Feature specific frequencies

Hey guys, first of all: thanks for this forum - it is a wonderful resource and I learned a lot here! I just finished my new small suit which is a room in room construction and all in all sounds great. Especially the low frequencies are extremely tight and even. However, weirdly enough, I just measured again (and again and again) and I do have two issue. 1) I got a drop of about 7dB around 100Hz, starting to fall down at about 70Hz and fully raised up at about 160 Hz again. 2) At about 300 Hz there is a major drop to its lowest response (minus 14dB) at about 1.1kHz, coming up at 2kHz again to naturally start falling down at around 12kHz again. Conclusion: I damped the high frequencies too much. My major question is: is there any way to raise specific frequencies up again in a room (100Hz, pretty narrow and 1.1kHz rather wide)? I am not sure what to do, if I should add some more diffusor (if so, what might help here) or if all that might not help at all. Thanks for any help and I hope I am in the right section of the forum.... Axel

Just a little description of the room: it is a room in room construction, the whole inner walls are covered with 10cm rock wool plus a rear wall up to 50cm deep, a front wall 60cm deep and the whole ceiling is filled with 60cm rock wool as well. The whole room is covered with textile. Two 750l Helmholz resonators are still untuned and closed (not sure if I even will activate them) and there are three HOFA Difusors above my head and four at the rear wall. The size after treatments is fairly small: about 16qm or a little bit less.

Hi there Axel and welcome! :) You didn't mention the dimensions of your room, but very likely the issue at around 100 Hz is one of several issues that are very common in small rooms. For example, it could be modal (corresponding to a null in one of the natural modes of the room), or it could be SBIR, due to the location of the speakers and walls. Plus, you only mentioned a frequency-domain response issue, but very probably what you are dealing with is also a time-domain problem. We'd need a lot more information on the room to be able to help you pin-point the issue, and then decide how to treat it. Please provide an accurate diagram of the room, done in SketchUp, and also run a REW analysis of the room as it is right now, then post the MDAT file from that here, so we can take a look at it. Photos of the room would help a lot too. About those Helmholtz devices you mentioned: You say they are "closed" right now, which implies that you have resonant boxes inside the room, and that could also be part of the problem. Helmholtz resonators are not generally used in studios, despite the hype you see about them all over the Internet: they are extremely hard to tune accurately, and even harder to place correctly in the room. They also have to be very large to be able to deal with the typical problems found in most small rooms, and that in itself defeats the purpose of having one: just by being there, it makes the room even smaller! So it would probably be in your best interest to just get rid of them completely. They are taking up valuable space that could be used to better effect by other treatment, and they might be causing one or more problems just by being inside the room, even though they are "closed".

Conclusion: I damped the high frequencies too much.

Quite possible, but I'm not sure how you arrived at that conclusion since you don't mention anything at all about high frequencies! :) You only mentioned lows and low-mids. If you did in fact over-damp the highs, that can be fixed easily, so I wouldn't worry about that too much. I would first figure out what might be causing the issues with the lows and low-mids, fix that, then see if the highs need dealing with too.

My major question is: is there any way to raise specific frequencies up again in a room (100Hz, pretty narrow ...

Yes, assuming that it is a modal issue, and that you have enough space in the room. You would deal with that by putting very large bass traps in the room, and/or by correcting your speaker/boundary geometry, if possible. However, after having said all that, there are certain limitations imposed on what can be accomplished in any give room, simply due to the size and layout. It is impossible to get flat response in a small room (or any reasonably sized room, for that matter), simply due to the laws of physics. Sound waves interact with the walls, floor, ceiling, and other objects in the room in such manner as to create a non-flat response, and there is nothing at all that can be done to change that: It's just a fact of life that you have to live with. The larger the room, the less terrible the artifacts are, but any room that is small with respect to the dimensions of the wavelengths of the sound inside it, will have non-flat response. If your room response is a long way from the theoretical limit, then yes, you can do things to improve that, but the closer you get to the limits, the harder it is to improve things.

if I should add some more diffusor ,...

