You are here

Low frequency noise analysis

For everything after the recording stage: hardware/software and how you use it.

Low frequency noise analysis

Postby forumuser919698 » Mon Dec 16, 2019 9:21 am

Hello again after a good 10 years :wave:

I'm experiencing some low frequency noise, not in the context of producing music (which I haven't been doing anymore for a while), but rather in the context of disturbance during sleep :headbang:

Now I still had my old equipment lying around and tried to see if I could record it to aid in making my point at the relevant authorities, however I'm not really able to make sense of it... (I played instruments rather than engineer sounds hehe). I mean authorities speak of 60dB threshold for it to be called disturbance. But how does this translate to the negative dB values I see in my audio editor spectrogram or the relative dB values between recording in my living room vs my bedroom?

Is there anything sensible to say about these two visualisations and recordings I made with my condenser microphone?

My unconscious me will be forever grateful!
:angel:

This recording is made from the desk in my living room:
Image
Source wave file: http://s000.tinyupload.com/index.php?file_id=03415861949458853175

This recording is made from the floor in my bed room:
Image
Source wave file: http://s000.tinyupload.com/index.php?file_id=14656658141866630779
forumuser919698
Posts: 1
Joined: Mon Dec 16, 2019 9:04 am

Re: Low frequency noise analysis

Postby ef37a » Mon Dec 16, 2019 10:56 am

I am sure Dr H will be along shortly but I would say first thing you need is a C weighted SPL meter and then perhaps a speaker that can deliver 40Hz or so to calibrate your recordings?

But, VLF in rooms is going to be like chasing a standing wave dragon!

Dave.
ef37a
Jedi Poster
Posts: 11298
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk

Re: Low frequency noise analysis

Postby Wonks » Mon Dec 16, 2019 11:22 am

Your graphs are all relative to a recording level, and not to an actual volume level in the rooms. As ef37a says, you really need an SPL meter, and a calibrated one if you want to make an official complaint (hiring one with a calibration certificate will probably be your best move).
User avatar
Wonks
Jedi Poster
Posts: 10742
Joined: Thu May 29, 2003 12:00 am
Location: Reading, UK
Correcting mistakes on the internet since 1853

Re: Low frequency noise analysis

Postby The Red Bladder » Mon Dec 16, 2019 11:36 am

As infra-sound is one of my pet topics, I leaped upon this thread with a nude vicar - but sadly the audio file you posted has a tiny amount of machine, mains hum and background noise on the left channel and not much else!

We have a saying in German - Wer zuviel misst, misst Mist. (He who measures too much, measures sh!t!) so bear that in mind - but at the same time, you can not measure infrasound using some commercial condenser mic that is capped at 20Hz and 20kHz and whose diaphragm is too small and too tight to be capable of moving with a very low-frequency wave.

Try using a soft-mounted speaker driver and as ef34a states, calibrate your measurements with a pukka sound pressure meter. Make a series of LF sounds using either an online sig-gen (https://www.szynalski.com/tone-generator/) or a physical sig-gen in the room and set a specific level using that meter and read-off what the test driver registers.

Also, remember that you hear infrasound with your balance organs which are also in the ear and as one gets older, one becomes much more sensitive to infrasound (which is why there have been so many complaints in Canada and elsewhere from old people living down-wind of large land-based wind farms). We are particularly sensitive around 6Hz.

The building you are living in may amplify infrasound and modern building with block-and-beam floors, concrete walls decked with plaster-board do this almost always, esp. blocks of flats. Traditionally built old houses with oak beams with lime fill-in between the beams are very good at eliminating external infrasound.
The Red Bladder
Frequent Poster (Level2)
Posts: 2420
Joined: Tue Jun 05, 2007 12:00 am
Location: . . .
 

Re: Low frequency noise analysis

Postby ef37a » Mon Dec 16, 2019 11:42 am

Thank you for the mentch' Red (typo excused. Crimble)

I would also suggest a 'silent' recording of the system with a terminated, screened XLR in the mic input, 48V on.

Many mic pres suffer '1/f' noise that gets filtered out for normal music purposes.

BTW, I dimmly remember a TV piece about a low frequency noise that bedevilled a town (oop north I think?) . This 'Newcastle Moan' could be heard in certain places around the place but not others. Don't think they ever tracked it down. LF can travel a very long way.

Dave.
ef37a
Jedi Poster
Posts: 11298
Joined: Mon May 29, 2006 12:00 am
Location: northampton uk

Re: Low frequency noise analysis

Postby Hugh Robjohns » Mon Dec 16, 2019 11:44 am

forumuser919698 wrote:Hello again after a good 10 years :wave:

Hello, and welcome back.

I'm experiencing some low frequency noise... in the context of disturbance during sleep

Oh dear, sorry to hear that. I know how frustrating that can be... and how even more frustrating it can be to track down the source and do so etching about it.

