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- Explanation of the Ambisonics technology, as currently employed in room
acoustics
- Brahma: the first underwater 4-channels digital sound recorder
- A tetrahedrical hydrophone array for
Brahma
- Sound source localization from Ambisonics
(B-format) recordings
- Graphical mapping of boat trajectory
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- Ambisonics was invented in the seventies by Michael Gerzon (UK)
- It was initially a method for recording a 4-channel stream, which later
was played back inside a special loudspeaker rig
- It is based on the pressure-velocity decomposition of the sound field at
a point
- It makes it possible to capture the complete three-dimensional sound
field, and to reproduce it quite faithfully
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- Reproduction occurs over an array of 8-24 loudspeakers, through an
Ambisonics decoder
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- Trinnov, DPA, CoreSound, Brahma are other microphone systems which
record natively the A-format signals, which later are digitally
converted to B-format
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- The A-format signals are the “raw” signals coming from the 4 capsules,
loated at 4 of the 8 vertexes of a cube, typically at locations
FLU-FRD-BLD-BRU
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- The A-format signals are converted to the B-format signals by matrixing:
- W' = FLU+FRD+BLD+BRU
- X' = FLU+FRD-BLD-BRU
- Y' = FLU-FRD+BLD-BRU
- Z' = FLU-FRD-BLD+BRU
- and then applying proper filtering:
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- A Zoom H2 digital sound recorder is modified in India, allowing 4
independent inputs with phantom power supply
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- The standard microphone system is usually a terahedrical probe equipped
with 4 cardioid electrect microphones
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- However the recorder is equipped also with a split-out cable, allowing
for the connection of other transducers, including microphones,
accelerometers and hydrophones
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- Brahma provides phantom power (5V) for transducers equipped with
integral electronics. Hence the ideal hydrophone is the Acquarian Audio
H2A:
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- A tetrahedrical assembly can be built for underwater Ambisonics
recording:
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- For underwater recordings, a special setup of 4 screw-mounted
hydrophones is available:
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- Due to the small size (like a cigarette packet) it is easy to insert the
Brahma inside a waterproof cylindrical container, sealed with O-rings
- An external lead-acid battery can be included for continuous operation
up to one week (in level-activated recording mode)
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- The probe can be mounted on a weighted base, allowing for underwater
placement of the recorded, inside a waterproof case. However, the cables
are long enough (15m) also for keeping the recorder on the boat
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- The system is aligned vertically by means of a bubble scope, and
horizontally by means of a magnetic compass:
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- Once placed on the sea bed, the system is usually well accepted (and
ignored) by the marine life:
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- Brahma records A-format signals. They can be converted to standard
B-format by means of the Brahmavolver program, running on Linux /
Windows / Mac-OSX
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- Sampling rates: 44.1 kHz, 48 kHz, 96 kHz (2 ch. only)
- Recording format: 1 or 2 stereo WAV files on SD card
- Bit Resolution: 16 or 24 bits
- 3 fixed gain settings, with 20 dB steps (traceable)
- Memory usage: 1.9 Gbytes/h (@ 44.1 kHz, 24 bits, 4 ch.)
- Recording time: more than 16 hours (with 32 Gb SD card)
- Power Supply: 6 V DC, 200 mA max
- Automatic recording when programmable threshold is exceeded
- The SD card can be read and erased through the USB port
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- At every instant, the source position is known in spherical coordinates
by analyzing the B-format signal
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- Employing several buoys, the complete trajectory can be triangulated
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- Impulse response measurements inside a large pool
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- Polar patterns at two frequencies
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- The Marine Protected Area of Miramare (Trieste, Italy)
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- Noise spectra (SAN and boat passage)
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- Vectorial analysis of a boat passage
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- Estimated boat trajectory
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40
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Notes
