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TOPIC: very high spl below 20hz in silent room

very high spl below 20hz in silent room 5 months 1 week ago #46334

  • Kokkalis
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Hello. I own a umik 1 gain 18. Recently i noticed that when i use RTA at very low hz the spl goes up to 90db
The house is not close to roads every machine is turned off and even tho it measures too high.
sometimes when i try to take speaker measurement at reference level the graph under 30hz is like waves

i have attached two pictures from rew
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very high spl below 20hz in silent room 5 months 1 week ago #46338

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It's normal. Bass waves travel VERY long distances. You are talking about a wave dozens of feet long. These extremely low frequency waves travel through just about anything. Walls, buildings and anything short of mountains. Those, they just bounce off of. This is part of the environmental noise around you. Most people can't hear much below 20Hz and so it's a non issue. If you want to eliminate it from your measurement, then set your sine sweep to something higher when you take your measurement. Say from 20Hz to 20kHz.
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very high spl below 20hz in silent room 5 months 1 week ago #46341

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I'm not sure what I'm meant to be looking at with those two graphs. Could you explain exactly what the measurement situation was for each please?

If the 'waves' one is meant to be background noise measurements (no sound source playing) then there is something very wrong with what is ultimately determining the displayed signal amplitudes. I'm talking across the whole frequency spectrum not specifically 20 Hz or any other low frequency.
Last Edit: 5 months 1 week ago by Ultrasonic.
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very high spl below 20hz in silent room 5 months 1 week ago #46344

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in the first graph you can see measurements of the subs. one as -15db volume to the avr and the other 2 at -10db. as you can see below 25 hz the result is different every time. The sub does not have that spl at 10hz.

In the second graph you can see that the measurement up to 80 hz is nowhere strait. This is not the response of my subs.

i have add also two other measurements. the first one is in a completely silent room RTA graph just recording with no sound playing.
you can see more than 90db peaks.

and the other the response of my subs before that problems
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very high spl below 20hz in silent room 5 months 1 week ago #46346

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Which graph is the measurement with no sound source playing?

I think you have a different issue here but for info if I measure the frequency response of my ported sub below its port frequency, I can get into a region where results are variable. I think this as it's getting into a region where the driver is essentially undamped. I now avoid measuring this low so as not to risk damaging the sub.
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very high spl below 20hz in silent room 5 months 1 week ago #46347

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The first one that says RTA with the red line (peak). the mic recording with all the machines closed in a dead silent room at midnight.
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very high spl below 20hz in silent room 5 months 1 week ago #46348

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Kokkalis wrote:
The first one that says RTA with the red line (peak). the mic recording with all the machines closed in a dead silent room at midnight.

So when you start the RTA the black line jumps about wildly, creating that red peak curve? If so that's wrong across the whole frequency range.

What confused me is your measurement sweeps don't demonstrate the same problem. I'm not at home to open REW to look but I'd be checking if there is a different gain or even mic associated with the RTA somehow.
Last Edit: 5 months 1 week ago by Ultrasonic.
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very high spl below 20hz in silent room 5 months 1 week ago #46350

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So when you start the RTA the black line jumps about wildly, creating that red peak curve? If so that's wrong across the whole frequency range.

yes exactly. it goes up wildly, then down but most of the time is about 70db.
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very high spl below 20hz in silent room 5 months 1 week ago #46351

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Ultrasonic wrote:
What confused me is your measurement sweeps don't demonstrate the same problem..

why, i think it is very obvious. below 20 hz the subs rolls off. at about 15hz sould be 20-25 db less than 20hz.
but as you can see some times the measurement is at the same level from 10hz with the signal measured as waves,
and some other times it stays at the same level down to 10hz. or even louder at 10 hz than 20 hz.
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very high spl below 20hz in silent room 5 months 1 week ago #46375

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Kokkalis wrote:
why, i think it is very obvious. below 20 hz the subs rolls off. at about 15hz sould be 20-25 db less than 20hz.
but as you can see some times the measurement is at the same level from 10hz with the signal measured as waves,
and some other times it stays at the same level down to 10hz. or even louder at 10 hz than 20 hz.
Again I'm not actually sure which of your graphs I should be looking at here? Please post a reply with one graph and point out what you think is relevant.

Edit: this was the graph I was referring as not showing anything obviously wrong in terms of its shape:

Last Edit: 5 months 1 week ago by Ultrasonic.
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very high spl below 20hz in silent room 5 months 1 week ago #46376

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Kokkalis wrote:
yes exactly. it goes up wildly, then down but most of the time is about 70db.
Being anywhere near 70 dB in a quiet room is nonsense though. There is some sort of fundamental gain problem here I think.
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very high spl below 20hz in silent room 5 months 1 week ago #46377

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In case it's informative, here's an example noise spectrum measurement from my room:

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very high spl below 20hz in silent room 5 months 1 week ago #46383

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Ultrasonic wrote:
Kokkalis wrote:
why, i think it is very obvious. below 20 hz the subs rolls off. at about 15hz sould be 20-25 db less than 20hz.
but as you can see some times the measurement is at the same level from 10hz with the signal measured as waves,
and some other times it stays at the same level down to 10hz. or even louder at 10 hz than 20 hz.
Again I'm not actually sure which of your graphs I should be looking at here? Please post a reply with one graph and point out what you think is relevant.

Edit: this was the graph I was referring as not showing anything obviously wrong in terms of its shape:


Yes this graph is normal, no problem, at all. these measurements where before some months.
Now as you can see at the other graphs the normal roll off below 20hz does not exist because umic measures something that do not know what it is.
and at 2or 3 hz goes up to 90db
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very high spl below 20hz in silent room 5 months 1 week ago #46385

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Kokkalis wrote:
Yes this graph is normal, no problem, at all. these measurements where before some months.
Now as you can see at the other graphs the normal roll off below 20hz does not exist because umic measures something that do not know what it is.
and at 2or 3 hz goes up to 90db
That makes a bit more sense if you thought that was sensible :) .

Please could you reply including the graph you are talking about and your specific concerns with it though. I don't want to keep guessing :)
Last Edit: 5 months 1 week ago by Ultrasonic.
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very high spl below 20hz in silent room 5 months 1 week ago #46386

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Thinking about this, if you previously managed to make some sensible measurements the thing to focus on is probably what might have changed? My gut feeling is this is a REW/PC issue rather than a microphone fault...

It may be that the best place for your query is the official REW support forum actually:

www.avnirvana.com/forums/official-rew-ro...rd-support-forum.10/
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