# 75 decibels...



## takman (Oct 13, 2009)

sorry to make a thread on this..but i have searched and searched....

When checking levels, at 75 decibels, i have to crank the gain on the mic up nearly to max before i can get REW to tell me levels are ok......I dont know if this has to do something with my soundcards pre-amp? I'm using the Behringer ECM8000 and the phantom power is on....

even though it tells me the check levels are in the green, im still usually 6 DB's or more off...when i try to get closer, i get that loudfeedback in the speakers, so i have to back it off....

My question is, can i run tests at 80-85 decibels? That way i can match the levels closer....and if i do, what effect if any is this going to have on my readings?

I had read that people who compose/mix/master music...do it around 80-85DB's

thanks


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## JohnM (Apr 11, 2006)

You can run the tests at a higher level if you like, it will improve S/N a little but otherwise makes no real difference - wear hearing protection though, especially if you are planning a long measuring session. Levels within 6dB is fine though, so I wouldn't measure louder just to alter that.



> when i try to get closer, i get that loudfeedback in the speakers, so i have to back it off


This is a problem, however. If you get feedback it means the input signal from the mic is being fed back to the output signal, so you need to check the playback mixer settings and mute the input your preamp is connected to or you will not get meaningful measurements at any level.


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## laser188139 (Sep 19, 2009)

I am a little confused about exactly what you are seeing and doing. 



takman said:


> ...
> When checking levels, at 75 decibels, i have to crank the gain on the mic up nearly to max before i can get REW to tell me levels are ok......I dont know if this has to do something with my soundcards pre-amp? I'm using the Behringer ECM8000 and the phantom power is on....
> ...


I'm with you so far. I use two different computers for my measurements. For one, running Vista, I can set the mic preamp on the Xenyx 802 at around 1:00 and the soundcard on the computer sees an adequate signal. On the other, which runs XP, I seem to have to set the mic preamp volume on the Xenyx802 to its maximum for the soundcard to see an adequate signal. 

So soundcards and soundcard drivers are different, versions of Windows are different, etc. Where the soundcard driver does not let REW control the input volume on the soundcard, you may have to play with the input volume level through the Soundcard Control Panel in Windows. 



takman said:


> ...
> even though it tells me the check levels are in the green, im still usually 6 DB's or more off...
> ...


Here I start to get lost. What is 6dB off of what expectation? If check levels says green, the signal coming into REW is adequately high for the measures to be reliable, without clipping. If it is typically showing -22dB, and you are concerned that the helpfile says the target is -18dB, don't be concerned. If the light is green, it is good enough. 



takman said:


> ...
> when i try to get closer, i get that loudfeedback in the speakers, so i have to back it off....
> ...


What are you doing to try to get closer? Obviously, feedback is a problem. I am not sure what you are trying on the mic/input side that is creating feedback that you are hearing in the speakers. Getting feedback does make one wonder what is happening. 



takman said:


> ...
> My question is, can i run tests at 80-85 decibels? That way i can match the levels closer....and if i do, what effect if any is this going to have on my readings?
> ...


The obvious effect of running tests at a higher volume level is that they will be louder, and the readings should be higher. Some people find 75dB uncomfortably loud. But 75dB is usually loud enough for the measured sound to be well above the background noise level and the measurements to be valid. 

But yes, you can adjust the output volume such that the sound level is 85dB. And you can identify this as the calibrated SPL so that the charts indicate what was measured. Or you can lie to REW and say the average level is 75dB when it is actually 85dB on the SPL meter, so the charts look comparable to what people expect. And there is a feature in REW to adjust the measured data later, so you can measure at 85dB but translate everything down -10dB after taking the measures. Once you get off the beaten path, you will want to make notes in your measurements to keep track of what was really happening. 

If you flesh out the information on what you were doing along the way, it might be clearer if any improvements can be made on the input side. Based on my guesses, it sounds as if everything was fine up until you Check Values gave you the green light, but that you became overly concerned with trying to get the level there exactly -18dB. 

Bill

EDIT: I see that John gave the quick version, while I was elaborating on the long version.


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## takman (Oct 13, 2009)

john first off, thank you for this great program, making it free and taking time to help others in learning it...

When i say im getting feedback, its only when the mic is about maxed out on the volume. you know, when you have a mic and you turn the gain up too much and you get that high squeal noise....

I should note that my set-up is for mixing music, not home theater. i have my speaker/listening position set-up in an equilateral triangle with 1.5 meters on each side. The mic is set at 1.2 meters height.

this is the highest level i can get before i get the feedback...with the gain almost at max on the mic...












> So soundcards and soundcard drivers are different, versions of Windows are different, etc. Where the soundcard driver does not let REW control the input volume on the soundcard, you may have to play with the input volume level through the Soundcard Control Panel in Windows.


I'm using Windows XP service pack 3. If i go into sounds and audio devices then in the audio tab, i get this:










the different HD audio volume setting is turned up for all of them, and i tried applying them just to see if it changed anything but it didnt...

And i also toyed with all the settings in the mixer screen for my soundcard..didnt change..



> Here I start to get lost. What is 6dB off of what expectation? If check levels says green, the signal coming into REW is adequately high for the measures to be reliable, without clipping. If it is typically showing -22dB, and you are concerned that the helpfile says the target is -18dB, don't be concerned. If the light is green, it is good enough.


ok, so its good enough , but i still would like the two levels to match, its supposed to be that way anyways correct?


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## takman (Oct 13, 2009)

now onto the graphs.....

I've been trying to find information on the net about how to understand what im looking at...but information is hard to find..and its getting a little frustrating! most of what i find is people on forums asking others to interpret their graphs.....i need something to read thats thorough and easy to understand.

The frequency response graph is easy... i understand it...its the waterfalls, impule, RT60, energy time etc that i want to learn...

heres my room at 80DB smoothed 1/24

left speaker:










right:










compared:










waterfalls. are the settings correct? dont i want to see the decay time until it ends? it seems as if the Low frequencies decay is too long?
for the DB slider on the left, where do i set it?


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## hybris (Jan 25, 2009)

Regarding the feedback: I suspect you may be playing back what is coming from the mic, which is not a good thing. 

You should mute the mic/line input under the playback mixer, or you could try looking for a option such as this, and making sure it is deactivated / unchecked:


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## glaufman (Nov 25, 2007)

Regarding the other graphs, waterfalls are simply FR plots with a third dimension of time added. What you're looking at is how evenly the sound decays over time. Impulse Response plots are the room's response to an impulse (also known as a Delta or Dirac Delta function, are you familiar with these?)... again you're looking at sound decaying over time, but you're also reflections to gate out so they don't corrupt you're other plots. RT60 shows the amount of time it takes each frequency to decay to -60dB from its peak. Energy Time and others you might need someone smarter than myself to explain to you, but it's another way of looking at IRs. Actually, many if not most of these are different ways of looking at the same data, it's just that the different pictures make it easier to pick out different characteristics.


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