# First results



## bob755 (Apr 14, 2013)

Hello,
I built this dedicated home theater over ten years ago after a bit of research on the subject and some advice from a professional, but never did any analysis before. 
Thanks to the help on this excellent forum, I made my first tests today. Now comes the difficult part of interpreting the results, so I’d be grateful for some opinions and advice.
Below is the first set of graphs whch show the three front speakers together and the sub separately.
Also some photos and plan of the room. The room measures a about 6m x 4m. 
The room is divided into roughly three sections: 
1.	The FRONT which is heavily absorbing. Various thickness plywood panels are fixed to a wooden frame. The frame is made of vertical and horizontal timber with varying spacing to provide different sized panels in order to resonate at different frequencies. Behind the panels is high density mineral insulation (Rockwool in France). The side walls are angled avoid parallel walls?. The screen is microperforated with the front left, center and right behind.
2. The CENTER section mixes absorbing plywood panels and reflecting surface.
3.	The REAR section is designed to mainly diffuse the sound and has home built Schroeder diffusers.

The rear surround speakers are bi-directional ceiling mounted.

Looking at the REW graphs, there is a peak at 30hz but I’ve no idea how to resolve this. What other conclusions can be drawn from these results?


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## sickboy013 (Feb 28, 2009)

bob755 said:


> Hello,
> I built this dedicated home theater over ten years ago after a bit of research on the subject and some advice from a professional, but never did any analysis before.
> Thanks to the help on this excellent forum, I made my first tests today. Now comes the difficult part of interpreting the results, so I’d be grateful for some opinions and advice.
> Below is the first set of graphs whch show the three front speakers together and the sub separately.
> ...


Nice room! That 30hz peak could easily be tamed with some EQ. I personally run the Behringer dsp1124. You can have REW set filters and EQ for you, then you just make the adjustments on your EQ. There is a good thread here for applying minimal EQ and using a hard knee house curve.

You could also address the peak at 55hz and things would look pretty good after that.


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## bob755 (Apr 14, 2013)

sickboy013 said:


> That 30hz peak could easily be tamed with some EQ.
> You could also address the peak at 55hz and things would look pretty good after that.


I've borrowed FBQ1000 that I can try. From what you say that might be a solution. From what I have read, bass traps don't seem to be effective below 100Hz.

Does the front responses look ok? They also show that 30Hz bump.


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## sickboy013 (Feb 28, 2009)

bob755 said:


> I've borrowed FBQ1000 that I can try. From what you say that might be a solution. From what I have read, bass traps don't seem to be effective below 100Hz.
> 
> Does the front responses look ok? They also show that 30Hz bump.


It doesn't look too bad, what do you have your crossover set at?


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## bob755 (Apr 14, 2013)

sickboy013 said:


> It doesn't look too bad, what do you have your crossover set at?


The sub crossover is set to 60Hz


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## AudiocRaver (Jun 6, 2012)

Conclusions? WOW! Nice job! Most of our rooms don't look that good AFTER we have done EQ.

Agree that a little parametric EQ could tame that 30 Hz bump nicely. If you tame that bump by about 8 dB and then bring up the sub level about the same amount, you will end up with a pretty flat LF range with minimal effort. The 30 Hz peak is mainly from the natural response of the sub, and with all the room design & treatment you have in place, EQ is a perfectly legitimate way to handle it.


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## bob755 (Apr 14, 2013)

Here are the filter settings that I'm going to load into the FBQ1000. I see that the front speakers also show a bump around 30Hz so should I force a dip in the sub response to compensate?

Is there a way to overlay the measured front responses with the targeted filtered sub response?


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## stevekale (Jan 19, 2013)

Nice. Can you post your decay waterfall and RT60 graphs? SPL Response is just one slice of a the puzzle. With that absorption I'd be interested to see how good the decay is. 

While I am not as experienced at this as some others, I think you may have trouble with the 30Hz unless you add at least another sub. My front speakers will drive down to 18Hz (-3dB). I have them running full range and have a similar low-end issue as you complicated by room modes. I am in the process of adding a couple of RPG Modex Plates (35Hz, Type 2 version) to help low end absorption and I'm also running a LP (via my Theta Casablanca) to a sub positioned to the rear of the listening position - i.e. LF goes to 3 speakers (in a 2-channel listening setup). While I can only independently set the LF volume on the sub, I'm getting close to a 3 sub setup. 4 would be even better. (Some good reading here: http://www.madronadigital.com/Library/BassOptimization.html )

(Interesting roll off in the high frequencies... I see similar in my measurements but not as severe. I've wondered why this is the case. I'd also be interested in your results presented with a more typical 45-105dB scale.)


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## bob755 (Apr 14, 2013)

Here are the RT60 and waterfall graphs without EQ. I'm loading the filters into the FBQ today. The roll off at high frequencies may be due to the microperf screen.


