# Help needed interpreting my graphs



## Chico Santarosa (Feb 25, 2012)

Hello all,

it's been very nice so far knowing the HTS forums and experiencing with REW. I've gone through some threads that helped me using REW and interpreting my measurements and also REW help files were of great value. Despite working with professional audio for 6 years I am a newbie with acoustics measurements. Therefore it would be nice to have some help and advice with my measurements results I have so far. 

I apologise in advance for any English mistakes. 

I am posting some graphs and hope that could inspire you to help me. :help:

To start I'll say something about my room. It's a irregular shape, but symmetric, wood covered (from floor to ceiling). Fairly big: around 20 feet to rear wall (the wall actually meets the ceiling which is splayed). The side walls are more or less 7 feet away off the speakers each side (the speakers are around 5 feet spaced), but this is a very rough idea because the ceiling there is also splayed and there are broken spaces (symmetric each side). My room is in the top floor of an old house and is all made of wood. Walls and ceiling are made of thin lath and stuffed with fibreglass, having the roof tiles on the other side.

That was made like that, it was not thought for acoustics. Anyway I though that would be a good recipe for good acoustics. But there is something there I cannot guess. I have a consistent dip in the region between around 100 and 160 Hz. All the measurements you see were made with the microphone in the mix position. I used calibrated SPL and the Neumann KM183 (which wasn't ruled off but showed similar results as the radio shack meter in the lower region). I am sending you only the low part of the graphs as this concerns me the most at first. I can then send some more measurements of the rest. The speakers are Acoustic Energy AE22 (very well regarded). Full range. The graphs will show Frequency and Phase followed by Waterfall and then Impulse ETC.

Left Speaker























Right Speaker























Both









That's it! Is it telling you something? My first guess would be a part of the ceiling which are splayed in a non good angle, just above the speakers in close proximity throwing reflections directly to the mic. Both sides are equally away from ceiling (around 3.5 feet). I thought that the wood, fibreglass and tile would be a good combination to kill heavy reflections.

If you have some ideas to help me what to do or how to interpret those results I would be very thankful.

Cheers
Chico


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## SAC (Dec 3, 2009)

For help with specular reflections please post the full range sweep REW .mdat files for each individual speaker.


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## Chico Santarosa (Feb 25, 2012)

Hi,

thanks for the reply. I am not completely sure if you want me to post the full range frequency response only or any other full range graph though. I am posting the full range frequency response and if you want others graphs I can post as well. These next graphs are 1/6 octave smoothing but if you want no smoothing info the first post has the low region without smoothing. The HF cut off you see there is because I pointed the microphone upwards (the KM 183 is a diffuse field one and has a peak around 10KHz that I tried to rule out the measurement). If you think I should point it towards the speakers I can make it and then send the results. 

This is the left speaker










This one is the right speaker









The next picture is the stereo setup at the mix position









That's it. I was also thinking that could be some coupling of the speakers with the desk they're on (not a mixing desk). This is a temporary set up cause I am planning to remove the desk and put two speaker stands (that's gonna take a little while though). 

Thanks again for helping me with that.

Best Regards,
Chico


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## SAC (Dec 3, 2009)

The mic needs to be an omnidirectional mic.

I am not asking or the posting of any particular convolved graph. 

I am requesting you post the .mdat file, which is simply the data file REW generates when it captures the data for each full range measurement of each individual source speaker so that we can open the file and massage it in REW.

A speaker will not 'couple' with a desk. But it will generate additional early reflections that will create polar lobing and comb filtering.

If you are planning to replace the desk with stands, I would suggest making the measurements with the stands, as the response will change significantly with both the stands and the changing relationship of the speakers relative to boundaries..


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## Chico Santarosa (Feb 25, 2012)

Ok, sorry I didn't get that at first. :whistling: 

I tried but couldn't upload the .mdat file. I did go to the "manage attachments", browsed for the file and clicked upload. The upload never finished though. Any idea of what could be going wrong?

Thanks for your patience.

Best Regards,
Chico


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## SAC (Dec 3, 2009)

If it is too large, you may have to post it in a 3rd party 'repository' (such as Sendspace) and link to it


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## Chico Santarosa (Feb 25, 2012)

Yes, that was it. I suppose I'm a bit clumsy with those internet stuff. I am sorry about that.

Now I've got that on sendspace. The link is 

http://www.sendspace.com/file/4byt89

Thanks again for the patience (again).

Chico


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## Chico Santarosa (Feb 25, 2012)

I have just checked to see if my REW file was still for download at sendspace. So if there's anyone interested in having a look at that and giving me some advice I would be very thankful.

The link is in the previous message.

Regards,
Chico


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## SAC (Dec 3, 2009)

I still have lots of questions.

The impulse still shows no time offset - with the IR arrival set to zero time. (not in preferences, but on the display)
But maybe sound just travels faster in your neighborhood. :huh:

And the frequency responses.......
Even with smoothing they look like there are multiple sources being driven simultaneously.

Personally I have too many questions about these measurements and about possible extraneous contributing factors ranging from the mic to procedure to configuration (seeing as how there is no time of flight still...). But ignoring that and responding to the displays as if all is well...

The ETC (which I am beginning to suspect are actually log squared responses - but which has nothing to do with these measurements) indicate a very small room with a short hot first significant reflection occurring at about 6ms on the chart - but seeing as we do not know the actual time of flight, its hard to even guess at the distance, with quite a few very sparse specular reflections - not what you would want to be left in that manner. To expand quickly, even in a casual listening environment such as is discussed by Toole, the specular reflections must be dense and without high gain sparse reflections - so you would want to damp or redirect the early arriving sparse reflections, and you would want to diffuse the later arriving sparse reflections so as to render the decaying soundfield into a more uniform spatially and temporally dense field sans the high gain sparse reflections.

