# 100 hz dip



## matthijs87

Hi guys, i'll introduce myself as most of you dont know me.

My name is Matthijs, 28yo and i'm from the netherlands. (so my writing might nog be perfect )


Im currently setting up my new room which is meant for professional mixing purposes.

The lengt of my room is 5.65 (Meters)
The width is 3.48 (meters)
The highest point is 2,75 (its not a flat ceiling)

So with this information i made up some calculations where to put my speakers 
and where would be my listening position. I had read the listening position should be around 30 till 38 % from your
front wall. So i did. 

I placed everything (besides the lengt of course) exact in the middle and have setup the triangle as precice as possible.

My listening position is now at : 2.05 from the front wall which is 36 %

Im giving all this information becouse my graph gave me a big dip around the 100 hz and im being told i should experiment with 
moving my listening position and or my speakers position. The only thing im concernt about is how to go about it becouse 
every inch would make a big change in the symetry and even possibilities of placement.

So my question is, what would be the best and most effective way to find out where the dip is comming from whitout constantly changing my whole setup just to test if something changed, i mean there should be some guidelines right which makes things easyer ? Isnt there a way to find the source with the information given in the graphs ?

Here are some photo's of my measurements and the room im working in.

As you can see the dip is around 15 db, tho this is probably 5 db to much becouse my spl reading gives 5 db lower when doing the subwoover test.

I tried diffrenct mic placements with symetric angle towards my speaker, and i tried putting a absorb panel in front of my tv just to see if it does something with 
reflections or buildups, and i checked to see if it makes a diffrence when i put my refference monitors away. Not big diffrences on all.


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## bpape

First, When you are making changes, all you move is the mic. If you move the speakers and the mic, you have 2 variables and no idea what changes are related to what. You always only want to only change one thing at a time. Also, you appear to have a very high level of smoothing applied which makes it pretty much impossible to determine really where things are. Turn the smoothing off when trying to identify where things are coming from.


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## matthijs87

Okay, thank you, that makes sense indeed. 

I removed the smoothing, and of all tests moving the mic was the one which gave the best results. 
i moved it 20 cm towards the speaker which gives me less spikes and is overall a bit better but it doenst look like the source of the problem.

Do you have any sugestions on how to find the source more accurate ?


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## tonyvdb

Just to understand a little more about your speaker setup are you using a small subwoofer?


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## matthijs87

No, im using Genelec 8050s , they produce more then enough sub so i dont need a subwoover


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## tonyvdb

Bryan would be the best person to talk to but until he responds Im thinking because you have many reflective surfaces in the room that could be causing cancellation. Have you tried putting down a thick throw rug on part of the floor?


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## bpape

How far are the speakers from the wall you are facing and what position is the mic in for this measurement?


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## matthijs87

Tonyvdb, thank you for your input, i tried the floor on diffrent locations but it didnt make any big changes, some frequenties went a tiny bit better but it also created extra spikes. 

@ bpape : from front tweeter to wall is 1 meter (diagonal) 
76 cm when measured from the back of the speaker. (diagonal)

Sidewall till tweater is 87 cm by the way.

My mic is pointing up.


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## lesmor

matthijs87 said:


> Tonyvdb, thank you for your input, i tried the floor on diffrent locations but it didnt make any big changes, some frequenties went a tiny bit better but it also created extra spikes.
> 
> @ bpape : from front tweeter to wall is 1 meter (diagonal)
> 76 cm when measured from the back of the speaker. (diagonal)
> 
> Sidewall till tweater is 87 cm by the way.
> 
> My mic is pointing up.


I hazard a guess that going by 87cm that it could be a sidewall 1/4 wavelength cancellation?
Caveat being I am no expert and just trying to learn


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## matthijs87

How is that a 1/4 wavelength cancellation ? woudnt 1/4th be 25 cm ? or maybe im missing the point , but from what i have read is that the sidewall length shoudnt be an equivalent of the length from the wall behind the speaker. so if im right, that would mean cancelations would be most probably at 25, 50 75 in my case or maybe i didnt understand this part well enough, im curious as well


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## lesmor

matthijs87 said:


> How is that a 1/4 wavelength cancellation ? woudnt 1/4th be 25 cm ? or maybe im missing the point , but from what i have read is that the sidewall length shoudnt be an equivalent of the length from the wall behind the speaker. so if im right, that would mean cancelations would be most probably at 25, 50 75 in my case or maybe i didnt understand this part well enough, im curious as well


395.4Hz = 87cm
395.4Hz/4 = 99Hz

or 

speed of sound 344.5/87cm = 396 Hz /4=99 Hz
or 
99Hz = 347.47cm /4 = 86.8cm

As I say I am a novice so could be looking at this completely wrong

Move the speaker and see if 100Hz changes.
Please let me know if it helps


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## matthijs87

Oh right, that makes sense, i dint looked at it at that way , thanks, will try that for sure.


