Accounting for room ambience irregularities?

dd4321

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Hi,

New to the forums.
O.k, shoot me down if you want to ( :-) ), but is there any way to account for any strange frequency response in the ambient noise itself?
By that , I mean, when I record SILENCE in my lounge the plot comes out with a heavy bass peak between 20-300 hz.
I don't know what it is yet ( maybe some appliance, or air-con ), but was wondering if it can be taken into account in the measuring?
I know people out there will be tempted to say I should investigate it, and I will try in the coming months, but first I am curious if I can take account for it?

Many Thanks

Keith
 

Wayne A. Pflughaupt

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Funny, I thought someone had posted a background noise graph the other day with a low-end rise, but I can’t find it now…

Regardless, background noise shouldn’t be a problem. Just make sure you play the sweep volume well above it (i.e. much louder).

Regards,
Wayne
 

EarlK

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I would add that since you now have a graphic ( starting point ) that shows your rooms background noise level, you have a bit of a leg-up on others who may not be aware of this as an issue.

I would advise, look at the lowest frequency of interest ( of your noise graph ) add 20- 30db to what's seen in your graphic + then make that level your test level.

This gives a 20-30 db signal to noise ratio for doing your tests ( in the LF range ) that will be a good starting point .

Obviously 40 db ( SNR ) is better, but maybe that gets you into an area of it just being "too loud" for comfort.

:)
 

dd4321

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I would add that since you now have a graphic ( starting point ) that shows your rooms background noise level, you have a bit of a leg-up on others who may not be aware of this as an issue.

I would advise, look at the lowest frequency of interest ( of your noise graph ) add 20- 30db to what's seen in your graphic + then make that level your test level.

This gives a 20-30 db signal to noise ratio for doing your tests ( in the LF range ) that will be a good starting point .

Obviously 40 db ( SNR ) is better, but maybe that gets you into an area of it just being "too loud" for comfort.

:)
 

dd4321

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In case anyone is interested, here is the plot of my room ambience.

I am not using REW for this but straight from my USB interface to a DAW for recording.
I am using a Sonarworks calibrated mic.
The usb interface (according to REW) response is a straight line, so shouldn't have any effect.
I have tried another mic - an old but good Shure SM81, which gives the same response.

Thats quite a boost below 500hz!

Funny heh?

Keith
upload_2017-12-4_21-18-37.png
 

Matthew J Poes

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A low frequency rise is normal in a room. Most all room's show this. John may be able to give a better explanation than I. As I understand it, there are a number of reasons. First, the noise from HVAC vents is largely low frequency. Motors, pumps, electricity all make low frequency noise as well. Noise that travels through large heavy floors like in a basement tend to be low frequency. In small rooms low frequencies are steady state meaning the microphone isn't picking up direct sound but sound that has bounced around the room quite a bit. It's had time to build up a bit, hang around, do its damage.

It's also common to see a low frequency rise in the noise floor of equipment. The best way to see what you have is with a loop back test, and some do show total flat silence, but when I've done loop back tests on cheap sound cards I often see this. High end devices are hit or miss.

Here is my noise floor captured two different ways. One is with the RTA and the other is with a normal sweep test, where the distortion plot now gives the measurement noise floor. The USB mics are so noisy that a lot of this noise, especially at high frequencies, is actually the ADC and not my room or computer.

REW FFT Noise Floor.jpg

RTA Noise Floor.jpg
 

Matthew J Poes

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Outside of an anechoic chamber there is no room that would measure dramatically different. The absolute level. An change and the high frequency is are typically lower for mics using an external interface, but the rise would exist in all rooms.

It wasn't that important to me to lower the noise floor too much more than I did. Too low and you can start to hear your own heart pumping blood through your ears, which is not comfortable. However If I did want to lower the noise floor further at low frequencies there are a number of expensive solutions. You can use a kinetic noise control product for floor soundproofing and a damped double layer subfloor to eliminate the noise coming up through the floor. You can use large expansion boxes for air handling that drop air at very low velocity by using very large openings with no screen or grate in front, optimized air floor paths with turning veins in the bends, and rubber isolators throughout. You would also need to isolate the HVAC system on spring isolators tuned to the fan resonance frequency. The electronics could be kept outside of the soundproof shell or inside their own soundproof box to isolate their electrical noise. This would drastically reduce the low frequency noise in the room. It's VERY expensive to do all this.
 
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