How to tame 50hz with a 1' depth

Hey guys, I could really use some help. I've got a control room being built, and at this stage, the room is entirely engulfed in insulation. It was a concrete room, measuring 14'w X 17'd x 9'h. We've constructed an inner wooden shell, with a 6 degree sloped ceiling, and both walls slanted towards the front. THe entire cavity between the concrete and studs is filled with insulation 16kg rockwool. The shallowest depth of rock wool is at the front and back walls which is at 1'. Where the walls and ceiling slope to the front, there's over 2' at the very front of the room. The idea was to install a combination of slats and drywall over the skeleton to tune the response. The problem is a very pronounced 50hz ring... not in frequency response, but in the waterfall measurement. Response actually looks really good... but with all theat insulation I guess it should. After manipulating the slat wall calculator, I came up with a ridiculous 3' wide slat and .2" wide slot to get down to 50hz. giving me what... maybe 3 slots on the back wall and 2 on the front... to try to curtail this frequency. (It is a length mode I'm dealing with I think, as an SPL meter reveals these two walls as the highest in sensitivity. I'm wondering, is there a different method... one that will fit between 16"oc studs, and maintain a depth of 1' that will help me hit this 50 hz problem?

First of all congratulations on doing things right, as in measuring the actual response of the room. If I am understanding your post correctly, you have installed the insulation but not the covering drywall and slats. Is this correct? If this is, bear in mind that as you add the reflective surfaces, the waterfall will become more ragged. Addressing your slat design question directly, one option is to use 2x4s or some other form of material in front of teh drywall to increase the depth of the slots. Good luck! Andre

john - you aren't going to treat 50hz with a slot resonator - as you discovered, you don't have the depth. can you post some pictures so we can see what you have?

hey guys, thanks so much for the quick response. Here is a sketch file of the room in it's current state... minus all the insulation. We added a wall on the right side to attempt to enforce better mode spacing. This wall is 2x4 constructed, filled with VERY dense rockwool, and sheathed with 2 layers of 5/8" mdf... all joints and seams caulked. and here is a waterfall plot of what we have after insulation is in place. we've since been able to attribute much of the tiny razor rings to a fan in an adjacent space.

here is a rendering of what i want aesthetically from the slats and fabric. My slat spacing was not figured mathematically at this point in the design... this is the stage I am currently working on.

here's a shot of the testing process

Hi Andre. I'm expecting the introduction of reflective surfaces to create new anomalies in the waterfall. What I am hoping to do, is tame the existing problem.... I'm thinking, if I were to build mdf boxes that can be mounted between the stud cavities, and utilize a perforated panel... though I'm unsure if this design will yield any lower frequency targets than the slats can.

Here is an additional sketchup file. This time with numbers indicating spl readings of two frequencies. 50 hz is in red 150hz is in blue speakers were emitting sine waves of these pitches, and the spl meter was walked around the room and thresholds recorded.

John - this is a mistake I keep seeing - you have wasted all the space behind your walls where you can use the depth to get decent traps and low frequency slot resonators to function. You've misinterpreted the design of control room shapes completely. I'd pull it down and start again. Sorry.

John Sayers wrote:

John - this is a mistake I keep seeing - you have wasted all the space behind your walls where you can use the depth to get decent traps and low frequency slot resonators to function. You've misinterpreted the design of control room shapes completely. I'd pull it down and start again. Sorry.

hi mr. sayers WITH RESPECT real world situations are different. as we can see and interpret that the room is dead now with good frequency graph there is only some liveliness to bring out and some specific frequency tuning. so in spite of what could be done it is better what can be done taking into account the current situation. so as expected from you, you should give advice with what already had. sorting out things is real result of your life time experience not discouraging.

johndykstra wrote:

here is a waterfall plot of what we have after insulation is in place. we've since been able to attribute much of the tiny razor rings to a fan in an adjacent space.

That 50 Hz resonance looks like AC power line hum because it never decays and doesn't seem to have an associated peak. Are you sure you don't live in Europe? :D --Ethan

Hi everyone. Well now that we're all here... Amit is the actual owner of this space. He had started a thread on a different forum looking for advice on his control room... which is in India... perhaps the ac problem is there as well Ethan? John, Is there any improvements that can be made given the shell of the space? Meaning, if we remove some of the insulation and employ a better thought out trapping system within? Initial thought had been given to a different shape of the shell. One in which splayed surfaces could take the place of 1st reflection absorption... Amit and I did not trust our abilities to predict a final speaker placement at the beginning of the design phase. Humbly, if there is any directions you can point me in to help Amit get the best room possibly, I'd really appreciate it. Thanks, John

In India? The continuous spikes are 50 Hz hum. It would be good to see the waterfall without the hum. Andre

Well, that's good news... right? What needs to be done to clean the power up and get rid of this interference?