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## stevekale (Jan 19, 2013)

When you have labelled the graphs "Center" and "Sub", are these with just these speakers running or is it front L/R + Sub in both cases? There are others far more expert than I am but I think one challenge you face is that 30Hz is not only resonating but taking a long time to decay (likely because of a room mode). You probably see some very long tails on your spectrogram.


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## bob755 (Apr 14, 2013)

stevekale said:


> When you have labelled the graphs "Center" and "Sub", are these with just these speakers running or is it front L/R + Sub in both cases? There are others far more expert than I am but I think one challenge you face is that 30Hz is not only resonating but taking a long time to decay (likely because of a room mode). You probably see some very long tails on your spectrogram.


The "Center" and "Sub" graphs are just those individual speakers running. 
How does one connect the sound card to test all 6 channels together?

I'm about to test a parametric equaliser to remove the subs 30Hz bump.

I've done some searching in the forum and Google and haven't found anyone that has a satisfactory solution for removing resonance at this frequency. Bass traps don't go down that far.
Perhaps with the equaliser, the audible results will be ok but if anyone has any potential solutions for removing 30Hz resonance, I'd be keen to know the method. That is, as long as it doesn't defigure the room. Question of finding a compromise between sound quality and esthetics.


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## stevekale (Jan 19, 2013)

I believe most people aim for decent 2 channel + sub results and let the 5.1 lie where it does or at least start with 2-channel. So observe L+sub, R+sub, L+R+sub and perhaps sub alone. That assumes the sub is being used to assist LF in 2-channel listening.

For 30Hz, the closest I have seen is the RPG Modex Plates Type 2 which I have just fitted. They're effective down at 35Hz. (Or perhaps a purpose-built / tuned Helmholtz resonator?) What are your room dimensions? 

This is very much worth a read:

http://www.acousticfrontiers.com/st...0681100537/acoustic_measurement_standards.pdf

I don't think parametric equalisation will help with your decay time challenge (or mine ;-) I'm battling even more compromises as I don't own my room and my wife thinks I'm nuts)

Another worthwhile read (as was suggested to me here) is The Master Handbook of Acoustics by Everest and Pohlmann. I have it on my iPad.


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## bob755 (Apr 14, 2013)

Interesting document. Thanks.

I believe that these RPG Modex Plates Type 2 cost something like $1000 don't they?
No way I'd spend that amount on something that may not even make a diffierence. 
I'm going to try the parametric EQ and moving the sub around before considering rooom modifications.

The room is about 6m x4m


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## stevekale (Jan 19, 2013)

In the UK they're just under £500. In all the reading I have done, a compound baffle (plate) absorber such as the RPG Modex Plate have been suggested as most effective at low frequencies. Otherwise I would wonder if a Helmholtz resonator can be tuned that low. Others will chime in. 

Ceiling height? You can enter your room dimensions in REW to calculate estimates of Room Modes (or use Bob Gold's online room mode calculator http://www.bobgolds.com/Mode/RoomModes.htm ). Below circa 250Hz your room dimensions will be playing a dominant role.

PS: you may get more help in the acoustic treatment forum than here in the REW section


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## stevekale (Jan 19, 2013)

Of course you should also experiment with sub placement.

BTW, here is an example of a Helmholtz resonator based product in the 25-60Hz range, the V6, http://www.diffusor.com/Basabsorbenter.htm often installed on their side where they can double as furniture http://www.performanceacousticslabs.com/PAL/Customers-Acoustics_Listening_Rooms.html


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## bob755 (Apr 14, 2013)

As you suggest, I might start a thread in the other forum but I'll do some experimenting first. I want to see what effect the filters have, try moving the sub arond and make sure that this 30Hz resonance is really impacting listening before undertaking time and/or money consuming solutions.

I could eventually install a resonator at the back of the rooom between the Schroder diffusers (see photo in first message).


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## bob755 (Apr 14, 2013)

Here are the results after installing the FBQ1000 and setting the filters to reduce the 30Hz bump. The audible difference is quite noticeable with a much clearer bass.
I'm probably going to experiment with a Helmhotz resonator to better trap the 30hz.


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## stevekale (Jan 19, 2013)

Can you post spectrograms for L and R channels with the sub operating for 15-250Hz, -100ms to 700ms? One thing to be a bit wary of re the Helmholtz resonators is that most have a very narrow absorption range. Broader band absorbers such as the Modex Plates are better for this reason. The good thing, however, is that if you did all that work in the room by yourself then constructing a Hermholtz resonator would be easy. It's getting the tuning right that can be the issue. This may help. You could even construct it in a manner that would allow variation of the size of the port...

http://www.mh-audio.nl/acalculators.asp#showcalc


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## Phillips (Aug 12, 2011)

I have a similiar shape room (6 x 3.7 meters), gee i wish mine had looked like that before EQ.

