There's a Persistent Hum in Windsor, Ontario, and No One Knows Why • Hacker News Board

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#191

Post by Johnythree » Wed, Feb 21, 2018, 2:53 AM UTC

Cookiesaurusbex wrote: post #53

ocdtrekkie wrote: post #32

Earlier quoted context omitted.

"Since reports of it surfaced in 2011, the hum has been studied by the Canadian government, the University of Western Ontario and the University of Windsor." Do you really think that between the Canadian government and two universities, over the past seven years, that you are the first person to suggest this "obvious first step"?

I went to school and worked for a while studying spectrum management. It boggles my mind to this day that drones get in the way of fire fighting and emergency services because an obvious first step world be to allow emergency vehicles & helicopters to jam the frequencies of drones. There are plenty of other vehicles with this ability (military, secret service, etc) so it's not like the technology to solve the problem…

The problem is that there's an infinite number of possible frequencies. Most Hobby drones use one of the ISM "junk" bands (eg 900MHz, 2.4Ghz, etc), but it's impossible to jam ALL of the possible frequencies without wiping out communications over a wide area.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#192

Post by Johnythree » Wed, Feb 21, 2018, 2:56 AM UTC

LeifCarrotson wrote: post #91

XR0CSWV3h3kZWg wrote: post #60

Earlier quoted context omitted.

typical triangulation would depend on the sound being heterogeneous in waveform/volume right? If it's a constant hum then you'd only have volume dissipate as you move away which would get a lot harder as sound has different loss functions as it moves through different materials.

No, the triangulation you would use for a project like this would look for time delay or phase lag between 2 (or more) adjacent microphones. The sound pressure wave is picked up by one, then some few microseconds later, it appears on the other(s) a few centimeters away. This is pretty straightforward with ordinary sounds that dominate the signal. You could pick out the wire which was struck in a piano, and even the n…

You can't do this with a sine wave. It's impossible to exclude waves caused by reflections.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#193

Post by Johnythree » Wed, Feb 21, 2018, 2:58 AM UTC

peter303 wrote: post #73

Seismologists now have some tricks for locating hum sources. Standard earthquakes have an impulsive start. So with four or more seismograms you can triangulate for the unknowns x, y, z and t0. Hum noises dont have an definitive starting time. But a version of seismology called inferometry can find the origin. You basically cross correlate all pairs of seismograms with signals. The peak lags define a hyperbolic wavefr…

Interferometry can't handle reflected sounds from more than a wavelength away.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#194

Post by blunte » Wed, Feb 21, 2018, 2:59 AM UTC

jjwiseman wrote: post #170

rocqua wrote: post #133

Earlier quoted context omitted.

Some googling yields this: http://earthworksaudio.com/microphones/m-series/m50/ which should be sensitive enough to pick it up. Next step would probably to get a few of these, see if you can record the hum on a few of them, and then start experimenting. The theory behind triangulation is simple. But in practice, it's probably really hard to make sure all microphones have the same sensitivity. The standard approach de…

You could start with two mics going into the same computer. Even a few feet of separation might be sufficient. Plug them into a mixer, one as the left channel and the other as the right channel of a stereo stream to keep their samples synchronized. Filter, then cross correlate the signals to find the phase difference/TDOA.

Since low frequency waves have a very long period, I don't think any mics near each other would give much useful location info. Depending on where a given mic was relative to the wave, I expect the recorded volume would be very different.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#195

Post by blunte » Wed, Feb 21, 2018, 3:03 AM UTC

monk_e_boy wrote: post #176

blunte wrote: post #101

As one who can hear these sounds, I can attest to how maddening it is. It's even worse when (most of) the people around you can't hear it, and they don't believe you. Earplugs only make it worse, because they filter out ambient noise that otherwise helps distract you from the incessant hum. Daytime is not as bad as night because of the natural activity sounds of a city. I have read that waves and underwater currents…

If I open the front window and back window in my car then drive at a specific speed I get a very weird woop-woop-woop and it feels like the air is pushing me back and forth in my seat. It pulses my head. Wonder if it is something similar.

Yeah, that's the same pressure differential, building and equalizing repeatedly. Same effect, and as most people agree, it's very unpleasant.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#196

Post by robertelder » Wed, Feb 21, 2018, 3:06 AM UTC

wl wrote: post #190

robertelder wrote: post #172

Earlier quoted context omitted.

