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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.

5. MEASURED PERFORMANCE

In Fig.11 through Fig.19 are plots of unequalized impulse and high frequency response 3kHz~30kHz of the prototype 4-channel PanAmbiophone, tested with a 1µs acoustic impulse (spark). The data provided clues to any errant reflections in the complex structure that might smear the response in time, and, therefore, degrade sonic performance. Of the total measurement series every 15°, the highlights are presented here.

In Fig.11, the impulse response (IR) of the front channels shows a risetime of 13µs with good settling. For frontal sources – usually the most important – this IR response is preferable to that typical of sphere microphones, where optimum performance occurs for sources at ±90° directly left or right [33-Fig2]. For reference, the high frequency response of the Ľ inch (6mm) microphone elements before mounting in the PanAmbiophone is in Fig.12, showing the response to be –2dB at 30kHz. Fig.13 through Fig.16 show, respectively, the unequalized high frequency response 3kHz~30kHz of front channels for a source at 0° directly front, 30° ipsolateral, 30° contralateral (showing lower level and greater tilt typical of the head shadow), and 60° ipsolateral. Fig.17 through Fig.19 show, respectively, the back channels for a source at 60°, 120°, and 180° directly back.



Fig.11 - Impulse response of PanAmbiophone front channels using 1µs spark.
Measured risetime is 13µs with rapid settling. Horizontal divisions are 50µs.

Fig.12 - Frequency response 3kHz~30kHz of the Earthworks
elements before mounting in the PanAmbiophone shows
response -2dB at 30kHz.

Fig.13 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone front channels for source directly front (0°).
Horizontal divisions are 2kHz with 30kHz at the extreme right.

Fig.14 - Unequalized frequency response 3kHz~30kHz of front
channels for a source positioned ±30° (ipsolateral).

Fig.15 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone front channels for a contralateral source
positioned ±30° - shows response in “head shadow” of the
microphone on the opposite side from the source.

Fig.16 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone front channels for a source positioned ±60°
(ipsolateral).

Fig.17 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone back channels for a source positioned ±60°
(ipsolateral)..

Fig.18 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone back channels for a source positioned ±120°
(ipsolateral).

Fig.19 - Unequalized frequency response 3kHz~30kHz of the
PanAmbiophone back channels for a source positioned 180°
directly back.

In the total measurement series every 15°, only one significant reflection error was found – measuring 0.235µs at source angle 45° on the contralateral microphone, down -9dB. The problem has been addressed with dampening compound within the structure. Also, using equalization, the high frequency response of the array has been corrected to within 2dB at 16kHz around 360° for a “perfect omni” result overall. Perfectly omni-directional high frequency polar response implies positioning the microphone at distances from the source that approach natural hearing, yet with with “reach” and “presence” obviating spot microphones. Below the cutoff frequency of the baffle, all four microphone signals are sufficiently correlated so that their signals add to a flat low frequency response. The overall frequency response of the PanAmbiophone is 5Hz to 30kHz. It is consistent to the extent possible with the spatial definition concept defined earlier in this paper.

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