# REW compared to room acoustics simulations



## Gerbrand (Dec 28, 2009)

Hello all,

As I wrote when introducing myself, I have been trying to match REW measurements with simulations using the CARA Room Acoustics Package (not free). I hope to be able to show you that you can actually use simulations to understand your room acoustics better. I appreciate your input on this.

*Description of the room*

My HT has a floor surface of about 28 m² (300 ft²). It is situated in an attic and the ceilings are slanted in a rather peculiar way. Below the floor plan as I defined it in the CARA software. The straight line above the listener is the ridge of the roof. The other diagonal line is also a ridge that runs down to wall level.









I have calculated the first couple of room modes from the dimensions of the room. They are 30 Hz (length), 35 Hz (width), 45 Hz (1st tangential mode).

*Measurement*

Below is my REW measurement of the room with the subwoofer in the location above:








*Simulated response*

Below are the simulated results. With the CARA program, you can increase the accuracy by increasing the number of reflections taken into account. For this simulation I used 12 reflections. 








As you can see the wide dip from 30 Hz to 40 Hz is explained, but the actual frequency seems to have shifted a bit (by around 5 Hz). Also the dip seem bigger in reality, however this may depend on the actual microphone location. 

*Sound fields*
The program also allows you to calculate the sound field for the whole frequency range. I have done this for 30 Hz. The graph is shown below:








This graph shows that the mode at 30 Hz is not a pure longitudinal mode. In fact, it looks like the slanted ceiling is also contributing, causing an odd shaped minimum.

On the other hand, the mode at around 40 Hz looks more like the room mode you would expect.









*Follow-up*

So now I think I understand what is happening in my room. As an additional check I had my subwoofer play a sine wave at 35 Hz and you could clearly experience the dip when walking around the room.

The question is: what can I do about it? The dip is really deep (around 30 dB from the dip at 35 Hz to the peak at 55 Hz), so I have my doubts about equalizing.

Do I need bass traps? A 2nd sub? Move the sub to another location?

Thanks for your comments,

Gerbrand


----------



## lsiberian (Mar 24, 2009)

I recommend you move the seat back.


----------



## glaufman (Nov 25, 2007)

It looks like you're sitting pretty much dead center in the room, which would automatically have big bass nulls. Now what's interesting is that the big null you're experiencing is due to the width of the room, not the length, so I'm not sure how much benefit moving back will give, but it's easy to move the mic, take a scan, and see...
As for CARA, I'm most intrigued by the "sound field" pix... what's the legend? What's it say about 35Hz? I'm not too sure how to interpret the green... is that a good area, middle of the road (i.e. target SPL), or a null?
Commercially available traps' effectiveness tends to roll off significantly under 100Hz... that being said they can help anyway, and can help more than just modes... There are also tuned absorbers. Placement and EQ can help as well. I'm trying to stay away from rule of thumb suggestions here because the room is irregular. If it was me, I'd want have REW "find peaks" and look at IRs and waterfalls next...


----------



## rogerv (Nov 8, 2009)

I'm also interested in how to interpret the sound field pix. I'm really a novice for these technical considerations, but I'm always interested in learning more. 

Regards,

Rog


----------



## Gerbrand (Dec 28, 2009)

Hello,



> It looks like you're sitting pretty much dead center in the room, which would automatically have big bass nulls. Now what's interesting is that the big null you're experiencing is due to the width of the room, not the length, so I'm not sure how much benefit moving back will give, but it's easy to move the mic, take a scan, and see...


Thank you for your suggestions. Actually I allready did those measurements, but forgot to mention it in the first post... :doh: I measured at distance of about 50 cm around the central seating position. Below for instance is the measurement when I move back 50 cm:








As you can see the dip becomes even more pronounced, so moving forward and back is not really an option. Actually if you move sideways, the plot improves, but then I no longer would be sitting in the center of the screen.

With regards to the sound filed plots: they basically show the intensity of a specific frequency at different locations of the room. Basically as if you were walking around with the microphone. 

Below I have shown a 3D version of the sound field at 37 Hz (for some reason 35 Hz is not calculated). The 3D makes it easier to see the dB scale and also clearly visualizes the dip.