Since you seem fairly knowledgeable about this subject, perhaps you could answer a related question I've been wondering: Would an effective low-cost way of measuring low frequencies (let's say ~20hz) be to use a subwoofer speaker (used as a microphone) hooked up to something that would amplify and recording of the signal? My reasoning is that they're naturally designed to work with low frequencies to begin with (more…

You're right that you can use a speaker as a microphone, because they're pretty much dynamic microphones run in reverse. However, speakers are exceptionally low sensitivity when used as microphones. You could amplify them and get a usable signal level out of them, but the noise would probably be much worse than a purpose built microphone. Microphone size doesn't really matter that much for low frequency sound. The Ea…

Cool, thank you for the link. To measure and correct for the frequency response of the microphone, can't you just play a range of known frequencies that each have known power levels, and then measure the gain/loss in power level that gets recorded by the mic, and then solve for a filter that gives you unity gain on the mic? I assume that you'd have peaks in the frequency response around the natural resonance and harmonics of the mic, and you'd basically just be adjusting for those.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#197

Post by jjwiseman » Wed, Feb 21, 2018, 3:09 AM UTC

blunte wrote: post #194

jjwiseman wrote: post #170

Earlier quoted context omitted.

You could start with two mics going into the same computer. Even a few feet of separation might be sufficient. Plug them into a mixer, one as the left channel and the other as the right channel of a stereo stream to keep their samples synchronized. Filter, then cross correlate the signals to find the phase difference/TDOA.

Since low frequency waves have a very long period, I don't think any mics near each other would give much useful location info. Depending on where a given mic was relative to the wave, I expect the recorded volume would be very different.

Yeah, 20 Hz = 17 m wavelength, so move the microphones to be at least 17 meters apart, or a bit more if you think the sound is below 20 Hz, which should be doable.

And yes, I don't expect comparing amplitude to be useful--sound localization almost always uses TDOA/phase difference for that reason.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#198

Post by wl » Wed, Feb 21, 2018, 3:20 AM UTC

robertelder wrote: post #196

wl wrote: post #190

Earlier quoted context omitted.

You're right that you can use a speaker as a microphone, because they're pretty much dynamic microphones run in reverse. However, speakers are exceptionally low sensitivity when used as microphones. You could amplify them and get a usable signal level out of them, but the noise would probably be much worse than a purpose built microphone. Microphone size doesn't really matter that much for low frequency sound. The Ea…

Cool, thank you for the link. To measure and correct for the frequency response of the microphone, can't you just play a range of known frequencies that each have known power levels, and then measure the gain/loss in power level that gets recorded by the mic, and then solve for a filter that gives you unity gain on the mic? I assume that you'd have peaks in the frequency response around the natural resonance and harm…

That's how the process works, but it's more complicated than you'd think. The room a speaker is in changes the overall frequency response. Speakers aren't flat, either. There are ways to get past these obstacles, but it can be difficult and expensive to get right. Going to the the world of infrasound just increases the complexity.

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#199

Post by 07d046 » Wed, Feb 21, 2018, 3:22 AM UTC

Johnythree wrote: post #181

blunte wrote: post #101

As one who can hear these sounds, I can attest to how maddening it is. It's even worse when (most of) the people around you can't hear it, and they don't believe you. Earplugs only make it worse, because they filter out ambient noise that otherwise helps distract you from the incessant hum. Daytime is not as bad as night because of the natural activity sounds of a city. I have read that waves and underwater currents…

I was plagued by the Hum for years. At times it became astonishingly loud. But I was able (eventually) to figure it out. Short version: I made many attempts to track it down. Used low frequency Microphones, filters, amplifiers, etc, as well as PC Spectrogram programs to try and and visualise and record it. Visited many nearby industries etc to try and find it. But none of my attempts did any good. As I'm a Radio Engi…

If I close my eyes, open my mouth and tilt my head back I can hear a hum. Is this The Hum?

Re: There's a Persistent Hum in Windsor, Ontario, and No One Knows Why

#200

Post by Mountain_Skies » Wed, Feb 21, 2018, 3:34 AM UTC

monk_e_boy wrote: post #176

blunte wrote: post #101

As one who can hear these sounds, I can attest to how maddening it is. It's even worse when (most of) the people around you can't hear it, and they don't believe you. Earplugs only make it worse, because they filter out ambient noise that otherwise helps distract you from the incessant hum. Daytime is not as bad as night because of the natural activity sounds of a city. I have read that waves and underwater currents…

If I open the front window and back window in my car then drive at a specific speed I get a very weird woop-woop-woop and it feels like the air is pushing me back and forth in my seat. It pulses my head. Wonder if it is something similar.

My car does this too. No previous car I've owned has had the same issue. Hopefully it isn't inherent to modern automobile aerodynamics because I'd like my next car to be one without it.