[drhoon]

Red is 1 foot out of the wall and the Blue is 38% off the wall.
seems like I have this crazy dip around 50hz...no matter speaker positioning..

is this something that can be salvaged with room treatment?

[JLM]

Fortunately narrow (frequency range) suck outs similar to your's normally can't be heard.  Can you hear this?  What is the room size/finishes like?  Where are you measuring from?  How did you accurately measure down to 20 Hz?

Unfortunately there is no easy answer to this.  Treatments take signal away (by absorbing or trapping).  To boost by 40 dB (as I'm able to read it) would take a 10,000 fold power increase (too much to ask of any amp or speaker).

[youngho]

Could this cancellation be from a reflection off the side wall or else the wall behind the listening position?

[drhoon]

Fortunately narrow (frequency range) suck outs similar to your's normally can't be heard.  Can you hear this?  What is the room size/finishes like?  Where are you measuring from?  How did you accurately measure down to 20 Hz?

Unfortunately there is no easy answer to this.  Treatments take signal away (by absorbing or trapping).  To boost by 40 dB (as I'm able to read it) would take a 10,000 fold power increase (too much to ask of any amp or speaker).

I can't hear anything in that range.
room is 16 x 12 x 9 feet. Concrete wall, wood floor.
Red I'm measuring about 10ft away from the speaker, Blue about 4 feet. Probably should ignore below 35HZ.

I do think its cancellation from the rear wall.

[Letitroll98]

From my limited knowledge on this subject, big honkin' bass traps worked for me in a similar situation.  And you can plot out your most significant room modes and place your speakers in one of the bass nulls.  When I did this for width and depth modes they came out approximately the same as the old 1/3 and 1/5 rules.  So now I just use that for initial speaker placement and adjust by ear from that point.  All of this only reduced my two bass suckouts, but it was a noticeable improvement.  As JLM noted they're hard to hear, easier to measure.  Such is my small experience in taming bass nulls.

[Mike-48]

It is a typical bass null -- I've had one in every room. In my current room, the only way to get rid of it is to put the listening chair either too close to the main speakers or along the back wall. Neither is acceptable.

The best solution will probably wind up to be a compromise between good measurements and seating/speaker positions that are appealing otherwise.

As others have said: Bass traps can make incremental improvements (and will probably improve articulation). Also: Narrow nulls often are not heard.

Another way to fill nulls can be by adding subs . . . but to do that right adds a lot of complication in setting levels, crossovers, etc. (That is the voice of experience -- I have two subs.) It can be worth it, but it depends on how fussy you are and how much work you want to do.

[ACHiPo]

Definitely check out the waterfall charts--the faster the decay the less noticeable these nulls are (IME).  Adding subs doesn't have to be that complicated or terribly expensive, and can significantly smooth bass response.

[Hipper]

See if you can adjust your frequency response graph to 'no smoothing'. This will show you how narrow or otherwise that dip really is. I agree with JLM and Mike that narrow dips are often not noticed.

You can use these free test tones to see if that dip really makes much difference:

http://realtraps.com/test-cd.htm

Also check out post 52 here:

http://www.audiocircle.com/index.php?topic=89655.40

[youngho]

Could potentially consider the Modex Plate 100/2.5 (http://www.rpginc.com/ProductDocs/MDXP_Modex%20Plate/Modex%20Plate_2%20Page%20Brochure.pdf) or active bass trap (http://www.stereophile.com/content/psi-audio-avaa-c20-electronic-bass-trap-specifications#DcvxvvvqvumBR8CA.97) directly behind the listening position.

[drhoon]

Thank you everyone for the help.
I'll try every possible speaker placement possible and go from there.
I don't think I would like to add a subwoofer. It's too complex for me. :p

I'll come back with results for my potential solutions I find in the near future.

Room  standing waves are much trickier than I thought. 

[JLM]

Try near-field setup.  I use it (7 foot equilateral triangle, nothing within 3 feet of a wall) which reduces room effects. 

[JRace]

Unfortunately there is no easy answer to this.  Treatments take signal away (by absorbing or trapping).  To boost by 40 dB (as I'm able to read it) would take a 10,000 fold power increase (too much to ask of any amp or speaker).

