I did buy some diffuser plans fro Acoustic fields, ...
Well, there's your problem! That's it, right there!
Your money would have been far better spent on correct information about diffusers, by buying the book "Acoustic Absorbers and Diffusers" by Cox and D'Antonio. Those are the guys who did decades of actual research on diffusers, in real acoustic laboratories, to discover the principles by which they work. And they don't work on fantasy. It would be nice if they did, but they don't. They work on phase changes, and that requires dealing with entire wavelengths, or half-waves in some cases. The guys who wrote that book derived the equations that describe how real diffusers work in the real world.
Buy the book: it's expensive, yes, and maybe the math is a bit intense, but the principles are easy to understand, and well laid out.
The placement of those are supposed to be at least 4-5 feet from the recording position. this is how I am understanding those things. So, I was figuring if I had a 10 foot long room i could use these at the back.
If you read the book, instead of the Archie comic version (!), you'll find that ALL numeric-based diffusers need to be at least ten feet from the listener in order for the artifacts to have smoothed over sufficiently, but that IN ADDITION the distance must be at least three full wavelengths. So if the low cut-off frequency of the diffuser has a wavelength of greater than about 3.3 feet (39 inches or 1m), then ten feet is not enough: you have to get further away than three times the lowest WAVELENGTH (not quarter wave: FULL wave). If you understand the principles of waves you'll get why it MUST be like this.
The basic issue is that all numeric sequence diffusers produce some form of lobing patterns and interference. Here's what they look like for a QRD sequence:
QRD-Diffusion-lobing--pattern-graph-SML-ENH.PNG
They MUST do this because they change the timing and therefore the phase of the sound wave, in addition to changing the direction and intensity. The incoming wave front hits different parts of the diffuser, and is firstly broken up like that, with each part of the wave going down the well that it happened to arrive at, hitting the bottom, and coming back up again. If the wavelength happened to match the depth dimension of the well, then it will come back out the top exactly out of phase with the way it went in. The wave that is four times the frequency will do the same, but not the wave that is one quarter the frequency. Nope. Sorry. It just doesn't. The cut-off frequency is set by the depth of the deepest well. The device will still SCATTER down to half of that frequency, but at a quarter of that frequency, it simply reflects the wave, without doing anything to it. Because according to basic acoustic theory, an object has to be at least comparable in size to the wavelength in order to have any effect on it. Waves that are considerably longer than the dimensions of an object will NOT be affected by it. Only ones that are shorter, or similar in size.
... has its deepest well as 26.5 cm. ... finding that 26.5 x 4 is a wavelength somewhere close to 100hz.
Ummm... Nope! The wavelength at 100 Hz is 11.3 feet, which is 135.6 inches, or 344 cm! Certainly not 26.5cm. A quarter of 100 Hz is 2.825 feet, or 33.9 inches, 86cm. You made a big mistake in your calculations.
In fact, the deepest well in a numeric-sequence diffuser needs to be a half wave deep (in order to get phase cancellation at that frequency). At 160 Hz, the wavelength is 84 inches, so the half wave is 42 inches. Therefore the device needs to be 42 inches deep.
If you use a QRD diffuser calculator (which are FREE!), you will find that for a typical N=13 diffuser for 160 Hz will need a maximum block height of 39 inches, and a minimum block width of 6.5 inches. If you made that with 8x8 lumber, which measures 7 1/4 inches on each side, the resulting diffuser would diffuse down to 160 Hz, it would scatter down to one octave lower, at 80 Hz, and the upper cut-off frequency would be 934 Hz. The minimum distance to the listener position would be 254 inches, which is a little over 21 feet...
Here are the actual results that I calculated for you, using a real diffuser calculator, and showing the cross-section of the 1D version (normal Schroeder diffuser), with the diagram that shows the lobing directions:
QRD-calc-for-160Hz-diffuser.jpg
Here's the actual design for the 2D version (Skyline style) of such a device, :
QRD-calc-for-160Hz-diffuser-2.jpg
And here's the diagram of block heights for those 8x8 lumber blocks that you would need to build the 2d version:
QRD-calc-for-160Hz-diffuser-3.jpg
You do NOT need to pay somebody to make the plans for you: you can do it yourself using this same program, called "QRDude", which is FREE. You can get it here :
https://www.subwoofer-builder.com/qrdude.htm If somebody made you pay for doing the same thing, then perhaps you've been scammed.
The placement of those are supposed to be at least 4-5 feet from the recording position.
21 feet, according to this ACTUAL calculator that uses the REAL equations. Take a close look at the calculate real results, above. Your room would have to be about 30 feet long to use such a device, since it is 3 feet deep itself, and you do need a bit of space to stand in. A 40 foot room would be better...
I had said, "If I enter my room dimentions in the Amroc calculator
https://amcoustics.com/tools/amroc?l=36 ... 40&r60=0.6 I see some frequencies below 80Hz (47.64 Hz, 57.17 Hz, 71.46 Hz, 74.41 Hz). Why worry about them? Won't they only come into effect if you make sound with those frequencies?"
Because the modal response of the room shapes the entire frequency response curve for the room, regardless of whether nor not you actually make any sounds at those frequencies.
