Desktop Reflection Revealed

[DanDan]

The Mirror is very useful. It has delivered a clear warning.
Change the position or angle of your rack.
However, you may want to go into detail here. This is where it's worth moving up a gear. Better viewing/measuring tools. Look for the Combing from your rack or look at the spike on the ETC representing the reflection from your desk. Move the rack a little, try the blanket, try a few books under the legs of the rack to change the angle.

DD

[BCglitch]

Hello all!

My very first post on GS, but I've been feeding for quite a few years!

I'd like some more informed individuals to weigh in on some thoughts about desk top absorbtion as a means of treating desk top reflections.

Firstly, I'd like to put aside arguments about testing/measuring first. Lets assume I just like the idea of treating the dam thing and being done with it. For starters, I won't have too much gear on it, but what I will have may move about from time to time and may vary depending on the situation, so its damage limitation in soe respects. I figure as needs evolve over time and equipment changes, at least i'll know that my desk surface is a clean starting point, waiting for gear to mess up!

Secondly, i want an area of desk top for many functions, but I see no need to compromise acoustics. My studio is newly built, and it is very well treated to a high standard, lets not bother going into details, lets just say I love the sound of the room as is without a dedicated work surface, but I need a work station.

I can post a proposed plan for my desk at a later date, but essentially, I was thinking of combining a 6 degree slant in the desk, with a some what absorbtive finish.

I've seen the thread where a guy used dressed 703 rigid rock wool, but I want something a little more durable, cleanable etc etc.

I had initially thought of using leather with some kind of foam underlay, the kind used in furnature that wouldn't dent and would also provide comfort.

I am now looking into a number of linoleum desk/floor/furnature finishes on the following website, my thoughts on them are pending some samples coming in the post:

Desktop Forbo

Such a surface would be durable cleanable practical etc. Paired with the correct underlay and a 6 degree slant in the desk this could do a serious amount of damage limitation.

I'm trying to think of these absorbtion issues in terms of the wave lengths involved in these problem reflections despite my limited knowledge. Heres why I think it is a good solution.

1. Though the linoleum surface may be some what reflective for very short wave lengths/higher frequencies, the 6 degree slant should go a long way to solving these reflections, say at a rough guess, reflections above 5 k, very roughly.
2. It is not as hard as wood, or a metal mixing console, so as the wave lengths get longer, it still shouldn't reflect too badly.
3. In the real problem reflection area (the 1 k to 3 k range) would probably pass through or lose power sufficiently with this linoleum finish. i see this as the most important frequency band to limit these reflections as it would colour critical areas for vocal mixing etc.
4. Having a stiff foam underlay presents a new medium for still longer wave lengths to enter, and it would have its own absorbtion benafits, along with being real comfy.
5. It would probably be preferable to stiff rock wool not just for practical considerations, but a dense rock wool might in fact begin to present a reflective property for certain higher frequencies itself, though most likely this wouldn't b a huge difference with a foam like I'm talking about.
6. Another advantage to all this is that these materials should be cheap easy to come by and long lasting/practical.

These are early plans, and I'm not positive on the linoleum solution, but it seems a good one at the moment. The link I provided above also has links to acoustic dampening linoleum products, intended to reduce impact noise for flooring, but it also tweaked my interest a little, though I'm sure their quoted 17 DB reduction in impact noise could hold a false allure for my needs.

Oh yeah, did I mention my speakers will of course be behind the desk on stands?

Please add any thoughts, but go easy on me, beyond a module on acoustics and psycho acoustics in college and my readings here I'm a beginner, all be it with a very definite goal.

[Demor]

I've got another question. What if if you make the desktop from some sort of perforated metal? Wouldn't the most irritating freq's go thru? Let's say a maze of 10 mm.? Maybe even with some sort of sequence in them?

[BCglitch]

Hey there,

Perforated metal has been addressed quickly in another thread, I'll try to do a search, however it is highly dependent on the type of metal and the kinds of holes etc....Eric Valentine, a mixer and producer of note is developing a console with such an approach for dealing with reflections. Personally I'm not convinced entirely, unless exicuted very well it could make things much worse.

Apparently Eric Valentine's company is lookoking into applying what they're developing to studio work stations though, so this may come to be a viable option in future.

