20 Physical Science Properties of Waves Notes 3 Chapter 12

Demo review constructive interference then tuning fork demonstration p.339

Demo beat frequency using audacity or tuning forks (old school) refer to p. 342 for explanation (formula is not required by curriculum but the simplicity is interesting)

Example a 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

Modes of Vibration

Illustration tune 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?

Demo standing 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.

Illustration p.348 - the “sound print” idea

Demo microphone hooked through oscilloscope & compare tuning fork and guitar patterns.

Mechanical Resonance

Illustration How 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 China my 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 on youtube

Resonance in Air Columns

Demo wine glass then gulf tube in graduated cylinder stress that as of yet it is only called resonance because sound is being amplified

There are two types of air colunms open and closed.

I) Closed Air Columns A 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 resonate 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.

Practice p. 351 # 1 - 3 p. 367 # 47 - 51

II) Open Air Columns An 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.

Practice p. 352 # 1 - 2 p. 367 # 52 - 57

Flight Related Topics

Lesson Mach 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 object note units must be the same speed of sound

Practice p. 317 (bottom) # 1,2 p. 335 #17

Demo So why is mach # important? Youtube simulation Doppler effect and then as this intereference phenomenon accounts for sound barriers and sonic booms

Doppler Effect

The apparent shift in the frequency of a sound due to the relative motion of the source

The formula is only for enrichment – not required by curriculum

f2 = f1 vs 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 # 59, 58, & 60