Auditory Theory: Acoustics

Lecture 004 Sound III

Reading Assignment for Lecture 005

Before next class please read Section

• 1.6.n Time and Frequency Domains and Section
• 1.7.n Analyzing Spectra

Pages 51 to 63 of Acoustics and Psychoacoustics. We may have a brief quiz on these sections at the beginning of the next class.

Brain Bullets

• Superposition
  • When sounds destructively interfere with each other they do not disappear. Instead they travel through each other

• Refraction
  • Sound bends towards the slower medium

• Absorption
  • Sound is absorbed when it interacts with any physical object.

• Reflection from hard boundaries
  • Reflection from a solid boundary results in a reflected wave that has the same polarity as the incoming wave.

• Reflection from bounded to unbounded boundaries
  • Reflection from a bounded to an unbounded boundary results in a reflected wave that has the opposite polarity as the incoming wave.

• Sound interference
  • Correlated sounds will cause interference in a room

• Standing waves at hard boundaries
  • Standing waves occur when sound waves bounce between reflecting surfaces.
  • f_lowest = v/2L

• Standing waves in a pipe open at both ends
  • Can have a node or an antinode at each end

• Standing waves in a pipe closed at one end
  • Will have a node in one end and an anti node in the other
  • f_lowest= ((2N+1) v)/4L

• Diffraction
  • Low frequencies turn corners while high frequencies don't.

• Sound Scattering
  • If the size of the object struck by a sound wave is significantly larger than the wavelengtgh of the sound then reflection will occur, however if the size of the object is smaller than the wavelength of the sound then some scattering will occur from the smaller object.