What Could Cause a Persistent 250 Hz Hum at Night?

[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

 

[Vanadium 50]

But 250/60 = not a small integer.

As far as diesels running at constant speed, if a tuning fork were on the train, I'd still hear its pitch change as it moved.

 

[DaveC426913]

Diesel-electric locomotives run constant revs on the diesel.

Mayhap, but they are still subject to Doppler effects, no?

 

[Baluncore]

Mayhap, but they are still subject to Doppler effects, no?

Doppler is noticeable only if it passes close by you, with rapid changes in azimuth.

For a heavy freight train, speed is low, and range changes only slowly, as it travels across your view, parallel with your horizon.

 

[berkeman]

250240/60

Fixed that for you.

 

[DaveC426913]

Doppler is noticeable only if it passes close by you, with rapid changes in azimuth.

Vanadium was asking about a change in pitch with the train's velocity as opposed to azimuth. I assume he knows that it would be proportionately small, yet he was still considering it a possible solution.

For a heavy freight train, speed is low, and range changes only slowly, as it travels across your view, parallel with your horizon.

Again, I assume Vanadium knows this and - at the risk of speaking for him - he may have been thinking of a train that is travelling parallel to his line-of-... hearing (i.e. head-on/tail-on) wherein a change in velocity would have its greatest effect on pitch.

 

[Vanadium 50]

Fixed that for you.

I don't think it was that low. That is a sharp B-flat. This was more like a slightly flat middle C.

 

[Baluncore]

Again, I assume Vanadium knows this and - at the risk of speaking for him -

The V50 posts are a bit too short and cryptic. It is hard to tell what point he is making, so there is little point in a reply.

Until a pitch measurement is made, we cannot assume the hum has a fixed frequency. If the hum was digitally recorded, others could process the sound file. We could then identify the frequency variation and the harmonic content of the sound.

Diesel engine RPM will vary slightly with load because the governor, that controls the injection pump, does not have infinite gain. The changes I hear are due to the driver adjusting the governor set point, from idle to power when climbing a grade.

 

[Averagesupernova]

The pump on a septic service truck that would VAC out a septic tank would probably run about that long and it would run at a steady rpm. At the beginning you may hear it ramp up in speed as well as ramp down at the end.

 

[berkeman]

I don't think it was that low. That is a sharp B-flat. This was more like a slightly flat middle C.

Okay, I'm clearly out of my league in this thread. I can barely tell the difference between a thump-thump and a ping! "Hey dude, did you hear that?"

https://www.hollywoodreporter.com/wp-content/uploads/2019/02/billandtedsexcellent_pub02-h_2019.jpg

 

[Vanadium 50]

This afternoon a truck drove by, carrying maybe a dozen sections of concrete pipe: a few feet long and a few feet high. It made a sound that was not completely dissimilar to what I heard, although the pitch varied.

I am wondering if the truck had been parked north of me, and if the wind was right, it would be like blowing over the top of a coke bottle. Resonance, but not the atmosphere's fault. The pitch seems higher than I would have guessed, but now I know there were likely pipe shaped objects near the pipe like sound.

 

[jrmichler]

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 lasted about 4 minutes.
• The frequency never changed - i.e. no Doppler effect. Not an engine revving either.

I have heard a similar hum outdoors at night more than once. The hum was always in an area with mosquitos. I never could tell what direction it was coming from. It sounded enough like a mosquito in a quiet bedroom that I have always wondered if a bunch of mosquitos synchronized their wing flapping.