Help with tracking down low frequency (~63Hz) noise

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Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by Spriter1275 » Fri Apr 18, 2014 8:58 pm

Hello,
I hope I'm posting this in the correct forum.........For the past 3 years I've been bugged by a low frequency (~63Hz) noise which I am assuming is coming from an adjacent hotel but have yet to track the source down. I've bought a Tascam DR-100Mk2 and using the spectrum analysis of the Audacity program have tracked down what I believe the annoying frequency to be ~63Hz. Quite often it is at a low enough level that I can sleep, other times slightly louder to make it annoying and difficult to sleep and on other occasions extremely annoyingly loud such that the only way I can get any sort of sleep is to play white noise though some noise cancelling earphones.

The hotel has an assortment of pumps, air conditioning units, air handling units, boilers,kitchen extract / intake fans etc. I have had an acoustics "expert" take readings of the hotel equipment but he was unable to pinpoint the source. By measuring the noise intensity he came to the conclusion that the noise was entering my house via the ground rather than through the windows / walls. The hotel is circa 40m away. I am trying to organise with the hotel another time when I can get another noise expert and my plan was to constantly record the noise from my house while turning the hotel units on and off and see if I can tie that in with the noise recordings assuming the frequency vanishes at some point.

I've attached below a number of noise signatures and was hoping that maybe somebody out there can recognise this signature is a characteristic of say a boiler / air con unit or something else. I was trying to think what would cause the noise level to get higher but the frequency stays the same?! Multiple units of something maybe ?!

I have carried out a number of continuous recordings over night and the noise level seems to go up and down periodically.

(Ignore the 50Hz as that seems to be the mains charger noise) Some of the recordings were taken with white noise being played through some ceiling speakers which accounts for the higher frequency noise.

Appreciate any help as this noise drives me nuts at times !

(I can't seem to upload the images, what is the best way to do this, I've tried .docx, pdf, .htm but it doesn't seem to allow it ?!
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by kozikowski » Fri Apr 18, 2014 10:48 pm

Images can be in PNG or JPG.

I don't know of anybody who can recognize equipment by the pump sound leaking into another building. You may be able to do it with cycle times. I once had electrical interference from a neighbor's refrigerator. It had the perfect refrigerator timing signature.

There was another poster who had sound transmission problems. After multiple forum chapters I think we got it down to kitchen equipment the neighbor had on the common wall of their semi-detached house. In his case, the separation was about a foot.

120 feet is a long distance for dirt to be transmitting sound (he said, typing in an earthquake zone). The pumps would need to be almost broken to generate that much vibration. Don't motors tend to rotate at their power line frequency 50Hz?

If it never goes away, that signals a roof vent fan or other motor that never goes off. You can make a stethoscope out of a piece of garden hose. Jam one end in your ear and touch the other end to your walls and floors. Maybe even the dirt between the buildings. Also if you have a long heavy screwdriver you can hold that against a wall or other structure and listen to it (attached).

Koz
Attachments
Screen shot 2014-04-18 at 3.46.04 PM.png
Screen shot 2014-04-18 at 3.46.04 PM.png (196.53 KiB) Viewed 1425 times
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by Trebor » Sat Apr 19, 2014 3:49 am

63Hz may be the resonant-frequency on the room.
Noise-source(s) may be producing a range of frequencies,
but the acoustics of your room could be tuning-in to the 63Hz component.

https://en.wikipedia.org/wiki/Resonance
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by Spriter1275 » Sun Apr 20, 2014 5:35 pm

Thanks for those replies. I’ve attached some of the Spectrum plots now plus a sketch of my house in relation to the hotel.

Your view on it being unlikely to be transmitted through the ground was the view of the noise expert I had after he looked at the equipment the hotel has. The only pumps they had were 7kW or so, he seemed to think it would have to be a pump hundreds of kW’s in power and suggested I look around to see if there is something in the neighbourhood but there are no pumping stations that I have found.

I guess it’s true pumps will be at 50Hz or multiples of what with blade passing frequency etc. I was thinking maybe something that operates off a variable speed drive (3,800rpm), I’m guessing at maybe an air conditioning compressor, or fan. Another thread I saw on another forum was that the firing frequency of a boiler was causing a resonance in the exhaust which was around the 60Hz value.

Trebor’s view of it being a broad spectrum of noise translating to a resonance within the house is interesting. When I’ve measured the spectrums outside I can’t distinguish a 63Hz frequency over the surrounding frequencies, but inside the house the other frequencies are attenuated leaving only the 63Hz. I can pretty much hear the frequency within all rooms of my house so think it is in the structure somehow. The noise expert didn’t seem to think it was a resonance saying that if a resonant standing wave was being set up then it would be quieter closer to the ceiling (or wall) and would vary along its length, must admit I haven’t carried out readings to see if this is the case. I’ve calculated that 63Hz equates to a wavelength of ~5.4m ?! The distance between my floor and ceilings are 2.5m so that doesn’t tie up exactly but maybe close enough ?!

The other thing I was wondering is whether a standing wave is being set up within the “L” shape of my house, see attached sketch.

