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Desktop Reflection Revealed Summing Mixers
Old 15th January 2011
  #1
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Desktop Reflection Revealed

Thought y'all might like to see this.
The Red line shows comb filtering as a result of a reflection from a desktop.
The Blue line shows how the problem is removed by slightly tilting the desk towards the operator. More comfortable too.
DD
Desktop Reflection Revealed-combover-2.jpg
Old 15th January 2011
  #2
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any chance you could throw a broadband absorber on top of the desk and take measurements (just for education / theory+practice sake?)

-thx
Old 15th January 2011
  #3
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Lost

A lost opportunity I am afraid. It was an on site measurement.
There were keyboards and control units all over the desk in any case, so the angle was the only game in town. It would have been interesting nevertheless and I will try your suggestion some day.


DD
Old 15th January 2011
  #4
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Desktop Reflection Revealed

Very interesting. How large was the desk and how much did you have to tilt it? Did you just put something under the table legs to tilt it?
Old 15th January 2011
  #5
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For the test we just put books or a piece of wood under the legs. I have a much smaller table in my White Room, (soundsound.ie). I intend to cut a little off the legs. The tilt was quite small, not enough to make things slide off.
10-15 degrees maybe? Here's the desk in situ. Don't know if this pic is before leg adjustment or after.
Desktop Reflection Revealed-10112010289.jpg

Glad you found it interesting. I found it a bit of an eye opener that such a smallish surface, not even flat really with all those things on it, can wreak such havoc.
DD
Old 15th January 2011
  #6
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Thanks for sharing the knowledge of your experience with us Dan.

Andre
Old 15th January 2011
  #7
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Interest

You are welcome Andre. It seems there is some interest. I will do some more controlled tests, with loopback correction, in my own room so that I can show a credible ETC and how much angle is needed.

Shortly, DD
Old 18th January 2011
  #8
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Done

Great Microphone heh
Perhaps this picture says Mirror Trick with no words needed.

Desktop Reflection Revealed-pc180007.jpg

There was a very significant difference between the Mirror and the Painted Formica Desktop, strangely. So the most spectacular comparison here is Table with Mirror on vs No Table at all.

Desktop Reflection Revealed-mirror-no-table.jpg

A sheet of Isover was tried on top of the table. The result was pretty much the same as no table at all but obviously not a practical prospect.
The front legs of the table were raised by 3 CD Jewel Cases.
A significant improvement was noted. Next a ProTools Reference Manual inserted. This was about as much tilt as would be practical without things sliding off. The legs of the table will be cut to make this tilt permanent.

Desktop Reflection Revealed-sc-table-vs-tilt.jpg

Lastly, when the table is present it appears to have some LF benefits. I expect it is blocking a LF reflection from the floor.

Desktop Reflection Revealed-sc-floor-null-.jpg

The Loopback Correction in FuzzMeasure causes the ETC to show a Time of Flight Graph. I have very quickly gotten used to viewing the paths in mS and have no desire to convert to distance. Why bother. Now if only I had a tape measure with mS printed on it.....
If anyone wants the FM files to play with, to try some ETC Geometry/Arithmetic or whatever, PM me.
DD
Old 18th January 2011
  #9
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Quote:
Originally Posted by DanDan View Post
Now if only I had a tape measure with mS printed on it.....
I seem to recall Genelec used to have such a tape measure as SWAG.

Andre
Old 28th January 2011
  #10
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Hi DanDan,

I'm in the process of building a composition desk for myself and would like to know what is the recommended/optimum angle for the desk acoustically speaking. I've read somewhere that 6 degrees will be enough, any comments on this?

Than you in advance!
Old 28th January 2011
  #11
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Quote:
Originally Posted by Sansodor View Post
I'm in the process of building a composition desk for myself and would like to know what is the recommended/optimum angle for the desk acoustically speaking. I've read somewhere that 6 degrees will be enough, any comments on this?
Sansodor,

Yes, a 6 degree angle seems to be a good general rule (see PDF) - but before you build, it might be good to confirm this by testing - to be sure...

-- Great topic DD!
Attached Files
File Type: pdf Speaker and Desk Relationships.pdf (33.0 KB, 1638 views)
Old 28th January 2011
  #12
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Desktop Reflection Revealed

Probably the best use ever for a ProTools manual! ;-)
Old 28th January 2011
  #13
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Geometry

Cheers Soren. That pdf looks interesting but I can't get at the layers. Maybe I need to update my Adobe reader? Does it have acoustic measurements at different angles?

This is a situation where compromise rules.
Most agree that the desk should be as small as possible. No desk is by far the best in my tests.
However, that floor bounce in my test suggests that we should at least consider having it wide enough and deep enough to interrupt that floor bounce.
Remember my floor has decent carpet and underlay.

I tried an angle by eye. 3 CD jewel boxes, i.e. about 30mm. The desk is 400mm deep. Someone could calc the angle from that.
It was not particularly impressive, so I looked up the ProTools Reference Manual.....heh 50mm or so.
This was much better, result shown above. This was about the strongest angle that was practical. More would have meant things tending to slide off.
I would take a look a some Mastering rooms also. Their angles are not shy.