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## bpape

I would get the speakers closer to the front wall. I still don't know where your head is.


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## matthijs87

Oh im sorry i didnt get that, i thought you where refering to pointing up or pointing at the speakers. My head or mic posistion is 1 meter 50 from tweaters till head . I tried allot of diffrent things today, I found that changing the length to sidewall mostly just shifts the frequentie dip. Placing it more to the front wall did made a change and even some out around that area, gained 6,7 db there so thats great but still not perfect. The subb is a diffrent story tho, i had found one position where the sub and the 100 hz where pretty good but i could only get this to work in a direction where its not pointing to the center of the room. Is this useless information or can i do something with it ? obviously i cant make a working triangle this way.


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## bpape

I need to know where your head is in relation to the wall, not the speakers. Don't worry about the sub not being pointed out into the room. Bass is omni-directional.


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## matthijs87

Ah okay, i posted this in my first post i think, its 2.05 from the front wall which is 36 % from the whole length of the room. I had read that this (1/3 till 38 %) would be the most ideal position for the listeing position to be in.


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## bpape

Sorry - didn't remember that. Honestly I would try to get the monitors as close to the wall you are facing as possible and then set up in an equilateral triangle about 3.3 meters.


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## matthijs87

Thanks ! I will try that as soon as possible


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## matthijs87

its not possible that way , The width of my room is 3,50 besides that the foot of the stand also takes some space and theres also a wooden box that covers some gas pipes. the best i can get is (from tweeter to tweeter) 2.50 at an equilateral triangle. maybe a little bit more if i buy diffrent stands and cut som of that box away. but 3.30 is pretty much impossible. 

If im not wrong this 3.30 would also be the minimum for it to work right ? it looks to me this settup is using the same principle as i aplied before only then backwards. like a third from the back wall in stead of the front, is that right? 

I did some measurements annyway to see what it does. And it helps at the 100 but it creates some other problems as well.


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## bpape

Get it the best you can and then treat the wall that you are facing with some panels that are 4-6" thick.


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## matthijs87

Thank you , i did some measurements already on some spots with the basstraps i have, and it does help. I just still got a question left concerning the equilateral triangle. I was just wondering how importand it is to stick on this rule when it comes to the width ( speaker to speaker) the thing is that i can get some pretty good results when the width is at a diffrent length. Also just for wanting to know how it works, how much and what does affect the width when talking about an equilateral triangle?. The same distance to the walls and the same distance to the listening postion already covers the speed and the frequentie response i would think.


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## bpape

Equilateral is just a starting point. No problem being inside or outside of it. Just a different presentation.


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## matthijs87

Oh really ? that makes it allot easyer. Thank you !


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## matthijs87

Im planning to treat the room a.s.a.p, but i dont want to invest more then necessary. What would you suggest ? If you look at the last photo you can see the front wall, it now exist only of a big carpet. Would you suggest treating the whole carpet (even behind the tv and the bottum) and what about above it ? what is your experiencie in what areas it is most effective ?


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## bpape

Something pretty thick on the rear - at least 6" and at least 2 2x4' panels.

For your side walls you'll want more surface area and thicker. Again 2 2x4's each side and at least 4" thick with a bit of a gap behind them.


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## matthijs87

Hi, im back for more questions :grin2:.

That it is no problem getting inside or outside the Equilateral triangle was realy helpful
This way i have bin getting much better results and after doing some minor changes with EQ it looks pretty good i think(frequentie response is taken at (1/48 smoothing)

all though the angles in my setup are looking quite extreme. (check photo's) 


Annyway my question is about the room acoustics, you suggested to get pretty thick panels' but does this still aply the same way now my frequencie response is okay ? You can check the photo's for my waterfall graph for more in depth info. I'd also like to know how to read this or better said "What is genneraly accaptable"? so i know how to meassure/read the graph.


