# Reduce Ringing/Decay times



## neo_2009 (Nov 13, 2009)

My home cinema room is in a loft, has 7x7m, hard wooden floor, and has very little furniture.
As such, its a very lively room, and as it can be seen in the waterfall chart, the ringing times are very long :


I already have an Antimode, but it isnt enough to counteract the room characteristics.

As such, i would like to ask your opinions, regarding this specific panels : http://www.amazon.co.uk/GIK-Acousti...&qid=1375053212&sr=1-3&keywords=gik+acoustics

They are reasonable cheap, and i could apply two of them (maybe four if two aren't enough), in the side walls of the loft, something like this :

Left Corner


Right corner


- Would this have any impact on reducing ringing levels?
- Are there better choices that can be applied ?
- What type of treatments should i apply in order to reduce these ringing times?

PS: Original post with all REW measures can be seen here : http://www.hometheatershack.com/forums/rew-forum/67952-subwoofer-positioning-choise.html


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## bpape (Sep 14, 2006)

In a 7m square room, you'll want/need a lot more than just 2-4 panels. Also, the 242 is not going to be thick enough to impact the bass frequencies. The 244 and Monster are more appropriate for that.

Bryan


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## neo_2009 (Nov 13, 2009)

Although the loft has 7x7m, the roof is very short, 2.2m on the highest part, and 1m on the sides. It's shape is like an inverted V.
Do you think that 2 boards of the GIK 244 would have a significant impact on the decay/ringing times?

http://www.amazon.co.uk/GIK-Acousti...=1375365770&sr=1-9&keywords=gik+acoustics+244


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## bpape (Sep 14, 2006)

It will make a little bit of a difference. The low ceiling is not benefiting you - just makes the reflections quicker and the height more of an issue. To get the most from the 244's, I would hang them on the ceiling straddling the peak.

Bryan


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## neo_2009 (Nov 13, 2009)

Bryan, 

you were right, the ringing times are much greater than 300ms :




What do you think that would be the benefits of adding 3 Tri-trap on the corners, two stacked on the left corner (moving the subwoofer a little btl to the right) and one on the right corner :

- http://gikacoustics.co.uk/product/gik-acoustics-tri-trap/

Left Corner :


Right Corner:


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## bpape (Sep 14, 2006)

The left corner will likely do little to nothing. Only 1 real boundary and a short wall. Do what you can on the right and maybe 2 a couple horizontal along the wall/floor junction.

Bryan


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## neo_2009 (Nov 13, 2009)

Something like this, on the floor, and the right corner? 



On the floor, maybe an 242/244, and on the right corner a 244, and a tri-trap?


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## bpape (Sep 14, 2006)

Sorry - let me rephrase. 1 standing up in the right corner, 1 on it's side on the floor under the screen. Also try pulling that sub forward a bit.

Bryan


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## neo_2009 (Nov 13, 2009)

Hello Bryan.

I've consulted the prices for GIK Europe, and although the prices are very good, the taxes and the shipping costs to Portugal are very expensive.
So, i'm decided to build my traps 

As my main objective is to reduce ringing times in the 30-200Hz region, i will try to address that problem in the first place.
As corners and intersections are the place where the low region is more excited, i think that the best chance to address the problem is using corner bass traps.

I've read this two articles, and i still have some doubts i don't fully understand:
- http://www.ethanwiner.com/acoustics.html
- http://www.ethanwiner.com/density.html

Could you please give me your inside on the following questions?

1 - Whats the best design for corner bass traps :
1.1 - Triangular shape, totally filled with absorbent material (fiberglass, rock wool, etc);
1.2 - Triangular shape, filled with absorbent material, but leaving a gap between the absorbent and the corner;	
1.3 - As a rectangular panel shape, as can can be seen in the second link.


2 - What is the effect of applying a reflective material (wood, aluminium, etc) on the panels face :
2.1 - On the surface facing the room;
2.2 - On the surface facing the corner;	

3 - What is the best way to avoid absorbing the mid-high frequencies?

