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Audio Engineering Society
Convention Paper

Spatial Definition and the PanAmbiophone microphone array
for 2D surround & 3D fully periphonic recording

Robert E. (Robin) Miller III ©2004
FilmakerStudios, Bethlehem, Pennsylvania 18018, USA

Presented at the 117th Convention
2004 October 28-31 San Francisco, CA

This convention paper has been reproduced from the author's advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for the contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 60 East 42nd Street, New York, New York 10165-2520, USA; also see www.aes.org. All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society.  For a PDF version of this paper (0.5 MG), click here.

4. DIRECTIONAL PANAMBIOPHONE

The results in Fig.6 through Fig.8 showing 360° polar characteristics and localization approached what was thought to be the ideal for horizontal (2D) surround. In practice, this characteristic is applicable mostly to anechoic environments, such as outdoors. Subjectively, good results were obtained recording a passing parade and natural ambience, such as for movies and nature recordings. However, in acoustic space, the original design reproduced too much ambience in front, as is common with sphere microphone recordings (where the balance of stage-to-hall is fixed by positioning the microphone, and is not electronically controllable, live or in post-production, as it is with the PanAmbiophone).

Especially for staged presentations, a different characteristic may be more desirable – one that focuses the front microphone pair more on the stage, allowing a deeper range of placement of the array farther from the source, such as an orchestra, than is possible with the original PanAmbiophone. Therefore, a new design was conceived that reproduced the 120° stage in front and Miller PanAmbiophone for 2D & 3D surround recording the 240° hall in back, while still being able to control stage-to-hall balance, live or in post-production.

4.1. Focused surround microphone for interior recording

Design criteria for the latest in the PanAmbiophone evolution intended for use in recording staged performances in acoustic spaces were:

  • Ellipsoidal approximation of human HRTF [32]
  • Simple barrier directionality (minimal artifacts)
  • Front stage 120°, independent back stage 240°
  • Control of ceiling reflections in front stage
  • HRTF-related ITD, ILD shadowing (no pinna)
  • Minimal degradation of mic. Impulse Response
  • Frequency response compensatible to 30kHz
  • Boundary layer operation (no comb-filtering)
  • SNR +6dB over microphone elements
  • 4.0 outputs (48V phantom power)

Achieving directional characteristics using mechanical boundaries has advantages in that small pressure (omnidirectional) microphone elements are used, with resulting SNR equivalent to larger microphones. With small diaphragms, high frequency phase response more closely approaches the ideal, along with the good low frequency response of omni-directional elements. However, designing the complex baffle had risks, and had especially to ensure that no reflections would smear the high frequency impulse response and cause comb filtering. As in the original PanAmbiophone, the four microphone elements simulate human “ears,” but are now positioned on a single head-shaped ellipsoid [32]. Now tangent to an acoustically hard surface, each diaphragm acts as a barrier microphone, with 6dB acoustic gain above cutoff frequency, resulting in a 6dB greater SNR. So as to avoid unwanted reflections causing comb filtering of direct sounds, each element had to be tangent to three planes – side baffle, top baffle, and the plane tangent to the ellipsoid. Unlike the thick barrier of the original PanAmbiophone above, in the new design, each pair of elements front-to-back had to be within 1 inch (25mm) to correlate in the listening acoustic “mix” as though a single “ear.” The design is illustrated in Fig.10 and the prototype is shown in Fig.9, with an attached discrete soundfield array for PerAmbio 3D recording (with height).


Fig.9 - New design of the Pan\Ambiophone. Two pairs of small pressure microphone elements are coincident at “ear” positions.
A discrete soundfield array is mounted atop – a total of eight recording channels for PerAmbio 3D (with height).

Fig.10 - Top, front, and side engineering views for the directional PanAmbiophone
show ellipsoidal head-shape, four microphone elements, and typical source angles.

The directional PanAmbiophone and PerAmbio 3D/2D, both 5.1 compatible, are Pat.pending.

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