Diffusers? What types of diffuser do you already have in the room, what frequencies are they tuned to, and how big is the room? Many types of diffuser cannot be used in small rooms, especially if they are tuned low, due to the lobing artifacts. The problems you mention are probably not related to that, but you might have other problems that are related to that, and once again if you do have diffusers in a small room, then they are probably taking up space that would be used better for other treatment. So please provide as much information as possible on your room, but an especially accurate diagram with dimensions, a full REW test, and some photos. Those will allow us to analyze your room and see what is wrong with it, then figure out hoe to fix it. - Stuart -

Hi Stuart, this is awesome, thanks for the reply! I am on a business trip and will answer on Monday providing all the information you asked for. Thanks! Axel

So, I hope I got all the information together... First of all, here is the measurement: http://www.axelrohrbach.com/downloads/R ... hrbach.zip Here is the SketchUp File. Actually the room is a tiny bit larger (2 or 3 cm in both directions). I hope that is OK. The outer walls are not included, but I guess that doesn't change anything anyway. The Inner walls are installed outside of the mainframe. http://www.axelrohrbach.com/downloads/S ... ach_3D.zip Here are some pictures of the current state (audio wise finished, but visually some small things are still on the to do list :-) Does this help? Dimensions of the room are about 4.02m (length) x 4.08m (width) x 3.28m (hight). There is a open ceiling filled with 60cm rock wool and about 5cm air gap. Pretty quadratic, which I know is bad for acoustic but I had no other chance. Thanks so much for help! Axel

Hey there again. I just couldn't sleep and tried several things. I removed the table and voilá, a lot of issues in above 200Hz are gone. REW measurement in the following zip: http://www.axelrohrbach.com/downloads/REW_NoTable.mdat.zip The problem is: I can not really work without a table. Any experiences with that or solutions? I tried to slope the table but it changes nothing (it only shifts the problematic frequencies a little bit up). I also measured without the monitor, but there were no reasonable changes, too. So it really seems to be the table. I might to try out a really extremely small table, only big enough for the computer-keyboard, a mouse and the fireface remote. The 100Hz room-mode still exists - so I would be very happy about any ideas on that, too. Thanks, Axel

Dimensions of the room are about 4.02m (length) x 4.08m (width) x 3.28m (hight)

Ouch! :shock: :!: That's not good at all. The room is almost a perfect square, which is the second-worst shape possible. The worst is a perfect cube. Plug those numbers into the famous Bob Golds' calculator, and see what the result is: http://www.bobgolds.com/Mode/RoomModes.htm Not very happy.... I downloaded your SkethcUp file, but I can't open it: it says it is the wrong version, but my version is up to date...

Here are some pictures of the current state

I'm curious about how you did those speaker soffits: from the photos, it looks like you just have cloth stapled across framing? Do you have photos or diagrams of how that was actually built? Also, you have several 2D Schroeder diffusers on the walls and ceilings: get rid of those. They are not doing you any favors at all, since the room is far too small to be able to use numeric-sequence diffusers, such as those. In a room that size, you are going to need a large amount of bass trapping, but you don't seem to have any at all? But curiously enough the REW plots aren't that bad: Are the walls built "inside-out? I suspect they must be. I also suspect the "wedge" shape on the rear wall must be mostly absorption? However, the REW data is showing some strange stuff: the IR is rather strange: it's almost as though the room is somehow coupled to two other rooms, one a bit larger and the other very much larger. The room is also very dead: too much absorption at all frequencies. You need to so something to return the mids and highs to the room. If you explain more about how the room was built, and provide more photos of the process of building it, then we can give you some options for how to fix these issues. What speakers and sub are you using in there? Also, what measurement mic did you use for the test, and where was it positioned? Also, which speaker did you use or the REW tests (left or right)? That is important to know. Finally, you are doing your tests at a level that is not high enough: it should be 83 dB for each speaker, but your tests seem to have been done at only 70 dB.

I removed the table and voilá, a lot of issues in above 200Hz are gone. REW measurement in the following zip:

Unforunately your two data sets cannot really be compared to each other, since you did the two tests at different levels and with different parameters set in the REW preferences. You also had 1/6 octave smoothing on for the second data set...

The problem is: I can not really work without a table.

The table is not an issue. Yes, it is causing some irregularities in the low mids, but that might be due to the room layout geometry, not just the tale by itself. Fix the geometry, and the low mids will improve all by themselves. They'll never be perfect, of course: that's just part of the compromise of using a small room for a studio.

The 100Hz room-mode still exists - so I would be very happy about any ideas on that, too

There's no evidence of modes at 100 Hz. (although you do have a pretty big one at 59 Hz!). The dip in frequency response at around 100 Hz. is probably SBIR, or some other form of phase cancellation, but I'd have to know all the details of the speaker geometry to say for sure. - Stuart -