Post a picture of your installed acoustical treatments.

[Rutgar]

You will need a measuring system (OmniMic, or XTZ or REW) to determine what surfaces are creating the nulls. In my case, while running sweeps (OmniMic), I moved the mic in all directions horizontally but the null only changed in Q, center point or magnitude but never went away. Only when I moved the mic up and down was I able to figure out where the problem was.

Hi Audioguy. Can you give a bit more detail on how you determined what was causing your nulls using OmniMic? For instance, what were you seeing when moving the mic up and down vertically, and how that indicated where to locate treatment?
Also what specific frequencies were the problem nulls occurring at?

 

[microstrip]

Our was a custom designed theater so there is no "before" pictures. What was there before was cinder block front wall, and drop ceiling that was open to the rest of the place. We built one room and then another inside it. The front wall is about 3-4 feet deep (or something like it). The room was designed by Keith Yates. He took advantage of that space to make a custom bass trap from the whole front wall:

Amir,
Can you give us some more details about this whole wall bass trap? It is something I am considering very seriously for my room.

 

[audioguy]

Hi Audioguy. Can you give a bit more detail on how you determined what was causing your nulls using OmniMic? For instance, what were you seeing when moving the mic up and down vertically, and how that indicated where to locate treatment?
Also what specific frequencies were the problem nulls occurring at?

My null was between about 100hz and 300hz and about 10 to 15 db deep. Using OmniMic, I placed the mic at the MLP and the null was very obvious as I used the frequency sweep. I moved it forward a foot or so and while the center frequency of the null changed a bit as did the exact frequencies over which it occurred changed a bit as did the amplitude, it was still there. I moved it further forward with basically the same results. I then moved it backward and again, while some parts of the null changed, it did not go away. I moved it further back and no improvement. That could only suggest that the null was NOT caused by either the front/rear wall interaction or the side wall interaction. (Just so you know that I was not smart enough to figure this out alone, I got the recommendation to do this from Mark Seaton).

I then started raising the mic up and down and I quickly determined that the floor/ceiling interaction was the problem as I was able to SIGNIFICANTLY improved the null. The rest was some educated guesswork: If I could make the floor or ceiling acoustically disappear, the problem would be solved. So I purchased the GIK Bass Traps and at the recommendation of the folks at GIK, I suspended them per my previous description. And viola, no more null. Had this not worked, my next step would have been to do the same thing to the rear ceiling (but fortunately that was not necessary as my projector is back there the implementation would have been a bit messier) . While I have not personally done so, I can see no reason why this same process would not work for other frequencies and/or caused by other room surfaces --- but there are a lot of folks a lot smarter than I am who post around here who might be able to give you better guidance.

 

[Rutgar]

Thank you for the detailed reply Audioguy! I have a couple of nasty nulls too (one at 30hz and one just above 100hz. Like you, horizontal speaker/listener position changes appear to have little affect on them. But if the problem is due to floor to ceiling reflections, then that would make perfect sense!

 

[amirm]

Amir,
Can you give us some more details about this whole wall bass trap? It is something I am considering very seriously for my room.

I can't discuss the details of anything custom designed by Keith. That know-how is one of the reasons you pay for Keith's services .

 

[microstrip]

I can't discuss the details of anything custom designed by Keith. That know-how is one of the reasons you pay for Keith's services .

I was not expecting specific details or instructions, just the general principles, but I understand. Did Keith require you to take a course on how to resist torture before the bass trap was built?

 

[audioguy]

Thank you for the detailed reply Audioguy! I have a couple of nasty nulls too (one at 30hz and one just above 100hz. Like you, horizontal speaker/listener position changes appear to have little affect on them. But if the problem is due to floor to ceiling reflections, then that would make perfect sense!

The one near 100hz should be addressable like mine was. You may want to call the folks at GIK (the guy in Atlanta is who I called) to get some guidance on the one at 30Hz since that may take a bit more work/treatment/expense.

