Archive for Acoustics

Throat whistling?

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Zoom time

I'm involved with several projects that analyze recordings from e-interviews conducted using systems like Zoom, Bluejeans, and WebEx. Some of our analysis techniques rely on timing information, and so it's natural to wonder how much distortion might be introduced by those systems' encoding, transmission, and decoding processes.

Why might timing in particular be distorted? Because any internet-based audio or video streaming system encodes the signal at the source into a series of fairly small packets, sends them individually by diverse routes to the destination, and then assembles them again at the end.

If the transmission is one-way, then the system can introduce a delay that's long enough to ensure that all the diversely-routed packets get to the destination in time to be reassembled in the proper order — maybe a couple of seconds of buffering. But for a conversational system, that kind of latency disrupts communication, and so the buffering delays used by broadcasters and streaming services are not possible. As a result, there may be missing packets at decoding time, and the system has to deal with that by skipping, repeating, or interpolating (the signal derived from) packets, or by just freezing up for a while.

It's not clear (at least to me) how much of this happens when, or how to monitor it. (Though it's easy to see that the video signal in such conversations is often coarsely sampled or even "frozen", and obvious audio glitches sometimes occur as well.) But the results of a simple test suggest that more subtle time distortion is sometimes a problem for the audio channel as well.

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The "Mosquito" in Philadelphia

Michaela Winberg, "Sonic Devices Target Teenagers In Philadelphia", NPR 7/5/2019:

WINBERG: If you look at the rec center building at Philadelphia's East Poplar Playground, you'll see a small beige speaker screwed into the wall. Every night at 10 p.m., that tiny speaker activates. And for eight hours, it plays nonstop. Here's what it sounds like.

(SOUNDBITE OF STATIC)

WINBERG: Didn't hear anything? If so, it's likely you are not between the ages of 13 and 25. That's the age group this sound is targeted toward. As we age, some of the cells in our ears start to die off. So when we get older, we have trouble hearing higher-frequency noises like the one that this device plays.

Philadelphia resident Lamar Reed is 17, and he hears the noise loud and clear.

LAMAR REED: It's so loud. Like, it can – like, what if it damages our ears or anything like – something like that?

WINBERG: It's called the Mosquito, and it's an acoustic deterrent device, technology used to keep humans or animals away from a designated area. It's usually used by law enforcement or the military. The Mosquito was manufactured by Vancouver-based Moving Sound Technologies. Michael Gibson is the company's president and says he has worked with about 20 parks departments in cities around the country to install his devices.

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