Determining the source of low frequency noise

[porina]

Putting it in here since it doesn't really fit audio forum...

 

Since about 2 weeks ago, my house has become subject to "the hum". It's kinda borderline between a noise and a feeling, but is noticeable in my main bedroom and prevents me from getting to sleep easily. I have a sound level meter from work, that also shows the sound spectrum, and there's a spike around 25 Hz. It is not very loud, around 40 dBSPL in my normal bedroom which should be well below the threshold of hearing. Maybe I'm feeling it? Or maybe I'm sensing harmonics. Oh, there's a 2nd component to the hum. I can't see a spike at 150 Hz, but if I hum what I think I can also hear, it's bang on 150 Hz, the 3rd harmonic of the mains power here.

 

Is one of the many PCs vibrating in my house? I turned everything off. PCs, aquarium filters, the refrigerator. At that point it was easier for me to flick the switch on the main fuse block. No electricity at all to the house. The noise remained. The source wasn't anything electrical I was doing.

 

Back to the sound level meter, I wandered around the house with it. Two observations, it seems strongest at the front of the house, ball park 10 dB or so higher than towards the rear. Also it seemed louder when close to a wall. This I found interesting, and kinda lends itself to the theory it is a mechanical vibration, not airborne noise. It is only becoming airborne from the walls.

 

Since my house is mid terrace, I spoke to both my neighbours today, while being social distancing compliant. Neither had noticed any noise, or had any explanation. One suggested their fridge, but it was located at the back of their house so it seemed not to correlate well. The other mentioned his wife was on a ventilator (pre-existing condition, not covid-19 related). I don't know, do the pumps on those vibrate much? I doubt it is the source, and even if it was, that's not something I can do anything about.

 

I'm struggling to measure airborne sounds outdoors as the ambient noise swamps it. My current best guess is there is an electro-mechanical source to this noise. Maybe a pump, motor, or similar. I wonder if it is ground propagated, but I don't have anything to measure that. I did a general search, and apart from the odd small news report I did find a place that was trying to research the phenomenon. Mine wouldn't be of interest to them, as they're focusing on sources that can't be reasonably explained, but they did have a guide suggesting that small motor devices could generate a subharmonic of mains frequency. That fits in very well with my observations. Locating it is the hard part, as is trying to investigate in a covid-19 compliant manner.

 

That or I'm going crazy. For now my only workaround is to temporarily sleep in the back of the house, sharing a room with my geckos. Let's not get started about a neighbour on that side with a wind chime in their garden...

[IkeaGnome]

https://www.adn.com/alaska-news/anchorage/2020/02/13/mysterious-mechanical-groaning-noise-haunts-southwest-anchorage/

 

Not by chance actually from up here are you?

[Canoe]

This could be rather difficult.

Low frequencies can travel some distance from their source before people hear them.

Like someone hating the boom of the stereo from the apartment above, but up to their door and no stereo, no boom.

Offending stereo was three floors up.

 

Given the harmonic to line mains, for safety you may want to do a walk about of your building to see where the transformer is and where the electrical line comes in. Can you detect any noise near or far from the transformer. A call to the provider may be in order. 

 

Sound travels through air easily, but vibration can travel through solids well, until the vibration modulates a surface, like drywall or plaster, and generates sound from that vibration. Motors love making vibration noise as they start to go. Or the bearings of something they're driving.

 

[porina]

On 3/26/2020 at 10:18 PM, Voluspa said:

Not by chance actually from up here are you?

There's various reports out there, but that's not the sort of noise I'm getting.

 

On 3/26/2020 at 10:42 PM, Canoe said:

This could be rather difficult.

Low frequencies can travel some distance from their source before people hear them.

Like someone hating the boom of the stereo from the apartment above, but up to their door and no stereo, no boom.

Offending stereo was three floors up.

Difficult is an understatement. I have a spectrum measuring sound level meter from work, but it only goes down 7 Hz. Obviously, it's only of use for airborne noise, not mechanical vibration. Often I can see a clear spike just under 25 Hz. Level goes up or down depending on what part of the house I'm in, and it isn't constant. I can't say I've observed any pattern to it either. I've also started using my phone since I can't keep that sound meter forever, and that seems to have a practical lower limit around 20 Hz, so 25 Hz is just about measurable. 

 

I'm not sure if what I'm chasing is that 25 Hz though. In areas of my house, the level doesn't appear high, but I can still "feel" it. More like a pressure in my ears than a sound.

 

On 3/26/2020 at 10:42 PM, Canoe said:

Given the harmonic to line mains, for safety you may want to do a walk about of your building to see where the transformer is and where the electrical line comes in. Can you detect any noise near or far from the transformer. A call to the provider may be in order. 

Houses here may be different than you're used to. Feed is by underground cable, that exits by the meter, then goes through a fuse box before the rest of house. The fuse box is literally that, a bunch of wires. Nothing magnetic. 

 

On 3/26/2020 at 10:42 PM, Canoe said:

Sound travels through air easily, but vibration can travel through solids well, until the vibration modulates a surface, like drywall or plaster, and generates sound from that vibration. Motors love making vibration noise as they start to go. Or the bearings of something they're driving.

