Course:PHYS341/2018/Calendar/Lecture08

Phys341 Lecture 08: Summary and web references

2018.01.19

Textbook: - 7.1-7.5

Slide List

1. Interference
   • A sound wave consists of successive crests and troughs of pressure
   • In general sources of sound are not point-like
   • Sound from different sources will result in a crest and troughs arriving at a point in space at the same time as crests and troughs from another
   • Two crests will make a bigger crest; two troughs will make a bigger trough
   • A crest and a trough will at least partially cancel each other
   • If the crest and trough are of equal magnitude, they will completely cancel each other out
2. Two point sources
3. Two point sources in and out of phase
4. Why sound does not behave like light
   • Light goes in straight lines, and can be blocked by solid objects
   • Sound goes around corners, and cannot be blocked by solid objects, unless they are very big and very solid
   • Sound and light are both waves, so why the difference?
   • Its all to do with wavelength
   • Light has a wavelength of ~ ½ μm (1/2000th of a mm), i.e. very small compared to everyday objects
   • Sound has wavelength of a few cm to a few m, i.e. similar to everyday objects
5. Diffraction https://www.youtube.com/watch?v=BH0NfVUTWG4
   • Light can be considered as rays, so long as it is interacting with objects much bigger than its wavelength (like us, for example)
   • When any wave encounters an object of similar or smaller size than its own wavelength, diffraction effects kick in – the waves bend around corners
6. Sound Waves approaching an aperture
7. Sound Waves striking a barrier
8. Directionality of a loudspeaker (or a musical instrument)
9. “Flashing brilliance” of a violin
   • Low notes (wavelength longer than the size of the instrument) emitted isotropically (i.e. in all directions equally).
   • High notes (wavelength shorter than the size of the instrument) emitted only certain directions.
   • Destructive interference in some directions, constructive in others.
   • In a concert hall you may only hear some high notes if they are reflected off one the walls or ceiling.
   • Thus different high notes sound as if they coming from different directions.
   • Rapid scales seem to come “madly in from all directions” (to misquote Stephen Leacock).