Interference of Sound Waves
Interference of Sound Waves
Demo interference and tuning fork demonstration p.339
Demobeat frequency
use the two signal generators hooked up to the ghetto blaster
refer to p. 342 for explanation
Examplea tuning fork with a frequency of 440 hz is sounded at the same time as a piano key with a frequency of 444 Hz. What is the beat frequency ? A: 4 Hz
Practicep. 343 # 1,2
p. 366 # 26 - 29(# 30 - 32 for more of a challenge)
Demobeat demonstration using OHP
Factors effecting the Frequency of Vibrating Strings
LengthL
TensionT
diameterd
density(or mass per unit length)
Experimentthe effect of length on frequency
done as a demonstration
using guitar , signal generator, ruler and graphical analysis
extension- show that f1L1 = constant and
f1L1 = f2 L2
ExampleA guitar string that is 80 cm long vibrates at 256 Hz. What would be its frequency of vibration if the string were shortened to 40 cm? A: 512 Hz
Summary
f 1/Lf1 L1 = f2L2
f 1/d f1d1 = f2d2
- note error in text p.344
Practicep.345 #1,2p. 367 # 35 - 40
p. 346 # 3 - 4 p. 367 #41 - 43 ( more Challenging i.e 2 variables changed )
Quizvibrating strings (on File - “strings Quiz”)
Modes of Vibration
Illustrationtune the signal generator (hooked through the ghetto blaster) to the same pitch as a guitar string. Ask a student to turn around while you pluck the string. Can the student tell the difference? How? What distinguishes the two sound waves?
Demostanding wave with a spring. Students imagine that the spring is a large guitar string. Emphasize different modes. Summarize on the board including overtones, harmonics and the relationships between the wavelength and the frequency.
Illustrationp.348 - the “sound print” idea
Demomicrophone hooked through oscilloscope & compare tuning fork and guitar patterns.
Mechanical Resonance
IllustrationHow many people have been stuck in the snow? How do you get out? Emphasize how natural it is.
Other examples: a child on a swing,dribbling a basketball
truck and Chinamy car’s rattle
motorcycle (speed wobble)
Hula - hoop
my producing a standing wave in a spring
The pendulum apparatus
Resonance- the response of an object (that is free to vibrate) to a force applied with the same frequency as the natural frequency as that object.
Tacoma Narrows- film if available
- see p. 349
- soldiers breaking step.
Resonance in Air Columns
Demowine glass then gulf tube in graduated cylinder
stress that as of yet it is only called resonance because sound is being amplified
Activityresonance in closed air columns
Prelab includes:
- define Resonant length (RL)
- specify that there is more than one RL for any one frequency - demo
- call shortest RL1 and so on
- indicate the variables involved RL,f,T
- data table (for fixed T) includes f, , RL1, RL2,RL3,RL1/,RL2/ etc (in practice , just find RL1, RL1/ for all)
- use the very large plastic graduated cylinders
- postlab - do it over as a demo with fixed values (in notes)
There are two types of air colunms open and closed.
I)Closed Air ColumnsA closed air colunm is an air column that is closed at one end and open at the other end. For a source of sound of a given frequency, resonance occurs (that is the sound is made louder) when the length of the air column (the “resonant length”, RL) is a certain length (or lengths because there can be more than one length for which resonance occurs). This length must be such that a node forms at one end and an antinode at the other. The length of a closed air column(s) is given by:
For a given resonant length, there can be several frequencies (the fundamental and overtones) that will
Demo- using a big graduated cylinder of a given length students can now predict what frequencies will result in resonance. Follow up with the solution on the board for those who have difficulty.
Practicep. 351 # 1 - 3
p. 367 # 47 - 51
II)Open Air ColumnsAn open air column is an air column that is open at both ends. For a source of sound of a given frequency, resonance occurs (that is the sound is made louder) when the length of the air column (the “resonant length”, RL) is a certain length (or lengths because there can be more than one length for which resonance occurs). This length must be a multiple of half the wavelength. The length of a resonating open air column is given by:
For a given resonant length, there can be several frequencies (the fundamental and overtones) that will result in resonance.
Demo using a hollow tube of a given length students can now predict what frequencies will result in resonance. Follow up with the solution on the board for those who have difficulty.
Practicep. 352 # 1 - 2
p. 367 # 52 - 57
Quiz
Flight Related Topics
LessonMach number is the ratio of the speed of an object to the speed of sound. Recall the speed of sound depends on the temperature.
Mach (M) = speed of objectnote units must be the same
speed of sound
Practicep. 317 (bottom) # 1,2
p. 335 #17
Demo transparent circles on OHP
- imagine a plane with a horn
- first stationary souce
- than sub-sonic source
- than trans-sonic
- concept - compressions superimpose in front of the aircraft (constructive interference)
- now supersonic - BOOMs - first parallel with a boat in water
Doppler Effect
The apparent shift in the frequency of a sound due to the relative motion of the source
Demo transparent circles on OHP
- this time with a car with a horn
- note and \ frequency in front and behind the car
- formula derivation included in notes
f2 = f1vs where f2 = the apparent frequency
(vs + vo) f1 = the actual frequency
vs = the speed of sound
vo = the speed of the object
Example
Practice- p. 368 # 58 - 60
Test