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.

 

[jon_eir]

Every outlet's recording has the same steady tone at 1147.6 Hz

That's a very interesting piece of spectral analysis, particularly the consistency of the 1,147.6 Hz frequency across the recordings.

I think there are two important distinctions worth exploring before ruling out electrical machinery.(I do have a electronic/electrical engineering background but havent really used it like this in a while so this took some thinking about and research, its also a bit long winded also)

1-The American electricity supply operates at 60 Hz, compared with 50 Hz in Europe (fyi European Debunkers). Conventional transformer hum is principally associated with twice the supply frequency.

The fundamental sound frequency is twice the power line operating frequency of the transformer.

Acme Electric — What causes transformer noise?

However, transformers also generate harmonics. These are whole-number multiples of the fundamental frequency, meaning a 120 Hz hum can have additional components at 240, 360, 480 Hz and upwards like 1200HZ.

But also

The noise spectrum is composed of multiple harmonics of the line frequency, 60 Hz

Transformer Technology — Quiet by Design: Understanding and Reducing Transformer Noise

The ninth and tenth harmonics of 120 Hz would be 1,080 and 1,200 Hz. Interestingly, the nineteenth multiple of 60 Hz is 1,140 Hz, just 7.6 Hz below the reported tone. Nevertheless, 1,147.6 Hz isn't an exact mains harmonic, so ordinary transformer hum remains an unlikely explanation.

I may need to think about this a bit more there is a ±0.03 Hz tolerance so the 19th harmonic of 1040 could go as high as 1040.57, @SkyAugur what was your actual lowest bound figure? Electronic motor controllers can generate other frequencies from the standard 60 Hz supply also.)

Could mechanical resonance be involved? Possibly, but resonance normally amplifies an existing excitation frequency rather than shifting a sustained mains harmonic to an unrelated frequency.

the natural response of the system will be damped out.

Penn State — Undamped Harmonic Forced Vibrations

This leaves the sustained vibration following the driving frequency. A separate resonant or nonlinear mechanism would therefore need to be demonstrated.

2- Electrical frequency versus mechanical noise? A second distinction is that ruling out direct electrical hum doesn't rule out mechanical noise from an electrically driven fan or pump.

A conventional two-pole AC motor has a synchronous speed of 3,600 RPM on America's 60 Hz supply, but induction motors run slightly slower under load.

Speed of an operating electrical motor with load is lower than the synchronous speed

Engineering ToolBox — Electrical Motors: Speed at Operating vs. Synchronous Load

The accompanying table gives approximately 3,450 RPM for a typical loaded two-pole motor.

More importantly, fans and impellers produce their own acoustic frequencies, determined by blade count and rotational speed.

Calculation: (Number of blades x fan RPM) / 60 Hz

AMCA International — Basics of Fan Noise, slide 54

For example, a hypothetical 20-blade blower running at 3,442.8 RPM would produce a blade-passing frequency of exactly 1,147.6 Hz, potentially accompanied by higher harmonics. That's remarkably close to a conventional two-pole induction motor's typical operating speed.

A fan or pump can also change pitch during startup and shutdown as its rotational speed changes. Admittedly, the reported upward chirp at cutoff would need further explanation, since a freely slowing fan would normally decrease in pitch.

So I think the absence of an exact 60 Hz relationship is useful evidence against direct electrical hum, particularly from a transformer, but it doesn't eliminate an electrically driven fan or pump.

The frequency stability, harmonics and precise startup/shutdown behaviour deserve closer examination before choosing between a rotating mechanical source and a pressure-driven whistle in my opinion.

Its tricky one, I may have over looked something and not a simple problem I think.

 

[Mendel]

A frequency of 1147.6Hz corresponds to a wavelength of almost exactly 30cm (a little under a foot).

That means air rushing past a pipe that's open at one end and 7.5cm (about 3 in) long, or open at both ends and 15 cm long.

Since a close-ended pipe only produces the odd harmonics, the resonator would be something like an open pipe, about 6 inches long.

I have no theory as to the frequency drop and rise at the beginning and end.

 

[Dave51c]

See: https://www.firgelliauto.com/blogs/calculators/resonance-in-pipes-calculator

 

[Dave51c]

The directionality of the microphone doesn't help much if the sound is already sufficiently loud

Not sure I understand this. With highly directional microphones, the rear is sensitive to a loss of directionality purely because most of its directionality is from the front. I mean, if you are so close to a sound source that both direct and primary/secondary reflections are creating an omnidirectional sound field around you, then yes agree. But then you are probably right on top of the very loud source anyway. Same effect as stuffing your head inside a bass bin at a Metallica* concert. (*Other rock bands available).

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.)

Personally I'd just keep it simple and work with bearings from a directional monophonic setup. Wide spaced microphones (in my experience anyway - YMMV) produce a smeared stereo image. Shotguns (tubes, but not simply a tube), did not work well as crossed stereo pairs (again, weird stereo sound stage) but one might work ok in a mid side configuration.

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

Yes, best case. I had one once, it was fun.

I have no theory as to the frequency drop and rise at the beginning and end.

A fan or heat pump would wind up and down over seconds and drop in pitch when it stops

Given the resonant pipe theory, if the air is provided by a pump, with the change in velocity affecting pitch, much like a flute being played, might we be seeing the modulation effect of the motor controller? Power is applied to the motor which initially is overspeed and then brought back to 'spec' (via a feedback loop of PRM). After time T the controller 'releases' the motor which races a bit until power it's is turned off. Wild guess of course, perhaps vintage control electronics and fading electrolytic capacitors.

All a bit academic now anyway.