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Acoustic treatment with slats - before-and-after measurements

Flaesh

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SPL into the absorber, then ... the wall, and then the remaining reflection passing through the absorber again, in the reverse direction
It is in the calculator:
1678645206889.png

are way more expensive
But polyester is completely harmless. Bulk PET for inner layer and thin, say 1-2", plates as coverage?..

I personally use double bass array for LF absorption)). But I wonder what the people can do with absorbers.
 

juliangst

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But polyester is completely harmless
And so is glass wool. The myth than rockwool or glass wool are harmful in any way has been debunked so many times by now.
 

sarumbear

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Thomas_A

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sarumbear

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Thomas_A

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Maybe you can show the test results?
My own panels and experiments are gone since 25 years. BBC has some reference material to those kind of panels.
 

sarumbear

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My own panels and experiments are gone since 25 years. BBC has some reference material to those kind of panels.
The panels work, certainly, but do they work at frequencies below 200Hz? All published test data, including the one you posted shows that they don’t. There can be no benefit by mixing multiple filters that is not effective in the range of interest.
 

Thomas_A

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sarumbear

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Not below 100 Hz. Use EQ or tuned traps for that.

The OP is asking how he can improve absorption in the 100-300Hz. They are using an absorber which is about an inch thick. According to the BBC tests, from the paper you kindly posted, even if they double the thickness there will be a small difference at that frequency range. They need to use almost 10x the thickness! Hence, I can’t see any benefit for the OP to use perforated panels.

23EB7948-6658-408B-B245-1124CFAAABFA.jpeg
 

Thomas_A

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The OP is asking how he can improve absorption in the 100-300Hz. They are using an absorber which is about an inch thick. According to the BBC tests, from the paper you kindly posted, even if they double the thickness there will be a small difference at that frequency range. They need to use almost 10x the thickness! Hence, I can’t see any benefit for the OP to use perforated panels.

View attachment 271258
If you cover the front wall with 95 mm fiberglass, covered with perforated board (5 mm holes, 20 mm c-c), and the board with 60 mm wedged acostic foam behind the speaker positions you will have quite broad-band absorption down to approx. 100 Hz. It will however take 16 cm of your wall/room space. The Dekustik site do not have those dimensions (they have for 5 mm holes, 16 mm c-c and 32 mm c-c and 20 mm c-c will be somewhere in-between).

Edit: You cannot do anything with one inch....but on the other hand you don't need very thick either. But you loose efficiency when you do them broad-band and with perforated board.
 
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Inner Space

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It is in the calculator:
View attachment 271240

But polyester is completely harmless. Bulk PET for inner layer and thin, say 1-2", plates as coverage?..

I personally use double bass array for LF absorption)). But I wonder what the people can do with absorbers.
That doesn't imply a two-way trip, with input and output measured on the same side of the material. That's a one-way trip, through material with a rigid backing.
 

Adhoc

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A helmholtz / slat absorber with narrow depth, like 10 cm / 4", can show good absorbtion performance in the lower frequencies if made correctly.

I posted the diagram below on a Swedish forum yesterday. It has been posted on the Gearslutz forum before by the acoustician Jens Eklund. The diagram shows the performance from actually made measurements in an echo chamber. (I tried to enclose the 74 page pdf as well for those who can probably understand Swedish, like Norwegians and Danes. The file was too large though, 12 MB. It was published for a Master of Science at KTH, -the Royal Instititute of Technology in Stockholm.) Setup was a sealed wooden frame, 70 mm high with area 10,5 m² (108 ft²) placed on the floor in an echo chamber, -to follow international standard for absorbtion measurements.

The frame was filled with glasswool ("Glasull" in Swedish), 70 mm thick (2,75”) no airgap towards the floor or slats, density 14,5 kg/m³. Wooden slats 70x16 mm (2,76x5/8”) where then placed on the top of the frame and the gaps between the slats where then varied before measurements where made. Density 14,5 kg / m³ and glasswool would equate to ”regular” house insulation in walls, probable gasflow resistivity should then be in the ball park of 12-14 kPa x s / m². Another insulation material was measured too, 50 mm thick rockwool with density 70 kg / m³ + 20 mm air gap. This showed only minor differencies versus the glasswool.

In the diagram the line without triangles, squares, x:es et.c. shows absorbtion of the glasswool itself, with no slats on top of the frame. So, a slat absorber only 8,6 cm / 3,4” deep can show good and fairly wide absorbtion in the lower frequencies with 2 to 4 mm gaps between the slats. If the slats where thicker or the insulation depth larger, this would improve some on the low frequency absorbtion. In general, one would prefer not to have ”extra absorbtion” from mid frequencies and upwards! That frequency span is easily modified with curtains, carpets, upholstered furniture et.c. Don’t make the room too ”dead sounding”!

To be of high value in the low range, the slat absorber should be large. Small sized absorbers will not accomplish much, considering the wave length of low bass notes. To avoid fibers in the room, I would consider placing a non woven cloth between insulation and the slats. (The type you use in the garden against weed, -very cheap and also easy to breathe through.) Absorbtion coefficient of 50% equates to 3 dB damping. I see 80% and above as good performance, about 7-8 dB damping / passage, if I remember correctly.
 

