Desk early reflections
Old 21st December 2012
  #1
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Desk early reflections

Hi! I'm confused. I have a issue with my ETC curves. I did some variations to my setup to try to get a better looking ETC curve, but I'm not seeing an improvement. What I did:

1) Replaced my big desk (160cmx80cm) for one three times smaller.
2) Reduced height of new desk in 5 cm and angled it by 6-10 degrees.
3) Reduced height of speakers by 5,5 cm so I have now my ears sitting just inbetween the tweeter and woofer level.
4) Placed main screen behind speaker level. On my desk remains the laptop.

The results.

Red= Before ; Blue= After





I can see on the blue curve that now the dips are bigger.

FR graphs:




Last edited by blizt; 21st December 2012 at 09:52 PM.. Reason: FR graphs
Old 21st December 2012
  #2
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The blue chart shows a 4db attenuation to the first spike in the ETC. A 3db reduction is in line with nulling one surface reflection. You might want to check what the floor is doing by laying down some foam or panels at the floor reflection point.
Old 22nd December 2012
  #3
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GIK Acoustics's Avatar
 

That time difference between the initial spike and the first reflection looks to be about 0.9 ms which looks to be about a 1 foot (or 30 cm) distance from the speaker. Could you provide a picture of your set up? Perhaps the reflections are from the sidewall, from the front wall (if the angle is steep enough), from the desk, OR it may be actually traveling to the microphone barely after the direct sound via the floor or other medium other than air.

Sound moves much faster on a surface (like the floor, a wall, etc) than it does in air. So you can actually signal to the microphone that isn't a reflection, but merely the sound traveling down the speaker stands, to the floor, to the mic stands, to the mic almost as quickly as the main sound through air (this isn't an odd occurrence either). This can be solved by proper decoupling. But these peaks could also be reflections.

If any objects are within one foot distance, remove them and test again. If that peak still remains, you'll know it's from something different.

Are your speakers on stands, or on the desk? Are they properly decoupled?
Old 23rd December 2012
  #4
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Quote:
Originally Posted by GIK Acoustics View Post
That time difference between the initial spike and the first reflection looks to be about 0.9 ms which looks to be about a 1 foot (or 30 cm) distance from the speaker. Could you provide a picture of your set up? Perhaps the reflections are from the sidewall, from the front wall (if the angle is steep enough), from the desk, OR it may be actually traveling to the microphone barely after the direct sound via the floor or other medium other than air.

Sound moves much faster on a surface (like the floor, a wall, etc) than it does in air. So you can actually signal to the microphone that isn't a reflection, but merely the sound traveling down the speaker stands, to the floor, to the mic stands, to the mic almost as quickly as the main sound through air (this isn't an odd occurrence either). This can be solved by proper decoupling. But these peaks could also be reflections.

If any objects are within one foot distance, remove them and test again. If that peak still remains, you'll know it's from something different.

Are your speakers on stands, or on the desk? Are they properly decoupled?
Took some measurements without the desk. The big spike went away. Looks like it's causing the early reflections.
green = L / purple = R

Desk = red / no desk =green


Is there a practical way to get rid of it? I need a desk at least for the laptop, mouse + keyboard

Edit: Just checked the FR graph and wow! It also look like its doing a lot of damage in the higher part of the spectrum. Without desk is a lot more smooth. How do I fix that?

Old 26th December 2012
  #5
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Bump.

Tips anyone?
Old 26th December 2012
  #6
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Beano Bean's Avatar
 

How big is your room?

Can you move your speaker monitors further apart and move the desk away from them towards you?

If you are sitting in your sweet spot can you move the speaker monitors further back instead of the desk forward?

I guess moving the speaker monitors away from the desk may help a little with this, it's a similar problem to what i have and i am going to try this myself in a couple of days ;-)
Old 26th December 2012
  #7
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DanDan's Avatar
ETC

The Blue After ETC shows an overall lower level and amount of very early reflections. I would call that a definite improvement.
A bigger angle on the desk should get rid of the reflection.
Desktop Reflection Revealed
DD
Old 26th December 2012
  #8
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As expected, the desk was the cause for the spike at about 0,8 ms (relative to direct). Possible solutions for this issue are:

Quote:
Originally Posted by Jens Eklund View Post
Assuming all early reflections from nearby walls/ceiling have been dealt with; increasing the distance between the speaker and desk, raising the speakers (and if so, a slight tilt so the acoustic axis is still pointing towards the ears), minimizing the desk size and also tilting it, redirecting the reflection away from head height, are some measures one can do to minimize these early reflections.
Old 27th December 2012
  #9
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Jens has posted some great info above. My suggestions would be the same. Note that small differences can make big changes (in terms of angles, etc) so I would experiment a lot to achieve the best results.
Old 27th December 2012
  #10
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The ETC looks okay, really. The desk reflection isn't a huge interference source at -15dB, but the FR did show comb filtering in a bad way. Did you take the FR measures with both speakers?

Besides reverb, I would get that bass sorted. Those swings are huge and mixing like that is torture. You might need to adjust your source/listener positioning, but surely add some bass traps.
Old 27th December 2012
  #11
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Mctwins's Avatar
 

Quote:
Originally Posted by blizt View Post
Took some measurements without the desk. The big spike went away. Looks like it's causing the early reflections.
green = L / purple = R

Desk = red / no desk =green


Is there a practical way to get rid of it? I need a desk at least for the laptop, mouse + keyboard

Edit: Just checked the FR graph and wow! It also look like its doing a lot of damage in the higher part of the spectrum. Without desk is a lot more smooth. How do I fix that?

Hi
Your ETC shows difference between Left and Right. Right shows some strange anomalies from 0ms to 30ms, you don't have this in the Left measurement. Some more distortion in the Right/measurement, maybe.
Old 28th December 2012
  #12
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@Dandan: Thanks, I'm now understanding better how to interpret ETC graphs.

@Jens @GikAcoustics: Yes this actually what I'm trying to do, but due to space restrictions I'm having hard time to move the desk without screwing my listening postion. I'll guess I will have to really fine tune the position if I want to get rid of it. Desk is already angled by 6,4 degrees. If I go harder things my start to fall from it Actually I cant tilt speakers.

@Opusoftrolls: Yeah I also percieved the comb filtering happening at the higher part of the spectrum. It reduces when i remove the desk, but this is not possible. Any solution?

@Mctwins: Maybe this is happening because left and right wall are not the same. On the left I have a big window with a curtain.
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