Currently I am doing some more simulations on different sub placements. From the simulations in the 1st post it looks like I am dealing with the mode due to the length of the room and due to the width of the room simultaneously. I think that moving the sub along the side wall, may help reducing the effect from the length of the room, so I only have one mode to deal with.

I will measure some IRs when I have time.

Gerbrand


----------



## glaufman (Nov 25, 2007)

Yes, in a rectangular room, I would expect that dip to improve if you move off-center laterally... that's classical modal influence... and confirmed by what I see in your CARA sound field of 37Hz.
Trouble is, CARA is saying that moving back should improve things at that frequency, and you're seeing it doesn't.
Another issue, of course, is that you're room is pretty close to square, the the length and width modes can reinforce each other.


----------



## patchesj (Jun 17, 2009)

I would be interested to see what the response looks like with the sub simply moved to the left side of center (just slightly under the diagonal ridge)


----------



## Gerbrand (Dec 28, 2009)

glaufman said:


> Yes, in a rectangular room, I would expect that dip to improve if you move off-center laterally... that's classical modal influence... and confirmed by what I see in your CARA sound field of 37Hz.
> Trouble is, CARA is saying that moving back should improve things at that frequency, and you're seeing it doesn't.


CARA does predict the behavior I observe, only at a slightly higher frequency (around 40 Hz). The problem with these simulations is that the number of reflections taken into account is quite low. For a soundfield calculation I typically use around 7-8 reflections, for a frequency response at one position typically around 10. If you consider that the RT60 at these frequencies is of the order 1 second, in which sound travels 345 m, you would really need to consider 345/5 = 60 reflections.

I did move the sub also slightly left off center. I am at work right now, but I will post the graphs when I come home later tonight.

Gerbrand


----------



## glaufman (Nov 25, 2007)

Well, perhaps the resolution isn't great enough, but even the 40Hz field looks to me like CARA says it'll get no worse as you move back, but it clearly gets worse. 
Are those RT60s you quoted calculated or measured? I think 1 sec in a room this size is pretty high, and you might as well place some absorption, might as well start with bass trapping.


----------



## Gerbrand (Dec 28, 2009)

glaufman said:


> Well, perhaps the resolution isn't great enough, but even the 40Hz field looks to me like CARA says it'll get no worse as you move back, but it clearly gets worse.


Well if you look closely at the measurements, you see that the dip not only gets deeper but also moves to higher frequencies. The following graphs show three measurements, one 50 cm closer to the front, one at the present seating position and one 50 cm back.

50 cm forward:








center:








50 cm back:








I think this can be explained, because the room gets narrower towards the back. So while the dip at one frequency may get shallower, as the sound field predicts, the dip at a neighboring frequency gets deeper.



glaufman said:


> Are those RT60s you quoted calculated or measured? I think 1 sec in a room this size is pretty high, and you might as well place some absorption, might as well start with bass trapping.


No, this number was an estimate and just meant to illustrate that you need quite a number of reflections to fully explain a sound field.

CARA predicts 0.4 seconds for T10 at 80 Hz (I think this is the same as RT60). I measured 0.35 s at 80 Hz, but I did not use the subwoofer but the front speakers for this measurement.

Regards,

Gerbrand


----------



## glaufman (Nov 25, 2007)

Gerbrand said:


> Well if you look closely at the measurements, you see that the dip not only gets deeper but also moves to higher frequencies.


I had missed that, you are correct. But at the same time, this implies what you're dealing with is not an axial mode, which would be a characteristic of the room and not the LP, and would therefore not shift in frequency. So is it the actual measured version of the 45Hz 1st tangential mode you calculated?


----------



## laser188139 (Sep 19, 2009)

I think you made a small mistake in your initial calculation of what you expect the modal frequencies to be. From your pictures, if I estimate the width as 5.0m and the length as 5.6m, I get 34.4Hz and 30.8Hz for the corresponding width and length modal frequencies, but the tangent is 7.5m with a half wave resonance at 23.0Hz. Your estimate of 45Hz was, I think, the 2nd harmonic of the tangential mode. (If you think about it, the tangent must be longer than the other sides so its modal frequency should be lower, not higher.)