Think of it this way: If you are standing on a hillside that is steeply sloped, do you have difficulty walking UP the hill or ALONG the hill? Even though you have no intention of going DOWN into the valley below you, does that valley still affect you? Yes, it does, because the valley is part of the shape of the hill. Even if it is night time, in total darkness, you can still feel that the valley is there, even if you can't see it, just by walking around. The slope cause by the valley is always there, and affects your life, even if you never see it, and never plan to walk down into it: it is still there.
Same with the hills and valleys in the frequency response of the room: they are sill there, all of them, and the "valleys" way down low still cause a slope in the response for the frequencies where you will be "walking around". Even if you never play any tone that goes that low, the valley is still there, and will still affect the tones that you DO produce in that room.
If you are walking around that hill, and get tired of limping off to one side all the time, then you can just fill in the valley! Dump a lot of dirt in it, to level it out, and it won't affect you any more: you can walk on level ground. Ditto with your room acoustics: if you want a level frequency response then "fill in the valley" with suitable treatment.
but I use a Low cut filter, I think it's called, to cut out the low base sounds that might be in a room when I record - oh, hold on
there it is, why would I do that if I am imagining there are no low frequencies to cut out.
Yup! If there were no lows in the room, you would not need to use EQ to filter them out!
All right I get your idea trhat a rooms 'signature' incudes low frequency reverberation, I just dont understand it? is it because outside sound waves that are so low you cannot hear them, get into your room from outside?
Very likely there ARE lows in the room, coming from outside sources, or even from the person in the room just being there. But even if there were NO lows at all (if you had perfect isolation), the acoustic response curve for the room is still shaped by the dimensions and treatment of the room. The ground you are walking on is shaped by the valley below and the hillside above, even if you never plan to go there. Those still affect the slope of the ground WHERE YOU ARE WALKING, as well as the parts where you are NOT walking.
What about electric hum at levels that wouldn't be picked up as a hum issue, but nevertheless seepoing in as low frequency, actaully direct sound?
It's a hard concept to get your head around, but there doesn't actually need to be any sound at those frequencies in order to affect the shape of the acoustic response of the room. The response curve is already shaped, sound or no sound. The tree in the forest still does make a sound even if you weren't there to hear it. The room response still has a shape, even if you don't make any sounds that would reveal it. It is still there. The hill and valley are still there, even if you don't shine any light on them to see them, or walk over them to feel their shape...
Ok, like Um, or Ah-hah, or a grunt maybe. Okay, so I go down to 50. Can I avoid going down to 40hz?
Nope! If you want to walk on level ground, you have to level out the ENTIRE valley, not just the bottom part. You have to fill it up to your own level, not just drop a few stones in the riverbed.
but you still have to make that lower direct sound, dont you?
Do you have to walk in the riverbed to known that the ground on the hillside slopes down?
I figured I could use a combination of superchunk corner traps, which I calculated* at 60% effectiveness if average 12" average thickness - not a disaster.
I'm not sure where you are getting your information from, but it is highly suspect. Insulation 12" deep might have a coefficeint of absorption of 0.6 at 160 Hz, yes, but that does NOT mean that it is 60% efficient! All that it means is that it can absorb 60% of the random incident sound in a purely diffusive and reverberant room. As long as your room meets those conditions, then you could expect it to absorb 60% of the energy that would otherwise be reflected by the wall behind it. Let's assume that it is 50%, to make the math easy. A drop of 50% is a reduction of 6 dB, so your device would be able to reduce the modal peak at 160 Hz by about 6 dB. If that peak happened to be 15 dB high, then it would come down by 6dB, and would now "only" by 9 dB high. If your mic happened to be in a null which was 30 dB deep, then that null would now be only 24 dB deep.
You need to put the numbers in perspective, to understand what they actually mean for you. To get more absorption, you would need to find a lighter weight (lower density) insulation and make it thicker. A deep superchunk could have a coefficient of over 0.8 a 160 Hz, and over 0.7 at 60Hz, for example.
If you have only one, where would you put it? Modes happen in all three dimensions, on all three axes of the room. So if you put your trap at a location where some of those modes never reach, it won't so anything to them. If you fill in the valley around the other side of the hill, it wont do anything to make the ground flatter on YOUR side of the hill...
There is one referenced there that goes lower, but I didn't want to make that because I would end up with too much reflection going on, because that design uses plywood facing.
That's a TUNED trap, and only absorbs at a specific frequency. Those are very god for dealing with stubborn modal issues, but not what you need for a vocal booth.
Using
http://www.acousticmodelling.com/porous.php, if I have an average 12 inch (~300mm) thickness of Owens Ccorning 705 (6.0 pcf = 96 kg/m3 = 30.000 MKS Rayls/m = 30.000 K Pa.s/m2) (lets say 17” deep to the corner) with no air gap, it shows over 60% efficiency at 160Hz - not a disaster.
Way too dense! You want LOW frequency absorption, not HIGH frequency absorption. Use 701, not 705. Even 703 would be better than 705! You need something around 7000 rayls, or less, to get down low effectively.
- Stuart -