Thank you very much in advance.


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## bpape (Sep 14, 2006)

Triangular shape, solid material, gap does very little with these. Do not use too much density contrary to Ethan's paper. Thicker absorbers like this actually work better with something around 2-3 lb/cu ft (sorry ,too early for me to remember the conversion to kg/m3


With no or little gap, no impact on performance with a hard material on the sides facing the wall. On the side facing the room, it depends on the density and flexibility of the material applied. Something like very very thin plastic starts to roll off in the kHz range. MLV starts to roll off between 2 and 300 as long as the cavity is not sealed air tight and the mass is limp.

Dense wood and or metal is very unpredictable and will roll off pretty low.


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## neo_2009 (Nov 13, 2009)

Thank you very much 
From your post, and to the purpose of minimizing the ringing times on the low frequencies, while minimizing the impact on the mid-high frequencies, can i conclude that the best approach would be :

*- Triangular shape* : Wood frame like this 









*- Absorptive material with 6mm / 3cu ft density;*

*- Side facing the room has a very very thin plastic cover ;* Maybe an plastic adhesive like this : 









*- Using a fabric to cover the trap* : 









- *Using this "design"* (note, the traps shape is incorrectly shaped, due to using 2D image, the correct is the wood frame above) : 









- I'm unsure regarding the gap, would i benefit of a gap between the absorptive material and the fabric (on the wall side)?

Are this assumptions OK ?


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## bpape (Sep 14, 2006)

That will work but if you want to restrict just the highs somewhat, then use something like a thin disposable 
tarp/dropcloth. If you want something that will restrict a bit lower, think about a sheet of pond liner.

No need for the gap

Bryan


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## neo_2009 (Nov 13, 2009)

Would a *perforated kraft paper* thin layer be ideal, to avoid the absorption of the mid-high frequencies ?:









Or, as you said, the *tarp *would be ideal ?:









Just one more clarification, what would be the ideal thickness of the panels?
I've incorrectly stated 6mm, but i was referring to 6 inches.
Is the *6" thickness / 3lb cu ft* relation ideal ?


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## bpape (Sep 14, 2006)

4" for reflection panels - 6" for bass control or rear wall.

Think of something like this - http://www.globalplasticsheeting.com/10-mil-plastic-sheeting/ for the facing. The Kraft Paper can work - try to find it with the foil layer also - commonly referred to as FRK or FSK.

Bryan


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## neo_2009 (Nov 13, 2009)

Brian, thank you very much for all the help.

I will make some experiences with several materials, and will document the whole process here, to be able to help others with the same questions as mine.

Best regards.


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## neo_2009 (Nov 13, 2009)

Brian, please forgive me, but can you clarify three final questions?

1 - Here in Portugal RockWool is more available than fiberglass.
Assuming the same thickness (6"), and the same density (3 lb/cu ft), does RockWool have the same absorption characteristics of fiberglass ?

2 - Would a ripped panel or perforated wood panel (on the room surface) be useful to avoid mid-high absorption?
Or using only the plastic layer is the best option?

3 - Is the wood type important? Should i use a solid wood, or can i use MDF?


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## neo_2009 (Nov 13, 2009)

I think i misunderstood the relation between Density and thickness.

Can you please clarify me this detail?
- If i'm building a triangle corner trap :



With this dimensions, its better to use 3lbs or 6lbs density, or is there an ideal density?