This seems the most likely situation. As said, I've eliminated anything inside my house as the source, and I'm unable to detect anything outdoors as the noise floor is too high. If I don't lose my sanity before the covid-19 situation is over, I could try asking my immediate neighbours (their houses are physically connected to mine) if they wouldn't mind me try a measurement. They have been receptive, but say they can't hear anything. 

 

What is making me wonder is what could be the source? Since I started noticing it, it is pretty much constant in presence, although there are variations in when or where I can sense it. Two locations are my front door, and what was my bedroom. Both are at front of the house, but on opposite sides.

 

Of common electrical appliances, I don't think it would be a refrigerator as that isn't constant, likewise for freezers. Mine throws out a pretty big spike at 50 Hz when it is on, but again not constant. A pump in heating or cooling systems, again, seems unlikely to be that constant. I have heard of a positive pressure air system, that could potentially be a cause.

 

I have had similar noises internal to my house before. One time it was found to be an AIO, another time it was an aquarium filter. As far as I know neither neighbour keeps fish, as I have done for a long time and we would have spoken about it. On one side, there is a teenager... dunno if he's into PCs though. 

[vf1000ride]

This may sound stupid but it's an old mechanics trick.  Use a screwdriver or anything with a blunted/round end on it to use like a stethoscope.  Put the rounded end against the side of you head were your jaw meets your ear and the other end against each appliance(adjust placement for best volume).  It will let you hear only what that one appliance is outputting for sound/vibration.  If you don't find the tone thats bothering you, it's probably like others are saying and coming from another part of the building.

[porina]

4 minutes ago, vf1000ride said:

Use a screwdriver or anything with a blunted/round end on it to use like a stethoscope.

Does it work on walls? As mentioned before, I had literally pulled the power to my house and the sound persisted. The source is external to my control. If I could even get some direction on it, it would be a starting point.

[vf1000ride]

1 hour ago, porina said:

Does it work on walls? As mentioned before, I had literally pulled the power to my house and the sound persisted. The source is external to my control. If I could even get some direction on it, it would be a starting point.

It might, I have never tried it while my heat is running to see if I could hear it but it can't hurt to try.

[porina]

To kinda give closure to this thread, the noise has been identified and ended without my intervention. It was indeed the ventilator one of my neighbours needed for pre-pandemic reasons. It turned out about the time I first noticed the noise, they got an upgrade to a more powerful unit than the one they had before. The day the noise stopped matched the day she was taken away by ambulance and didn't return.

 

A possible new problem is I'm now hyper-sensitised to any low frequency noise. While 25 Hz has gone, I currently have a new friend around 30-40 Hz, and an occasional one around 90 Hz. 

[porina]

I'm on a roll here! One noise I hadn't mentioned in this thread is at 90 Hz. It doesn't happen often, but when it does, it is quite noticeable. Only using a phone app, and moving it around my desk area, I noticed it was strongest on one wall in my front room. Was my neighbour doing something the other side? Couldn't pick up anything if I went to the other end of the house, or directly upstairs. I walked around some more and noticed it was coming from the area around my TV quite strongly. This was at the front of my house by a large window. Was it outside? Nope. Nothing detected if I go out. Ok, I tried flicking the house power, noise went away. Power on, noise came back. Repeated to check. It was being generated inside my house. But what was it? The TV?... nope. Unplugged it, and noise remained. I was then suspecting maybe an old AV amp was going bad. I don't use it any more, but I needed its sound delay to match my old TV which had massive latency. Moving the phone near the speaker... it didn't really change. Then I saw the TV box from my cable company. Could that be it? Pulled the power, noise gone! Thinking more, I think I can explain the noise. At 90 Hz, that's 5400rpm, the typical rotation rate of a cheap hard disk. Like you would find in a TIVO, which my TV box supported. I guess the disk in that is going wobbly, crating a vibration on the cabinet top it sat on. Now, I don't watch broadcast TV but I have to have it as part of my broadband bundle. I'm not going to miss it powered off. That's one more noise I can cross off the list.

[nick name]

On 4/21/2020 at 7:21 AM, porina said:

-snip-

 

The day the noise stopped matched the day she was taken away by ambulance and didn't return.

 

-snip

We're gonna need to know your whereabouts between the hours of blank and blank.  And if anyone else can confirm that.

[Teddy07]

Are you low frequency sensitive?

 

My uncle has a device that sends out low frequency noise to scare away weasel and so on. Humans usually donˋt hear it. I am the only one in the family that hears it even through a wall because I am sensitive. Maybe a neighbor has a similar device in use?

[porina]

28 minutes ago, Teddy07 said:

Are you low frequency sensitive?

Possibly. I don't know if there is a medial description of it, but I would describe my hearing as sensitive but not selective. I can hear the faintest noise, but not tell you what it is. In the quiet of night, I can hear the faintest of noises and depending on their characteristics, it can affect my ability to sleep. Hotels are hell for me.

 

Quote

My uncle has a device that sends out low frequency noise to scare away weasel and so on. Humans usually donˋt hear it. I am the only one in the family that hears it even through a wall because I am sensitive. Maybe a neighbor has a similar device in use?

I haven't heard of using low frequencies before. Any idea what actual frequency range they operate in? I have heard of devices that send ultrasonic for a similar reason. I'll try looking it up.

 

Edit: in a quick search, I've only come across ultrasonic devices.