# Reflections



## michael123 (Jul 26, 2008)

Hello

reading various sources (http://www.hifizine.com/2011/12/listening-room-reflections-and-the-energy-time-curve/), I understand that the ETC plot with these peaks shows the existence of strong reflections..

1) Is this a correct conclusion?
2) Are there any means to treat these electronically?


I believe my target is to drop these peaks, except for the one at 0ms to drop by at least 10db across 40ms

How can I determine which frequencies are these?


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## JohnM (Apr 11, 2006)

michael123 said:


> 1) Is this a correct conclusion?


Yes



> 2) Are there any means to treat these electronically?


No



> How can I determine which frequencies are these?


They are not frequencies. The ETC is a time plot, it shows how sound arrives at the listening position as time progresses. The peaks show when strong reflections arrive, and the time after the initial peak at which they arrive shows how much further the sound had to travel than the direct sound from the speaker. You can get a direct reading of the extra distance by putting the cursor on the initial peak, holding down the ctrl key, then right click on the mouse and drag to the peak you are interested in. Knowing how much further the sound has travelled gives a hint about which surface the sound might have bounced off, putting some absorption on that surface could reduce the level of the reflection.

Best advice on how to treat the room and how to position the speakers and listening position to reduce the level of the reflections can be found in the Home Audio Acoustics forum.


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## michael123 (Jul 26, 2008)

Thanks, John

On both channels, I see that the two strongest peaks show 0.5 meter and 1.0 meter.
While it seems logical to me for the right channel (that is close to window), I do not understand this for the left channel, that has no wall by its side, but rather opening to the kitchen space..
Not so clear to me what that might be...


I may place something like this by the window:
http://abstracta.se/web/Produkt1_1_9_2_4.aspx

or, in the short term, I will try to put a thick bed cover along the window..


It is a living room, neither me nor wife will want to put some "old-school" panel.
I have two SonicPrint by Auralex on the back wall, and Vicoustic Varibass in the corners (that are not so useful as an electronic EQ for me)


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## michael123 (Jul 26, 2008)

michael123 said:


> Thanks, John
> 
> On both channels, I see that the two strongest peaks show 0.5 meter and 1.0 meter.
> While it seems logical to me for the right channel (that is close to window), I do not understand this for the left channel, that has no wall by its side, but rather opening to the kitchen space..
> Not so clear to me what that might be...



John,

I am reading here
http://www.acousticfrontiers.com/Acoustic-Measurements.html
that the frequency can be determined from the spectrogram..

How can I know what is the direct sound and what are reflections?

More specifically, 
will this work for me?
http://www.texaa.com/products/vibrasto/vertical-blinds/acoustics/


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## Barleywater (Dec 11, 2011)

Resolving reflections in spectrogram display:

The default settings are geared to bass range. Opening the graph controls and changing settings by increasing Scale Range (dB) from 40dB to approximately 60-90dB, reducing Time Range to 50ms, and Window to 3-7ms will result in display with horizontal banding corresponding to walls, floor, ceiling, and intersections of walls. Tweaking the gamma setting also has effects similar to increasing Scale Range.

These settings are at expense of low frequency content which becomes smeared blob below about 3x the Freq. resolution displayed on graph controls.

-Andrew


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## michael123 (Jul 26, 2008)

Something like this?


So, the reflections, in my case, are having frequency of 200Hz..1KHz ..?

And having the blind with the following acoustic NRC data will help ..?


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## Barleywater (Dec 11, 2011)

Hard smooth surfaces reflect sound similar to mirror with light. Longer wavelength sound, is lower frequency, and requires bigger surface to get echo type reflection. Human perceptual mechanism tends to integrate reflections <6ms with source, causing increase in perceived loudness and size of source. Our ability to discern distinct echos starts on the order of 25ms. For example, how far back do you have to stand from large concrete wall so that hand clap has distinct echo? 

Your new spectrogram is a little "hot", but shows broadband reflections at about 3,5,7,11,17,22, and 35 milliseconds. For left speaker I am guessing that you placed microphone at seated position for couch. The 3ms reflection would be wall behind couch. The next reflections are floor and ceiling, then floor to wall behind couch, ceiling to wall behind couch. 35ms is likely wall that was behind you for where living room photo was taken.