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## neo_2009 (Nov 13, 2009)

*Materials*
Mineral Wool - (< 5kpa) - 135cm x 60cm x 60cm (due to being a little compressed") - 21m2 of mineral wool
RockWool - 150kg/m2 - 120cm x 100cm x 16cm - 4.8m2 of Rockwool

*Placement*
As you can see by the pictures, the placement was not ideal

*P1 - Mineral Wool*


*P1 - RockWool*


*P2 - Mineral Wool*


*P2 RockWool*


*All SPL P1*



*All SPL P2*



*Results position P1*

Mineral Wool - No positive effect, it even induces a null on 120Hz
RockWool - A 12db's effect on the null on 92Hz


*Results position P2*

Mineral Wool - None
RockWool - None

*Conclusions*
Honestly, I am quite disappointed with the results. 
The effect of the Mineral Wool was virtually nil. 
Even taking into account that the placement wasnt the ideal (i should have simulated a corner superchunk) it had no effect.

In the case of rockwool, there was a positive effect on the null of 92Hz. 

Given the placement was far from ideal, I do not know if i can conclude anything from this tests ...


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## bpape (Sep 14, 2006)

First of all you're only looking at frequency response. If you looked at the waterfall, you'd notice a reduction, though not certainly not going to fix everything with a couple of bales.

Second, you need to find out where the specific problems are coming from and treat there. Corners are efficient but not a be all, end all solution to every problem.

Third, some of the issues you're having may have to do with seating, sub and speaker positions where moving one or more will help the problems or at least shift them in center frequency to identify where they're coming from so you know where to treat.

In general, once you get to 5" or more, 3lb is more ideal, cheaper too. Below that, if reflections, use 3lb. For other, 6lb can work find but honestly, not that much better than 3lb. You can make more improvment lower with 6" of 3lb vs 4" of 6lb and it's cheaper too.


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## neo_2009 (Nov 13, 2009)

Brian, thank you very much for you reply.

I've generated the waterfall charts and they indeed show a few reductions on the decay times :

*P1 - Mineral Wool VS Rockwool*



*P2 - Mineral Wool VS Rockwool*



Well, its more encouraging, as soon as i receive my umik-1, i will make a few more tests


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## Babak (Mar 20, 2009)

Hi 

I don't think waterfall diagrams show the right picture. 

You want to improve the response of the room, or to be more exact, the decay of reverberant sound. 
Waterfall diagrams don't only show the room response, but a sum of direct sound and reverberant sound and it's decay.
So each frequency starts off at a different sound pressure level and it is difficult to compare the decay times at different frequencies. 

I would recommend to measure the room reverberation time alone - a classical RT60 or RT30 measurement in third octave bands. 

This shows how long it takes for the reverberant sound to decay by 60 dB or 30 dB, respectively. 
That's independent of the sound pressure level at the beginning of the measurement.

Cheers 
Babak


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## bpape (Sep 14, 2006)

T30 or 40 can be used. T60 is for large spaces. I would agree that you want to address both decay time and decay. That said, if you cannot for whatever reason tame a particular resonance, you would still want to bring it down within the same time frame so it's not masking other things even though it started at a higher point.


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## Babak (Mar 20, 2009)

Even better than to tame /bring down a resonance, it is better not to let it come up in the first place.
If there is no resonance that is being excited nothing needs to be absorbed. 

Placing full range Loudspeakers / subs on null points of room modes prevents that those modes are excited. 
Placimg the listening position on a null point reduces the effect of that mode (low sound pressure from the mode resonance). 

Floyd Toole and Sean Olive provided excellent publications on that. 

Cheers 
Babak


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## bpape (Sep 14, 2006)

Agreed if it can be done. Sometimes it cannot without causing other issues - some of which have nothing to do with acoustics (walkways, wife acceptance, overly large screen sizes already installed, built in cabinets for looks, etc.). Not everything is done in a perfect world where we have 100% full reign over everything.


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## Babak (Mar 20, 2009)

That's true. 

Here I read something of a 7x7 (!) metres home theatre. 

So chances are good that the room is dedicated for listening to music and /or watching movies. 
Then one could convince the boss (aka wife) that speakers and seats need to be moved. 