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## bpape

Your decay time is still very long all the way through the range. I would do the thicker corner traps and on the rear wall and ceiling. The side walls you could probably get by with a little thinner.


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## matthijs87

Thanks!

My ceiling is kindof triangler shaped (you can see it at the photos in my first post. ) How would you go about that ? Is there a way to know where to place it ? like the mirror technique for the side walls for example? or would this be more of an trial and error thing ?


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## bpape

The mirror method would be best.


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## matthijs87

Great!, last question, how big and thick would you suggest for the ceiling as we didnt discuss this in the other post where you named all the dimensions. again 2 2x4's ?


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## bpape

At least 4" thick with a 4" gap.


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## matthijs87

Thanks allot


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## matthijs87

Hi again , Im doubting between doing the whole ceiling with 6 inch glasswool or doing it with the 2 panels you described. Would treating the whole ceiling kill to much of the space ? or would it be a good thing ? i am not sure in this case. What is your experience with this types of ceilings if i may ask?


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## bpape

I wouldn't kill the whole ceiling. Actually 4" with a 4" gap is as good or better than a 6" panel flush with the surface.


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## matthijs87

Great, good to know. Your talking about the whole ceiling surface with 4 inch gaps between them right ? Becouse we talked about 2 panels for the ceiling with the mirror trick before this..


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## bpape

Yes - just cover the reflection zone - not the whole ceiling.


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## matthijs87

Okay thank you, i was also asking about the panel flush because i forgot to mention there is no proper isolation at this moment. It might look like it on the photos but its actually not doing much, behind the grey surface you can see on the photo's is just wood. This is still no issue now i mentioned it? 
this whole project has bin a bit delayed but today i'm finally gonna buy all the stuff i need so i thought lets double check just to be sure. 

Front wall : 4”-6”. ( x 2 )
Rear wall : 6” ( x 2 )
Side walls : 4” with a bit of gap behind it ( x 2 )
Ceiling : 4” with 4” gap ( x 2 )

So this is a summary of what you advised me to get. Only at the ceiling and side walls
you talked about “gaps”. For the side walls a bit of gap and for the ceiling a 4 inch gap. 
This is just to get more out of it right ? so it also will absorb more bass frequencies ?
Shouldn't i then do all of them ? or do the front and rear wall don’t need the gaps somehow ?

I'm planning to do them all 6 inch to be sure. The only thing is when doing all the 6 inch panels with 4 inch gaps i get 10 inch thick panels, that's kind of huge, that won't even fit behind my TV on the front wall without changing my listening position. Also is it still effective if the tv is in front of the panels ? the tv(50”) is already a reflective surface of it self i can imagine. 


For the ceiling i will be using the mirror technique but what about the front and the back ?
is there a rule about where to place them ? 


For the sidewalls you talked about 2 panels on each side. But with the mirror technique i will only find one reflective zone, so where then should i place the second ?.


And final question : How important is it the panels are 2x4' ? I can get rockwool at sizes of 100 cm by 61 cm (15 inch thick) for way much cheaper then the 2x4.


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## bpape

4" on the front wall is fine. No need for 6" and 4" gap. 6" with a couple inch gap on the rear wall. The rest, 4 and 4 is fine.


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## matthijs87

Oke, it won't even matter if i use 4 or 6 inch for the rest :O ?


So how can i find the second spot for the side panels ? i can only find 1 with the mirror trick
Im also asking because there might not be enough space for another panel next to it (depending on how for away from each other) because there's a window on one side prettu close next to the reflection point and a door on the other side. 

And where to place the panels on the rear and the front ? Is there a rule on where to place them ?

And how important is it the panels are 2x4' ? I can get rock-wool at sizes of 100 cm by 61 cm (15 inch thick) for way much cheaper then the 2x4. in ft that would be 3.3'x2'. I can still make it work by cutting them of course if it will really make a big difference.


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## bpape

In the reflection zone the more surface area the better. 6" for behind you and any corners and 4" for the rest. Make the 2nd panel smaller if needed to fit with the window interference. Find the 2nd point the same way you did the first - just have someone keep moving the mirror closer to you til you see the opposite speaker. 

