of two sounds played together Humans can distinguish beat frequencies of seven or less The beat frequency is equal to the difference in the frequency of the two sounds. What is the beat frequency when a 262 Hz and a 266 Hz tuning fork are sounded together?