Auditory Theory: Acoustics

Lecture 005 Sound IV

Reading Assignment for Lecture 006

Before next class please read Sections

• 2.0.n Introduction to hearing,
• 2.1.n The anatomy of the hearing system
• 2.2.n Critical Bands

(pages 65 to 79) of Acoustics and Psychoacoustics. We will have a brief quiz on these sections at the beginning of the next class.

Brain Bullets

• Time domain
  • Plots amplitude against time.

• Frequency domain
  • plots amplitude against frequency.

• Fourrier synthesis
  • states that all complex waveforms can be built up by using an appropriate set of sine waves of different frequencies and phases.

• Calculating Harmonics
  • The frequency of a harmonic can be found by multiplying the f₀ frequency by the harmonic number
  • The Amplitude of a harmonic for a pure tone can be calculated by multiplying the amplitude of the fundamental by 1/harmonic number²

• The effect of phase.
  • In addition to the pitch and amplitude of a harmonic the phase of the harmonic can have a radical effect on the shape and often the sound of a wave

• Lowpass filters
  • Allow frequencies below the cutoff to pass

• Highpass filters
  • Allow frequencies above the cutoff to pass

• Bandpass filters
  • Consists of a lowpass and a highpass in series with the lowpass cutoff set above the highpass cutoff.
  • Allow frequencies between the HPF and LPF cutoffs to pass

• Band Reject filters
  • Consists of a lowpass and a highpass in parallel with the lowpass cutoff set below the highpass cutoff.
  • Allow frequencies outside of the HPF and LPF cutoffs to pass

• Fourrier Analysis
  • A spectrum can be derived from a sound by running it through a bank of bandpass filters

• All filters have a response time which is a function of:
  • The frequency range they are tuned to
  • The bandwidth of the signals they past