Reflection Point Video - http://www.gikacoustics.com/video-early-first-reflection-points/


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## matthijs87

bpape said:


> Something pretty thick on the rear - at least 6" and at least 2 2x4' panels.
> 
> For your side walls you'll want more surface area and thicker. Again 2 2x4's each side and at least 4" thick with a bit of a gap behind them.


I'm sorry, i dont get it, maybe its becouse im dutch and there is some misscomunication, but you where speaking here about 2 panels on the left side and 2 panels on the right side, correct ? 

Do i need the second panel on the same side to mount right next to it, above it, or would there be another way to check where to place? for as far i could check there is not a second reflection point on the same wall to find with the mirror technique.


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## bpape

Yes - 2 on each side wall forming essentially a 4x4' square. Same thing in the rear but 6" thick instead of 4"


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## matthijs87

Wow, now i'm really getting confused. :dizzy:

I think its because of the differences in how you are used to describing measurements in the US. It's diffrent in Europe.
Though i thought i understood. So essentially this is what your talking about ? (check pic)

I thought all this time you where talking about feet. like 2 feet by 4 feet when talking about 2x4' which would be panels from 1.20 by 60 cm converted into European standards.



But if im understanding you right this time you meant with 2 2x4's pannels  4 pannels from 4 ft by 4 ft each ? which would be 4 times 1.20 meter to form a square. 
Thats impossible, that would be a square of 2.50 meters by 2.50 meters. Thats huge for this room. Where am i going wrong here in my calculations ?

Thanks for your patience by the way :grin2:


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## bpape

No. You're right. 2x4' panels. 2 each side.


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## matthijs87

Oh! Oke. I will try to get it to work then. So correct me if i'm wrong but 2x4' times 2 in the form of a square is just the same as 4x4' in a square right ? this must really sound silly but for a European that's really confusing to say it like that. But im glad i'm understanding now ). So i guess the gaps you where talking about where based then on in between the 4 and not behind the panels right ?


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## bpape

Yes - sorry to be confusing. The air gaps are between the wall and the panel to get the front of the panel farther from the wall.


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## matthijs87

No that's not on you, that's more my understanding of U.S measurements i guess .

Yes I know the air gaps are between the wall and the panel. But are there supposed to be gaps between the 4 panels as well ? 

For example when you say : "At least 4" thick with a 4" gap." Then are you talking about a air gap or a gap between the 4 panels like the picture above ?


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## bpape

No. We're talking about the gap between the panel and the wall.


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## matthijs87

Oke, thank you, then i know everything i need to know. I will be back with some updates and tests when its all finished )


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## matthijs87

bpape said:


> Yes - 2 on each side wall forming essentially a 4x4' square. Same thing in the rear but 6" thick instead of 4"


Man, im sorry, but i am still not sure if i am understanding this right. last time i was sketching two situations at which your response was, yes you are right.. but which situation was right i am now wondering ? 

As you can see in the quote your saying 2 on each side wall forming a 4x4' square. 

*So when talking about 2x4' are you talking about the size of the panel ? (2 feet by 4 feet) or are you talking about the amount of panels ? 2 TIMES 4' panels (so 2 panels of 4ft by 4 ft) . which would take 8 panels per side in total to get 2 squares. This makes a huge difference*


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## bpape

Yes. The panels are 2x4'. So put 2 side by side and it makes basically 4x4'.


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## bpape

10" to do 100 Hz? Don't think so. 4" across a corner will go 60 ish easily.


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## matthijs87

Thank you for your comments, 

I did use REW to get my frequency response as good as possible, that took quit some time before i found the right spot, i found only one position that worked for my frequency response, so as bpape recommended i looked for that and am now trying to fix the acoustics from this starting point. I guess that room simulation option would have done that job faster, but yea i found it manually, so no problem. 

Yes to much isolation would sound to dull i can imagine that's why i am not isolating everything but just covering the direct and early reflection points. The grey stuff which is all over my ceiling is not actually doing very much, it may look like isolation but behind it is wood, so that might still compensate for the dullness and if its really needed i will ad some diffusers later on for sure but i first want my room to sound acoustically good. 