Another thought was maybe the noise is being transmitted through the drains that connect me to the hotel’s drains, plus my house has a network of drains surrounding it which takes the run off from neighbouring houses. Maybe a resonance could be formed in one of these lengths and then transmitted through the ground to my house ?!
The houses next to me are also about 20m away so don’t think it can be any domestic appliances that they have.
Attachments
spectrum 1.PNG
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spectrum 2.PNG
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spectrum 3.PNG
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spectrum 4.PNG
spectrum 4.PNG (145.6 KiB) Viewed 1236 times
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by Spriter1275 » Sun Apr 20, 2014 5:38 pm

further attachments as seem to be limited to 4 per posting.......
Attachments
spectrum 5.PNG
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spectrum 6.PNG
spectrum 6.PNG (134.32 KiB) Viewed 1237 times
HOUSE sketch.jpg
HOUSE sketch.jpg (46.54 KiB) Viewed 1237 times
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by Trebor » Mon Apr 21, 2014 7:19 pm

Spriter1275 wrote:... I’ve calculated that 63Hz equates to a wavelength of ~5.4m ?! The distance between my floor and ceilings are 2.5m so that doesn’t tie up exactly but maybe close enough ?!

wikipedia.org/Room_modes wrote:Room modes are the collection of resonances that exist in a room when the room is excited by an acoustic source such as a loudspeaker. Most rooms have their fundamental resonances in the 20 Hz to 200 Hz region, each frequency being related to one or more of the room's dimension's or a divisor thereof...
http://en.wikipedia.org/wiki/Room_modes

If it's audible in all rooms, which are of different dimensions, then maybe the the roof-space [attic] or cellar is the resonant-chamber ?.

If windows of the same size are in all the rooms, which may differ in size, the resonance of the window unit [pane(s) of glass] will be the same for every room. In that case the noise would be louder in the rooms with windows facing the noise source ...

Sound Transmission through Glass Panels - A Building Case Study, (page 2).png
http://sem-proceedings.com/25i/sem.org-IMAC-XXV-s01p03-Sound-Transmission-Through-Glass-Panels-A-Building-Case-Study.pdf [window-resonance]
Sound Transmission through Glass Panels - A Building Case Study, (page 2).png (66.78 KiB) Viewed 1229 times

http://sem-proceedings.com/25i/sem.org- ... -Study.pdf [ see page 16 of that pdf for an ingenious solution to window-resonance
]
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by hellosailor » Tue Apr 22, 2014 12:59 am

If this is in a US location, [strike]53[/strike] 63 Hz. sounds suspiciously close to 60Hz, our nominal AC line frequency. Air handling equipment, air conditioning compressors, large motors of that sort often are improperly mounted on hard fixed mounts and transmit large amounts of very low frequency noise that way. "Proper" mounting is on rubber sound isolators when noise is a concern, but I doubt the business would retrofit that.

If they are being co-operative, you might start wandering around their mechanical floors and equipment, testing for the same noise pattern. But tracking down anything at nearly subsonic frequencies can be a long and difficult process. Most equipment isn't really set up to deal with frequencies that low, and many people physically can't hear that low.

You might also try putting a frequency counter on your AC line, and theirs, to see if it comes up with the same odd 63 Hz. I don't know if "bad utility power" at 63 instead of 60 might be causing a problem but that's worth finding out. The local utility company should be able to check the powerline frequency for you--although they often stonewall on any "quality" complaints.

Sadly, the only practical way to get quiet may be to soundproof the bedroom, which can mean essentially building an isolated "room within a room". But that WILL solve the problem. (And if the problem is from that business, that solution still might be cheaper to them, than fixing an equipment problem.)
Last edited by hellosailor on Tue Apr 22, 2014 6:14 pm, edited 3 times in total.
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by kozikowski » Tue Apr 22, 2014 6:45 am

(Ignore the 50Hz as that seems to be the mains charger noise)

So "50Hz Mains" is the clue to the location. Probably not Arizona.

It keeps you up, so you can hear it. It never goes away? Can you hear it if you jam a screwdriver against your ear and touch the door frame like that illustration? If you can do that it's the best thing. That means you have a means to tell which parts of the house are louder than others. The short garden hose stethoscope will let you inspect the pipes, vents and drains. This is how car mechanics find noises that seem to be coming from everywhere at once.

Motors are most efficient when they're running at or near the power line frequency. Variable Speed motors would be insanely expensive in an industrial environment or installation. That's why your air conditioner shakes the house when it starts. It's cheap to do that. Some Very Definite reason would have to be found to install a variable speed motor, which makes 63Hz very unusual.

Koz
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by flynwill » Wed Apr 23, 2014 1:41 pm

Well actually variable speed motor drives are very common in commercial scale HVAC equipment these days. Particularly for things like fans and pumps. I could easily believe that Hotel might have an intake or exhaust fan with a 4-blade prop running at 945 rpm. However, as Koz points out 120' is a long way for such noise to get transmitted through the ground.
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Re: Help with tracking down low frequency (~63Hz) noise

Permanent link to this post Posted by hellosailor » Wed Apr 23, 2014 5:44 pm

"120' is a long way for such noise to get transmitted through the ground."
Assuming it is through earth and dirt, but not something else. I've heard relatively small hot water pumps (pressure boosters) cycling in the night, traveling up nearly 100' of masonry-over-steel. I've heard construction work, with handtools, appear to come from directly under a corner or wall when the work was up to 200' away but on the same slab flooring. The corner creates a resonance, which amplifies the noise, making it seem like the source.

Maybe there is something else, a conduit, a roadbed, or simply the same shallow bedrock that both the home and source are in contact with?

Noise is funny that way, very deceptive sometimes. It can't be de-coupled until the real source is found, which either means footwork, or some exotic equipment. (Time-delay measurements with multiple microphones, etc. to directionalize it.)
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