Course:PHYS341/2018/Calendar/Lecture 16

Phys341 Lecture 16: Summary and web references

2018.02.07

Textbook 12.1-12.3

Slide List

1. Quiz - A pipe 34.4 cm long plays at what frequencies (approximately)?
   • 1000, 2000, 3000 Hz... with open ends; 500, 1500, 2500 Hz... with one closed end
   • 500 , 1500, 2500 Hz... with open ends; 1000, 2000, 3000 Hz... with one closed end
   • 500, 1000, 1500 Hz... with open ends; 1000, 3000, 5000 Hz... with one closed end
   • 500, 1000, 1500 Hz... with open ends; 250, 750, 1250 Hz... with one closed end
   • 250, 750, 1250 Hz... with open ends; 500, 1000, 1500 Hz... with one closed end
   • 250, 500, 750 Hz... with open ends; 500, 1500, 2500 Hz... with one closed end
2. The fundamental frequency of the acrylic tube used in class is 215 Hz
   • = (344 m/s)/(2)(215 Hz) = 0.80 m, 80 cm
   • But: the tube is only 74 cm long.
   • The standing wave is leaking beyond the confines of the tube (3 cm at either end).
   • The wavelength is longer than the simple theory predicts.
   • This is why we can hear sound radiation from an open tube.
3. Radiation from the end of a pipe or cavity
4. With an open end, sound is radiated to the environment: we have the makings of a musical instrument.
5. Edge tones (a.k.a. air reeds)
   • Modes in real air-reed instruments
   • You cannot drive a standing wave precisely at a node.
   • Wavelengths in a real pipe driven by blowing are longer than the simple theory predicts.
   • Fundamental frequency also varies with how hard you blow (pitch “bending”).
   • Karman vortex street (ignore math, look at images and videos) https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street