I have read allot and seen allot of youtube vids about the subject but i am not sure anymore. I thought diffusers would be best for the more bigger rooms. My main concern now is to get the waterfall graph good. There is still to much going on in this room allot of high as well by ear, so i guess that will be taken care of with the panels i am making now. The sub and bass i am not sure about, i still don't know exactly how to read this waterfall exactly so i am putting my trust on bpape on that one. 

i am using 6 inch for the corners so that's already 15 cm, that's pretty thick right ? and with the gap behind it it should do some extra isolation if i am right. 

But i am a bit confused now, would diffusion be enough ? or should it then be a combination of the both ?. or can either do the job at its own way under certain circumstances ? See this is actual the same what you find on the internet if you look long enough, different opinions different arguments, different situations. I would like to do what is best for my room of course. :smile:


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## bpape

puukorva said:


> 100hz has the wavelength of 135,6 inches. In order to effectively absorb sound with an absorbent material you need to have at least a quarter wavelength amount of the material. Soft absorbers fail at low frequencies, it's simple physics.


Theory vs testing.

http://www.gikacoustics.com/product/gik-acoustics-244-bass-trap-flexrange-technology/

Standardized testing at Riverbank Labs.


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## bpape

6" across the corner will do very well. Down into the high 40's.


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## bpape

That's why you use an independent lab like Riverbank.


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## matthijs87

33 inch ? thats insane. I thought this gap is just needed to let it more effectivly bounce back into the absorbtion panels, 
why should the gap then be a quarter ? and if this theory is right should you then not also measure the thickness of the panel within that calculation ? i mean if the gap should be 33 inch then the total is 39 inch thats indeed eating up allot of my room and not realy an option.

But what about the rear, front and ceiling ? would this be enough or better with diffusion ? is it situation dependend ? (size of the room/irragular shaped room ? and what is your point of view on this bpape ?


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## bpape

Here is the actual test report and without the range limiter.

http://www.gikacoustics.com/wp-content/uploads/2013/11/RAL-Report-Full-Range-244-Bass-TrapNEW.pdf

Also, all of our panels can be ordered with or without the membrane.


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## bpape

If you do the 33" all that does is create a hump in the absorption at that frequency. The gap is to get the leading edge farther from the hard surface. Generally the max you do is the same gap as the thickness of the absorber to keep things linear.


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## matthijs87

bpape said:


> If you do the 33" all that does is create a hump in the absorption at that frequency. The gap is to get the leading edge farther from the hard surface. Generally the max you do is the same gap as the thickness of the absorber to keep things linear.


Yes that makes sense, changing the frequency response "drasticly" is not that much needed, how the room sounds is way more important. 

So does this mean you where talking about two diffrent things ?altering frequency response vs acoustic response ?


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## bpape

Frequency response is important - but so is impulse response, a balanced decay time curve, etc.

The trick to frequency response is not just what you use. It's more importantly knowing where the problem is coming from so you know how and where to treat.


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## matthijs87

Yes, but for as far as i understand my frequency response is quite oke, not perfect, but my impulse response , decay etc is less oke. 

Wanting to change the frequency response needs more isolation i can imagine as aposed to changing the decay, impulse etc. am i right ?

So besides that, what do you think about puukorva's statement about using diffusers for the rear and front ?


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## bpape

Absorption can impact all 3 pending what it is and where it is. Diffusion does not change decay time. Diffusion on the rear wall is personal preference. On the front wall the only time I use diffusion is when using an open baffle or panel dipole type of speaker to deal with the comb filtering from the rear wave but doing so without removing more energy from the room. If a typical monopole speaker, you're more concerned with is SBIR which must be deal with via absorption as a diffuser that would go low enough to address it would be unreasonably sized.


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## matthijs87

The advice was not given i think becouse of the 100 hz null, "that" problem was already kindof fixed at page 3 by finding a better listening posistion. you can check the waterfall and frequency response on page 3.


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## matthijs87

puukorva said:


> Oh, ok. I didn't read the previous posts word for word, I just wanted to mention the REW room simulation because I think it's an awesome feature in the awesome software.


I agree, it's verry awesome ) , 

* The title is a bit misleading by now, so i understand, i can't find an option though to change the title. Is this possible? somebody ?


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## bpape

puukorva said:


> You're correct in that if you want to maximise the effect of a small diameter absorber, the best way to do that is to leave a gap of a similar size to the barrier. That however, is no rule of a thumb on setting up absoroption.


Other than it's impractical to try do do porous absorbers to tune narrowly.

