I used REW to identify my problem areas first and determine what frequency ranges to address. Based on the delay lengths and some debugging, I discovered what reflections were causing the issues. Thus, this was a targeted fix for a specific problem and not a random "catch-all" fix.
REW (vertical spectrogram plot) showed I had strong reflections ranging from [1-7]kHz at 4.5ms and 9ms which is in the range of smearing the direct signal versus being perceived as an echo.
Once they were identified as side and cross-side reflections, I realized it was also what was causing the sound stage to shift when tuning one's head because the leading ear would switch between the side and cross-side reflection. The polys fixed that leading ear sound stage shift. I also used REW to validate the results.
4ms OC-703 reflection shown here in red, blue afterward with poly.
My commercial broadband traps were 2'x4' so I made my DIY polys similar in size.
To get a 180 degree dispersion, you need a 1/4 slice of a pie, or 90 degree arc from a circle. Other dispersion patterns can be made by changing the shape of the arc.
Below is a link to an ARC/CORD length calculator. If you enter in the height of your material, say 48" and then enter 90 degrees, it will give you a CORD of 43.2" inches which will be the vertical height of the poly or semicircle. Since 43" is longer than the [1-7]kHz frequency wavelengths (13" and 2" respectively), this size poly is suitable for the problem at hand.
Frequency Wave Length chart:
https://www.jdbsound.com/art/frequency%20wave%20length%20chart%202013.pdf
I have line arrays centered on the vertical height of my room. They do not exhibit much floor/ceiling bounce so that is not an issue. I centered the horizontal polys to the height of my room so it covers both sitting and standing positions.
There are multiple DIY examples of making polys on the web. I tested with solid (non-perforated) surface and also cloth covered. Both worked according to REW before and after measurements.
Another thing about polys is that they do not distort/scatter phase which can be beneficial for certain applications. QRDs scatter phase and tend to work best in the back of the room.
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2022/03/30 Historical Records CENSORED
