Second opinion on my measurements

[pollock0424]

Hello,
I did some analysis of my 2 channel setup, soon I would also like to use it as a theatre setup also. Room is a 12x16, there is a serious destructive interference between 45hz and 60hz, I think one way to solve this is using 2 x Subwoofers. Let me know what you think ?

Here's a description of my space:

On the left side I have my foldable workbench, desk for my office work and I think this is why I see more reflections in the IR for the left side. On the right side I have a footon. Common to both sides are a stack of 3, 2 inch foam (6 inch depth) approximately covering the first reflection points and second reflection points. Speakers are placed along the the 12ft wall approximately 5.5 feet away from the front wall and I sit about 8 feet away from the speakers.

Right next to the first reflection points are windows, so I made 4 sqft 4 inch depth panels using the above foam and inserted between window and the curtain, I also have blackout/sound deadening curtains on the windows.

Here are some measurements from REW and DIRAC. I am using a miniDSP SHD as a preamp/DSP. Also, what do you think about these measurements for my room ?

 

[amirm]

For frequency response, you need to do two measurements or at least two settings for the same measurement:

1. From 20 to 200 Hz to determine room modes with 1/12 or 1/24 octave smoothing (or none).

2. From 20 to 20 kHz for overall tonality with 1/6th smoothing.

 

[Rednaxela]

a serious destructive interference between 45hz and 60hz

You can say that again.

Are all these measurements with two speakers playing? Also, what speakers if I may ask?

 

[radix]

A sub or two would probably help. I didn't see what your gear is, but it looks like you have full range speakers? If you just do 10-100 Hz sweeps and move the speakers around, you could see how sub positioning could solve the nulls at 55 and 75. If you want to see how two subs would work, put your system mono and move both speakers around with 10-100 Hz sweeps. Its a bit of labor, but free before you invest in subs. You could then find subs that physically fit in the spots that solve the nulls.

 

[neRok]

Your speakers are just in a really bad spot, like REW Room Sim shows. Notice your dark green trace has all the same ups and downs.


The good thing about that big null is that it is the speakers interacting with the room mode, not the listening position with relation to the room mode, so it looks like a sub can fix it nicely.

The speakers must be sitting right on a room mode null, and so the result is like the animation "Monopole located at the first null from the left wall" in the thread Visualizing How Different Loudspeaker LF Directivity Patterns Couple to Room Modes

Or you can move things around and try get better results from just your speakers. They seem quite capable on their own (flat to at least 30Hz).

 

[dominikz]

Room is a 12x16, there is a serious destructive interference between 45hz and 60hz, I think one way to solve this is using 2 x Subwoofers. Let me know what you think ?

It seems to be SBIR related, probably caused largely by the reflection from the wall behind the loudspeakers. These kind of dips are worst if the front of the loudspeaker driver is at a similar distance from floor, ceiling, side-, and back-wall, in which case SBIR dips caused by each align and sum.

To test this you could push the speakers closer to the wall behind them and remeasure - the dip should go up in frequency (or at least be less severe if you have the alignment of dips I described).

Subwoofers could probably help as well as you could physically place them elsewhere compared to mains (e.g. corners), where the first SBIR dip would be out of their operating range.

 

[pollock0424]

@amirm
Look! The CFO himself, I'm pleaseantly suprised! Here are the plots for 20Hz to 200Hz (1/12th) and 200Hz to 20kHz (1/6th).

Also, including the secondary listening position. Primary and Secondary positions are 8ft and 7.5ft approximately from the middle of the mid/tweeter respectively.

Here's a plot for 200Hz to 20kHz

And here's the secondary position as well.

@Rednaxela : Data was collected by individually playing the speakers and made sure that the AC was turned off too! These are Thiel CS3.6 Speakers.

@radix : I have tried moving the speakers from about 2.5 feet from front wall and found 5.5feet is the best so far but I agree with you, if I can get away by finding a sweet spot then that will be a perfect solution. The only issue is that these speakers weigh close to 110 pounds!

