5 ms·

Some googling yields this: http://earthworksaudio.com/microphones/m-series/m50/ http://earthworksaudio.com/microphones/m-series/m50/ which should be sensitive e

by rocqua 9y ago

Some googling yields this: http://earthworksaudio.com/microphones/m-series/m50/ 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 depends on sounds being quieter when they are far away, but if one mic is naturally quieter (deeper inside the house, or more absorbant material, or just more obstacles in the way) that no longer works. The best approach would be using a phase difference to measure. Exploiting the delay due to the speed of sound. That means you need some way to synchronize the recordings. Presuming a 20hz signal, you'd need your synchronization to be better than 0.05 seconds. This also requires the hum waveform to be easy to match. That is, you'd need to be able to tell the phase offset by matching parts of the sound. You could also try a 'big data' approach, just cataloging all reported cases and looking for patterns. That requires dealing with very noisy data though, as reported cases will be inconsistent. This is due to people not always being home, not everyone hearing it, and no verification of reports. edit: Interesing idea: use the 'bit data' approach together with wind speed and direction. The sound should be easier to hear when the wind blows it towards you. With enough data and corresponding wind data, you might have something. Does require compensating for correlation between time of day and the wind. If wind is west in the night, and less people hear things when the wind is west, that might just be because they are sleeping.

wl 9y ago
I wouldn't recommend the Earthworks M50 for this application. Earthworks' design approach for these microphones is to maximize time domain performance by having excessive bandwidth, minimal phase distortion, and flat frequency response. In order to achieve this, they have to use a tiny condenser element, which has relatively low sensitivity and thus relatively high noise. If you're looking to measure things near or below the limits of human hearing, I'd recommend looking at microphones designed for infrasound rather than low sensitivity microphones designed to go into the ultrasonic range. PCB and Brüel & Kjær are the big names here. Measuring infrasound is pretty difficult, unfortunately. Most audio gear has a high pass filter, which makes sense for most uses but not this. Also, pseudo-noise from air currents is often a huge source of error.
- robertelder 9y ago
  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 so than a typical microphone anyway). I would also reason that you could also do a frequency response test over a range of known frequencies and power levels for the specific subwoofer to begin with, and then write a specific software filter to normalize whatever you record from it to get the 'true' sound. Does that make sense? Edit: Also, note that I mean doing hardware amplification of the electrical signal in the wire coming directly from the motion of the magnet of the speaker.
  - Johnythree 9y ago
    Yes, sub-woofer loudspeakers make excellent low-frequency microphones when used with a suitable pre-Amp. And using a PC with Spectrogram software is a great way to record and visualise the sounds. However, please read my post above on what I actually found.
  - wl 9y ago
    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 Earthworks M50 suggested above? I wouldn't recommend it for this application because if you're going to spend a lot of money, it might as well be on something well-suited to the application. However, the approach it takes works and can be done for much less money. Cheap omnidirectional electrets go down pretty far[1], and you don't need the ruler flat response of the M50. You just need amplification circuitry that doesn't filter out the low frequency stuff you want. DSP is commonly used to correct the frequency response of microphones. However, measuring frequency response is difficult and expensive. [1] http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_list.html http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_l...
jaequery 9y ago
This strange sounds have been going on for many years now. It is really a strange phenomenon that's been occurring globally. On Youtube, it yields several people video recording them, from Russia to Asia: https://www.youtube.com/results?search_query=strange+sounds+around+the+world https://www.youtube.com/results?search_query=strange+sounds+...
- rainbowmverse 9y ago
  All of these that I checked are either obviously fake or have simple explanations. One is a sound familiar to anyone who's played around with a cheap microphone, for example. Some are simple waves anyone can generate. This reminds me of a YouTube video I found from someone who thought they found a UFO near the Wal-Mart parking lot one night. It was the red blinking light on top of a water tower. People are very good at convincing themselves of things.
  - jaequery 9y ago
    some are fake for sure. but this is more akin to the likes of strange occurrences of sinkholes than ufos. its not just one person reporting it, but an entire neighborhood.
    - rainbowmverse 9y ago
      I don't doubt that some people experience hard to explain sounds. I do doubt that any of the sensational headlines in those videos were written by people who were prepared to consider reasonable explanations. Unfortunately, they were the only people there to look down/around to see if something obvious was making the sound instead of some ethereal oscillator. We only have their word to go by, and it's not much use given their leaning toward supernatural and conspiracy.
jjwiseman 9y ago
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.
- blunte 9y ago
  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.
  - jjwiseman 9y ago
    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.

