Reverberation

Slides

Class slides.

Materials

go to wikipedia...

very good stuff about artificial reverberation by Julius Smith

  • Real Analog Reverbs:
    • A real room: echo chambers
    • Spring Reverbs:

      a spring with torsional transducers at both ends, one drives the spring, the other acts as a microphone and picks up the transmitted sound and its reflections at both ends:

      Information about spring reverbs

      You can build your own.... get the springs at Accutronics, or get a complete PAIA kit

      (problems: not a clean sound, suceptible to hum, don't move them if you don't like eathquake sounds, but has its own "character")

    • Plate Reverbs:

      transducers drive a metal plate with sound, contact mics pick up the direct sound and its reflections, it is a two dimensional system that much better simulates the dense reflections of a real room.

      • Pictures of an EMT 410 Planet Reverb unit
      • Or you can get a plugin...
      • or get the impulse responses and cheat... (the web site is currently being reorganized and the impulse responses are not there yet)

  • Digital Reverbs:

    • Schroeder Reverbs:

      a good description can be found here

      basically an array of comb and allpass filters that creates a high density of echoes of the incoming signal

      from Julius Smith's Introduction to Digital Filters, lots of mathematical details:

      or in easier to digest form:

    • Convolution Reverbs:

      see what wikipedia has to say about convolution

      if we have the impulse response of a reverberant space then we can use convolution to superimpose those reverberant qualities in any sound. If so, why don't we always use convolution to create reverb? It is very expensive computationally.

      Measuring impulse responses: Aliki by Fons Adriansen, (LAC 2006 paper, manual)

      Designing a low latency convolver, also by Fons

    • More on allpass/comb reverbs:

      • the original Schroeder allpass and comb filter reverbs are not enough (not enough density)
      • added lowpass filtering in the feedback paths (Moorer)
      • improved by simulating the early reflections through tapped delay lines
      • still not very good response to impulsive sounds

      still, limit cycles, overflow oscillations are problematic...

    • Rooms: image source method

      • creation of virtual sources
      • models walls as acoustic mirrors
      • find reflections up to a maximum distance
      • non rectangular rooms?
      • computationally expensive

    • Rooms: ray tracing method

      • random rays are traced from the source
      • they reflect on the walls and other obstacles
      • if they land near the listener they are counted as contributions to the perceived sound
      • builds an accurate representation of the room
      • very computationally expensive

    • Waveguide Reverbs

      2 or 3d meshes of digital waveguides can be used to simulate room acoustics including reverberation

      • basic properties
      • basically a bidirectional delay line
      • scattering junctions connect them
      • loss is added to the junctions to define a decay time for the energy in the mesh
      • filtering is added to the junctions
      • energy is conserved in the mesh
      • "connectivity rich" environment which creates a dense late reverb
      • solves "overflow oscillations" and "limit cycles" problems

    • Granular Reverb

    • Distributed Rooms

  • "A duck's quack doesn't echo"??

    one answer... or check out this mythbusters episode

  • Reverbs you can use...

    • Freeverb (an implementation in faust)
      See an analysis of it here
    • Gverb
    • JConvolver: a stand alone, very efficient convolution engine for Jack (by Fons Adriansen)