@neRok : Yes, I have tried simulating and preliminarily found that this spot is "ok". My room has a vaulted ceiling and the volume is about 1700 cu.ft. Here are some measurements of the room and the speakers:

Floor to higest point : 9.5 ft
Floor to lowest point: 7.5 ft
Length: 16 ft
Width: 12 ft
Center of the speaker to the side wall: 2.2 ft
Primary listening position to the front wall: 12.8 ft
Primary listening position to the speaker mid/tweet: 8 ft
Top of the speaker to the ceiling: 4.4 ft
Woofer (center) to the floor: 1.6 ft
Passive (center) to the floor: 2.4

@dominikz : The woofer is 1.6 ft from the floor and passive is 2.4ft from the floor, also the woofer plays upto 500hz.

Here's the room/setup:
From the "front"

Aimed at left side: (Shamelssly showing off the IEEE Signal Processing Society Chair creds )

Aimed at right side:

 

[radix]

@radix : I have tried moving the speakers from about 2.5 feet from front wall and found 5.5feet is the best so far but I agree with you, if I can get away by finding a sweet spot then that will be a perfect solution. The only issue is that these speakers weigh close to 110 pounds!

If you have any speaker that goes down to say 40hz, you could use those to find where the subs would work best. The room modes don't care if it's a $100 speaker or $100k speaker, it's all frequency and geometry. Often it is mid-wall, or off-center on a wall, or use side-firing (i.e. try positing a regular speaker sideways). You can get a rough model of how the room would work.

Room wizard, as another poster used, is also a good tool to get an idea of what might be good places for the subs.

 

[pollock0424]

@radix
I will do that, I have a pair of Thiel 1.5s, I will play with these, and get an idea of the room. Fingers crossed!

 

[AdamG]

@amirm
Look! The CFO himself, I'm pleaseantly suprised! Here are the plots for 20Hz to 200Hz (1/12th) and 200Hz to 20kHz (1/6th).
View attachment 308992
Also, including the secondary listening position. Primary and Secondary positions are 8ft and 7.5ft approximately from the middle of the mid/tweeter respectively.
View attachment 308993

Here's a plot for 200Hz to 20kHz
View attachment 308994
And here's the secondary position as well.
View attachment 308995


@Rednaxela : Data was collected by individually playing the speakers and made sure that the AC was turned off too! These are Thiel CS3.6 Speakers.

@radix : I have tried moving the speakers from about 2.5 feet from front wall and found 5.5feet is the best so far but I agree with you, if I can get away by finding a sweet spot then that will be a perfect solution. The only issue is that these speakers weigh close to 110 pounds!

@neRok : Yes, I have tried simulating and preliminarily found that this spot is "ok". My room has a vaulted ceiling and the volume is about 1700 cu.ft. Here are some measurements of the room and the speakers:

Floor to higest point : 9.5 ft
Floor to lowest point: 7.5 ft
Length: 16 ft
Width: 12 ft
Center of the speaker to the side wall: 2.2 ft
Primary listening position to the front wall: 12.8 ft
Primary listening position to the speaker mid/tweet: 8 ft
Top of the speaker to the ceiling: 4.4 ft
Woofer (center) to the floor: 1.6 ft
Passive (center) to the floor: 2.4

@dominikz : The woofer is 1.6 ft from the floor and passive is 2.4ft from the floor, also the woofer plays upto 500hz.

Here's the room/setup:
From the "front"

View attachment 308997

Aimed at left side: (Shamelssly showing off the IEEE Signal Processing Society Chair creds )
View attachment 308998

Aimed at right side:
View attachment 308999

The problem is simple to diagnose once you provided pictures of the room. The sound emanating from your speakers is wildly running away from the pink noise wall color.

 

[pollock0424]

The problem is simple to diagnose once you provided pictures of the room. The sound emanating from your speakers is wildly running away from the pink noise wall color.

Haha! Actually, it is Jaipur Pink Noise, not just any pink noise!
@AdamG247

 

[neRok]

My room has a vaulted ceiling

That could be a problem. The way my room is, one of my speakers fires towards "two back walls", so it's like there are 2 reflections , and the effect is apparent on the plot (those reflections arrive in phase and boost). Perhaps your ceiling is doing something similar, but the ceilings reflections are out of phase and causing cancellation? If you use Filtered IR screen with 50Hz 1/3 setting (just for 1 speaker/measure is enough), you might see some evidence of the problem. If it is the ceiling, I'm not sure what would help. Perhaps minor adjustments towards/away from the side walls would be enough to change the reflections geometry and thus the phase timing? Being right up against the walls might help too. You would only need to move/test 1 speaker to check this affect.