Having said and done all of that.....the earlier post showed the result of a reasonably small tilt on a reasonably small desk. Furthermore this desk was covered with different control devices. One might have regarded it as a diffusor scatterer. So, go figure.

I suggest that you try some sort of mock up. It is possible to get a much more comfortable key and mouse scenario and to improve the reflection problem. This is well worth it.

In any case testing seems to be the only sure way.

DD
Old 28th January 2011
  #14
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There really can't be a "guideline" angle. Speaker height, and distance, desk height... it's all very situation specific.
Old 28th January 2011
  #15
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Quote:
Originally Posted by DanDan View Post
Cheers Soren. That pdf looks interesting but I can't get at the layers. Maybe I need to update my Adobe reader?
Works fine for me, file should be fine... I get at the layers, get me?

Quote:
Does it have acoustic measurements at different angles?
Not as far as i can see, no.

Quote:
In any case testing seems to be the only sure way.
Quote:
Originally Posted by johndykstra View Post
There really can't be a "guideline" angle. Speaker height, and distance, desk height... it's all very situation specific.
Although a 6 degree angle probably would be better than a flat surface in any situation (hence the general recommendation i think) i believe you are both right.

Cheers,
Old 28th January 2011
  #16
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Old 28th January 2011
  #17
Old 28th January 2011
  #18
SAC
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Quote:
Originally Posted by johndykstra View Post
There really can't be a "guideline" angle. Speaker height, and distance, desk height... it's all very situation specific.
What am I missing here?

DD has already done an admirable job of illustrating the concept.
And John is right on the money describing the myriad site specific situational variables that contribute to the behavior that prevent one making a static cookie cutter statement sufficient to eliminate the need for basic thinking and analysis.

What is left to prove???? Do some still doubt the basic concept of superposition?

The simple step at this point is for one to employ basic geometry and to make (a) simple site specific ETC response(s) in order to ascertain the specific site specific relationships in that particular setting and to possibly modify them as necessary in order to re-mediate the issue.


(So, next does one have to document every potential reflective path in order to describe, illustrate, and validate the concept of the superposition of specular reflections in a room?)
Old 28th January 2011
  #19
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Quote:
Originally Posted by SAC View Post
site specific ETC response(s)
This is important... and not mentioned enough. Your head is going to move while mixing. As should your measurement mic.
Old 28th January 2011
  #20
SAC
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Quote:
Originally Posted by johndykstra View Post
This is important... and not mentioned enough. Your head is going to move while mixing. As should your measurement mic.

For you maybe!

But some of us have solved that problem!
As I, like many others, no longer use our heads! heh
Old 29th January 2011
  #21
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Off Topic

Sorry, but a bell just rang inside my head!! The city of Cork and the name Fitzgerald...

DD are you by any chance related to a local table tennis player by the name of Brian Fitzgerald? Ranked (or at least was) as the Irish No.1?

Old 31st January 2011
  #22
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Ping Pong

Soren, yes I heard that bell in your head. :-) No relation.
gullfo, thanks for calculator. On a brief look I am a bit confused by it.
I must say, visually it looks steeper, maybe due to the now angled legs on the little table. I would take it from all of this that 6 degrees is plenty.
EDIT just checked the table and that calculator.
The table top is 400mm. The drop in height front to back is 50mm. I am getting 7.19 degrees from the calc. I am usually wrong in calculations but again this suggests 6 degrees is on the money.
DD
Old 1st February 2011
  #23
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another option - in SketchUp - using the protractor/rotate tool you can use ratios in the VCB - entering 15 results in degrees, but you can enter 4:12, or 30:400, etc to get a ratio to display then simply measure that angle using the protractor (it would be nice if the entity info displayed the bounding bound or line angle but it doesn't). 50:400 works out to ~7.125° in SU...

agreed, 6° would be a good starting point in most cases
Old 1st February 2011
  #24
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You guys all forgotten your highschool math ?
Old 1st February 2011
  #25
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You mean, like this?

Old 1st February 2011
  #26
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Quote:
Originally Posted by PaulP View Post
You guys all forgotten your highschool math ?
math? you mean like on a calculator?
Old 1st February 2011
  #27
SAC
Registered User
 

It scares me more that right angle geometry and such Basic trig is, for some, apparently a "High School" subject!

That would make long division a 7th grade (Jr. High) subject!
Old 1st February 2011
  #28
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Quote:
Originally Posted by SAC View Post
It scares me more that right angle geometry and such Basic trig is, for some, apparently a "High School" subject!
So when did you learn trig ? For me it was 11th grade, so last year of highschool.

We used tables
Old 1st February 2011
  #29
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I've got a desk that I can't tilt...it's way too heavy. But, I figure that I can build some 1" absorbers that I can custom fit onto the horizontal areas adjacent to my control surface during mix time. During tracking, these areas are needed for note taking, drum machines, and other odds and ends

1" isn't very thick...but it looks like the desk reflections are mostly above 1kHz.

I'll be adding that to my 'to do list'

Cheers

Kris
Old 1st February 2011
  #30
SAC
Registered User
 

Quote:
Originally Posted by PaulP View Post
So when did you learn trig ? For me it was 11th grade, so last year of highschool.

We used tables
:-O

7th grade was Algebra 1 and 8th grade was Algebra2/Trig!
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