Check out my thread/post, i have done more work since then but gives you a good idea.
http://www.hometheatershack.com/forums/rew-forum/66651-please-check-my-measurements.html

I have a room mode of 37hz which is a more of a sharp peaks than yours.

I have Velodyne Digital Drive 15 with its PEQ. This dealt with the mode very well, very easy with REW, enter the filters then re- measure. 

I have Energy Veritas that i have always run with bass management through the Yamaha Z9 due to this room mode.

What i now run is the Veritas full range (with no bass management) and have got a DSpeaker Antimode Dual Core 2.0 (that has multiple setups). I tend to flick back from the Velodyne which will eventaully deal with LFE. Currently run 2 x REL Strata 5s which i have had for sometime. The room mode has been severely dealt with both the Velodyne and Antimode.

One thing to remember that you won't be able to totally get rid of that Room Mode, 

Trey other crossovers such as 80hz see how that looks.

When looking at Waterfall Graphs limit to 300hz ,above that there is no use.

You will be able to use a couple of filters to deal with that easily and like in one of the previous post suggested (AudiocRaver) just bump that sub up to taste and you are good to go.


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## stevekale (Jan 19, 2013)

I still think, rightly or wrongly, that one needs to fix decay first before equalisation. A flat response that lingers will just muddy the sound.


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## AudiocRaver (Jun 6, 2012)

Not disagreeing in principle with anything that has been said, just trying to see it in perspective...

This room has more treatment and absorption and trapping in place than 90% of the rooms we talk about, and users go after modes like that 30 Hz with EQ and get great-sounding results all the time. The whole area of room treatment involves diminishing returns, and the usual advice is "treat the room _within reason_ and EQ what is left." I would say this room as been treated _beyond_ the degree that many of us would ever dream of pursuing before resorting to EQ. From that perspective, the room, including the 30 Hz mode, is entirely ready for EQ. My opinion.:innocent:

OTOH: It all depends on where you set the bar, the standard of "good enough." The perfectionist in me is also curious just what it would take to tame that ONE LAST MODE.

Again, not being argumentative, just giving a perspective.


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## bob755 (Apr 14, 2013)

AudiocRaver said:


> Not disagreeing in principle with anything that has been said, just trying to see it in perspective...
> 
> This room has more treatment and absorption and trapping in place than 90% of the rooms we talk about, and users go after modes like that 30 Hz with EQ and get great-sounding results all the time. The whole area of room treatment involves diminishing returns, and the usual advice is "treat the room _within reason_ and EQ what is left." I would say this room as been treated _beyond_ the degree that many of us would ever dream of pursuing before resorting to EQ. From that perspective, the room, including the 30 Hz mode, is entirely ready for EQ. My opinion.:innocent:
> 
> ...


Thanks for this comforting opinion.
Before any EQ, I was globally satisfied with the acoustics but somtimes there was this boomy bass that became irritating on certain sound tracks. After recently discovering REW and making som measurements, I was able to identify the 30Hz mode as the likely cause. As you say, there is alreay quite a bit of room treatment and from what I could see from the results, only this 30Hz bump needed solving. It therefore seemed that some EQ would help and it has. Measuremnts, responses, spectrograms are useful indicators but what counts for me is whether my ears are happy!

After installing the FBQ1000 and applying the filters , the response is much better and I can hear a big improvement but the boomy bass is still present in some cases and the spectrogram confirms that the 30hz mode is still there.
Conclusion: EQ has improved things but not resolved everything.

Today I built a Helmholtz resonator tuned to 30Hz and installed it behind de sofa with the port facing upwards. 
Listnening tests are surprisingly satisfactory. Even me wife, who was very sceptical about this huge box with a hole in it, heard the improvement.
I'm also happy (my wife too) not to have disfigured the room.

I haven't had time to do the REW tests again yet but I'm very pleased with the results so whatever the graphs show I won't be looking to make any drastic changes except tweaking the filters again now the resonator is installed.

I'll post some photos of the and results soon.

This thread has probably deviated from RAW but thanks to everyone for your comments and advice.


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## AudiocRaver (Jun 6, 2012)

Cool. You have a VERY nice room. Looking forward to those details.


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## stevekale (Jan 19, 2013)

Well done. I wish I could just pop into the garage and make a Helmholtz resonator! Unfortunately my carpentry would, without doubt, let me down. Please post the 15-250Hz spectrograms when you have run your tests. Congrats!


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## bob755 (Apr 14, 2013)

Here are some before and after measurements with the readjusted filters and resonator.


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## stevekale (Jan 19, 2013)

The decay (or lack thereof) around 30Hz is better but still not "great". (Also, it seems to have sucked a hole out at 65Hz. I wonder what's up with that.) That very low-end is still lingering a little. 