LF dips are caused by room nodes cancelling certain freq at certain locations in the room. Bass traps can help and so can multiple subs.

OP: this may be helped by changing both seating location and speaker location.

[JLM]

How can absorption or traps increase sound pressure levels?  Perhaps a Helmholtz resonator, designed for the problem frequency could be made to work, but that's tricky to do.   

Yeah the room is almost twice the length as height and the width is 3/4 of the length (you should try to avoid simple ratios of room dimensions), so not good.  Another idea is to shorten the room with a false wall or built-in casework.

Yes a "swarm" of subs is probably the best way to even out in-room frequency response (look up Duke LeJeune, AudioKinesis, and swarm here at Audio Circle), but to make up 40 dB will take incredible subwoofer capacity.  But if you can't hear it, why worry about it in the first place?

[youngho]

JLM, it's called destructive interference. If it's due to a reflection off the rear wall (behind the listening position), the wavelength is reflecting off the rear wall and returning out of phase relative to the in-phase signal from the speakers. Absorbing or reducing the amplitude of this reflection results in less destructive interference (aka cancellation), so the net effect is reduction of the null, aka increase in SPL at that frequency.

[youngho]

Bass traps with good measurable absorption at 50 Hz are relatively less common, hence my specific suggestions to consider.

[triumph]

Room nodes are very easy to pick up. Just walk to a corner of the room and the bass will increase to almost booming. Yes, the reflections in opposite phase will cancel the frequencies that were amplified near the corners.

I have done the experiment. I also have a concrete house, and, compared to the same room treated and untreated room (with or without bass traps in corners).  With bass traps, the bass got better and cleaner.  No longer bouncing off corners, it became tight and dips like the one seen in the OP message are greatly diminished.

It's crazy that so many are willing to spend thousands on hi end equipment, but unwilling to spend a few hundreds on room treatment that will provide a much more impactful return on the investment.

[JRace]

Bass traps with good measurable absorption at 50 Hz are relatively less common, hence my specific suggestions to consider.

And usually large,

These would work great:
http://www.gikacoustics.com/product/gik-acoustics-244-bass-trap-flexrange-technology/

[drhoon]

Thank everyone. After walking around trying to find the best position with least amount of null and peak, I found these two spots. This seems more reasonable to treat especially the 70 Hz peak.



Thanks I've been looking at GIK stuff. What's a respectible Sabin value? I am not too failiar with it. Only absorption coefficient.

bigger picture:
http://imgur.com/v0oCPXb

[youngho]

It may be difficult to compare different measurements taken in different ways at different facilities. A simple, rule-of-thumb conversion between Sabins and absorption coefficients is shown at http://realtraps.com/art_measure.htm, but obviously some caution should be used in interpreting the results. 70 Hz should be easier to treat, though I believe that the Modex 100/2.5 would seem to be one of the more promising candidate products still.

[Hipper]

Do you mean you left the speakers in one place and walked around the room to get those measurements? If so, that is not the best way to do things.

The beat way is to make a rough guess initially to finding the best locations for your speakers and listening chair (ears). You can use 'The Thirds' or perhaps 'Fifths' for this.

The Thirds is having the speakers one third in from each wall, and your chair too. With your room of 16' x 9' that would be 3' out from the 9' wide walls and 5' 4" out from the 16' walls. Measure to the centre of the drivers. Your ears would be 3' or 5' 4" out from the rear wall. It looks silly but may well sound pretty good. If The Thirds is not practical use The Fifths.

Having set up the speakers, put the microphone where you ears will be (including height) and measure. If your room is symmetrical and you can measure one speaker alone that will save you time. Get one right and the other should be the same, although you should check it anyway once you've found the best location.

Now draw on paper a grid and perhaps put tape on the floor with measurements on it. Then move one speaker about, both sideways and backwards and forwards, measuring each time and noting the position on the grid. Try big movements first - 6" say - then select the best result (s) and use 1" increments in that region.

Remember also that you can move you chair/ears. It's tedious work but should pay dividends.

The idea of The Thirds is to reduce room issues. That's why it is a good starting point. It can also give a remarkably good 3D image with the right recordings.

Once you've done this you can start looking at room treatment, or if that is not practical, DSP (Digital Signal Processing).