I think your 200Hz+ plot is quite nice BTW, as it has good decay and seems flat enough (might look great with Psy smoothing applied). So there's probably not much point tweaking that freq area before solving the main problem in the bass area.

 

[pollock0424]

@neRok

I will try testing your hypothesis here

Audieum - Acoustic Design for Critical Listening Spaces

Audieum is the new generation suite of software tools for acousticians and end users to design, simulate and analyse the acoustics of critical listening spaces.

audieum.com

And of course I will try to do get the data that you're asking for, can I reuse the existing measurements to gate the IR curve for a specific time?

 

[neRok]

can I reuse the existing measurements to gate the IR curve for a specific time?

Yes, just open the overlay screen and use the impulse tab there. You should be able to see the problem after looking at just 1 measure. Your room problems seem symmetrical, so both speakers should exhibit the same problems when compared at the listening position.

Here is my room with the filtered IR showing the "double back wall" problem the left speaker has. You can see all the extra energy in plots 1 and 2 (I prefer using plot 2=dBFS instead of %, because having it un-normalised gives a better comparison), and how much SPL that equates to on plot 3 (at ~80Hz and ~160Hz).

You can see on my plot 3 that I have an SPL null at 200Hz, so on plot 4 I filtered at that frequency. You can see that the first waves (direct waves and some early reflections) build up nicely, but then some out of phase reflections arrive after ~18ms and cause the cancellation. I reckon your filtered IR's at 50Hz will look similar.

 

[ozzy9832001]

The null at 46 and 58 are both modal. A subwoofer or 2 would be the easiest fix if you can't move the listening position. If the speakers are positioned on the 12' section that would be right in the null. If you are sitting opposite them, you'd be in the null as well, since they run length-wise in your room. The area of loudest sound or maximum pressure would be along the long wall. If you stand along the 16' wall, you should hear those frequencies quite loudly. Though, moving the position would introduce another set of possible nulls, but possibly easier to navigate.

 

[dualazmak]

And, just for your reference, I know that "enough" sound deadening spaces (or treatments) behind the SP system and behind our ears/head would be critically important for better total sound quality, as I shared here and here.

 

[pollock0424]

@dualazmak
Thank you, I will take a look. Here's how my behind the head looks like.

 

[pollock0424]

@neRok :
I clicked on Overlay option and then on "Impulse", I couldn't see any option to filter the IR. So, I went into "EQ" and then applied the LP at 50Hz and this what I have for the IR.

This is how my EQ settings look like:

REW version is 5.20.13 and I'm using a free version. I looked around but couldn't generate the plots that you have.
EDIT: @neRok
Nevermind, I was able to filter the IR, there is a separate Filtered IR tab:

 

[neRok]

REW version is 5.20.13 and I'm using a free version.

I am on on 5.20.14. There must have been many changes between our versions, including the filtered IR on overlays option. Also your whole EQ screen looks different (it is the old layout).


You can "bandpass" the result like you have done, but the "Filtered IR" preset does it with 1st order (6dB/oct) filter at 1/3 octave points. So for 50Hz I guess that's 41.6Hz HP and 66.7Hz LP (1st order BW). Once you have set the "EQ" there is a button "Created measurement from predicted", and then you can look at that on the Impulse screen (where you can zoom and normalise it etc).

 

[neRok]

Nevermind, I was able to filter the IR, there is a separate Filtered IR tab:

Cool. It looks pretty weak because usually there are more strong waves (3 up and 3 down on the % plot seems to be "good"), but I don't have a big sample size of "good". The first reflections arrive very quickly (lets say its 3m up and 3m down, that's 17msec travel), and so it can still be the ceiling. Maybe its that you are actually in the node of the ceilings reflection?!

Could you also post a filtered IR of 100Hz (a frequency that appears good), and a screenshot of the Decay tab with "Fill Slices" off, and smooth=1/24 (make sure you turn off the filtered IR first)?