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14th September 2011
#61

Quote:
Originally Posted by Jens Eklund
There’s no definite answer here. Even if the with of a plate was infinite in length (and width), it would still not reflect completely (assuming plane wave) since there will always be some losses due to diffraction at the edges.
Just for the fun ... I wonder what the edges of an infinite plate would be
Yes I know a purely theoretical question, with no practical interest
14th September 2011
#62
Lives for gear

Quote:
Originally Posted by mhch
Just for the fun ... I wonder what the edges of an infinite plate would be
Yes I know a purely theoretical question, with no practical interest
The distance between the edges is infinite … heh
14th September 2011
#63

Quote:
Originally Posted by Jens Eklund
The distance between the edges is infinite … heh
That, I understood

But how would mathematical formula for a finite width slat - assuming they could be created - would behave when pushing the width to the infinite limit is something else.

Put another way, I would believe that the ratio of reflexion to diffraction goes to infinite, conversely diffraction to reflexion goes to zero.

Again this has no practical interest. My mathematical mind was just tickled
14th September 2011
#64
Lives for gear

ok, but setting that aside and looking at the center of the reflector/slat it seems obvious that if it is bigger than the sound wave it will reflect.
If the wave gets bigger it will bend around, thats what i have read. This is a bit harder to imagine because one could think that the bigger wave hitting more than one slat will still be partly reflected.

On the other hand it seems that long and short waves are looked upon differently sometimes. Of course they are nothing but preassure changes, but it sure matters if this happens on the space of an inch or in an area several meters in diameter.
High frequencies are described like a ray of light, whereas low frequencies can be considered as air preassure rising and falling.

I have come across many posts in a german forum where people use slats of a certain size, say 34cm, with 1cm gap.
This is said to be a "tuned reflector" for anything above 1000hz.
I don´t see how an array of much smaller slats could have any effect on this frequency.

Slightly off topic but maybe of interest to some:
Another option i have read about is using strong foil to reflect the highs.

mf=90/F

F is frequency passing the foil with 80%
mf is Mass of the foil in kg/sqm

For example to reflect above 500hz a foil with 180g/sqm is needed.
This is easy to calculate and easy to set up. And you don´t need a table saw...
14th September 2011
#65

Quote:
I have come across many posts in a german forum where people use slats of a certain size, say 34cm, with 1cm gap.
This is said to be a "tuned reflector" for anything above 1000hz.
I think this is partly true for sound coming from a perpendicular angle 90 degrees, in other words at the opposit wall facing the sound source, placed at other surfaces I think the frequensy will change, depending on angle of incidence.

Some sound will be scattered (in an area around 1000hz, when the sound comes from a perpendicular angle, not shure excatly what the bandwidth of scattered frequensies is).
14th September 2011
#66
Lives for gear

i think this is also true for sound arriving at an angle other than 90°.
Why shouldn´t it?
14th September 2011
#67

frequensy of scattered sound will change with angle of incidence ,at 90 perpendicular angle most diffraction will happen around 1000hz, I would think that absorption coeff will change when frequensy of diffraction change, because diffraction plays a central role detirmining the absorption coefficient at different frequensies.

P.S I see a possible problem with all panels having same width, it can emphasize certain frequensies (diffraction grating effect).

Last edited by hsal; 14th September 2011 at 03:13 PM.. Reason: Didn`t understand my own sentence...
14th September 2011
#68

Quote:
at 90 perpendicular angle most diffraction will happen around 1000hz
That would be "diffraction scattering sound back towards the room will happen around 1000hz".
14th September 2011
#69

This thread is becoming, for me at least, most of the most interesting & informative on this forum...
15th September 2011
#70
Gear Maniac

Quote:
Originally Posted by jrp
I have come across many posts in a german forum where people use slats of a certain size, say 34cm, with 1cm gap.
This is said to be a "tuned reflector" for anything above 1000hz.
I don´t see how an array of much smaller slats could have any effect on this frequency.
Interesting. This is exactly what I built for my studio. It was supposed to reflect above 1 kHz and absorb below. But it didn't work this way, so I had to remove it. Measurements showed that frequencies as low as 200 Hz were still partly reflected by this "tuned reflector". I suppose it would work this way if the slots were much wider than 1cm. Somehow 1cm is too narrow to really break up or absorb the low frequencies.

