Forest Grove, OR "mystery sound" (Feb 2016): a steady 1147.6 Hz tone, looks like a pressure whistle

[SkyAugur]

In Feb 2016 a loud high-pitched noise kept Forest Grove, Oregon up at night for a few weeks. The city, the gas company and a university crowd-mapping project never found the source, and it stopped around Feb 27. The fire marshal's guess at the time was an attic fan or heat pump.
Washington Post: https://www.washingtonpost.com/news...g-experts-and-terrifying-an-oregon-community/
NBC: https://www.nbcnews.com/news/us-news/mystery-forest-grove-oregon-noise-may-never-be-solved-n579456
KOIN clip:


I ran the news clips through a spectrogram. Every outlet's recording has the same steady tone at 1147.6 Hz (all within 2 Hz of each other), with clean exact multiples above it. It holds within about 1 Hz for 15 seconds at a time, and it isn't a multiple of 60 Hz, so probably not a motor or transformer.

The interesting part is how it starts and stops: the pitch settles down a few Hz when it starts and chirps up 10-15 Hz when it cuts off (chart below). A fan or heat pump would wind up and down over seconds and drop in pitch when it stops. This looks more like something whistling from gas or air pressure, like a valve or regulator.

Can't place the actual device from the clips, but it narrows it down.

 

[Dave51c]

like a valve or regulator.

It does have that characteristic.

Hard to imagine a 1KHz single source acoustic signal couldn't be located. A supercaridiod microphone and calibrated VU meter would be a good starting point. Something like the MKH 8050 super-cardioid by Sennheiser as an example of highly directional microphones.

 

[SkyAugur]

It does have that characteristic.

Hard to imagine a 1KHz single source acoustic signal couldn't be located. A supercaridiod microphone and calibrated VU meter would be a good starting point. Something like the MKH 8050 super-cardioid by Sennheiser as an example of highly directional microphones.

View attachment 94230

Agreed, a shotgun mic and a walk around the neighborhood would have found it in an evening. Shame it stopped on Feb 27 before anyone tried. Now all we have is the news audio (as far as we know I guess)

 

[Mendel]

Agreed, a shotgun mic and a walk around the neighborhood would have found it in an evening. Shame it stopped on Feb 27 before anyone tried. Now all we have is the news audio (as far as we know I guess)

The directionality of the microphone doesn't help much if the sound is already sufficiently loud, unless your technique is to identify the dropouts at 120⁰ that should give a fairly narrow angle.

A better technique might be to use two microphones mounted on a wide bar, fed into headphones for one ear each, to improve directional hearing. (Could also use tubes.)

If you want a highly directional reception, use a parabolic reflector to make a parabolic microphone.