Help: 40 Hz Problem

JLeeMD -- Thu, 11/05/2009 - 17:12

I just bought Dynaudio Confidence C1 speakers for my small-medium sized listening room (14'X14'X10').  I now have a 10 db bump at 40 Hz.  What can I do to attenuate this?  (The good news is I get -0 db at 31.5 Hz and -10 db at 20 Hz ...with a standmount!)

Tom Martin -- Thu, 11/05/2009 - 17:39

I suppose that you realize that you have two primary room modes at 40.3 hz with a room this size? And I suppose that "get a different room" isn't an option?
First, have you run a speaker placement program to find speaker and listener locations likely to minimize the bumps?
Another step of course is equalization, for which there are many options depending on your budget. The good news here is that the modal nature of your room seems pretty simple (you should also have bumps at 80, 120, 160 and 200 hz, etc).
Not all EQ,  sadly, will deal with time effects. Ample use of tube traps can help with that, if you have the space and budget. It is possible to get traps tuned to 40 and 80 hz.

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JLeeMD -- Thu, 11/05/2009 - 22:10

Thank you so much for responding.  Using the Stereophile Test Disc, the bass response in the room is pretty linear  (+/- 5 db) at all bass frequencies except the 10 db bump at 40 Hz (the room is not square). 
a) what do mean TWO room modes at 40 Hz?
b) I didn't know speaker placement can attenuate this.  I thought there was nothing I can do about a "room node"...I will certainly try moving my speakers around.
c) will buying bass traps tuned to 40 Hz help?
d) and no, getting a different room is not an option, at least not until my oldest son goes off to college :-) (not verified) -- Fri, 11/06/2009 - 01:53

To answer A , you have two demensions in your room that are the same , by having two demensions be the same you will have a two room modes at the same freq ( which is bad)
To answer B , Speaker placement makes a HUGE difference how it interacts with the room
To answer C , Thats what a membrane/Panel trap is . It is very hard to tune it to a very narrow freq and depends how much insulation is used inside to get the proper Q
To answer D , kick him out!! :-) (not verified) -- Fri, 11/06/2009 - 01:47

There are many ways you  can fix this problem.
1. ( the best choice ) ROOM TREATMENT , you will need to treat the room with bass traps , not only will it help lower the bass hump but also make your system sound ALOT better overall.
2. Speaker positioning is important , you may need to " fine tune " the loudspeaker in your room by moving it back and forth from the front wall and side walls.
3. EQ
4. Similar to choice one is what is called a membrane bass trap or simply panel trap. You can tune the freq to any freq by adjusting the denisty and thickness of the front panel and by the depth of the trap also. Although i will say that broadband bass traps would still be my first choice.
5. Last but not least is moving your listening postion. This also makes a big difference

Tom Martin -- Fri, 11/06/2009 - 10:00

toology86's suggestions are good. I would add two thoughts:
1. To help with positioning the speakers and the listener (you), a speaker/listener placement program saves a lot of time. I use the one from RPG Diffusor Systems:
I think Audio Advisor, Music Direct and maybe others sell this. There may be similar programs; I have the RPG software so that's what I use. Note that in a small room, the listener is often near the boundary and in such cases is often sitting in an area of major bass emphasis. As a result, we are emphasizing listener placement as much as speaker placement because a small movement of the listener can do a lot in such situations (and because of the small room, often small movements are all that is possible).
2. Some of the bass trap companies show data for the frequencies where the trap operates. I would simply point out that 40 hz is fairly low, which is tough, so I'd want to get traps that work there for sure. You will probably need more than a few.

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Robert Harley -- Fri, 11/06/2009 - 11:32

I would try moving the listening seat forward and backward a foot or so; it's possible that you happen to be measuring and listening in a postive-pressure area of a standing wave. Where your loudspeakers are located has a large effect on how room modes are excited. The next step is to try 16" Full-Round Tube Traps from ASC. They are broadband bass absorbers, but will knock-down that troublesome frequency.
It is possible to build an inexpensive trap tuned exactly to your room's resonant frequency. Making it look good is another matter. I've built these into the walls of recording studios. You build a frame of studs, attached it to the wall, and cover it with a thin material such as Masonite through which you drill small holes. The device's peak absorption frequency can be tuned by varying the depth of the frame, the panel thickness, and the percent-perforation of holes. The Master Handbook of Acoustics includes complete instructions for building these. They are extremely effective, inexpensive, and can be tuned to problem frequencies. But, it's a challenge to make them look good.

JLeeMD -- Mon, 11/09/2009 - 13:50

Thanks so much to all.  First, I completely reconfigured the location of the speakers with guidance from ASC's web site.  I now have a 6 db "dip" at 80 hz and a 5 db "bump" at 40 hz...better than before and...WOW...the soundstage dramatically expanded in all directions!
I don't want to stop here.  I get it now: the speaker-room interface is a huge am looking into ASC Tube Traps.  A couple of questions:
a) Will the Tube Traps help with the "dips" as well as the "bumps?"
b) Since my room tends to accentuate 40 hz, it seems I will need 20" (!) Full-Round Tube Traps...does this sound correct?

k5E0o2eA -- Fri, 11/13/2009 - 13:27

Your listening position can also affect what you hear.  There are very good papers on the web by Floyd Toole of JBL/Harman with understandable explanations of the issues; his book has also gotten good reviews.  Moving my chair away from the wall behind me 1/4 of the front-to-back length of the room made a big improvement, and it was free!  I also use a Velodyne SMS and like it a lot.  If you already have a good way to measure your system's room response, the Audio Control Richter scale may be a more affordable alternative.  Having a graphic display of room response makes it much easier than using an SPL meter.  Have fun!