Glenn Kuras
President, Acoustic Professional
glenn.k@gikacoustics.com
Phone: 770.986.2789

 

[LenWhite]

The acoustic design for the listening room area was designed by Rives Audio based on optimizing stereo reproduction. A local carpenter implemented the Rives Audio design plan. (4) 6' X 6' diffuser "clouds" utilizing (4) RPG BAD panels each are suspended from the media room ceiling. LED lighting strips are mounted behind the crown molding in the upper decorator setback areas, and around perimeter of center opening in each of the (4) diffuser "clouds".

An absorber "cloud" is suspended from the hallway ceiling at rear of the listening room designed to acoustically isolate the kitchen area.

Absorber panels are mounted to the rear walls of high plant decorator areas in the kitchen and the listening room areas.

First reflection points are controlled by an ASC Picture panel hung on left side wall and Echo Buster floor standing absorber panels on the right side wall.

Echo Buster wall absorber panels are used at rear hallway wall and breakfast nook areas.

RPG BAD panels are positioned in front of the plasma HDTV and on the kitchen countertop during stereo listening.

Additional absorption is provided by insulated shades in front of Breakfast Nook butt-glass windows & the French door at FR room entrance.

Wool area rugs partially cover the hard wood floor surface between the sitting position and front speakers, and the tile surfaces in the breakfast nook and FR rear hallway.

Wood shutters cover French doors on the right side wall and (3) arched windows on the front wall of the listening room.

http://home.comcast.net/~lwhitefl/Pictures.html

 

[audioguy]

The acoustic design for the listening room area was designed by Rives Audio based on optimizing stereo reproduction.

Nice room.

Mine was also a Rives Design but following the plans he gave me provided far less than desirable results and the room has had to be heavily modified because it had (and still has) some very serious issues. While my room has become much more enjoyable due to the effort of (1) some of Rives competitors (GIK in particular), (2) Mark Seaton, and (3) information I have gleaned from forums like AVS, it still needs work. I'm glad you got good results as I'm sure it sounds very nice.

 

[audioguy]

The room is a challenging 27’x28’x8’ - the four corners are “superchunked” 34” in width to ceiling - OC 703 was used for chunks/absorbers - absorbers are mostly 2’x4’ varying from 4” to 6” thick - in total there’s 14 absorbers throughout the space - diffusion panels are Auralex T’fussors (cavities are OC 703 filled) and Metro fusors.

I like this room and I bet it sounds terrific.

 

[Rutgar]

The one near 100hz should be addressable like mine was. You may want to call the folks at GIK (the guy in Atlanta is who I called) to get some guidance on the one at 30Hz since that may take a bit more work/treatment/expense.

Glenn Kuras
President, Acoustic Professional
glenn.k@gikacoustics.com
Phone: 770.986.2789

Yeah, the 30hz will take a lot of effort, and won't be inexpensive to address.

Out of curiosity, can you post a photo of your GIK traps as how you have them placed on the front ceiling of your room?

A couple of things that would get in the way on my ceiling are can lights and ac vents. I most likely would have to space any traps around those, which I why I am curious to see how you placed yours. From what I can see, it looks like they are up in a soffit. So, I am guessing you have you traps grouped together and centered in that?

 

[audioguy]

Out of curiosity, can you post a photo of your GIK traps as how you have them placed on the front ceiling of your room?

A couple of things that would get in the way on my ceiling are can lights and ac vents. I most likely would have to space any traps around those, which I why I am curious to see how you placed yours. From what I can see, it looks like they are up in a soffit. So, I am guessing you have you traps grouped together and centered in that?

Taken with my iPhone so pardon the quality, The front portion of the ceiling between the left/right soffit is about 15 feet and the distance from the front soffit to the one that runs across the middle of the room is about 10 feet or so. The traps, as you can see are hung toward the soffit that runs across the middle of the room. I selected that spot as it is the center of the first point of reflection on the ceiling. So the entire trapped section is 2 feet by 8 feet.

