RE: Looks Very Interesting

Posted by emailtim (A) on September 21, 2023 at 15:25:31

In Reply to: Looks Very Interesting posted by Barry on September 21, 2023 at 13:12:35:

The polys in the 1st picture above are in a studio, not mine, that I used to show their usual horizontal dispersion orientation. I made my own DIY polys based on information from the web and in-room testing.

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" (ARC) 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 vertical dispersion polys to the height of my room so the 43" CORD 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

ARC Length/CORD Calculator (Open in New Window)

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Follow Ups Full Thread

Topic - Polys revisited - emailtim 19:41:06 09/19/23 (24)

One wonders if the sound have more air if the panels were removed. Nt - geoffkait 04:14:19 09/27/23 (4)

RE: One wonders if the sound have more air if the panels were removed. Nt - emailtim 08:05:12 09/28/23 (3)

RE: One wonders if the sound have more air if the panels were removed. Nt - geoffkait 09:51:26 09/28/23 (2)

Didn't you just say you used an SPL to measure room nodes ? - emailtim 14:16:23 09/28/23 (1)

Yes, I use an SPL meter to map out the room. - geoffkait 10:07:58 10/05/23 (0)

Plants are not good for the sound. - geoffkait 04:28:01 09/23/23 (11)

RE: Plants are not good for the sound. - Scholl 05:20:21 09/24/23 (6)

RE: Plants are not good for the sound. - geoffkait 07:54:13 09/24/23 (5)

RE: Plants are not good for the sound. - Tre' 17:55:43 09/26/23 (4)

RE: Plants are not good for the sound. - emailtim 08:21:11 09/28/23 (2)

must be he has a magic nasty plant too - Story 08:42:23 09/28/23 (1)

RE: must be he has a magic nasty plant too - emailtim 09:20:10 09/28/23 (0)

The only thing to fear is fear itself. - geoffkait 03:01:44 09/27/23 (0)

Those are not plants - emailtim 11:16:44 09/23/23 (3)

The Carpenters and the Beatles? Yup, that's Japan. Nt - geoffkait 12:27:17 09/23/23 (2)

Do your really think that this is the best place to air your prejudices? Nt - Tre' 17:37:15 09/23/23 (1)

You're right - geoffkait 03:15:45 09/27/23 (0)

Looks Very Interesting - Barry 13:12:35 09/21/23 (6)

RE: Looks Very Interesting - emailtim 15:25:31 09/21/23 (5)

RE: Looks Very Interesting - Barry 09:26:35 09/22/23 (4)

Mounting orientation - emailtim 11:24:00 09/22/23 (0)

RE: Looks Very Interesting - emailtim 10:36:06 09/22/23 (2)

Thanks so much! - Barry 15:26:54 09/22/23 (1)

REW Early Bird Release - emailtim 15:38:11 09/22/23 (0)

Follow Ups

RE: Looks Very Interesting - emailtim 15:25:31 09/21/23 (5)

RE: Looks Very Interesting - Barry 09:26:35 09/22/23 (4)

Mounting orientation - emailtim 11:24:00 09/22/23 (0)

RE: Looks Very Interesting - emailtim 10:36:06 09/22/23 (2)

Thanks so much! - Barry 15:26:54 09/22/23 (1)

REW Early Bird Release - emailtim 15:38:11 09/22/23 (0)

RE: Looks Very Interesting

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