I'll try dig up the threads I'm talking about when I've time if you'd like.

[Demor]

I'm not saying it will be cheap. It has to be "Heavy Metal" and probably has to be CNC worked to get an even transparency. But, if made in serial production it can be a very good option for a lot of people.

[BCglitch]

I agree, but I think for my own needs and with my resources I'm much less likely to screw things up completely with a design based on the more well known area of absorbtion...I think with out testing propperly etc a metal solution could introduce ringing, could reflect certain elements and absorb others etc etc...I think the possability of getting it wrong is much greater without propper R and D + triall and error. That's not t o say my plan isn't fail safe, but hopefully someone will tell me where i'm going wrong here with it!

Wish I could find that post about the perforated metal...

[BCglitch]

Its in this thread:
Goodbye HF reflections..Hello to my 703C desk

[Demor]

OK, so if you have a firm sheet of metal (26 gauge as suggested seems a bit thin concerning ringing etc) and you put a dampening back on it like some 3 mm. EPDM and then have it CNC worked....... This could be it for our side desk that will not interfere with the clients listeningposition in the back of the controlroom.

[BCglitch]

That does make it sound quite simple, though I must say I'd be worried about its implimentation still.

Also, I intend to have a more unusual shape to parts of my desk and some holes for cable routing through the surface, which all adds metal working complexity, but I guess its all very doable for the right price.

Just FYI, for me at least, and I think most that have been on this thread, the issue being addressed isn't so much inaccuracies for listening from the back of the control room as it is negative interference from reflections off the desk top for the engineer in the mix position.

[Demor]

In our control room the mains are a 6 feet behind the console so I have no problems with reflections but the clients do.

[BCglitch]

Sounds like you have nice space to play with. Unfortunately I'm not quite 6 feet from mine. You must have some fairly picky clients if they're worried about those early reflections!

[Demor]

Originally Posted by BCglitch

...must have some fairly picky clients if they're worried about those early reflections!

I never heard any of them complain, but I'm a bit neurotic about it.

[BCglitch]

Well why not, leave no stone unturned, or should I say, no frequency distorted.

Do you find the imaging to be off towards the back of the room when you listen to where your clients would be?

At the moment in my room, with no desk the stereo imaging and general balance actually holds up quite well even when sitting on the couch at the back of the room, and I'd like to keep it that way. Things do seem to vary in the lower end as I move back in t he room a bit though probably due to the differences in treatment overhead. Over all though I was surprised that the sweet spot didn't completely collapse on me when I was l istening from where a client would sit, so this is what I'll try to maintain. I'd even be happy enough doing elements of a mix from the couch at the back of the control room. Is this kind of increased sweet spot, or less dramatically reduced one what your hoping for?

I should point out, that my control room is not huge, but is highly treated....just in case someone thinks I'm meandering back fiteen feet and claiming my stereo image is still rocking!

[Demor]

Stereo imaging is OK in the back as well as the balance but when moving to the left and you get near the lefthand desk you get more treble. Its a tiny difference but it would be cool to get this fixed. If a whole band is in for a listening session the two couches are filled the people on the left don't hear the same as the rest.

[BCglitch]

Ah cool. Yeah, my control room is a one couch at he back kind of size, so clients can take what they get in that respect! haha!

[DanDan]

I reckon the desktop surface could be constructed with laths/gaps. Particularly if one is using a Trackpad or such. This plus an angle should surely lose it enough. We do advocate the smallest possible desk, but I have not seen any testing to show the balance between the advantages and dis.
A reasonably large dense work surface can be used to block the floor reflection. I know of no other practical way to eliminate the floor issue which can cause a nasty null.
DD

[BCglitch]

Hi Dan,

I'm not positive what your refering to as the cause of a null due to the floor boundary. Are you talking about standing wave cancelations or something summing from above/below out of phase? Personally, I've over head treatment, and I'm not sure how much information might have oppertunity to do serious damage from the floor boundary regardless? Obviously, since my control room is a pro design rather than my own DIY, I on one hand hope/don't think such a problem exists in my room at the moment, and on the other don't understand why it might.

