Array Microphones: Not Just for Birds

Posted by Sam Churchill on

Transit authorities in cities across the country are quietly installing microphone-enabled surveillance systems on public buses to record and store private conversations, reports Wired.

The systems are being installed in San Francisco, Baltimore, and other cities with funding from the Department of Homeland Security, according to the Daily, which obtained copies of contracts, procurement requests, specs and other documents.

The use of the equipment raises serious questions about eavesdropping without a warrant, particularly since recordings of passengers could be obtained and used by law enforcement agencies, says Wired.

The IP audio-video systems can be accessed remotely via a built-in web server and can be combined with GPS data to track the movement of buses and passengers throughout the city.

The RoadRecorder 7000 surveillance system, made by SafetyVision, allows “remote connectivity via the Gigabit Ethernet port or the built-in 3G/4G modem. A robust software ecosystem including LiveTrax vehicle tracking and video streaming service.”

Audio and video can be monitored in real-time, but are also stored onboard in blackbox-like devices, generally for 30 days, for later retrieval. Four to six cameras with mics are generally installed throughout a bus, including one near the driver and one on the exterior of the bus.

Cities that have installed the systems or have taken steps to procure them include San Francisco, California; Eugene, Oregon; Traverse City, Michigan; Columbus, Ohio; Baltimore Maryland; Hartford, Connecticut; and Athens, Georgia.

Here’s a quick overview of the power of the spectrogram on Audacity, the free, open source software for recording and editing sounds.

Spectrographic software, combined with array mikes, are commonly used to track animals.

PMEL’s Ishmael acoustic monitoring software tracks whales by their acoustic footprint. Spectrograms in Ishmael show time on the horizontal axis and frequency on the vertical axis. A call is “detected” when the sound template exceeds a threshold match for a set length of time.

Microphone arrays, such as the ones made by Wildlife Acoustics allow passive unattended recording for hundreds of hours. Then you can search for a particular “bird song” using their Song Scope spectrogram software. It works similar to facial recognition, matching a template to the recorded data. Cornell’s open source Raven software also can be used to match bird song spectrographs.

A spectrogram application for the iPhone uses the iPhone’s built-in microphone to create realtime scrolling spectrograms of any sound you’re hearing, letting you see spectrograms in the field, at the very same time that you’re listening to the bird sound.

Acoustic Magic’s Voice Tracker Array Microphones automatically locate the active talker and electronically steers a “listening beam” in that direction.

Smartphones may make large array mikes cost/effective. Microsoft Research has more info and the Acoustical Society of America has more links.

Google’s Nexus 10 comes with a blazing fast dual-core ARM Cortex A15 processor. The Tegra 4, expected at CES next month, is another version of the A15, while Samsung’s S4 smartphone will likely use another variant, Samsung’s Exynos 5450. Samsung’s $250 Chromebook features an ARM Cortex-A15 processor, 2GB of RAM, and 16GB of storage. All have real DSP firepower.

Cloud services such as Puppet Labs and Amazon’s EC2 could crunch spectrograms in real-time, identifying, tracking and recording audio events, such as automatically identifying overhead airplanes.

Skybox Imaging, which plans high-resolution imaging using microsatellites, will use Puppet Labs to help manage the cloud-based image processing.

Woody Norris showed off two of his ultrasound inventions at a Ted Talk.

Related Dailywireless articles include; Sound Cannons as Tsunami Sirens, Ultrasonic Teen Repeller, Titan Supercomputer Tour

Posted by Sam Churchill on Monday, December 10th, 2012 at 6:03 pm .

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