But to show you "worse than not great", I am battling a similar low-end issue. Here's my spectrogram (just for left speaker plus sub - I need to redo them for both L/R and sub...all my speakers drive below 20Hz).










Your's is so much better, much smoother, less-ragged, especially above 80Hz. Although, even in my relatively untreated room (just two Type 2 Modex Plates) the low-end doesn't linger for as long. (Note the differences in vertical scale. Also, this is a window of 300ms, frequency resolution of 3.3Hz.) That should give you some confidence of how good your room is. 

From another forum, I received this spectrogram of a "good" room from Nyal Mellor (the author of the article I referred you to earlier in this thread):










I am jealous of what you have achieved!!


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## stevekale (Jan 19, 2013)

BTW what were the dimensions of the resonator? (Also, have you experimented with placing a gauze over the port? It will broaden the operating frequency (although lower the intensity of impact). Some dampening at the bottom of the resonator is worth trying as well depending on your propensity to fiddle ;-) )


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## bob755 (Apr 14, 2013)

stevekale said:


> The decay (or lack thereof) around 30Hz is better but still not "great". (Also, it seems to have sucked a hole out at 65Hz. I wonder what's up with that.) That very low-end is still lingering a little.


I doubt that the 30Hz decay could be improved further unless I put another resonator at the other end of the room but it would really spoil the esthetics of the room and I’m satisfied with the audible improvement.
The gap around 65Hz is probably the resonator absorbing a harmonic.


I see from the “Good room” that you posted that my results after correction are not too bad.

There are a lot of nasty peaks in your spectrogram. You have work to do!
Can you post some details of you room? It’s also probably a good idea to continue this thread in another forum section as it’s not directly related to REW.



stevekale said:


> BTW what were the dimensions of the resonator? (Also, have you experimented with placing a gauze over the port? It will broaden the operating frequency (although lower the intensity of impact). Some dampening at the bottom of the resonator is worth trying as well depending on your propensity to fiddle ;-) )


The external resonator dimensions are 110cm x 55cm x55cm. (281 liters). The inside is lined with several cm of insulation to widen the bandwidth. I haven’t tried putting gauze over the port but that will reduce the port area won’t it?


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## stevekale (Jan 19, 2013)

bob755 said:


> I see from the “Good room” that you posted that my results after correction are not too bad.
> 
> There are a lot of nasty peaks in your spectrogram. You have work to do!
> Can you post some details of you room?


You have a achieved a fantastic balance between acoustics and aesthetics. I rent my space and so everything has to be removable. Also, my walls are Victorian brick and plaster - very solid and not at all absorbing. My next step is put up some first reflection to tame 80Hz and up (without too much absorption of higher frequencies critical to speech). So I'm thinking 4in BAD Panels from RPG. (These are like your hidden fibreglass panelling only perforated to act like Helmholtz resonators for more efficiency.) I have been holding my discussion here http://www.whatsbestforum.com/showthread.php?10307-RPG-Modex-Plates-(35Hz)-placement-question/page7




bob755 said:


> The external resonator dimensions are 110cm x 55cm x55cm. (281 liters). The inside is lined with several cm of insulation to widen the bandwidth. I haven’t tried putting gauze over the port but that will reduce the port area won’t it?


Master Handbook of Acoustics: "The width of this absorption band depends on the friction of the system. A glass bottle offers little friction to the vibrating air and would have a very narrow absorption band. Adding a bit of gauze across the mouth of the [resonator] or stuffing a wisp of cotton into the neck, the amplitude of vibration is reduced and the width of the absorption band is increased. For maximum effectiveness, Helmholtz absorbers should be placed in areas of high modal sound pressure for the tuned frequency."

What were your measurements for port opening and neck (including the neck created by the thickness of the material used to make the cabinet)? (I believe only internal measurements matter and that if all of the resonator is lined then this affects the depth of the port. It is more typical, I believe, to add the absorbent to the bottom of the resonator (opposite the port).) Out of interest, what happens if the resonator is rolled 90 degrees so the port faces the sub rather than points up in the air.


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## bob755 (Apr 14, 2013)

> What were your measurements for port opening and neck (including the neck created by the thickness of the material used to make the cabinet)?


Port is 9.5cm diam. The port length is 1.8cm which is just the thickness of the panels. For the current volume, this gives a frequency of around 30Hz



> It is more typical, I believe, to add the absorbent to the bottom of the resonator (opposite the port).


I've seen resonators of this type 30% filled with absorbent. 



> "...For maximum effectiveness, Helmholtz absorbers should be placed in areas of high modal sound pressure for the tuned frequency."


Yes. I found this to be at each end of the room, where the resonator is currently placed.



> Out of interest, what happens if the resonator is rolled 90 degrees so the port faces the sub rather than points up in the air.


It's right behind the sofa so the port would be blocked if I rolled it over.


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