Hum in the Air

[Vanadium 50]

TL;DR Summary

Can someone help me understand a hum I heard?

Last night I head a very odd hum. I believe it was atmospheric, but did not see the source.

• It was at 10PM
• It came from the north. It did not noticeably move.
• It was about 250 Hz.
• It was not a perfect sine wave - there was some distortion. More or less like an engine.
• It lasted about 4 minutes.
• The frequency never changed - i.e. no Doppler effect. Not an engine revving either.
• The amplitude fluctuated by about 10 dB, irregularly, but with a period of 5 to 10 seconds.
• Weather was partly cloudy. Air was cool and dry.

I would have guessed a plane except for the lack of frequency shift. I am wondering, could it be very far away, and the sound been channeled through some sort of atmospheric ducting? VHF radio does this sometimes. Could it be some resonance excited by a broadband noise source? Something else?

 

[fresh_42]

For those who are as curious as me how 250 Hz sounds:
https://www.szynalski.com/tone-generator/ or https://onlinetonegenerator.com/

My first thought was a defective starter in a fluorescent tube or some other coil.

 

[Vanadium 50]

My first thought was a defective starter in a fluorescent tube or some other coil.

Interesting, as I do have one. However, it's in the other side of the house. The sound appeared to be coming from outside.

Most outdoor lighting is either gaslamps or LEDs. Winter is too cold for CFL's.

And yes, I went to that link and it was definitely close to 250 Hz.

 

[Baluncore]

If there is a rail line within 10 km, heavy diesel engine(s), starting to move a train.

 

[Flyboy]

Interesting, as I do have one. However, it's in the other side of the house. The sound appeared to be coming from outside.

Having tried to triangulate odd sounds both indoors and out, it can sometimes be deceptive where it’s coming from, especially if you don’t have multiple chances or an extended period to triangulate it.

Has it ever happened before, or was this a freak/unique occurrence?

 

[Vanadium 50]

If there is a rail line within 10 km, heavy diesel engine(s), starting to move a train.

Interesting. There is a line about a mile away. It is mostly commuter, but there is some freight. And trains do stop there - it is one of the longest segments between at-grade crossings.

However, why wouldn't I have heard the pitch change as the train's velocity changed?

Has it ever happened before, or was this a freak/unique occurrence?

First time I noticed. However, last night there was a similar sound, only with Doppler, and it was a plane.

 

[DaveC426913]

Could it have been a generator? Maybe one of those road-side construction types?

A crew rolls up to fix some part of the highway, erects a set of lights powered by a diesel generator, gone in a few minutes.

 

[Vanadium 50]

Could it have been a generator?

Maybe. Four minutes at night? Kind of an odd time for it, but it could be.

 

[DaveC426913]

Maybe. Four minutes at night? Kind of an odd time for it, but it could be.

This wouldn't be scheduled work; it would be a reported incident.

Some hazard on the road is reported by a motorist - downed branch, flotsam or roadkill. A highway crew shows up, fires up their diesel-powered god lamps, clears the obstruction off the road in ten minutes, and then spends two hours filling out reports.

 

[Baluncore]

However, why wouldn't I have heard the pitch change as the train's velocity changed?

Diesel-electric locomotives run constant revs on the diesel. The injected fuel volume is adjusted to regulate RPM, as power is extracted by the field controlled alternator.
I hear the exhaust pulses from trains a few miles away at night, when they work hard. The pulse rate for a two-stroke exhaust would be ( Ncylinders * RPM / 60 ).
Six cylinders at 1500 RPM = 150 Hz.

There is also a distinct rumbling sound from aircraft cruising above 30,000 feet. I hear that inside my house. I expect to hear LAN805 overhead, in an hour or two, on its 14-hour flight, from Santiago Chile to Melbourne Australia, over the Pacific-Southern Ocean.
https://www.flightaware.com/live/flight/LAN805

 

[berkeman]

It was about 250 Hz.

You probably don't, but if you happen to have an oscilloscope at home that has "Line" as one of its trigger options, and a way to monitor a microphone pickup circuit, then you could tell right away if its source was AC Mains or some other source like an ICE engine on a generator, etc.

I don't have that stuff in my current house, but when I did EE consulting out of my home, I had those things in my "lab"/spare room. Line sync is handy for looking at signals that have some origin in the local AC Mains.

https://ueeshop.ly200-cdn.com/u_fil...x-oss-process=image/resize,m_lfit,h_752,w_752