John and Amit - yes I was being a bit hasty in making my judgement. It appears that your 50hz problem is what has been mentioned. i would imagine the room sounds very dead and lifeless. try this

Wow, that was quick what is the reasoning behind adding the second wall at the back of the room, and what is it's construction method?

The reasoning is to create depth - insulation off the wall absorbs lower frequencies than having it flat on the wall. http://johnlsayers.com/Recmanual/Titles/Acoustics1.htm

Anyone care to comment on a solution to the grounding issue? I'm no electrician.... I've seen the term "star grounding" kicked around, but never seen an explanation. Is this something I should be researching?

john - a quick search for 'star earthing' returned 12 pages on star earthing!

Mr Sayers, Thank you. I'm familiar with the search function. For all I know, star grounding is an effective implementation regarding eliminating rf interference, or single coil guitar hum, or cross talk... As I stated, I'm not an electrician... I was simply wondering if star grounding would eliminate the problem Amit is having. I have no problem researching a subject as long as I know I am going down the right path of action. I was hoping to hear this from you or Andre, as the two of you are people I have faith in trusting. THis is, after all, the internet... I'd hate to trust the word of a stranger and waste Amit's money... that is all.

Middle Atlantic just released a new version of their pro-audio paper: Power Distribution & Grounding White Paper http://www.middleatlantic.com/power.htm The star grounding discussion starts at page 56.

I was simply wondering if star grounding would eliminate the problem Amit is having.

First it would be good to know what problems he IS having! It seems that there is a problem with 50Hz mains hum getting into your measurements, but if grounding is the issue, then I'd expect that you would be able to hear that hum quite audibly from the speakers, just from having the system turned on, without playing any sound on it. Is that the case? Can you hear hum from the speakers? Also, how is the system grounded now, and what is the voltage measured between ground and neutral at the power socket where you have the system plugged in? In any event, regardless of whether or not you have a hum problem now, star grounding is still the best method to ensure that you never do have such a problem in the future. If you install a good star grounding system now, at the start, then it pretty much guarantees a hum-free future for your entire studio. So I'd certainly say that you should consider star grounding when it comes time to install the electrical system in there. Another possibility is that the interference is coming in from an adjacent source, outside your studio: is there any heavy industry close to your studio, such as maybe a welding workshop, heavy duty electrical machinery, an electrical substation, or something like that? Is your electrical system connected to any other electrical system? For example, is this studio part of a house or office, and the studio system is just a branch circuit from the main feed to the rest of the building? There are so many possibilities here that more detailed information is needed to track down the problem. The bad news is that this is never easy: I've occasionally spent many days trying to track down hum in installations I've done, and on a couple of occasions I've had to resort to the "nuclear option": rip out the entire existing system, and do it again, from scratch. I hope it doesn't come to that in our case, but it might. - Stuart -

Stuart - This really is the wrong forum to talk about star grounds. But, good as a star ground is, it's not the only good ground system and in some studios it may not be the best choice.

Hi Stuart. Thanks for taking the time to join the discussion. I'm going to contact Amit and have him come on over and answer some of these questions. Being I'm on the other half of the world, I really couldn't tell you. But again thank you.

India runs at 50hz like Europe. I'm in the middle of a class and we are learning about "dirty electricity." I live in Idaho and we have notoriously "dirty power." To make it simple if you looked at the sine wave of the two power legs they are out of phase in the case of 220-240. You need a line conditioner to correct this if it is coming in that way. There is a certain amount of distortion that is "to spec" per the power company so they won't do anything about it so that's why we are installing a HUGE conditioner where I work. First we had to determine if the distortion was coming from the power coming in or was one piece of our equipment causing the problem. Some power companies will come in and determine that for you and then help you find the source if it is your problem. The equipment is pretty spendy to DIY although some places you can rent it. Anyhoo.....I doubt it's a grounding issue since it's at 50hz so conditioning the power coming into your building may be the only way to go.