Quote:
Slightly off topic but maybe of interest to some:
Another option i have read about is using strong foil to reflect the highs.
I don't know how thick the foil is that I used to wrap up my rockwool chunks, but they do indeed reflect very high frequencies. Even the ones that I additionally covered with cloth. I was quite stunned to find out, because I was assuming those chunks would absorb everything.
15th September 2011
#71
Lives for gear

A perforation percentage of around 3% should have an fc of around 100 Hz depending on depth, wool, panel thickness etc, but yes; it should be quite reflective above 200 Hz.
15th September 2011
#72
Lives for gear

So perforation percentage of around 3% comes from 1cm/30cm gap/panel?
What formula are you using to get those 100hz?
The idea i described was just covering the mineral wool traps, not placing them into an air tight container with an perforated cover. So no Helmholz...
To me right now using strong foil seems the easiest and most predictable option.
15th September 2011
#73

Quote:
Originally Posted by jrp
Slightly off topic but maybe of interest to some:
Another option i have read about is using strong foil to reflect the highs.

mf=90/F

F is frequency passing the foil with 80%
mf is Mass of the foil in kg/sqm

For example to reflect above 500hz a foil with 180g/sqm is needed.
This is easy to calculate and easy to set up. And you don´t need a table saw...
15th September 2011
#74
Lives for gear

it´s from the german book "Studio Akustik" by Andreas Friesecke.
So i belive that it´s true in theory.
I would be very interested if someone is actually using this method and can give us a hint if it is working well.
It is cheap and easy and i could at leas imagine that it works.

I packed my absorbers in very thin foil to prevent mineral wool from getting into the air. I noticed no reflections in the highs in my measurements. All is dead right now.
15th September 2011
#75
Lives for gear

i decided to start a new thread on this, sice it is a bit off topic here.
15th September 2011
#76
Lives for gear

Quote:
Originally Posted by Jens Eklund
I’m also having trouble believing that the ”diffuser” in the ceiling seen in the pic in post 50/51 is a “diffuser” since it will absorb a lot, if not most of the energy hitting it except the very high range but still a lot even in the highest range. Is there some measurement data on this kind of device one can have a look at?
Anyone?
16th September 2011
#77
Gear Guru
Words

I thought Jens Boggy and I had decided not to called these things diffusors?
How about Amplitude Reflexion Grating? after Newell and RPG?
I would expect some 'scattering' in the returned reflections, and a little temporal messing about for low angle waves, however nothing like the full-on spatial and temporal cluster of a real diffusor. IMHO they are not diffusors, nor does anyone intend them to be.
Boggy and I have repeatedly stated the goal and expected result.
i.e. A little HF bounce and nothing dramatic.
Plus, no downside, anyone can build them, can't really go wrong, they look good and they stop drummers from falling into the fibre....

I too would like to see some tests. I will take a look over some of my own. I seem to remember a measured increase in HF decay time. Will post again if I find it.
EDIT Page 314 MHOA. Not exactly on point but take a look at fig 14-20 Flat panel with distributed absorption.

DD
17th September 2011
#78

Quote:
A perforation percentage of around 3% should have an fc of around 100 Hz depending on depth, wool, panel thickness etc, but yes; it should be quite reflective above 200 Hz.
I stand corrected.
18th September 2011
#79
Gear Maniac

Quote:
Originally Posted by Jens Eklund
Sorry Andre but I think that’s a tad unfair.

RPG (Dr. Peter D’Antonio) is after all an authority when it comes to diffusers:

RPG Diffusor Systems
Acoustic absorbers and diffusers ... - Google Böcker
While they may be authoritative, they also have products and services to sell and will construct "examples" that shine the best possible light on their product compared to others. Marketing 101.

-Bruce
14th April 2016
#80
Gear Maniac

Sorry to use this old thread, but I think my question fits here pretty good:
If I use a binary sequence and would like to have a little more reflective area than 50%, what would be the effect of simply reducing the width of the zeros? For example use a 4cm wide slat for the ones and 3,5cm slots for the zeros?
16th April 2016
#81
Gear Maniac

Quote:
Originally Posted by andow
Sorry to use this old thread, but I think my question fits here pretty good:
If I use a binary sequence and would like to have a little more reflective area than 50%, what would be the effect of simply reducing the width of the zeros? For example use a 4cm wide slat for the ones and 3,5cm slots for the zeros?
Anybody?
10th May 2016
#82
Lives for gear

I am also interested in this. Thanks in advance!
17th May 2016
#83
Gear Maniac

You will be fine with 0,5 cm... If you want you can stack an horizontal mls patern. Boggy has done that and looks great!. You will have 75% reflection then
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