I mean authorities speak of 60dB threshold for it to be called disturbance. But how does this translate to the negative dB values I see in my audio editor spectrogram or the relative dB values between recording in my living room vs my bedroom?

That's where references and calibrations come into it.

I presume the threshold they mention is in terms of sound pressure levels dB SPL... But there's probably also a 'weighting' specified as well, such as A or C weighting. You'll need to check the relevant documents or talk to the authorities to find out.

Once you know the reference, you can, in theory, calibrate your system to make measurements relative to that reference. But that's what a proper sound level meter is designed to do, so as wonks says, it will probably be more sensible to hire a professional SPL meter. Many have built-in logging facilities too. But talk to your local authorities to see wha they suggest. Most are willing to help...

At the moment, your measurements are relative to the peak level of the digital converters in your interface, which is why all the numbers are negative, counting down from 0dBFS. The problem is that we have no means of relating the converters reference to an acoustic sound pressure level because we don't know the sensitivity of the microphone (although you can look that up) or the amount of gain being applied to it, or the converter alignment.

These last two things are sometimes specified, but usually need to be measured with test equipment, or a known acoustic sound pressure level could be generated and that used as an acoustic reference.

Bear in mind, also, that the microphone, preamp and even the converter will all have a low frequency roll-off and so lose sensitivity at the lowest frequencies anyway. The mic is probably the most significant, especially of it is a directional (eg. Cardioid) mic which may drop off steeply from 40Hz or so. But most audio equipment is designed not to capture infrasound.

Is there anything sensible to say about these two visualisations and recordings I made with my condenser microphone?

Not really I'm afraid. The general downward slope is entirely normal. It does look like there's a peak around 150Hz in the first plot, but the frequency resolution isn't fine enough to see what that really is. It could be an acoustic sound... or it could be some mains hum getting into the mic cable or preamp, or even a ground loop problem...

Low frequency noise can be caused by all sorts of different things. There are physiological issues that can cause it, such as various form of tinnitus, or just natural internal biological sounds which may not be apparent during the (noisy) day.

Then there are noises due to mechanical sources, both within the property and without. A very common source in houses is the fridge, for example, but anything with motors or pumps can generate low frequency noise. Often the vibrations from the pump/motor will be coupled mechanically into the floor, or even pipework, and be conveyed mechanically to a completely different part of the house or building. Shared properties (flats etc) can be a real pain for that kind of thing.

You might be able to prove whether it is a local source or an external one by switching off the mains electricity as the consumer unit for a few hours when you go to be and see if that makes any difference. If it does, switch everything back on and then try switching off individual appliances to track down the culprit.

If its external noise, there's not going to be much you can do, as just trying to identify the source will be a major challenge. The problem is that low frequencies travel a very long way, and bounce off and around buildings in complicated paths, setting up interference patterns.

All manner of industrial systems will generate low frequency noise, including air conditioning plants, water, sewage, and gas distribution pumps, industrial chillers, traffic, backup power generators, and so on and so forth. There can also be LF noise from wind turbines, and even resonances caused by wind moving between buildings, or blowing over chimneys.

It might help in the identification process if you can estimate the frequency range that's troubling you. It is a clear 'hum' in the 50-100Hz kind of zone? Or is it much lower in the infra zone range below 20Hz -- the kindof sound you feel more than hear?

As low frequencies tend to increase in sound pressure level in the corners of rooms and near room boundaries (walls), it can often make a big difference to move the bed away from walls and especially corners, or to sleep with your head at the opposite end of the bed -- at least for a trial to see if it helps!

H
User avatar
Hugh Robjohns
Moderator
Posts: 26401
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Low frequency noise analysis

Postby The Red Bladder » Mon Dec 16, 2019 11:47 am

ef37a wrote:(typo excused.
Sorry, I nudged the fader and reduced your power by half - or -3dB.
The Red Bladder
Frequent Poster (Level2)
Posts: 2420
Joined: Tue Jun 05, 2007 12:00 am
Location: . . .
 

Re: Low frequency noise analysis

Postby The Red Bladder » Mon Dec 16, 2019 11:59 am

It is going to be A-weighted (for nearly all council districts - this is an H&S issue, so levels are in theory at least set locally). 'A-weighting' means that it is -50dB at 20Hz, so you can be shaken out of bed at three in the morning by passing lorries and still not have reason to complain!

Infrasound is just one of those pestilences that we are expected to live with in today's society, so one has a choice - either put up with it and grit your teeth and wait (or weight) for death, or do what I did and move to live in the middle of nowhere and own all the land around you, so that nobody can come and start annoying you with their noises!