Also, in both ETCs there is a high gain very early (~.28ms) reflection the MUST be addressed. This is most likely coming off the speaker mount. On some speakers I am familiar with this could also be due to driver alignment requiring precision microsecond delay to affect proper signal alignment. But this detrimental source Must be tracked done and addressed. This is not an option.


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## Chico Santarosa (Feb 25, 2012)

Hi SAC,

first of all thanks a lot for your help. 

I am having a difficult time measuring and understanding what's going on. After your last post I moved things a lot here. I put the speakers in a completely different place in the room. I also used a stand (not the definite one, but it's not a desk, as it used to be).

That anyway didn't changed anything. The response was very similar. I've got a dip in the MF around 180 Hz. And the impulse response showed also that very early, much less than 1 ms, high gain "reflection". And also other two sparse reflections a bit like the measurements you have seen (with different times though).

The room is not small and it's not squared. Apart from the floor I haven't got any close direct boundary reflecting from the speaker to the measurement microphone. 

I am starting to think I have some problem with my set up and REW configuration, despite having a good knowledge about setting up audio equipment. I also set up and put REW and my system configuration to work quite easily. Anyway something could have gone wrong.:dontknow:

About the impulse showing no time offset I thought that was set on preferences. I didn't understand what you meant about the zero time on the display though. Isn't it the same thing?

Thanks again for your help. And if you have any other idea of what to do to check whether I am proceeding right with my measurement I would appreciate that.

Best regards,
Chico


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## SAC (Dec 3, 2009)

I understand your frustration.

A few comments...

I cannot explain the issue with the direct signal set to T=0. Sound does not travel from a source to the mic in zero time. 

Do not simply move things around hoping to serendipitously resolve reflections!

Also, I should mention a bit of context to the focus here.

Dealing with the specular issues assumes that one has resolved the modal issues first. If they are a factor, one should map the space to determine the peaks and nulls in the region about the listening position(s) creating a grid like map. You can use a combination of sweeps and tomes to identify the resonant peaks and nulls. After which you can adjust your speakers and repeat; and or you can move your listening position forward or backward so as to move to a spot between peaks and nulls. Also, if bass traps are used, you should repeat this as much as is necessary to verify if the listening position is in any peaks or nulls and adjust accordingly.

Once that is done you can then move on to the specular issues.

You need to resolve each high gain reflection to its path and point of incidence (on whatever it is reflecting off). Since you do not have an accurate total time of flight for the sound to travel from speaker to mic (due to T=0 for the direct signal), you must use the blocking method, whereby you place an absorber between the mic and the boundaries at differing angles in order to intercept the reflection path. When it is blocked, the particular spike will be diminished. You do this one spike/reflection at a time. Once the angle is known, you repeat moving the absorber further away from the mic and close to the boundary, observing that you are following the path until you get to the boundary, where you can mark the spot with a piece of blue painters tape and label the spot and a printout of the ETC identifying each reflection for later reference.
_
"That anyway didn't changed anything. The response was very similar. I've got a dip in the MF around 180 Hz. And the impulse response showed also that very early, much less than 1 ms, high gain "reflection". And also other two sparse reflections a bit like the measurements you have seen (with different times though).

The room is not small and it's not squared. Apart from the floor I haven't got any close direct boundary reflecting from the speaker to the measurement microphone. "_

The grunge below 1ms may be diffraction from the speaker cabinet or the speaker stand. Do not let the shelf on which they are mounted protrude. It should be flush with the speaker baffle.
Also, the driver alignment may be off which will result in multiple successive very early signal arrivals - note, this is a speaker problem. But in any case, there is obviously something creating this. It may take a bit of investigating, changing ONLY 1 thing at a time!!!, to determine the source.

And in all cases, do not "move things a lot here". By this I mean, in trying to troubleshoot and identify causes of behavior, you carefully change one thing very deliberately to uncover a very specific behavior. For instance, you do not simply 'move a speaker'. Instead you move it slightly , say, to the left, keeping all other relationships the same. And you not the specific changes or lack thereof. Or you move it forward or backward, keeping all other relationships the same, and observe how that modified the response. 
In other words, you design the experiment to uncover a specific issue. Moving the speaker with respect to several boundaries simultaneously simply results in 'a bunch of stuff' changing, but as there are too many variables, you don't know anything more specific as to what change affected what specific behavior.

So, to recoup, the very early reflections can be a combination of diffraction, driver signal alignment, and possible boundary reflection.(Close micing each driver , being careful to maintain the same vertical mic position for each drivers IR/ETC can help identify signal alignment offsets if no other sources of very early arrival are present.)

OK, enough of my generalized rambling as to a few things to be aware of. I am not sure what is happening in REW as to the T-0 at IR start remaining even when it is not specifically selected in the Preferences -> Analysis menu. Assuming modes have been addressed, take it slowly, isolating one reflection at a time and using the blocking method to identify the vector path from speaker to boundary to mic. Then based on their arrival time, gain, etc., one can design if they are best absorbed, redirected or diffused... But as more quantified information is available as well as the paths and points of boundary incidence, further steps can be decided upon....And note that all of the reflections, etc., that appear have causes that can be granularly identified.

And if its any consolation, with the practice and discoveries, the process becomes easier! and this usually works out that at the moment you fix every problem, you are finally knowledgeable enough to do the entire process in a Much faster manner!


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

Chico Santarosa said:


> That anyway didn't changed anything. The response was very similar. I've got a dip in the MF around 180 Hz.


The only thing that’s going to improve that is bass traps. Electronic equalization should also help (assuming the measurement position is also the listening position), as the dip appears to be a trough and not a null.

Regards,
Wayne


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