FREE Soundproofing Workshop: www.soundproofyourstudio.com/...
QRD Calculator:
www.mh-audio.nl/Acoustics/diff...
One of the most effective and efficient types of diffusers are reflection phase grating diffusers otherwise known as Schroeder diffusers. A type of Schroeder diffuser is the Quadratic Residue Diffuser (QRD) and in this article I will teach you how the diffusers work, the theory behind them, and how to build one.
1) How Does a QRD work?
Manfred R. Schroeder developed the quadratic residue diffuser by using a series of wells of different depths with a constant width that are separated by thin dividers to evenly disperse sound back into the room. (Everest, 265). He found that a number sequence based on prime numbers can be used to calculate the depth of each well. This number sequence can also be repeated over and over (in periods) to expand diffusion across a wall or ceiling.
When designing a QRD the maximum frequency for diffusion is determined by the well width and the minimum frequency is determined by the well depth. For example a QRD that needs to cover most of the audible spectrum would need to have very deep wells up to several feet and very thin well widths to cover say from 60 Hz to 10,000 Hz. This is one of the design limitations of QRD and phase grating diffusers in general.
2) The Math Behind QRD?
So don't get scared, but I am going to talk about math. I am not a big math genius, so I always need to break down the numbers in simple terms, ideally, this makes me a better teacher for you! So, the equation behind the number sequence used to build the QRD is:
Well depth proportionality factor = n^2 modulo p
where n = integer greater than or equal to 0
p = prime number
If you are like me, you had no idea what the heck modulo meant. Modulo is a calculation in number theory. It is a short cut for the following equation
n^2 / p = x + remainder or residue
n^2 modulo p = remainder or residue
The modulo operation says take the n squared in our equation and divide so there is a whole number and a remainder. It says that the modulo will = the remainder not the whole number. If that still seems confusing let's do an example.
Let's say we want to build a QRD with a prime number of 11. Now lets pick an integer of 5 in our number sequence from 0 to 11 for the diffuser. Again, I just chose 5 randomly for this example, when building the diffuser you would do 0 then 1 then 2 and so on up to 11.
So, in our equation:
Well depth = 5^2 modulo 11
Well depth = 25/11 = 2 with a remainder of 3
Well depth = 3
11 goes into 25 two times, which equals 22 and to get to 25 we have 3 left over. The remainder and residue terminology is interchangeable. Confusing, I know!
So, the QRD are usually made from the following prime number sequences: 5, 7, 11, 13, 17, 19 and 23.
Below is a chart from the Master Handbook of Acoustics that shows the well depth ratios for all of those prime number diffusers.
You will also notice the diagram showing one period or one diffuser length. Notice this period goes from 0 to 1 in the prime 17 sequence. This shows how diffusers can be placed side by side to continue the sequence across a wall or ceiling.
3) How To Design Your QRD
So, first we must choose a prime number to build our QRD. Each prime number has some benefits and limitations and ideally in a room design you might use a combination of different prime numbers to achieve a diffuse sound field across as much of the audible spectrum as possible. All that is a bit beyond this article. For this example, I will be building a QRD 13 based on the prime number 13 sequence.
Now, the chart above is great in theory, but it is hard to imagine translating all those ratios to our actual physical diffuser. So, I recommend using a handy QRD calculator. There are a lot on the web, but this one does the trick: www.mh-audio.nl/Acoustics/diff...
I will be doing this example in metric since my design is for a client in New Zealand, but you can do yours in imperial and all the concepts stay the same.
Read the rest of the blog article here: www.soundproofyourstudio.com/...
Work Cited:
Everest, Frederick A., and Ken C. Pohlmann. “Schroeder Diffusers.” Master Handbook of Acoustics, McGraw-Hill, New York, 2015.
0:00 - Intro
1:56 - How QRD Works
5:03 - The Math Behind QRD
11:05 - How To Design Your QRD
16:51 - How To Build Your QRD