Low Frequency Absorption vs. High Frequency Reflection in a Small CR

[Jens Eklund]

4m x 5m x 2.5m room - modal resonance problem

Your room is too small for traditional diffuser placement on the rear wall. Perhaps (since you already have them) you can place them on the rear side walls but rotated 90 degrees so they don’t partially scatter back to the sweet spot, and use broadband absorption (deep panels so you´re not only removing the highs from the reflections) and pressure based absorbers on well-chosen areas (check the ETC and try to extend your ISD-gap as long as possible).

[diggo]

Originally Posted by Jimmy Mansfield

I have been re-treating my CR for the past few days and wanted to get some insight as I am knee deep in the process at the moment.

I have a small mix room 10x10 with 8' ceilings. I first treated the room about a year ago by adding triangle super chunks from floor to ceiling in all corners, 2x4 4" baffles at the first reflection points as well as 2 clouds above the mix engineer, and 3 QRD diffusors along the back wall.

QRD diffusors need bigger rooms than 10' to work. Get rid of them - they arent doing anything useful in your room.

(I'm repeating what Jens has posted, due to typing this before he posted his response)

Originally Posted by Jimmy Mansfield

My two big problems with the room is it does not translate well, and I could never hear low frequencies (it was always difficult to mix the kick and bass) so I just added a sub and am in the process of reworking my original treating.

I am using Room EQ Wizard with the behringer test mic and am trying to get all frequencies from 20hz-20khz within 5 dB of each other.

There is no need to do this from 20Hz - 20kHz....and "within 5%" is an impossible goal in a room of the dimensions you describe. It's largely unattainable across the entire spectrum in any room which isnt purpose-built from the ground up, much less very small rooms like yours.

You need to control modal and SBIR issues at low frequencies. Worrying about nearly inaudible high frequencies is way down on your list of priorities - you are worrying too much about the analysis results, partly because you arent fully understanding what they mean and how to interpret them.

Originally Posted by Jimmy Mansfield

I needed to get some high end back in the room (specifically at 15k and up according to REW) so I put 1/4" plywood in front of ALL my baffle frames/ clouds and also double bagged the insulation in 3 mill heavy plastic trash bags. This seemed to help my dip at 15k by 2 dB or so, but I have been reading that you only want absorption at your early reflection points. So I was thinking about taking the plywood reflection out. I have the same concerns with the two clouds above me. Not sure what to do because I am also trying to get my high frequencies back.

You havent lost your high frequencies. The highs are being swamped by resonances from the lows ("one note bass" etc).

The wavelength of a 15k sine is roughly 0.9 inches (i.e. 0.075 feet). Move the mic a few inches and you'll get a completely different result above the upper mids, much less at frequencies at the limit of your hearing. Dont worry about high frequencies at this stage.......the most useful thing you can do is reduce the low frequency ringing from modal resonances and reduce SBIR nulls where possible. This will have a profound beneficial effect on the high frequencies.

Originally Posted by Jimmy Mansfield

The other issue is my two big problem peaks are at 140-150hz and 700-720hz. I am in this frustrating battle of adding more trapping in the room to manage low and mid frequencies while managing not to loose the "air" in the room from around 15khz and up.

Forget about this idea you are somehow losing the "air in the room from 15k up". Although it seems intuitive to you that ultrasonic highs are a problem, dont worry about it - the highs are the least of your concerns.

A 10' square room with 8' ceiling is gnarly to sort out and it is certain that velocity absorption will not suffice without additional methods.

You need to differentiate between static frequency plots and other methods which derive better results across the time domain (such as the waterfall chart). You havent provided enough info to tell us how you derived your conclusions......

Attach your REW mdat file to this thread so others can load into REW and interpret the data.

Originally Posted by Jimmy Mansfield

I am also running out of room to put more bass trapping. I am considering loosing the 3 QRD's on the back wall (as I have come to find out they are not as relevant in a room as small as mine) and replacing them with more 2x4 4" baffles. I have also considered mounting the baffles off the wall to soak up more low end (although I am not sure how far off to hang them or how much more it will help). I am even considering wrapping the super chunks in the corners in 3 mill plastic and putting 1/4" plywood in the frames if necessary.

You need to seriously consider a pressure-based solution like the RPG Modex Plate. This will nail your problems without consuming precious space, in a manner simply impossible to attain with velocity absorption.

