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Have you ever been outside when an ambulance is passing by? • Does the pitch of the ambulance sound higher as it approaches you and lower as it moves away? • Why? • http://www.phschool.com/atschool/conceptual_physics/active_art/doppler_effect/index.html
Imagine a bug that is jiggling its legs and bobbing up and down in the middle of a pond λ (wavelength)
Now the bug moves across the water to the right. The wave pattern is no longer concentric, when the bug was at the center of that circle.
This apparent change in frequency due to the motion of the source is called the Doppler effect • Can be observed for Water wave, sound wave and light waves
In this case, the moving ambulance is moving away from the person. The distance between the ambulance and the person increases, the wave fronts reach the person less often – frequency is lower – sound is softer.
With your partner: The fire engine begins moving toward you. Does the frequency that you hear increase, decrease, or stay the same? Does the wavelength that reaches your ear increase, decrease, or stay the same? How about the speed of sound in the air between you and the locomotive?
Light • Doppler effect can occur for light as well. When a light source approaches, there is an increase in its measured frequency, and when it recedes, there is a decrease in its frequency. • Blue shift = increase in frequency because the increase is toward the high-frequency or blue end of the color spectrum • Red shift = decrease in frequency referring to the low frequency end of the color spectrum
Sound Intensity and Loudness • Which creature produces the loudest sound?
Sound Intensity and Loudness • http://www.nidcd.nih.gov/health/education/decibel/pages/decibel.aspx • As sound waves travel farther from their source, the more spread out their energy becomes. In the diagram you can see that as the distance from the sound source increases, the area covered by the sound waves increases. The same amount of energy is spread over a greater area, so the intensity and loudness of the sound is less.
Unit of intensity for sound is the decibel (dB), after Alexander Graham Bell, inventor of the telephone. • Since the range of intensities that can be detected by the human ear is very large, a logarithmic scale, based on powers of ten, is used to measure intensity levels. This intensity level is measured in decibels (dB).
Sound Intensity • Hearing threshold is 0 dB. • Sound intensity increases by 10. • So a sound of 50 dB is 10 times more intense than 40 dB. • If a sound is 50dB it is 100 times more intense than 30 dB.
Practice Questions • How many more times intense is a 60 dB sound than a 40 dB sound? • How many more times intense is a 70 dB sound than a 30 dB sound?
Loudness • Sound intensity is objective, while loudness is a physiological sensation and is sensed in the brain- it can be different for different people. • Loudness is subjective but is related to sound intensity
Natural Frequency • If I was to drop a baseball bat on the floor and then a wrench do you think the sounds would be the same or different? • Do you know that the sounds of a couple of pennies dropped on a hard surface one dated before 1982 and one after. The old penny is made of 95% copper and 5% zinc, and sounds noticeably different than the newer pure zinc core pennies plated with copper.
Natural Frequency • When any object composed of elastic material is disturbed it vibrates at its own special set of frequencies, which together form its special sound. • An object’s natural frequency the frequency at which an object vibrates when it is disturbed. An object’s natural frequency depends on the elasticity and shape of the object. Natural frequency is one at which minimum energy is require to produce forced vibrations requires least amount of energy to continue this vibration
Homework • Read Chapter 26.8-26.10 • Complete questions on Homework sheet due Wednesday 12 March