# Interpreting REW Charts



## tane0019 (Jul 15, 2010)

Make some measurements using REW + X-Spectrum UMIK-1 mic this morning.

Below are the charts that I tried to understand & self-interpreting.
But the RT60/Waterfall/Spectrogram charts - I really don't understand that need some help.

This chart: From 20Hz-200Hz, it seem that my room has some form of bass boom.
Why is there a sudden drop in dB from 4KHz onward ? Too much adsorption in my current room at the direct sound path ? 









Now the most confuse charts: RT60/Waterfall/Spectrogram: How to interpret if the current room reflection & etc ... is good, OK or bad ?


























I also perform another set of measurement with the mic pointing toward the ceiling (with the respective mic cal file loaded).
Would expect them to be almost the same (or at least the trend). But to my surprise, between 500Hz to 3KHz, the curve seem to be "out of phase". Is this common or expected ?


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## Wayne A. Pflughaupt (Apr 13, 2006)

tane0019 said:


> This chart: From 20Hz-200Hz, it seem that my room has some form of bass boom.


That’s right. And very poor extension. Are you using a subwoofer?




> Why is there a sudden drop in dB from 4KHz onward ? Too much adsorption in my current room at the direct sound path?


Not likely too much absorption at the direct sound path. You’d have to have someone holding a blanket in front of the speaker to cause something like that. If that’s not the case (and I’m sure it isn’t  ) then it’s just the way the speaker is.

Another possibility is that you measured with both speakers playing. That can cause cancellation in the upper frequencies, due to the mic not being perfectly centered between the speakers (something that’s difficult to achieve), which would show up on a graph as a droop in high end response.




> Now the most confuse charts: RT60/Waterfall/Spectrogram: How to interpret if the current room reflection & etc ... is good, OK or bad ?


The waterfall shows low frequency signal decay. Peaks and especially room modes will show up with decay times much longer than the average. The Spectrogram is the same information as the waterfall, only viewed from overhead. Note that both show the same “hot spot” and long decay times at 50 Hz. Don’t know enough about RT60 t advise you on that.




> I also perform another set of measurement with the mic pointing toward the ceiling (with the respective mic cal file loaded).
> Would expect them to be almost the same (or at least the trend). But to my surprise, between 500Hz to 3KHz, the curve seem to be "out of phase". Is this common or expected ?


Unexpected, can’t say I’ve ever seen that. I generally have less trust in 90° frequency response measurements, but the mic is omnidirectional down in that range, so there theoretically shouldn’t be any difference.

Regards, 
Wayne


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## willis7469 (Jan 31, 2014)

Good points from Wayne. 
The absolute first thing I would do is find a new place for the sub. I can't believe that Rythmik sub is falling on its face. I'm gonna guess the "room boom" you mentioned is obnoxious, and distracting. Do you have placement options?


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## tane0019 (Jul 15, 2010)

Actually, for this measurement, I'm only using the front Left & Right speakers (ie Stereo) only.
Sub is not involved.

Bass boom portion I can understand cause I'm using a floor standing speaker in quite a small room + I do not have any treatment done (ie no bass trap or panel). Should be simple to solve if I'm going to introduce a DSP in between the pre and power amp in the near future.

Worrying is the dip at the HF portion.


> You’d have to have someone holding a blanket in front of the speaker to cause something like that. If that’s not the case (and I’m sure it isn’t ) then it’s just the way the speaker is.
> Another possibility is that you measured with both speakers playing. That can cause cancellation in the upper frequencies, due to the mic not being perfectly centered between the speakers (something that’s difficult to achieve), which would show up on a graph as a droop in high end response.


Some one was suggestion that maybe the tweeter/mid speaker fault causing the dip 
If so, then I'm in big trouble cause I seem to like the way this pair sound at the mid/high :gulp:
In fact the bass region as well as long as it is not turn up that loud.