Many wives enjoy good sound themselves. 
So one can try to convince them by letting them hear the results. ;-)


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## neo_2009 (Nov 13, 2009)

I do have some freedom to move the speakers, but i cannot change the listening position, as its the only place where the sofa can be.

I've received my UMIK-1 and i was planning to take new measures of each individual speaker.
Looking to the room schema, do you have any suggestions for the placement of the main speakers?


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## neo_2009 (Nov 13, 2009)

Regarding the measure of each individual speaker, is it possible to clarify some doubts i still have?

1 - The speaker distances in the AVR should be inserted, or the measure is done with the distances to 0?
2 - I have a Sub + Sat configuration. Should the speakers should be measured in full range, or at a 80Hz crossover?

Thanks in advance.


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## Babak (Mar 20, 2009)

Hi 

Thanks for the floor plan. 

Could you tell us what the circles, ellipses, the green box, P1 and P2 mean and where the screen is (and if it's possible to move the screen)? 


Regarding the measurement... 

More interesting than to measure the speakers would be to measure the room response. 
This means to excite the room with noise signals and measure the decay rate of the room response, e.g. with a RT30 measurement 


Cheers 
Babak


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## neo_2009 (Nov 13, 2009)

I've updated the room schema 

Regarding the "symbols" : 

- the gray line represents the screen,
- The green box is the Subwoofer (SVS PB1000),
- The 3 ellipses near the "screen" are the LCR speakers,
- The 2 circles at the sides are the surround speakers.

- The screen cannot be moved as its the only wall available.
- The sofa cannot be moved as it is the only acceptable place in the room (behind the sofa is a glass wall that covers the stairs to the down floor).

I will try to produce the ETC charts this weekend :T


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## Babak (Mar 20, 2009)

Hi

thanks for the update.

Regaring the placement of the speakers you do nor have many options with the sofa and the screen fixed.

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I see some issues with the asymmetry of the room and the position of speakers and the listening spot.

The right speaker is near the wall (early and more intense reflections) and the left speaker stands an open area (late and less intense reflections).

I think that those issues could be reduced if you toe in the speakers so they point right to the listening position.

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Ideally all speakers should have the same distance to the listening position.
So you could draw a circle with the center at the listening position and with the radius being the distance to the two front speakers.

Then you will see that the centre speaker is too close and the sub is too far away.
So, I would move them to a position on the circle.

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Also your surround speakers are not at the right positions.

The standard for 5.1 surround places the speakers as follows (on the circles):

Centre: 0° (straight ahead)
Left front: 30° left
Right front: 30° front
Left surround: 110° to 120° left (so al little behing the listening position)
Right surround: 110° to 120° right

If one of the surround speakers is not on the circle, you can correct that with the setting in your AVR (that adjusts the delays).

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Another question would be how the boundary to the stairs looks like.
Is it a solid wall or is it a handrail or something else?
That's important for the correct position of the left surround speaker.

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It is also difficult to tell the right positon of the subwoofer from the scheme.

That's a rather easy task for a box-shaped room.
But your room has got the staircase.
That opening makes it difficult to determine how the room modes look like, where the peaks and nulls are.

The easiest (but rather time consuming) way would be to triy out.
Put your subwoofer into a room corner (so it excites all room modes).

Then play some test signals containing low frequencies (music will do for getting a firts guess), walk around in the room and mark those positions where you hear the bass louder (peaks) and where the bass is the weakest (nulls).

Put coloured spots at the scheme.
That will help to find the best position for your subwoofer.


Cheers
Babak


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## neo_2009 (Nov 13, 2009)

Babak,

thank you very much for such a detailed response 

I will take some photos of the space to provide a better understanding of the room.

Regarding the speaker placement, i suppose this picture illustrates your points:









I have the freedom to move the LRC speakers, but not the surround speakers, as they are placed in a flat wall. But, i can place them further behind, and as they are dipoles, it should work.

The boundary to the stairs is a glass handrail, i will post some pictures to clarify it.

Thanks once again :T


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