Course:PHYS341/2018/Calendar/Lecture08
Phys341 Lecture 08: Summary and web references
2018.01.19
Textbook: - 7.1-7.5
Slide List
- 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
- Two point sources
- Two point sources in and out of phase
- 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
- 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
- Sound Waves approaching an aperture
- Sound Waves striking a barrier
- Directionality of a loudspeaker (or a musical instrument)
- “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).