"The Hum"

[Bob Scott]

Is anyone else in this message base familiar with the phenomenon? Have you heard it at night in your location?

It is a very low frequency audible annoyance that is currently the subject of discussion in our local newspaper. A beating VLF hum so low my wife didn't hear it. Information is available on Wiki:

http://en.wikipedia.org/wiki/The_Hum

It has kept me awake some nights for over a decade and now that it is in the newspaper it is apparent that many other people are annoyed and sleepless all over the north shore too. I now use Zopliclone to get to sleep.

I heard the same noise some nights in San Jose. I am positive that the noise source was locomotives idling in the train yard a few miles away. I'm quite sure because I heard it increase in frequency. Then I heard a locomotive air horns blasting at level crossings. It drifted away. Those low frequencies can travel through the ground for miles.

Thanks to Engineer1138 for pointing me to BG Micro where I just bought a pair of seismic geophones. I'll separate them a few feet so that I can use the pair to detect phase differences. That way I can triangulate the signal to find the source or sources. I don't know if the frequency response is suitable but I'm going to find out.

Bob

[Bigglez]

Greetings Bob,

Is anyone else in this message base familiar with the phenomenon?

Sorry, that doesn't ring a bell...

I'm more concerned about "The Nack".

Comments Welcome!

[haklesup]

If you are indeed sensitive to ground vibration then the geophones should pick up something but I think you will have a hard time picking the signal you want out of all the other noise you will get. It should be an interesting experiment if nothing else with other applications possible. I think you need three phones to triangulate but you can move two around and eventually come to some conclusion.

If the annoying VLF is Acoustic in nature than of course the geophones won't work but there are some VLF microphone designs if you search the net. Might even be able to use a sub-woofer baffle (as an acoustic filter) and a good microphone to go down to 20Hz at least.

I'm not particularly sensitive to either so I don't have any direct experience with the phenominon though I have heard trains at night in San Jose all the time, there are plenty of long heavy ones. The freeway can spit out quite a rumble too especially at night when speed is high and other noise sources are low. I find that humidity has a major influence on how far low freq sound can be heard.

One way I see it, wind itself can be seen as a VLF audio source. At just a few herts (equivelent, non periodic noise) can you imagine what the equivelent SPL in dB would be (nobody actually measures it that way). Below 15Hz (or so) the ear filters sound anyway, you would need to "hear" it with your body.

[philba]

I'm sure it's caused by global warming. or aliens. maybe both.

Seriously, I'm pretty sure that this is a very disparate set of phenomena. For example, I live on a bluff above railroad tracks. Some times the trains will idle on the tracks, even miles away. You can hear them but the sound is pretty indistinct and has some pretty low frequencies. I've also heard VLF coming from ships (I also live near a fairly busy shipping channel) - engines, I think.

An interesting project would be a VLF direction finder. This might entail construction of a sensor (I hesitate to call it a microphone). It would require some signal processing - FFTs, filtering and so on. I'll have to think on this topic.

[dyarker]

????? Hum, 3 sensors in equilateral triangle at least 100 yards on a side, equal length cables, and measure phase difference ??? just brain storming, sounds interesting

[Bob Scott]

Speed of sound is close to 1130 fps. A time delay of a single 1/4 cycle should be very apparent on a scope. So if the noise is around 30 Hz or so, the wavelenth is 37 feet. I'm going to set the detectors apart by 1/4 of this, around 9 1/2 feet.

I'd like to make the VLF "audio" hearable by anyone with ears with a normal frequency response range by electronically processing the signal up in frequency. Heterodyning will have two frequency products. I only want one so I won't use heterodyning. Too bad. I'd like to play with analog multiplier chips. I need to digitally record the signal and play back at a higher clock rate.

It's my opinion that people can clearly tell the direction of sounds only (when signal level is equal at both ears) within a narrow range of frequencies where 1/4 wavelength is close to the distance between the ears: around 7 inches. So by multiplying a 30 Hz signal's frequency by 16, the 1/4 wavelength becomes 7 inches and the frequency will be converted to an easily audible 480 Hz. The direction of the noise should then be apparent by using stereo headphones.

Heterodyning can convert the 30 Hz to two audible frequencies 60 Hz apart, but can't do anything to the phase to make the direction discernable by stereo hearing with stereo headphones.

Oooh, I wish they still made bucket brigade sound delay ICs. Does anyone know of a source? Otherwise I'll have to use A/Ds, memory, D/As. Too much work!

[jollyrgr]

I have known about this for years; but don't hear it myself. This is communications for submarines from the US Navy and most likely those of other countries. There was a TV show on one time YEARS ago (pre 1990) when someone was talking about this noise. The guy that could hear it started tapping on something to simiulate the sound; which sounded very much like RTTY. I said "this is submarine communications". My parents and brothers looked at me like "what????". The TV show then went on to theorize the same thing.

