The following was seat of the pants done solely for S's&G's .. it was a first time playing with the "simulator" in REW V5.. I only wish i had taken measurements when the room was totally bare to compare during riser construction...
I know much of this is obvious and old hat to many of you but it may get someones wheels turning...
The following is an excerpt from a recent post I made in a thread regarding some REW measurements, that sent me down this path..
Originally Posted by KNKKNK
SNIP.... particularly your readings for the first 2 length modes, which should be fairly predictable regardless of changes in width/ height, soffit to soffit, floor to soffit, soffit to riser etc...
I cant see the short face of the riser or even the entry impacting these modes to the extent that it would shift the 1st or second length mode significantly from the predictions. These should be the most predicable and easiest to correlate with even seemingly substantial changes.... Ie if your room measured shorter by 10% or longer by 10% to account for the face of the riser/ entrance etc.. SNIP...
After posting I really started thinking about what the actual response my room would look like and how it would compare to the predicted responses from the calculator/simulators.. since it varies somewhat significantly from the rectangle room models.. So I set off looking for my mic again...
MY Room Basics
The interior dimensions of the completed shell are 25' 8" long, 14' 7" wide, 9' 8" tall.
Harmon's mode calc is really quick and easy for generating axial mode prediction:
These calculators represent theoretical modes/waves based on perfectly reflecting walls and as such we can expect reasonable variations due to actual construction, real wall impedance etc...
Expecting these variations I wanted to create a window I felt my modal response should reasonably mimic.. So as a total WAG I used the calc to see what effect a +/- 2ft window for room length would have:
so 1st axial length mode should be between 21-24hz, 2nd between 41 and 48hz.. etc..etc..
Other Room factors worth mentioning...
Ceiling has 6 different heights elevations ranging from 9'8" to 8' to Finished floor..
3 changes from 6'6" to 7'10" to the top of a 22" riser..
4 foot deep/12" high stage at the front..
and the baffle wall (rigid) is ~18" from front wall and roughly 60% open for 1/2 of its height
quick and dirty setup..
EMC8000 mic.. with generic Cal file
quick sound card cal
Used a Triad Bronze IW 6 for the sub
No SPL cal .. just wanted to see "Shape" of response < 100HZ
Measurement positions.. Since the initial focus was on the axial length modes, the Mic was positioned @ mid room width and mid height which should minimize the 1st width and height modes (mid height of "shell" with no consideration for risers or ceiling elevation)
I Ran a piece of tape down the center of the room and marked of the positions based on the dimensions from the Harmon Calc from the back to the front wall...and then placed the triad as close to real front corner as possible to excite the modes...
Again since I didnt really calculate the SPL for this single woefully under powered sub, I was primarily interested in the shape of the response and the locations of constructive interference...
First measurement at Center Height and width ~ 1" off of rear wall... all length modes should be excited..
Although not exact.. It was fun seeing the FR Peaks come in close to what the Harmon calculator had predicted for the length modes, and the imaginary window I had established...
Then I looked at a comparison of the same FR compared to the Room simulator in REW V5
The FR in the room simulator makes it easier to see the generic shape compared to the REW results.
The room simulator also shows a graphical representation of the SUB and MIC position in the room.. that can be moved to see the impact position has on the simulated FR (pasted in red box)
Below the top FR represents actual
The bottom FR represents the predicted.. (oblique and tangential modes were turned off in simulator)
Red lines are length modes, Green are width modes, and blue lines are height modes in the simulator
I aligned 20hz and 200hz for comparison..
I like the basic shape and the alignment with the predictions.. There is a slight frequency shift between the 1st and 2nd-3rd modes for the length but that's for another discussion.. one interesting observation is the peak around 80hz.. Although the mic position was intended to mitigate the 1st width and height modes this corresponds to the peak of the 2nd width mode (2nd green line) which should be ~ 77hz ..
Just a couple more comparisons to the simulators at different positions...
This one is with the mic at a quarter point which should be a null for the 2nd length..
Again.. I like the shape, it's as predicted relatively speaking, with the various considerations..
The same ran true for the entire length of the room.
One last comparison to share...showing the impact of moving the sub to mid wall and measuring at mid room...
again a fairly solid correlation with the general shape between the simulator and actual s..
Not sure what is happening in the 60-80Hz range on the actual FR compared to the simulator but I think it may be related to the sub position (~ 2') to the front of the 22" riser..
The simulator allows for varying placement of up to 4 subs.. I have 1 more IW 6 Bronze that Im considering hooking up to see how close the FR can correlate to the simulator with 2 subs going.
You can also offset your measurements in the simulator ie left, right, front, back to check response over a given seating area. It allows you to adjust absorption coefficients for the various surfaces to see the impact on your peaks/nulls, and many other variables... if this interests you.. the simulator is fun to play with, and a hell of a good starting point to play with sub positions without crawling around on the floor..
I found it to be useful from the perspective of forcing me to look for "Why" the differences between the calculated/simulated and actual measurements...
and its FREE