# REW Question RT60, EDT, .....



## Micheallong (Jul 8, 2007)

Hi Forum,
love REW, and read a lot here in the forum. 
I have some questions and it would be nice to get a little help and support from you. Thank you.

1)Is there a way to get a more static reading of REW RT60 graph? Everytime i measure it is really fluctuating a lot. If i measure 4 times at the same micposition with nothing else changed, "topt" changed everytime for 0,2 to 0,4 or 0,5 in different frequency ranges. How could i generate a more static measurement. I have really low ambient noise.
Is a longer sweep with multiple sweeps avareging better? It still fluctuates when i measure like this. But i think a little bit less, than with shorter sweeps.

2)I tried to find infos about "correct" rt60 measuring. Is it more significant to measure with different mic and also different speaker position, i read its even better to put the speaker in one corner and the mic in the other corner and than to avarege the measurements by eye?!?!?

3)When i measure a friends studio. The EDT curve in the rt60 graph (manual illustrates it shows the rt60 for the first db decay) shows a really odd graph. Where the lower frequency region 80-400 is at 0,15s and the higher region 1-8khz at 0,4sec. Does this illustrate a possible problem in the first 20ms? Speakers are soffit mounted. I know "topt" gives a more solid reading here, but in general the trend is the same as from the EDT curve.

4)Oh and a simple one....does the RTA-Analyzer (in combination with the generator) make use of the mic calibration file i set in the preferences?


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## Micheallong (Jul 8, 2007)

4) I tested it allready with an extrem wrong tect *.cal file. And the RTA makes use of the cal file. Thats nice.


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## Micheallong (Jul 8, 2007)

No advices for RT60?0


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## JohnM (Apr 11, 2006)

The RT60 results should be consistent for successive measurements, if they aren't there may be some other issue with your measurement setup. Easiest way to check that would be for you to post some of the measurements where you see variations.

RT60 is most appropriate for large spaces. Putting a speaker in a corner and a mic in another corner is a way to excite and measure all the modal resonances of the space. Not necessarily of much relevance for a studio.


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## Micheallong (Jul 8, 2007)

Hi John, 
thank you so much. 
Here is a group of measurements with the mic at the other side of the room.
http://expirebox.com/download/3172409ca43b2feee97aab45398e5a10.html


BTW. Do you have a suggestion on my 4) question from the firt post? Or do you also need a measurement.
Thank you.
Bye


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## JohnM (Apr 11, 2006)

Q 4 was does the RTA use the cal file. The answer is yes.

That's an odd set of measurements. The largest peak occurs after several other peaks have already arrived, suggesting the direct sound is lower than the reflected sound, which is very unusual. There are very, very large 50 and 80 Hz resonances. What kind of environment is it measured in? What is in the room? Where were the speaker and mic placed and what kind of speaker and mic are they? Some of the measurements in that group also have significantly higher noise levels than others, is the location affected by traffic noise, or trains, or some other noise source?


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## Micheallong (Jul 8, 2007)

Specs: Room 4x5x2,5m, speaker KRK vxt8, living room with some furniture tables absorber etc., mic dayton
Measuring: speaker near frontwall, and mic near the rearwall floor corner.....i think the earlier peaks are caused because the mic was a little bit of center and one speaker arrived earlier.

So in conclusion i need much more direct signal for RT60 measuring and the mic centered? I thought i should be more in the diffusefield a little bit offcenter to catch more of the ambience/verb to measure it? Perhaps thats a wrong guess?
You mentioned the difference in noise levels. Did you meter (see) it from the lower end of the impulse-graph? Or is there a better way to look at noise floor? I not sure what could cause the differen noise levels you mentioned because it was really quite and all doors and windows were closed. 

Here is another small measurement set: with the mic centered 50cm in front of the rear wall. No prereflections this time. But still a lot of variation in the rt60 graph.
Thanks

http://expirebox.com/download/c86160797b54366ea47e559b18a2b0c2.html


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## JohnM (Apr 11, 2006)

There isn't a meaningful reverberant sound field in such a small space. The RT60 results will be fairly consistent above a few hundred Hz, but they are still not reverberant decay in the sense for which RT60 is intended. In small spaces the results will vary significantly with mic position, unless you plan to listen with your head stuck in that corner I suggest you make your measurements with your mic where your head would be when you do listen 

It was the variation in the impulse response noise floor I was noticing in the previous measurement set. If there is no acoustic reason for that the cause might be electrical - for example, WiFi activity can interfere with measurements.


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## Micheallong (Jul 8, 2007)

Okay, thank you so much for helping me. Is there another type of measurement to proof if a small room is too dull or to measure high and mid frequency absorber in smaller room. I see on most measurements a slight dip at the upper end. In the Early Decay Plots there is also a clearly dip in the high end. Is this an indication for dullness or is this also not enough information within small rooms? I´m interested in measuring with and without carpet or curtain, or shelvs.So i thought RT60 would be the best choice to even out the rt60. Its more a question in generall.


Sorry John, i meant question 3 before. May i post a measurement of my friends studio? Perhaps you have an idea where i could find the error.


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## JohnM (Apr 11, 2006)

EDT will not give meaningful information in small rooms and it is completely useless below a few hundred Hz in such rooms as the response there is dominated by modal resonances. To look at the early behaviour of the impulse response use the envelope (also called Energy time Curve).

Above a few hundred Hz the RT60 plot will give a reasonable indication of relative absorption levels. For more advice on studio treatment in general gearslutz is a good resource.

It is entirely normal for listening position (as opposed to near field) measurements to roll off at the high end, don't be tempted to try and boost anything to change that.


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