If you insist on pursuing the "DIY velocity absorption" route as your only solution, put velocity absorption in the corners, particularly the 8 tri-corners. Remember that corners extend horizontally as well as vertically - put superchunks in all 90 degree corners.

Use velocity absorption in areas of the room you can afford to lose (panels across corners or superchunks). But dont expect miracles from velocity absorption in a small room. It will help, but it wont solve everything. Focus on the important stuff first (ringing is your enemy).

If you can live with a 7' 2" ceiling, consider building a false ceiling made entirely of 4" velocity absorption hung 6" below the ceiling (ie total including absorption and air gap = 10 inches). This has the effect of increasing the perceived size of the room (i.e. even though the ceiling seems lower, it sounds much higher). This is a pragmatically effective way of productively using space you can afford to lose.

Sean

[boggy]

Originally Posted by Jimmy Mansfield

I.........................
I have a small mix room 10x10 with 8' ceilings...................... My two big problems with the room is it does not translate well, and I could never hear low frequencies (it was always difficult to mix the kick and bass) so I just added a sub and am in the process of reworking my original treating.
................................

I had a similar request, couple of years ago, and my answer was: MyRoom Acoustic Design

So, if you have time, please read this thread, with attached documents... I hope you will find some leads for your problems solution.

After that, when you have specific questions, please ask it here.

Good luck!

[ciro]

Hi Boggy:

The treatment on rear wall (Pressed Lizard) is the same we see in the pics (side and front?) I´m thinking about something like for my rear wall (also very small room, a bit less than 7' from mix position).

Thanks
Ciro

[DanDan]

Jimmy, the Behringer mic is not at all flat at HF.
http://www.cross-spectrum.com/cslmic...mic_report.pdf

Depending on where you point it, there can be quite a drop off at 15K and above. And a boost around 10K.
Furthermore many of us feel that a HF roll off is desireable in a small highly damped room. It aids/enables translation.
See understanding RTA at studiotips.com
And Monitor response curve

Etc. etc. Flat may not be a useful goal.
But if it is, you will need a flat reference. A correction file applied to the behringer might help you. Although to be honest I think HF is best done by ear. Speaking of which, it is useful to know if one can hear 15K and above at normal listening levels.
This will tell you. Hearing Test Products and Tinnitus Test Products

DD

[jwl]

Some good advice above. I doubt that a small, nearly-cubic room will ever be flat within +/-12dB, much less 5dB. But, every room can be improved!

For details on our strategy, see: RealTraps - Acoustic Basics

[Glenn Kuras]

Originally Posted by jwl

Some good advice above. I doubt that a small, nearly-cubic room will ever be flat within +/-12dB, much less 5dB. But, every room can be improved!
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And just to add it will come from both treatment and proper treatment.
Needless to say I would look at pretty much decay time vs frequency response.
Bass Traps and Understanding Low Frequencies

[boggy]

Originally Posted by ciro

Hi Boggy:

The treatment on rear wall (Pressed Lizard) is the same we see in the pics (side and front?)

There are different details, no loudspeakers... etc, but basically yes, they are the same

Originally Posted by ciro

I´m thinking about something like for my rear wall (also very small room, a bit less than 7' from mix position).

It is good idea.

Cheers!

[Jimmy Mansfield]

You guys rock! This information is helping tremendously. Attached are pics of the room as well as ETC, Waterfall, FRC, and a .mdat file. Also wanted to add, length and width of the room is 12x12' not 10x10' sorry for not double checking before posting.

I am doing some reading at the moment on all the links that have been posted. I will ask more questions later today after I am up to speed on this situation.

Thank you Jens, Sean, Boggy, DanDan, jwl, and Glenn. I appreciate your time!

P.S. On the graphs:

100dB is the reference level that I am using to determine what is "flat"

The ETC graph has .1 ms smoothing

I will also add that the only graph I understand completely is the FRC and Waterfall. Doing more research on interpreting ETC as we speak.

[Jimmy Mansfield]

added .mdat in a zip file and rt60 graph

[DanDan]

I reckon those speakers would sound a lot better in terms of horizontal off axis response, stereo image etc. if you made them vertical.
From the photos I would expect a secondary decay tail from the side space, which might cause some confusion.
Full range FR (if not all) measurement is safer done using one speaker only.
Two tweeter signals can fight when they arrive at the mic at slightly different times. Leading to....combing and rolled off HF.

Two speakers or one speaker for ETF,REW etc?

DD