Below about 1.5kHz your speakers become highly omnidirectional and reflections involving wall behind speakers come into play. The general boost strongly suggests that your speakers crossover and equalization design are optimized for free field listening, as in placed well away from walls and corners. A general bit of equalization can help this.

Seated at center of couch reflections off of sliding glass door in part make right speaker sound louder, and garbles locational cues relative to right speaker resulting in reduced sound stage resolution. From room photo looks like left glass panel is operational? If so you should notice different sound imaging with it open v closed. If this is the case, then the acoustic blinds may help.

What were your listing impressions/complaint prior to investigating with measurements?

-Andrew


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## michael123 (Jul 26, 2008)

Thank you, Andrew,

I did not publish before, I was advised by JohnM to 'zoom-in' on the ETC graph,
it shows me that the peak of reflections is at 0.5m and 1.0m - for both channels

I also add the spectrogram for the right channel (did not notice I added left channel twice)

I do use EQ (and these are the results), however I used only for the 24-200Hz frequency range.

Comparing sound before and after is night and day.

Regarding the sliding window - you're right, I indeed shall try (listen + measure) the sound with the window open (I can move all three segments there)

What is a mystery to me are these 0.5m and 1.0m.. Why are these the same for both channels?


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## JohnM (Apr 11, 2006)

michael123 said:


> On both channels, I see that the two strongest peaks show 0.5 meter and 1.0 meter.
> While it seems logical to me for the right channel (that is close to window), I do not understand this for the left channel, that has no wall by its side, but rather opening to the kitchen space..
> Not so clear to me what that might be...


Could be a reflection from the piece of furniture in front of the sofa.



michael123 said:


> How can I know what is the direct sound and what are reflections?


The first peak is the direct sound - it travels the shortest distance so arrives first, and reflections travel further and so arrive later.


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## michael123 (Jul 26, 2008)

I have a calibrated Dayton EMM-6 mic from Cross-Spectrum Labs (but not Earthworks), is it good enough for these measurements at this scale?


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## Barleywater (Dec 11, 2011)

Calibrated EMM-6 ought a due just fine.


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## michael123 (Jul 26, 2008)

Hey guys,

thanks for your help.

Today, I did some experiments.

I tried to figure out that 1 meter peak on the ETC graph.. that turned to be the back wall (the wall by the couch). I brought two pillows from the bedroom, put them on top of couch.. and the peak - disappeared.

Then I opened a window (actually, 2/3 of it, I pulled two segments to the right). Reflections were reduced, and also I discovered that 60..200Hz region was boosted by 5-7db (!) by the window for the left channel, for the right channel, it is about 3db for all region < 150Hz.


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## michael123 (Jul 26, 2008)

Since I already know that the (strong) reflections are caused by the wall near the couch, what would be the best way to cancel them/reduce by 15-20db?

A Diffuser?
Panels?
Will anything of these work for 100-200Hz?

I read for some diffuser @ Auralex website that it works down to 800Hz.. 
How come that simple pillow removed me a peak @ 200Hz?

I need something that does not look like an acoustic treatment.. 
Acoustic blinds was a nice idea since we planned adding blinds anyway.. Sadly, it won't do much..

??


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## Wayne A. Pflughaupt (Apr 13, 2006)

michael123 said:


> Since I already know that the (strong) reflections are caused by the wall near the couch, what would be the best way to cancel them/reduce by 15-20db?
> A Diffuser?
> Panels?
> Will anything of these work for 100-200Hz?
> ...


You’ll probably be hard pressed to find some acoustical treatment that doesn’t look the part, but for a definitive answer to that and your other questions, I’d refer you back to Post #2:


JohnM said:


> Best advice on how to treat the room and how to position the speakers and listening position to reduce the level of the reflections can be found in the Home Audio Acoustics forum.


That said, what exactly is the cause of your concern? Does your room sound bad to you, or are you merely concerned because some article said it sounds bad? Did things sound audibly better when the 1-meter peak disappeared from the ETC graph after you brought in the pillows?

Regards,
Wayne


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## michael123 (Jul 26, 2008)

Wayne A. Pflughaupt said:


> what exactly is the cause of your concern? Does your room sound bad to you, or are you merely concerned because some article said it sounds bad?