Robert Harley -- Mon, 11/09/2009 - 10:53

The larger the Tube Trap diameter, the lower the frequency it will absorb. I have never used the 20", but I imagine them reaching lower than the 16" ones I've used for years. The Tube Traps will smooth the frequency response by absorbing excess energy as well as by preventing constructive and destructive interference between direct and reflected energy.
It's interesting how fixing what appear to be frequency-response problems improves soundstaging.

JLeeMD -- Mon, 11/09/2009 - 14:12

I'm actually ectstatic that I discovered this problem because the improvements from simple manuvers to address this have been huge and I'm sure I'm only a fraction of the way there.  I'm hoping the ASC Tube Traps will result in another big leap foward.  Lastly, Mr. Harley I just read your blog re: setting up the Wilson Alexandrias.  You mention timing delays quite abit...can you explain what that's about or guide me to a site where I can read up on this as I'm guessing from the emphasis you place on these measurements that they are an important determinant of speaker-room interface/performance. 

Robert Harley -- Sat, 11/14/2009 - 17:17

I'm not sure what you mean by timing delays. Perhaps it was the Alexandria's ability to move the various drivers forward and backward to time-align the outputs of each driver at the listening position. (not verified) -- Mon, 11/09/2009 - 14:16

ASC tube traps are great for corners and also help with back wall reflections by diffusing down to around 500hz which will make your room appear " larger " . Side wall reflections need to be controled by aborption. Some people use diffusion which will help but in your size room , i dont think it will develope enough by time the energy reaches you. Try putting some absorbing material ( thick blanket ) at the first reflection point on the side walls. You can find them by having a friend move a mirror along the side walls flush and you sitting in the listening chair and anytime you see the louder speaker on the wall , thats where absorption goes , you will want to upgrade to fiberglass after useing a blanket because fiberglass will absorb much lower than blankets or any open cell foam. Heres a link about the basics on room treatment and all things that make the biggest difference . Any more questions feel free to ask.

ScottB (not verified) -- Mon, 11/09/2009 - 20:12

 I have lots of experience with room treatments of various kinds, culminating in a very extensive, expensive, and effective custom treatment from Performance Acoustics Labs. I've made quite a few expensive, time consuming errors along the way. The bad news is, Tube traps, Real traps, and other bass treatments which rely on damping air motion will not be very effective at damping 40 hz resonances unless placed so far out into the room as to be impractical. Efficiently dealing with frequencies that low requires treatments that respond to pressure rather than air motion, such as Helmholtz resonators or panel traps. Note - tube traps act pretty much as velocity rather than pressure absorbers, design claims notwithstanding.
The good news is, treating modes at higher frequencies (like that 80 hz mode) is just as important to the overall subjective impression a room makes as dealing with the fundamental modes. And simple absorptive treatments can be quite effective above 80 hz or so. There are other treatments besides Tube Traps which are very effective (and also, less expensive) - see the Real Traps and GIK acoustics web sites:
Warning: don't over-treat the room. Too many absorptive treatments of any kind (even Tube Traps and Real Traps, which have partly reflective facings) will excessively shorten the reverb times in the upper octaves, and take away too much ambience and life from the music. In a room your size, I would start with no more than 6 large traps (Real Traps, if you can afford them) across the vertical corners (2 in front, 4 in back). Then, treat the first reflection points on the side walls and ceiling with absorption or diffusion (see the GIK diffusors), and the point on the back wall directly behind the listening seat with absorption if the wall is close, diffusion if the wall is further out. If all that works out, try diffusion on the wall behind the speakers.
Good luck - the room is perhaps the most important, certainly the most variable, component in your audio system, and the hardest to optimize. (not verified) -- Mon, 11/09/2009 - 20:32

I agree 1000% , it is very easy to have to much full range absorption in any room. I myself am not a fan of consumer level room treatments. I made all of the 42 panels i have in my room and which 90% of them are hybrid absorption/diffusion. My RT-60 is pretty much perfect at all freq's. My Magico V3's measure +/- 1.2 db's from almost 20hz to beyond 20khz with a high rez sweep no 1/6 octave stuff. Proper room treatment is a must but again like ScottB said , dont go overboard without having the required equitment to measure it with ( laptop , calibrated mic and so forth )

vlach (not verified) -- Fri, 11/13/2009 - 14:05

I've achieved decent audible results (not measured), for a similar bass "bloat" problem using a foam plug in the rear port of one of my speakers, the one closest to the corner of the room.

johnk (not verified) -- Mon, 11/16/2009 - 14:07

I have used panel absobers with success in the past but there is another option.  I have a friend tht had MBL speakers that have difficult to control bass.  He tried acoustic solutions but they were difficult to tune in the the exact frequncey.  What workded was a Rives Audio PARC.   If was sonocally tranparent, exept for the notch filter.  It is costly but you don't have to waste money with  trial and error acoustic designs and can dial it in exactly with no loss of bottom end.

JLeeMD -- Mon, 11/16/2009 - 14:59

After a complete rethink of speaker positioning using guidance from ASC's web site, I am now getting +/- 5 db all the way down to 31.5 hz with audible bass at 25 and 20 hz.  The sound and especially the soundstaging improved so dramatically that I am now asking myself if "the enemy of good is better."

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