I would have thought a dense and large work surface would simply create a more problematic floor boundary closer to the ceiling giving more oppertunity for higher energy waves to reflect and sum/cancel. Hence why I want a good system of absorbtion with my desk. I was also on the lines of thinking that a less dense but non resonant work station desk would be a benafit in this situation, as I've described, I hope to absorb and deflect any possible harmful reflections in the higher mids/ higher frequencies, however, if the desk presented a very solid boundary, this would be reflective for mid/low frequencies would it not?

Hence, my thinking was that as long as it wasn't resonant, a desk that could simply reduce the energy of any lower or mid frequencies on their way to the floor boundary would be my best bet.

I'd appreciate any clarifications/thoughts on my theories.

Unfortunately, I don't have the means to test these myself, but I think they're based on sound enough fundamentals. But that's why i'm posting here, to be found out, if there is a flaw!

Cheers,

Brian.

[DanDan]

I refer to the reflection from the floor. It causes a deep null which changes in frequency as you move back. Same as the side wall and ceiling reflections.
As the floor is typically hard and untreated the effect can be quite strong.
I have not seen test or even hypothesis to compare the problems vs issues with a large-ish angled solid surface. Even a large desk is not big enough to be a boundary, so maybe the LF part of the reflection gets 'broken' while the MF and HF are nicely redirected. Such redirection can be as or more powerful than absorption.
However, in the absence of more evidence, instinctively I would go for the minimal desk.
As small as possible. Laths and gaps perhaps. Angled.
Note the angle increases comfort when using computer keys and trackpad IMO.
DD

[BCglitch]

I think your definitely on to something o n a slatted desk design...I can't really think of a down side to it anyway, and without testing to see how much of a difference it might make it would still be practical to have gaps in a desk for cables coming from moniter controlers, control surfaces etc racked else where to thes ide of the sitting position.

I think I might even try and combine all these ideas i.e. a sloped desk, slatted with relatively sizable gaps and I might try to treat the slats to be more absorbtive.

I still want a good size of a desk, of course if I wasn't sure I can minimise the problems it might cause I'd go with a tiny desk surface, but I think I'm on to something here with his combination of ideas.

I mus tnow think about the method of constructing this thing.

I'm still not positive why these room nulls would happen when there is significant treatment on the ceiling. Would you not need too opposing reflective surfaces?

Its probably also worth mentioning that my control room is carpeted. I have noticed in consistancies in the bottom end in my live room with a hard floor and more exposed ceiling, however much less in the control room.

[DanDan]

Nulls can be caused by a single reflection or a resonantly repeating one.
SBIR vs Modes.

SBIR & Floor Bounce calculator

I did some tests in a thread way back. I had a carpeted floor. The null was deep when viewed with no smoothing, maybe 9-10dB. However in third octave view it became 1-2dB. Significant or not is a matter of opinion.
If I had a mixing desk I would make sure I got the free blocking by checking with ETC. Otherwise minimal desk.

DD

[BCglitch]

What kind of frequency/harmonics of it was that focused around as a matter of interest...presumably 80-100 hz?

I should clarify in saying I wouldn't have thought a carpet to solve the problem, but I suppose it can help take some energy from a reflection.

Luckily in my control room I can maintain an equilateral triangle with my speakers of about 4 feet with my nearfields pointing from the corners. The side boundaries i.e. walls aren't a problem and as said before I think I've sufficient absorbtion over head.

What I'm getting at is I think the right kind of desk design might help attenuate reflections in the lower end also, by not appearing as a boundary, but by taking energy from the waves as they pass through it. Of course, this must balance with ensuring the desk doesn't resonate too much. So in this situation, I feel a desk could be beneficial, as long as the early reflections in the higher frequency range are dealt with by sloping the desk as you originally prove in this thread, and as I'm trying to further improve with a more absorbtive approach to desk construction.

If a desk could be non reflective and offer even modest broad band absorbtion it would be better for the listening at least than no desk at all. This kind of desk construction shouldn't be so difficult.

Unfortunately, measurement and test beyond my own comparative tests with my ears aren't an option, as I'm visually impaired and all test software and other methods are completely graphical.

Hence my need to think out loud and at least come with the best theoretical approach before committing to a design.