It also means I can listen to Rammstein at ear-bleed volumes!
The Red Bladder
Frequent Poster (Level2)
Posts: 2420
Joined: Tue Jun 05, 2007 12:00 am
Location: . . .
 

Re: Low frequency noise analysis

Postby The Red Bladder » Mon Dec 16, 2019 12:16 pm

P.S. and whilst I am still here - even top-quality calibrated SPL meters use Mickey Mouse electret or dynamic microphones and they are 100% useless below about 20Hz. I actually built a couple of 4" ribbon mics to experiment with infrasound, as there is nothing out there that goes below 10Hz in any meaningful manner.
The Red Bladder
Frequent Poster (Level2)
Posts: 2420
Joined: Tue Jun 05, 2007 12:00 am
Location: . . .
 

Re: Low frequency noise analysis

Postby Hugh Robjohns » Mon Dec 16, 2019 12:21 pm

The Red Bladder wrote:... or do what I did and move to live in the middle of nowhere and own all the land around you, so that nobody can come and start annoying you with their noises!

Nice if you can... But you'd need to own a very great deal of surrounding land to be sure of silence.

We live in a small rural village, and most of the time it is very quiet indeed. But if the wind is blowing 'the wrong way' I can just hear the LF noise of the pumps and chiller machinery at a dairy farm 3/4 mile away, and/or the local sewage treatment plant nearly a mile away in the same direction. And very occasionally we can hear the LF tyre noise from the M5 motorway about three miles away in the opposite direction.

As I said, VLF sound can travel a very long way... And doubly so if you're near large areas of water.
User avatar
Hugh Robjohns
Moderator
Posts: 26401
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Low frequency noise analysis

Postby The Red Bladder » Mon Dec 16, 2019 3:12 pm

small rural village, + noise of the pumps and chiller machinery at a dairy farm + the local sewage treatment plant + noise from the M5 motorway

That sounds like the joys of present-day rural Englandshire!

Noise pollution here is when one of my neighbour's sheep farts.

We had a girl from Virginia here on work experience and she was reading a book in the sunshine, when one of the horses came around the corner of the studio, looked at her and breathed out loudly. Other than that there were no noises - the sheep were having their siesta and the tweety-birds had stopped tweeting for the afternoon.

She looked at me and said, "Goddam, it's noisy around here!"
The Red Bladder
Frequent Poster (Level2)
Posts: 2420
Joined: Tue Jun 05, 2007 12:00 am
Location: . . .
 

Re: Low frequency noise analysis

Postby James Perrett » Mon Dec 16, 2019 3:41 pm

I remember seeing a schools TV programme many years ago where they showed some university researchers measuring low frequency rumbles by using a turntable with the stylus placed on a stationary platter.
User avatar
James Perrett
Moderator
Posts: 8921
Joined: Mon Sep 10, 2001 12:00 am
Location: The wilds of Hampshire
JRP Music - Audio Mastering and Restoration. JRP Music Facebook Page

Re: Low frequency noise analysis

Postby Hugh Robjohns » Mon Dec 16, 2019 7:44 pm

That's a very clever idea. I like that. (But you'd need a solidly mounted turntable, not a sprung mounted one!). :clap: :thumbup:
User avatar
Hugh Robjohns
Moderator
Posts: 26401
Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
Technical Editor, Sound On Sound

Re: Low frequency noise analysis

Postby Martin Walker » Tue Dec 17, 2019 12:49 am

I used to perform loads of acoustic feedback tests on Garrard's turntables when I worked in their R & D lab donkey's years ago, by resting the stylus on the turntable, which in turn was standing on a vibration platform whose frequency could be swept.

This is how we determined where best to add struts in the turntable moulding to damp down any resonances.


Martin
User avatar
Martin Walker
Moderator
Posts: 15204
Joined: Wed Jan 13, 2010 9:44 am
Location: Cornwall, UK

Re: Low frequency noise analysis

Postby CS70 » Tue Dec 17, 2019 12:36 pm

Martin Walker wrote:I used to perform loads of acoustic feedback tests on Garrard's turntables when I worked in their R & D lab donkey's years ago, by resting the stylus on the turntable, which in turn was standing on a vibration platform whose frequency could be swept.

This is how we determined where best to add struts in the turntable moulding to damp down any resonances.

Martin

Ah! Just last week I acquired and used a Garrard turntable for next to nothing, to use as a prop for the lyrics video of my band's next song, due next Friday.

I have no idea which model it was other than was old (it took a while to clean it up to presentable form) and it looked very nice, with that retro vibe that fitted the song perfectly.

I was tempted to keep it but in the end I just gave it away, because I have zero use for a turntable. But how cool it was!
User avatar
CS70
Jedi Poster
Posts: 4983
Joined: Mon Nov 26, 2012 1:00 am
Location: Oslo, Norway
Silver Spoon - Check out our latest video and the FB page

Next