BBC PRD Repetition--am I allowed?

[wesarvin]

So, I'm converting my 2'x4' gobo's into PRD diffusors on one side. I've heard that repetition is bad, but what if I stack two of these 18"x18" designs (http://www.pmerecords.com/Diffusor.cfm) to make a vertical 18"x36" rectangle?

If doing this is a no-no, then what design (mac-friendly) can I use to build two 2'x4' PRDs to screw onto my gobo's?

[wesarvin]

SAC, Andre, Ethan, Glenn?

[SAC]

First, I would personally use QRDude to generate an optimal response pattern.

For 2- 2'x2' panels, you should be fine with the pattern repeated.

Another option would also be a GOBO utilizing a QRD that would provide for diffusion on a horizontal plane and avoid additional floor (and to a lesser degree - ceiling) reflections - which might possibly pose potential phasing issues when micing in the near field.

[wesarvin]

Here's the first of two beasts. The backer is 1/2" MDF with 4in absorption on the opposite side. I'm going to stain it cherry and probably replace the casters. It weighs about 90lbs.

Instead of 18"x36", I think I'll make the next one a full 2'x4'.

[PaulP]

Originally Posted by guitarwes

Here's the first of two beasts.

Nice thumbsup.

I look forward to reading your report on their acoustic properties.

Paul P

[wesarvin]

Thanks. In my 12'x11' control room, the sweet spot seems a little bigger with a little more depth. Also, strumming an acoustic guitar into it seems like I'm listening to a bigger version of the same guitar.

I'm having trouble finding an acoustic calculator for Mac, or in imperial measurements. Does anybody have some plans or could post a screenshot of results for a 2'x2' PRD with a 6-7" depth?

[Nordenstam]

Looks great!


Online PRD calculator: Calculate a Two-Dimensional Primitive Root Diffuser (Skyline)

A 23x12 grid can be bult using prime 277 and primitive root 5. Here's a suggestion with every parameter quantized to inches (2.54cm=1 inch):

Speed of sound: 343 m/s
Quantize well heights within: 2.54 cm
Lowest frequency: 964.5669291 Hz
Highest frequency: 3375.9842351 Hz

Prime number P: 277
Primitive root of P: 5
# columns: 23
# rows: 12

24*46 inch grid, 2" widths and 1" depth quantization(7 inches total depth).

The depths are listed as:
2.54 cm: 40
5.08 cm: 39
7.62 cm: 40
10.16 cm: 40
12.7 cm: 39
15.24 cm: 40
17.78 cm: 19

Which is 1/2/3/4 etc inches written in centimeters.

Here's a post with more details on the parameters for the calculator.

[wesarvin]

Thanks Andreas. I noticed that post before, but didn't quite understand the conversions or which primes to use. Beast #2 will be underway soon.

[wesarvin]

Uh oh.

I just realized that our American 2x2's are actually 1.5"x1.5" That would leave an 18" wide grid. Whoops.

After rereading your earlier post and re-inputting information into the calculator, I'm still having trouble getting the numbers correct on the diffusor. Any help anyone?

[Nordenstam]

I'll run through how I do it and you'll hopefully be able to get something useful out of it. Scroll down to the last link if this math stuff is making you drowsy!

First, lets assume you want a 16x32 grid since (16*32) with 1.5 inch units gives a 24x48 inch size.

This doesn't work for two reasons. 16*32 is not coprime, they both have 2, 4, 8 and 16 in common. The sum of those 16*32=512 is not one less than a primenumber either, which is the other criteria for the grid. I don't know any more elegant way of doing it than to hit the calculator with various suggestions and see if the sum is prime minus one. 16*33 is not, 16*35 is not, but 16*37 gives 592 which is one less than the prime number 593. 16 is also coprime to 37 since there is nothing in common for those numbers except 1 and itself.

This will give you a 24*55.5 inch total build size. If it doesn't work for you, search for another grid that works better!

Time to input the numbers into Oliver Primes calculator. First, go to this page, enter the prime number into the "primitive root modulo N" field and write down the result. In this case, it's 3. So we already have the most important parameters for the calculator:
Prime number P: 593
Primitive root of P: 3
# columns: 37
# rows: 16

That's the important part. The frequency stuff is just for convenience, making those numbers easier to use in the real world. The calculator is in meters. A hundred of meter is centimeter, with 1 inch being 2.54 cm and 1 cm being 1/2.54 inch. The speed of sound is set to 343 meters per second, or 1125 feet per second. It doesn't matter what the value reads, it's just a reference.

The build uses 1.5" inch widths, so the high frequency have to be set accordingly if you want the calc to display well width and final size in the correct measures. It doesn't change anything, practically speaking, but it looks better in the calculator. The corresponding high frequency cutoff is speed of sound divided by size of object divided by two. The size is 1.5(inch)*2.54=3.81(cm). The speed of sound is 343 meters, divided by 0.0381 meters (3.81cm) divided by two. In numbers: (343/0.0381)/2=4501.31233595800 Hz. <- that's the high frequency cutoff that'll give you 1.5 inch divisions.

The low frequency cutoff is the depth of your build. A nice looking example is to use the prime number divided by four as the starting point. This is 148.25. Again, the relevant frequency is found by using speed of sound divided by depth divided by two. If depth is 14.825cm's, equation is: (343/0.14825)/2=1156.82967959527824620 Hz. The reason do this is obvious when looking at the intermediate results in the calculator: Calculate a Two-Dimensional Primitive Root Diffuser (Skyline) As can be seen, each step have it's own unique number and it's all integer numbers or nice looking rationals. You don't need to use that number, but it's a neat way to check that you've got everything right. If there's something wrong in prime number, coprimes etc, it'll be very clear as the numbers will look messy.

You want 7 inch depth, so it's 2.54cm times seven, 17.78 cm's depth. That's (343/0.1778)/2=964.5669291338582 Hz. The quantization should be set as fine as you can be bothered. BBC states a reason for using only 5 different depths. I didn't get it and neither did my PRD diffuser hero around here , GE. If you want to be safe, use more quantization steps if you can be bothered to have more different depths. Quantizing with 2.54cm gives inches, half that gives half inches etc.

All in all, with 7" depth and quarter inch quantization: Calculate a Two-Dimensional Primitive Root Diffuser (Skyline)


Is this better?

[wesarvin]

Thank you for your patience with my lack of math ability. I earned a "C" in calculus in college. I called it quits in math after that.

My trouble was in finding a prime minus 1. It makes sense now, to just play with different grid sizes until a correct number is reached. I'll resume building beast #2 as soon as our Texas snow ceases. Hopefully this thread will serve as inspiration to others.

Here is a stained and trimmed Beast #1 and Beast #2 in progress: