Axiom Home Page Forums General Discussion Home Theater Fred's Excellent (hopefully) Calibration Adventure

(is there an echo in here?)

Re-posting with hopefully working picture links... sorry for the spam. [Google web albums changed the way you embed images stored there and it looks like I guessed wrong the first time]

RE-POST FOLLOWS

Fred - I'm sure you remember I went through the same fun (I think?) with my own poor room modes when I first picked up the EP600.

This was what my raw output looked like where I first had my sub:



I had some ugly dips at around 35 and 60 hz. I ended up getting the Behringer Feedback Destroyer (fondly known as the BFD) to smooth responses, but what I found was that these types of dips (-20 dB or more) weren't correctable through any other means than moving furniture or moving the sub itself. The best I could do in that position, even with the BFD, was to get the curve to about -12 dB at 60 Hz (and other ugly problem spots as well), which was still way too much of a drop off for me to be happy with.

Fortunately in my case better results were achievable without moving walls.

Now take a look at what happened just by moving position of the sub, this is still before using the BFD in the new position:



Moving the position caused the response to move from +/- 40 dB to +/- 10 dB (and my satisfaction level with my EP600 to go from 0 to 100).

Then, finally, I applied the BFD adjustments to my curve and achieved the flattened curve I was looking for:



The moral of this long story filled with squiggly lines and charts? Even though I know you've said you have a tough room, definitely experiment with different locations for the sub to see if you notice a difference (primarily with your ears, but also with the REW curves). Also, like you said, adjust the phase options to see if it makes a difference, though in my case even phase adjustments in the first position were not terribly helpful. It was just a very bad location in the room. In the second position though, and especially after adding a second sub, the phase adjustments became critical to smoothing response throughout the room.

Have fun, keep us posted!

Jason

do you have any smoothing applied to those graphs? The last one is very impressive!

I know, I couldn't believe the last one either because the room is totally non-treated (i.e. no traps, etc.). I used the BFD (see above) to equalize the curve, but even so, I never expected it to get that flat.

I later added a second sub into the mix and could never get it this flat again, however, the response in general in multiple listening positions is better with two subs, it's just not as perfectly flat in the primary listening position (maybe it could have been, but I gave up tinkering at some point)

I keep not seriously tinkering just because I know my system is going to change. I swear I'll put in the effort once I have my final two surrounds and my multiple subs and it's in the room I want it in.

Unless you really enjoy the process of tinkering, if multiple subs is in your future, I'd wait until you do that too... it adds such a ridiculous amount of complexity to bass tuning that you'll likely want to pull your hair out... at least if you wait you'll only have to do it once.

And the adventure continues.

Unfortunately, moving the sub around did not yield a nice curve like Jason's, no mater wher I put it.

This is about as good as it gets. Sub at the side of the room facing forward:



That rather nasty peak at 52Hz (+15db) is killing my bass. The question is, what do I do about it?

Equalization via BFD is an option, but that only fixes the main seating position and it could well make other positions worse.

I went out and did a bunch of reading on various traps, positioning and effectiveness. I also went back to re-visit room modes and mode calculators.

So, plugging my room dimensions into a calculator I see that I have room modes at 28, 56, 84, 112Hz relating to room length. Those match up quite well with what I see in my graphs.

I also have a vertical room mode at 70Hz that sort of matches with the nasty null I see in the 70-75Hz range depending on where I measure and sub position.

It seems that the fiber traps that most folks use are not all that effective down low unless they are very thick and well out from corners or walls.

That leaves resonating absorbers, which work on sound wave pressure rather than velocity. They work well in corners and against walls. Sigh. I can't believe I am contemplating playing with plywood to make my HT sound better.

Concrete sucks!!

As long as this fiddling acomplishes something.

Its been interesting to watch things change as I moved sub and microphone positions. Peaks and nulls seem to move up and down, and shift in frequency, but the same general pattern appears no matter what. For giggles, I will try measuring at a different vertical location (something other than ear height) next time I fire up REW.

Just because I can:


I got bored and decided to play with google sketchup. Thats a neat tool. I remember being at a trade show in the 80s where a rep from Alias was doing similar stuff... on a $10,000 SGI graphics workstation.

Another 10 years and we'll be modeling dinos on our desktop.

Anyway, I thought a 3d layout might give a better idea of just what the limitations of my setup are.

Thanks Fred - I love Google Sketch up. Totally addictive little tool. I obsessed over it when building my riser (and it paid off), so it's fun and effective.

I realized that after doing the QS8s.

I'm still working through the basics of room acoustics to get a handle on what I can do. Outside of the usual broadband traps there is not a lot of info. available online. I'm gonna pick up the Master Handbook of Acoustics sometime this week.

I got one good tip for getting a handle on room modes and where they come from. I will be rigging up a couple of broadband absorbers that I can move around to see what effect they have. By placing them near the spl meter and changing their orientation in the room, I hope to get confirmation of what peaks and dips relate to the different room modes. Hopefully that will help me to figure out where to place any absorbers I build (aside from the obvious corners).

I am hoping to confirm that the 52Hz mode is related to room length. I want to try a panel resonator at the front of the room above the aquariums.