Which means that your first picture, with the green dip diagonal across the room, probably is showing the dip in the tangential mode from the upper left to the lower right, as the sub is nearer the upper left corner so it probably feeds that tangent and not the other one. 

Confirmation of this should be visible if you run waterfall charts from your REW measures. But if your simulation is accurate and the listening position is near the mid room null of both these modes, it may be hard to see the longer decay time due to the lower level. 

By the way, I love your three REW graphs showing how the null in the dip in the frequency response moves as you moved the mic forward and backward. On first glance, the third picture looks the most even, as the peaks are balanced. But of those three, your center placement is really the best; the common advice is that it is better to remedy the null with placement, as the dips are easier to attack with equalization. 

Bill


----------



## JohnM (Apr 11, 2006)

laser188139 said:


> If you think about it, the tangent must be longer than the other sides so its modal frequency should be lower, not higher.


Doesn't work like that, you need to fit wavelengths between the surfaces along the path, so the important dimension is the one between any pair of surfaces on the path. The various resonances are given by f = (c/2)*sqrt((p/L)^2+(q/W)^2+(r/H)^2) where c = speed of sound, L,W & H are length, width and height (and should use the same length units as the speed) and p,q,r are the mode numbers. For example, for the length modes p = 1,2,3 etc and q=r=0. The lowest frequency corresponds to the 1,0,0 mode.


----------



## glaufman (Nov 25, 2007)

So John, (I haven't gotten this far in my studies), that would mean that ALL modes are PURELY characteristic of the room, and should not vary (significantly) in frequency with placement changes?

(I've known this to be true of axial modes, but wondered about others...)


----------



## JohnM (Apr 11, 2006)

Yes, the modal resonances are properties of the space and their frequencies are the same everywhere in the space, but their amplitudes vary (to zero at nodal points) so the overall shape of the frequency response also varies. That can make it appear as if peaks or nulls are moving in frequency as you move about but it is only the amplitudes that are changing, the frequencies of the underlying resonances are unchanged.


----------



## glaufman (Nov 25, 2007)

Right. Saw that a week ago in the Acoustics forum.


----------



## laser188139 (Sep 19, 2009)

JohnM said:


> Doesn't work like that, you need to fit wavelengths between the surfaces along the path, so the important dimension is the one between any pair of surfaces on the path. The various resonances are given by f = (c/2)*sqrt((p/L)^2+(q/W)^2+(r/H)^2) where c = speed of sound, L,W & H are length, width and height (and should use the same length units as the speed) and p,q,r are the mode numbers. For example, for the length modes p = 1,2,3 etc and q=r=0. The lowest frequency corresponds to the 1,0,0 mode.


Well, isn't that neat. Thank you, John.

If I had thought a little more about it, I should have figured that out myself. That is so clear.


----------



## Gerbrand (Dec 28, 2009)

JohnM said:


> The various resonances are given by f = (c/2)*sqrt((p/L)^2+(q/W)^2+(r/H)^2) where c = speed of sound, L,W & H are length, width and height (and should use the same length units as the speed) and p,q,r are the mode numbers.


This is exactly the equation I used to calculate the room modes. I got the equation plus a good explanation from this website. 

Of course I assumed that the room is rectangular, which it is not. Because the room is narrower in the back the first axial mode in W is expected to increase in frequency. 

For instance: in the front W = 4.94 m. So you expect the axial mode to be 343 [m/s]/2 / W = 35 Hz. In the back W = 4.43 m and the expected axial mode frequency is 39 Hz. 

Gerbrand


----------



## laser188139 (Sep 19, 2009)

So looking at the pictures again, the first picture with the diagonal dip looks even more odd. 

The second, with the dip across the room, similar to the dip in the 3D chart, makes a lot of sense, given the modal freq of 35Hz along the short, width wall. It looks like what one would expect of a standing wave along the short wall. 

Using that as a model, one would indeed expect the first one to show a vertical dip, especially as the freq of the calculation is almost exactly the freq you calculated for the modal freq along the long wall. Your hypothesis that this is the effect of a slanted ceiling is intriguing. Have you tested this hypothesis by running the calculation assuming a flat roof, instead? 

The CARA software does look fun to play with.
Bill


----------