 

[Nyal Mellor]

Thank you for the detailed reply Audioguy! I have a couple of nasty nulls too (one at 30hz and one just above 100hz. Like you, horizontal speaker/listener position changes appear to have little affect on them. But if the problem is due to floor to ceiling reflections, then that would make perfect sense!

These nulls are likely speaker boundary interference.

- figure out the path length difference of the sound reflected from the major first reflection points using measurement, cad model, etc.

- figure out the path length difference of the null frequencies using the formula in the link.

Correlate the two, and use appropriate acoustical treatment to solve.

 

[Rutgar]

Interesting Nyal. I will play with that tonight.

Thank you for the photo Audioguy. And the iPhone quality was just fine.

Follow Up Edit: After reading additional information on GIK and RealTrap's sites on SBIR, do I understand correctly that the way to address this issue, is to place traps on the walls directly adjacent, above, and behind the speakers?

 

[Rutgar]

Playing with the SBIR measurements, it appears that the distance of the speakers to the side walls is contributing to the just over 100hz null. The calc'd frequency of the front and back walls are 73hz and 70hz respectively. Both of those frequencies are in a broad band peak I have from around 40hz to 100hz.

The 30hz null doesn't add up to looking like an SBIR issue (as far as I'm understanding SBIR), since the feet measurement difference of 30hz appears to be about 18 feet. And I don't see any speaker to listener distances, which that correlates to.

Finally calculating SBIR from the floor or ceiling; Since my speakers are a WMTMW config. I don,t know where to measure from. I'm assuming the other measurements (side, front, and back walls) are from the front the speakers for determining the SBIR frequency.

 

[Nyal Mellor]

Playing with the SBIR measurements, it appears that the distance of the speakers to the side walls is contributing to the just over 100hz null. The calc'd frequency of the front and back walls are 73hz and 70hz respectively. Both of those frequencies are in a broad band peak I have from around 40hz to 100hz.

The 30hz null doesn't add up to looking like an SBIR issue (as far as I'm understanding SBIR), since the feet measurement difference of 30hz appears to be about 18 feet. And I don't see any speaker to listener distances, which that correlates to.

Finally calculating SBIR from the floor or ceiling; Since my speakers are a WMTMW config. I don,t know where to measure from. I'm assuming the other measurements (side, front, and back walls) are from the front the speakers for determining the SBIR frequency.

In reference to your question from the previous post: the solution for SBIR is to put acoustic treatment at the reflection point, not directly adjacent to the speaker (not sure what this would do?).

30Hz SBIR null would indeed be around a 19ft path length difference (i.e. the reflected sound travels 19ft further than the direct sound). I have not seen your measurements but chances are at 30Hz this null could more be just a hole in your frequency response where there are no supporting room modes (since the spacing between room modes is quite large down that low, in the 'sparsely populated modal region').

With WMTMW you have to consider which drivers are emitting the frequencies. Likely with SBIR they are coming from the woofer parts which means you have two SBIR nulls from the ceiling and floor (since the path lengths are different for both drivers). I am doing Chris Venhaus's room at present and he has a similar speaker layout (Legacy Whisper XD) with two distinct ceiling SBIR nulls. With the vertical SBIR nulls you really have to put everything in a 3D CAD model to understand what is going on as the sound is traveling in two distinct planes (horizontal and vertical). With the other lateral reflections you are generally on the same vertical height more or less as the drivers so a 2D model is sufficient.

 

[Rutgar]

Thank you for the helpful information Nyal. It looks like nailing down floor to ceiling issues is bit more complicated to figure out, than I can do with a piece of paper and a ruler! Especially when adding an extra woofer into the mix!

 

[rbbert]

The MSR Spring Traps do seem to have significant effects at 30 Hz while at the same time being relatively small volume (about 4 cu. ft. each)

 

[Nyal Mellor]

The MSR Spring Traps do seem to have significant effects at 30 Hz while at the same time being relatively small volume (about 4 cu. ft. each)

Any measurements before / after?

 

[rbbert]

Any measurements before / after?

See the Website and/or the latest Stereophile; I think Google will also return a number of reviews/measurements from the pro audio literature.