Discussion “If a tree falls in the forest and there is no one to hear it, does it make a sound?" - what is meant by this question? - is there any one that believes it doesn’t - what's the best answer to this question? - parallel this idea with other phenomena that we perceive (i.e. the color red)
The idea here is that there is more to the real world around us than what we perceive and in fact what we perceive is biased. Afterall we're only human!
Models for Sound
- the slinky but its 1 dimensional - concentric spheres of compressed air separated by spheres of decompressed air (these are slight pressure fluctuations) radiating outward from a source - but this is static
Source of Sound - vibrating objects (demo the pith ball and tuning fork)
Demo amplitude is volume & frequency is pitch using the signal generator hooked up to stereo.
Demo Range of transmission & reception using the signal generator hooked up to a stereo
Demo sound waves in a bell jar SOUND WAVES ARE MECHANICAL
Practice p.335 # 18 - 21 (review of wave equation)
Exercise Using the speed of sound at different temperatures data, graph the speed of sound versus temperature and derive the equation
Observe the speed of sound in other media table p. 317 The speed of sound is finite meaning it takes time for sound waves to travel a given distance. This allows echoes and thus SONAR.
Practice p. 317 #1,2 (S.O.S)
Example What is the temperature if a 440 Hz whistle 1.2 km away is heard 3.4 seconds after it is blown.
Practice p. 334 # 8 - 16 (S.O.S) p. 335 # 22-24
Sonar devices, such as those in depth finders, send sound waves to the bottom of the ocean. The waves reflect and travel back to the source. The Sonar device measures the time the wave takes to complete the round trip. With the speed of sound known, the depth of the ocean can be calculated.
depth = vt/2
Example A boy yells across a canyon and hears his echo 1.6 s later. If the temperature is 25 C, how wide is the canyon?
Exercise Mapping the ocean floor; handout
Activity Investigation 11.1 “speed of sound”
For other applications see p.327
Practice p.328 # 1,2 p.336 # 29-33
Energy - the ability to do work - the unit is Joule (or kgm2/s2 )
Power - the rate at which energy can be used up - the rate at which work is done - the amount of energy used up per unit time
Power = Energy / time
- the unit for power is Watt ( kgm2/s3 )
Illustration If a task can be accomplished in less time than there is more power exhibited. Also compare two cars, two light bulbs and two speakers.
Intensity - the amount of power in a given area - the power per unit area
Intensity = Power / Area
- the unit for intensity is watts / square meter (W/m2 ) - or kg/s3
Conclusion: if the distance is doubled the intensity is 1/4 if the distance is tripled the intensity is 1/9 if the distance is quadrupled the intensity is 1/16
I α 1/r2
Illustration Plot hypothetical data (intensity for various distances) based on a speaker with a power of 3.0 Watts. Plot I vs first r than 1/r, than 1/r2 . Find the slope and come up with an equation relating I and r. Prove that this equation is the same as the one we came up with earlier i.e I = P/A
Example - Assuming that the maximum intensity that the human ear can withstand is 5.0 x 103 W/m2 , what is the maximum power that the ear drum can withstand. Assume that the surface area of the ear drum is 0.40 cm2. Answer 0.20 Watts
The sound energy from 50,000 fans at a football game that lasts 90 min is