It was not bad, actually since I re-positioned the speakers and re-calibrated DEQ it was much better.
Yet, I know that my speakers can do better.. 
I would like to improve a bit the focus and add some depth. 




Wayne A. Pflughaupt said:


> Did things sound audibly better when the 1-meter peak disappeared from the ETC graph after you brought in the pillows?


I just measured with REW, I will try actually to listen..


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## aackthpt (Jan 24, 2011)

michael123 said:


> Since I already know that the (strong) reflections are caused by the wall near the couch, what would be the best way to cancel them/reduce by 15-20db?
> [snip]
> I need something that does not look like an acoustic treatment..
> Acoustic blinds was a nice idea since we planned adding blinds anyway.. Sadly, it won't do much..


Take the Auralex SonicPrint panels and put them behind the head positions on the couch. They will do more for your sound that way, and still reduce the reverberation time the same as they do up high--if that even matters here. Not sure what the white bumpy things nearer to the camera are, but if they are not moveable simply work around them. If they are moveable, move them. If you can mount the Auralex panels with air gap equal to their thickness behind them, do so as it will make them more effective.

A diffuser is not desirable this close to the listening position.



> I read for some diffuser @ Auralex website that it works down to 800Hz..
> How come that simple pillow removed me a peak @ 200Hz?


You can't directly go from what frequencies a device absorbs to whether it will be effective in changing the frequency response. That's because the change in frequency is being caused by this effect. Just ignore the "speaker" part of it and consider it boundary interference response--note that it is related to the room boundaries, speaker and listener positions. Change any one of them, and the resulting response changes. The typical first thing to do in such a situation is to move the LP and speaker positions if they can be until the best is found, then treat. Determining if the treatments are effective is generally best done by build and check, because (for a start) absorption varies by incidence angle for absorption panels. Calculating such effects is possible but requires a fairly high degree of acoustical science sophistication, so it will be easier for you to just put something there and test that it works--and if it doesn't then build something better.

You could always just line the top rear of the couch with pillows, too, since the one you put there was effective. They don't look like acoustical treatments, and are easily procured. You will need to put them the entire couch width if you want the improvement to exist for all possible listening positions on the couch.


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## michael123 (Jul 26, 2008)

aackthpt said:


> Determining if the treatments are effective is generally best done by build and check, because (for a start) absorption varies by incidence angle for absorption panels. Calculating such effects is possible but requires a fairly high degree of acoustical science sophistication, so it will be easier for you to just put something there and test that it works--and if it doesn't then build something better.


I try to conduct a small project with a professional, I want a guarantee.. 
My "try and error" list already includes Vicoustic Varibass and these SonicPrint panels..



aackthpt said:


> You will need to put them the entire couch width if you want the improvement to exist for all possible listening positions on the couch.


Yes, I was also thinking about custom-made RPG BAD panel, something like 20" x 125", 3" thick


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## aackthpt (Jan 24, 2011)

michael123 said:


> Yes, I was also thinking about custom-made RPG BAD panel, something like 20" x 125", 3" thick


What effect do you hope to gain from diffusion in that location? For talking purposes let's say you put an adequate absorber there (or a thick pillow) you will have the (evidently offending) reflection reduced to 10-20% of original strength. If you use a BAD panel you can expect it to be reduced to only about 60-70% of original strength. If you use curved BAD panels then it might be a bit better, let's say 40-50%. It seems clear what the safe option is.

Also I'm not sure a small amount of diffusion so close to listeners is likely to get you.

There are many places your room might (might) benefit from diffusion, but I don't think this is one of them.


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## michael123 (Jul 26, 2008)

but isn't BAD panel also a good absorber?

So, what do you suggest then?


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## aackthpt (Jan 24, 2011)

BAD panel is an absorber with a mask over it with 50% openings. As I said before I think you should use a pure absorber which is why pillows are a good choice, or moving the auralex panels.


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## michael123 (Jul 26, 2008)

aackthpt said:


> BAD panel is an absorber with a mask over it with 50% openings. As I said before I think you should use a pure absorber which is why pillows are a good choice, or moving the auralex panels.


thanks, understood.


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