Stereo and surround recording techniques

In this entry we are going to recall the main techniques for stereo recording:

  • Coincident microphones or intensity stereophony. It uses level differences between the two microphones, with no time differences.
    • XY pair. Consists in two directional microphones at the same place with opening angles which typically go from 90 to 150º.
    • Blumlein pair. Consists in two figure of eight microphones angled 90º.
      Blumlein -Stereo
    • Mid/side pair, or MS pair. Employs a bidirectional (or figure-of-8) microphone pointing sideways (side, or S) plus either an omnidirectional or a variant of a cardioid microphone pointing forward (mid, or M). L and R signals can be formed by a linear combination of S and M.
      MS stereo
  • Spaced microphones, time-of-arrival stereophony or AB technique. This techique consists in having two omnidirectional microphones separated a distance which goes from a few tenths of cm to a few meters. Signal levels are mostly the same, at least with small separations, and stereophony is due to time-of-arrival differences only.
    AB Stereo
  • Near coincident pair techniques, the most common of which is the ORTF system. It consists on two cardioid microphones opened 110º and spaced 17 cm.

By checking some references on stereo microphony, like

try to answer the following questions:

  • What are the advantages and incovenients of each one of the techniques?
  • How is the stereo image perceived in each one of the cases?
  • What kind of directivity patterns one can get with the MS technique? How to go from MS to LR signals?
  • Is it always the optimal thing to place two microphones near the stage in the approximate positions the playback loudspeakers would be? Why?

Also, there are microphone configurations directly suitable for recording directly in 5.1 stereo surround. See for instance:

Surround sound recording techniques are based on the same principles (summing localization — time and/or intenisty differences). They try to capture the main soundstage in the L, C, R channels and leave ambience and reverberation for Ls and Rs. In general, listening to surround sound reduces the stereo separation because of the center channel, and surround recording techniques try to counteract this effect.

Physical acoustics overview

In order to study the spatial properties of audio we need to know what is sound and how it behaves in space: how it propagates, what energy it does carry, how it does interact with other sound sources.

The first lecture of the course is therefore a 2h introduction to acoustics, and therefore it is a summary of what you already know from previous acoustics courses:

The topics we will treat will be:

  1. Waves on the air
    • The wave equation
    • Plane waves
    • Spherical waves
    • Energy and intensity
    • Decibel scale
  2. Features of the acoustic field
    • Frequencies and energies
    • Coherence and incoherence. Interference
    • Reflexion and reverberation
  3. Waves in a room
    • Room modes and geometric acoustics
    • The impulse response: direct sound, early reflections, reverberation
    • Reverberation time
    • Diffuse sound field
  4. Green function method
    • Wave equation in presence of sources
    • Diffraction and the Huygens principle
    • Green function in the free field

As references, any good acoustics book will do, like Kinsler‘s book, or even basic physics textbooks such as the one by Tipler. You can also check your lecture notes for previous courses.

Longitudinal wave traveling through a material medium. Extracted from Dan Roussell’s Acoustics and Vibration Animations

Also you can get good acoustics simulations in Dan Roussell’s Acoustics and Vibrations Animations web page. See in particular the following animations:

New course 2015-16: Introduction to Spatial and 3D audio

Welcome new students to the blog of the subject “3D audio”. We hope you will find this resource useful.

By spatial audio we refer to the investigation of techniques for sound

  • recording and encoding
  • transmission
  • manipulation (postproduction)
  • exhibition

taking into account the spatial properties and the spatial nature of sound. Spatial audio would be a term of widespread academic usage.

By 3D audio we mostly refer to the spatial audio techniques which  go beyond traditional stereo or 5.1/7.1 surround and which normally include height. It is rather a commercial term.