Is there a possibility that there is something wrong with my notebook sound card ? (I'm using 3.5mm headphone jack to LR RCA into the pre-amp). Got VIA HD Audio Deck + also got Sound Blaster Cinema 2. So many setting, also don't know which one to set. I do set the playback at 24Bits/192Khz on the VIA HD Audio Deck & set the mic-in on REW to 48KHz (I think).

Or can it be something wrong with my mic and/or calibration file by X-spectrum ??

How can I check all these one by one to determine the actual problem/fault ??


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## Wayne A. Pflughaupt (Apr 13, 2006)

The chances of the mic or calibration being bad is practically zero. After all, Herb had to determine the frequency response of the mic in order to create a calibration file for it. Your best bet to chase down the problem is to get some different speakers and measure them. If you get the same results, I’d be looking at the sound card.

Alternately, you specifically mentioned “left and right speakers (i.e. stereo)” but did not confirm if you were measuring them independently or simultaneously... 

Regards, 
Wayne


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## tane0019 (Jul 15, 2010)

They are measured simultaneously.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Wayne A. Pflughaupt said:


> Another possibility is that you measured with both speakers playing. That can cause cancellation in the upper frequencies, due to the mic not being perfectly centered between the speakers (something that’s difficult to achieve), which would show up on a graph as a droop in high end response.


Regards, 
Wayne


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## tane0019 (Jul 15, 2010)

Thank Wayne,

Think there is no other way to really find out but to try all possible measurements 

Use another notebook, borrow another mic, measure another set of speakers & etc ...


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## tane0019 (Jul 15, 2010)

Tried to made a few rounds of measurement last night.

Only have one pair of speaker + one mic, but did managed to use two notebook for REW.
Regardless of what setting used -> the results were almost the same.
That more or less conclude no problem with REW setup as well as soundcard problem.

But what was interesting is as follows:-


As long as I don't touch the mic postn - the measured data is quite similar.
Once I touch the tripod + mic & start shifting them, the reading is quite different.

Below curve were taken one by one without touching the Mic/Tripod positn. Only changes are take both speaker Grill off-on plus change input at pre-amp from SE1 to SE2.









The next 3 curves were taken much later (after I move the mic/tripod around to measure the tweeter/mid driver). 
BLUE - Tripod/mic was place quite close to back wall (this look like my pervious measurement).
RED - Tripod/mic was place on my sofa (this look like my 3 earlier test above).
BLACK - Tripod/Mic was place in front of my sofa (less than 1.5 meter in front of the LR speaker center-line)
All measurements were taken with mic level at the tweeter height (or at least at best that I can adjust).









seem like the cancellation in the upper frequencies or even with the reflection from back wall play some part in it.

BTW, what did you guys recommend to hold the mic during measurement ?
The provided stand is so short, cannot do anything at all.
And the thread/hole is not the same as my Camera Tripod - so cannot use it to hold the mic properly.
So I have to slot in one of the 3 mic stand leg into the Tripod handle "hole" (like below pic).


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## willis7469 (Jan 31, 2014)

I use this tripod with my audyssey mic. I think a camera shop(or Amazon) could find an adapter for the different thread sizes. I ended up using the included tripod. Not convenient but I stack pillows. I have some of the memory foam/gel ones. I think this simulates a person in the LP and translates to a more accurate measurement. I also wonder if the longer arms/legs on the tall tripod in your pic would resonate and transfer into the mic. I have one like it but haven't tried it.


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## Wayne A. Pflughaupt (Apr 13, 2006)

tane0019 said:


> BTW, what did you guys recommend to hold the mic during measurement ?


A traditional mic stand with a boom attachment works best, but most people don’t want to spring for one. They can be pretty reasonably priced used, however.

Regards, 
Wayne


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## tane0019 (Jul 15, 2010)

Thks.

Think this is the only proper way of doing it.

Actually, did found out that locally in Singapore, a new one can be quite reasonably priced.
http://list.qoo10.sg/item/BOOM-STAGE-STAND-FOR-MICROPHONE/430986506
That is about US$23.75


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