Also, the test results you mentioned earlier you're misreading. 8 of the 12 pieces were done leaning across the wall/floor junction. That's a corner just like any other vertical corner. The other 4 were on a wall. That replicates the way those panels are used in many rooms - 8 in corners, 2 side wall reflections, 2 ceiling reflections.


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## robbo266317

puukorva said:


> Sorry but you're trying to explain physics can be broken. Clearly it isn't so. The tested elements were not comparable to DIY soft absorbers but used multiple technologies combined (according to their website).


What is the link to their website?


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## bpape

Trust me. Those did not have the membrane. If they did, the high frequency absorption would be close to zero which it's not. We were there when the testing was done.


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## bpape

Again, theory vs data. Bees shouldn't be able to fly according to physics either. But they do.


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## matthijs87

I'm currently bussy making the soundpanels and all is going fine. But i got another question about the room acoustics. Im planning to buy a diffrent floor. I have read a bunch of diffrent articles saying diffrent things about it.
Im wondering, what would be the best floor to get ? 

Carpet 
Laminate
Vinyl

For carpet it absorbs high frequency's and it might sound dull and boomy in the end is what i found
For laminate it can sound bad by the reflective sound it produces
For vinyl i couldnt find an awnser. 

Also i read that an hard surface would work better with soundpanels and carpet would make it sound boomy

Source

Im planning to get Vinyl though, but i thought lets check what the experts know about it before buying it


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## bpape

Boomy is there regardless of the floor type until treated. Yes - carpet is high frequency only absorbing. That said, in a home theater situation, I would use carpet. You want a more damped room in a multi-channel environment. Now for 2 channel, Wood can be nice. I would not use tile, concrete, or vinyl.


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## matthijs87

Okay, I'm using 2 main speakers and i want to get the most reliable/real sound as its meant to mix music. What is important for me is that it gives me the most accurate sound (not the best warm etc sounding). 

The answer i'm looking for is "why" do you suggest wood ?. "Nice sounding" is not an priority if it ads an acoustic sound to the room which is not in the sound of itself . I need it to be as transparent as possible. Why do you think vinyl is a bad choice as aposed to wood/laminate ? wouldn't wood give extra reflections as well ?


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## bpape

Wood doesn't tend to be as hard/harsh sounding. If you want to use a laminate, go for it. Just giving my opinion.


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## matthijs87

I'm just trying to find out what would be best in my situation. I understand wood will sound better then laminate , but it is also allot more expensive. 
So instead of going for the harsh sound of laminate i thought vinyl would be a good in between choice as its allot thinner and less reflective (though i'm not that sure about the reflections or if it sounds harsh) or do you think vinyl is just as bad sounding as laminate or carpet for a 2 channel mixing-room situation ? 
I do have wood under the vinyl i'm using right now by the way, so no concrete if that might change anything.

For as far as i understand your experience is that vinyl/laminate/tile all produce a to harsh sound ? and that carpet wouldn't be an option for an 2 channel situation ?
leaving actually only wood as an option ? Have you done tests in the past if i may ask ? or is this only by ear (which i would find equally valid if you knew the floor was for a fact the source of the harshness) Harshness is not good in any way , so i wouldn't call that an opinion .


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## bpape

Over wood, vinyl would be slightly better than laminate but not a ton. It's just experience of hearing a ton of rooms over the years with pretty much any flooring type you can think of.


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## matthijs87

Ah yes, I figgured i have still a wooden floor beneith my currently vinyl so i will use that and paint it to look nice again.


Thank you for the advice


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## matthijs87

I'm back after a looooong time, it took some while to find the time and do everything the right way but i think i got it right this time... i hope. 
I think it sounds allot better already and i did bought a wooden floor, it realy gives allot reflections but when tamed down with the soundpanels it sounds warm and nice 

Could you check my measurements to see if its improved to an acceptable standard ? I'm still not sure how to read the waterfalls. As you can see in the first picture it looks like the average is 400 (zoomed out) while the second picture looks like the average is around 200 ms. 

The frequency response is quite oke i think, it has some spikes though around the 2, 4 and 600.

For the basstraps in the front corners i got also traps behind the carpet wall which you can't see on the photo's.

What are your thoughts about it ? im realy curious


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## matthijs87

bpape , could you take a quick look to see if i did it right this time  ? Would mean allot to me.

Thanks


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