forumuser919698 wrote:Hello again after a good 10 years
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