The funny thing is mostly men hear this. And many times people that can hear this cannot do so near the station but can many hundreds of miles away.

Here is a link explaining the low audio frequency system:


http://www.vlf.it/submarine/sbmarine.html

[Dean Huster]

As I read this thread, I'm also listening to this very thing. It is a railroad diesel locomotive. Depending upon the weather, humidity, etc., the sound level varies and it is a low frequency. The rails are about 4 to 5 miles away. I rarely hear the horn even though many crossings are within that radius.

Dean

[Bigglez]

Greetings Dean et al.,

As I read this thread, I'm also listening to this very thing. It is a railroad diesel locomotive.

I'd have to agree, that's a very distinctive sound.
When I was living near Chicago Illinois, in the 1980s,
the company was across the street from the Canadian
Pacific roundhouse. During the year every locomotive
came in for service.

On the weekends Anoraks (train spotters) would use
our parking lot and collect numbers as the locomotives
arrived for service.

What irony, the work we were doing was with lasers!
So the optical tables were placed on small inner tubes
as isolation.

Comments Welcome!

[Bigglez]

Greetings Phil et al.,

I've also heard VLF coming from ships (I also live near a fairly busy shipping channel) - engines, I think.

On a road trip through the north woods of Wisconsin (in the 1980s)
I saw a utility wire unlike any other, it went on for
miles! I was told it was part of the US Navy VLFstation
antenna, used to send coded messages to subs and spooks.

Here's a list of VLF stations.

Comments Welcome!

[philba]

I'm not sure that the "HUM" is due to VLF comm. The link that pete gave says that the base frequencies as 3Khz to 30Khz and the 2 modulation techniques used, fsk and msk, send 50-75 bps and 300 bps, respecitvely. msk is used for sub comm so it's unlikely that a steady 30-60 hz hum would originate from that.

By the way, I'm kind of surprised that no one mentioned power transmission as a source of "THE HUM".

[Karl Williams]

I live about a 1/4-mile from a railroad switching yard and although I can't hear the "hum" I can often feel the house vibrating at a very low frequency. Sometime all the windows vibrate. I've gotten so used to it that I've tuned it out. There have been many times that I've been awakened out of deep sleep when the rail cars crash together.

Maybe you could build a device that picks up the VLF with a sensor/microphone, inverts the phase, amplifies the signal and then outputs it to speaker in your bedroom at the same level as the incoming signal - sort of like the phase cancellation headphones available commercially.

[dacflyer]

you guys are touching on a subject that freaks me out sometimes at night.. i cannot hear anything,, but when i am on my bed in a deep sleep, or about to doze off, i can slightly feel my bed vibrate. i tested if i was going nuts or what ever,, i had a glass of water on the headboard, and it never made ripples, but i swear i could feel it.. well later on i discovered, that when laying on my left side, my heart beat was causing it..seems if i have my hand on my left side and laying on my right side, my heart has such a strong beat. but if i lay on my right side i barely can feel it...
ya i know, i am strange but i had to share this with you all..

[Bob Scott]

I'm not sure that the "HUM" is due to VLF comm.

By the way, I'm kind of surprised that no one mentioned power transmission as a source of "THE HUM".

I'm sure it's not VLF electromagnetic. I can't hear radio waves. If I could hear a 5 watt VLF transmitter way out in Wisconsin, The local 60Hz power hum would explode my ears. BOOFF!

It is not power hum. Anyone in electronics would recognise 60Hz hum. It's the wrong frequency. I think possibly the hum is creating standing waves between floor and ceiling. 8 feet is a quarter wave of 35 Hz.

It does not sound like it is modulated by code. It is continuous wave. It sounds like two or more BIG V8 diesel boat engines idling and creating a very slow beat because they are not phase locked.

I still think it's locomotives idling. I searched for information and found out that there are 12 and 16 cylinder diesel locomotives and they have different idle RPM settings for different ambient temperatures. Below 50F (10C) the idle RPM must be increased. And seeing how I googling gets me a whole range of idle speeds from 335 to 550, I will have to wait until my geophones arrive to measure the frequency and possibly direction, and see if the frequency changes with the temperature at night.

In the meantime I have a Tek function generator and frequency meter. I'll couple that to an old Wharfedale 8" woofer from my junk box. It has a 23Hz free air resonance. Adjust frequency until I hear another beat.

Bob

[Robert Reed]

Althogh no one ihas mentioned it, pity the poor guy that lives within earshot of the wind driven power plants. I have been told that spending any amount of time in the center of activity will drive one crazy from the drone they produce. Any one living near one of these?