Help needed interpreting my graphs - Page 2 - Home Theater Forum and Systems -

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post #11 of 12 Old 03-23-12, 01:35 PM
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Re: Help needed interpreting my graphs

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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post #12 of 12 Old 03-23-12, 01:41 PM
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Re: Help needed interpreting my graphs

Chico Santarosa wrote: View Post
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 thatís 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.

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