by rocqua 9y ago

wl 9y ago

I wouldn't recommend the Earthworks M50 for this application. Earthworks' design approach for these microphones is to maximize time domain performance by having excessive bandwidth, minimal phase distortion, and flat frequency response. In order to achieve this, they have to use a tiny condenser element, which has relatively low sensitivity and thus relatively high noise. If you're looking to measure things near or below the limits of human hearing, I'd recommend looking at microphones designed for infrasound rather than low sensitivity microphones designed to go into the ultrasonic range. PCB and Brüel & Kjær are the big names here. Measuring infrasound is pretty difficult, unfortunately. Most audio gear has a high pass filter, which makes sense for most uses but not this. Also, pseudo-noise from air currents is often a huge source of error.

robertelder 9y ago
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 so than a typical microphone anyway). I would also reason that you could also do a frequency response test over a range of known frequencies and power levels for the specific subwoofer to begin with, and then write a specific software filter to normalize whatever you record from it to get the 'true' sound. Does that make sense? Edit: Also, note that I mean doing hardware amplification of the electrical signal in the wire coming directly from the motion of the magnet of the speaker.
- Johnythree 9y ago
  Yes, sub-woofer loudspeakers make excellent low-frequency microphones when used with a suitable pre-Amp. And using a PC with Spectrogram software is a great way to record and visualise the sounds. However, please read my post above on what I actually found.
- wl 9y ago
  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 Earthworks M50 suggested above? I wouldn't recommend it for this application because if you're going to spend a lot of money, it might as well be on something well-suited to the application. However, the approach it takes works and can be done for much less money. Cheap omnidirectional electrets go down pretty far[1], and you don't need the ruler flat response of the M50. You just need amplification circuitry that doesn't filter out the low frequency stuff you want. DSP is commonly used to correct the frequency response of microphones. However, measuring frequency response is difficult and expensive. [1] http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_list.html http://volcanomodels.sr.unh.edu/jbj/MICROPHONES/microphone_l...

jaequery 9y ago

This strange sounds have been going on for many years now. It is really a strange phenomenon that's been occurring globally. On Youtube, it yields several people video recording them, from Russia to Asia: https://www.youtube.com/results?search_query=strange+sounds+around+the+world https://www.youtube.com/results?search_query=strange+sounds+...

rainbowmverse 9y ago
All of these that I checked are either obviously fake or have simple explanations. One is a sound familiar to anyone who's played around with a cheap microphone, for example. Some are simple waves anyone can generate. This reminds me of a YouTube video I found from someone who thought they found a UFO near the Wal-Mart parking lot one night. It was the red blinking light on top of a water tower. People are very good at convincing themselves of things.
- jaequery 9y ago
  some are fake for sure. but this is more akin to the likes of strange occurrences of sinkholes than ufos. its not just one person reporting it, but an entire neighborhood.
  - rainbowmverse 9y ago
    I don't doubt that some people experience hard to explain sounds. I do doubt that any of the sensational headlines in those videos were written by people who were prepared to consider reasonable explanations. Unfortunately, they were the only people there to look down/around to see if something obvious was making the sound instead of some ethereal oscillator. We only have their word to go by, and it's not much use given their leaning toward supernatural and conspiracy.

jjwiseman 9y ago

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.

blunte 9y ago
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.
- jjwiseman 9y ago
  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.