[vettett15]

Nyal,

    Appreciate the responses, one could spend a lifetime learning all the ins and outs of REW it certainly helps to get some guidance.  I see now the problem with the right speaker, i kept looking at the non-banded ETC, this is where I see a large spike in the left speaker.  Is it too close to the direct sound for it to be as big of a problem as the right speaker? I believe the spike in the banded ETC for the right speaker is the 500Hz band which make sense with the large dip I have.  I'm almost positive this is a room issue b/c I have done close sweeps of the speakers and the dip isn't there.  Really the only way I think i'm going to be able to tame the sub 50Hz issues is with my sub equalizer (I usually cross over 80Hz), so there may need to be some main eq needed also.

[Nyal Mellor]

Vettlett, I would suspect some form of floor or ceiling cancellation or both as being responsible for that wide dip you are seeing. There may also be other boundaries causing cancellation too. Check out this my http://www.acousticfrontiers.com/Speaker-Boundary-Interference.html page for more. My first take on your LF response would be to add bass trapping first, then go to EQ. The room modes are very 'peaky' at this point, it would be good to round and flatten them, which bass trapping will do. In regards to the ETCs, I'd be more worried about the peak in the right speaker on the band filtered ETCs than the one in the left.

[vettett15]

Nyal,

     Thanks againf or the response.  I had measured my old speakers (Dali Ikon 6) in the same room and never got that large null.  I'm hoping it is largely fixed by speaker/listener position and room treatment.

Here is the info for my hunt on the right etc spike, let me know what you think.

Guys,

      I've finally gotten some time to try and hunt down my rather large null in the 250-500Hz range.

Here is the graph of the left and right full range response (1/3 octave smoothing):


As you can see there is about a 10db drop in the 250-500hz range.

I have done close speaker measurements to ensure that this isn't the speaker.

I did an ETC to see what reflections I have going on, this is left and right overlayed:


As you can see there are a good amount of reflections.

So I went ahead and looked at filtered reflections:

Left:



Right:


Looks like a large reflection in the 500Hz band, so I took a closer look.

Here is the 500Hz band using the distance function:



I used the timing back loop reference to ensure the distance is meaningful.

Here is the room layout:




So I took some string, measured the distance between the right speaker and mic and added 13'.

Taped the string at the seat and at the right speaker, and manipulated the middle of the string to see what I hit.

Well, it turns on the string hits perfectly on the window you see on the right hand side.  Furthermore I looked at the 500Hz band ETC for the left speaker and sure enough there seems to be a nice reflection that is about 20-21', which makes sense as the speaker are about 8' apart. 

So currently this window (kinda large at 5' wide by 6' tall) has those fake wood blinds and curtains.  One thing I noticed is my fiance has moved the curtains to the side so all I really have are the blinds blocking the window.

My real question is what is the best way to get rid of this null?  I'm guessing it will be a combination between moving speaker, furniture and room treatments.  I was wondering if I moved the couch on the right towards the window if that would help me.

I know it will take some actual movements and measurements to know for sure, I would just like to go about it a smart way.

Thanks,
Pete

[Nyal Mellor]

Hi Pete

Sorry for the delay in responding. CES last week threw everything into a crazy whirlwind!

I would look at speaker boundary interference as the culprit. The way to deal with it is to either move things (speaker / seat) so that the interference null appears at another frequency or absorb it. To absorb a window reflection with curtains you need to be looking at heavy thick velour stage type curtains with a lot of spare fabric. Most window 'treatments' don't do anything useful with regard to absorbing lower midrange reflections. Did you check the floor and ceiling bounce distances too? Quite often a null at the frequency you have is related to those bounces.

Thanks - Nyal

[vettett15]

Nyal,

    No problem, appreciate the help.  I messed around a lot with the speaker and listening position (i.e. couch) placement, those furniture mover pucks are great btw.  I smoothed out the response a lot (some of which was due to me flipping the polarity between the woofers and mid/tweeter.  Basically i ended up pulling the speakers out into the room a little more, and the couch away from the rear wall.  here are my results (old vs new).  I didn't check for floor and ceiling bounce, is there a good calculator out there for this?

Right old vs new:




Left old vs new:




I looked at the filtered ETC for the frequencies in the dip and thought what I saw was interesting:

250hz, smoothed 1ms:




315hz, smoothed 1ms:




400hz, smoothed 1ms:




These ETCs seem to have very large reflections right after the direct singe and in some cases the reflection it just as large in amplitude as the direct single.  Am I reading too much into this or maybe just incorrectly.

In the mean time I also picked up the following:

2" Absorber panel, I was planning on using this for the first reflection for the left speaker

GIK Soffit, stick this in one of the corners.

With the REW RTA I plan on trying many different spots for these products, hoping it will help out this dip and bass response.


Thanks,
Pete

[vettett15]

Sorry to keep dragging this on, I just keep finding interesting things.

Nyal,  thought a little more about your comment on floor and ceiling bounces.  Didn't find a calculator but I kept playing with the ETC and the IR windows to see how shortening/lengthening the IR window changes the response.  I noticed right away that both speakers started showing the dip at the same time (12-14ms).  I thought that was interesting since my room was not symmetrical with regards to walls and speakers the only thing shared in common is the floor and ceiling. 

I used the string trick, used a length corresponding to 12ms for the left and 16ms for the right.  The left speaker string didn't quite reach the ceiling while the right one was a touch to long.  It seems a little difficult to determine which length is the correct one because the dip gets progressively worse, starting with 12 and stops progressing at 16. 

So assuming the ceiling is the culprit, whats the solution?

[Nyal Mellor]

Sorry to keep dragging this on, I just keep finding interesting things.

Nyal,  thought a little more about your comment on floor and ceiling bounces.  Didn't find a calculator but I kept playing with the ETC and the IR windows to see how shortening/lengthening the IR window changes the response.  I noticed right away that both speakers started showing the dip at the same time (12-14ms).  I thought that was interesting since my room was not symmetrical with regards to walls and speakers the only thing shared in common is the floor and ceiling. 

I used the string trick, used a length corresponding to 12ms for the left and 16ms for the right.  The left speaker string didn't quite reach the ceiling while the right one was a touch to long.  It seems a little difficult to determine which length is the correct one because the dip gets progressively worse, starting with 12 and stops progressing at 16. 

So assuming the ceiling is the culprit, whats the solution?

The string trick doesn't give exact answers, if it's within 1ft of what the graph is telling me that's close enough in my book.

The exact answer to the ceiling would depend on your speakers and how live dead your room is / you want it to be (which is dependent on your choice in music, personal preference, etc). A ceiling cloud like the Primacoustic Nimbus spaced off the ceiling 2-4 inches is a good solution.

[jtwrace]

A ceiling cloud like the Primacoustic Nimbus spaced off the ceiling 2-4 inches is a good solution.

Could a GIK 242 be used in the same fashion with similar results?

[Nyal Mellor]

Yes any similar thickness and density fiberglass product should be suitable. The Primacoustic Nimbus comes with specific hardware for ceiling mounting, if you buy from elsewhere you might want to consider getting some Rotofast Cloud Anchors (this is the easiest way to mount panels to a ceiling with an airspace and is what comes with the Nimbus.