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  • Spaltpanel 16x70 mm + olika glipbredder + 70 mm glasull d = 14,5 kg per m3.png
    Spaltpanel 16x70 mm + olika glipbredder + 70 mm glasull d = 14,5 kg per m3.png
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kipman725

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Adhoc

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Below; another diagram of measured absorbtion from that thesis for MSc at KTH. Setup: The same 70 mm high sealed wooden frame, 10,5 m², on the floor. Slats 70x16 mm on top of the frame with 4 mm gaps in between, below the slats the filling: A) 70 mm glasswool (”Glasull”); B) 50 mm rockwool ("Stenull") + 20 mm airgap towards the floor; C) no filling = only air (”Luft”) under the slats. From the measurements glasswool has slightly higher absorbtion in the lowest frequencies, stonewool slightly better than the glasswool from 315 Hz and upwards. With no filling the slat absorber is quite useless with a peak of 40% at 315 Hz.

(So, checking the 2 diagrams, I would disregard Sarumbear's remark in post #114 ”Hence, I can’t see any benefit for the OP to use perforated panels”, if one wants to improve absorbtion below 300 Hz.)
 

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  • Spaltpanel 16x70 4 mm gap 70 mm glasull + stenull samt tom på isolering.png
    Spaltpanel 16x70 4 mm gap 70 mm glasull + stenull samt tom på isolering.png
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napfkuchen

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Currently I'm still crazy about any guides regarding room acoustics. Jesko (Acoustics Insider Youtube-Channel) gives a lot helpful insights in his videos. Not sure if this already has been posted here but ...
 

ABuen

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Recently, I installed some acoustic panels in my listening room of the "slat wall" type, and I took the opportunity to make some before-and-after measurements. I'm posting it here as a service to the community, in case it is of interest.

For some background, I live in an apartment with an L-shaped living room as my listening room. The front wall is plaster/drywall, the back wall is concrete. Open on the left side, windows with curtains on the right side. I have a NAD M33 running Dirac Live. Two subs. The sound was quite good, but I had some issues with reverberation. Also, the speakers and my head are a bit too close to the walls, so there are - I assume - boundary effects in the 100-300 Hz region. This is what I wanted to fix.

Why slats? I first considered conventional acoustic absorbers from RPG, GIK, etc. Maybe mounted behind the speakers and behind the sofa where my listening position is. However, I was troubled by uncertainty about how effective they would be - at least in my use case. Besides, I find this kind of panels extremely ugly... Then somehow I came across slats. You can find a number of these slat panels on the internet, marketed as acoustic panels. Mine are from Fibrotech. The panels are 9 mm thick felt mats with slats in MDF glued on. A test sample I ordered looks like this:
View attachment 268873

Panels like this don't seem to be much discussed for audiophile purposes compared to conventional absorbers. The few discussions I have seen have been mixed. Anyway, even if they would not be as effective, they cover more wall - and look a lot better. So I figured that even if they would end up doing nothing for the acoustics they would look good enough to be worth it. Here is how my living room looks now:
View attachment 268874View attachment 268875

OK, enough talk, lets see some measurements. Here is a graph of the RT60 before and after (Topt is shown):
View attachment 268870

The only things that changed from before to after are the wall panels. The position of furniture, mic, etc. are essentially the same. Dirac Live was used with the exact same filters for comparison. As you can see, the region from about 300 Hz and upwards has significantly reduced decay time. Now it doesn't exceed much above 500 ms.

Here is the frequency response (both measurements are volume adjusted to roughly 75 dB on the REW SPL meter):
View attachment 268876

Unfortunately, the troublesome region from 100-300 Hz is still troublesome, or maybe slightly less so. I'll know more when I get around to re-running Dirac and do the sub integration again from scratch. (The response looks somewhat bass heavy, but actually it doesn't sound as bad as it looks.)

And here are left and right speaker ETC curves:
View attachment 268877View attachment 268878

So how does it sound subjectively? First of all the level of sound is lower. I need to turn up the volume a bit to play at the same perceived level. This makes sense to me, because some sound energy is removed by the absorption. Second, reverberation is perceivably lower. I would subjectively describe it as being "closer to the music" - especially for vocals. This also makes sense, I guess, since reverberation may be associated with large rooms, thereby distance. Finally, I think I can hear more details now, like in drumming and other "rapid impulse bursts". The first two of these effects, I feel certain to pick out in a blind test - the third, I *think* I would, but I don't know.

To conclude, I'm very happy with my new slat wall panels. They look good, and I think they solved at least some of my acoustic problems. They may be slightly less effective than conventional absorbers, but they work. My impression is that they are not highly regarded by audiophiles, but I really don't understand why. I think at least they should be in the toolbox.

OK, that's it. I hope this was useful.
Hi!
Intereseting measurements because there are not much data to find on slatted panels with polyester backing.
Would you be willing to sehare the room volume and size of the panels as well as the details for the panel dimensions? That would make it possible to asess the field absorption coefficient for the panels based on the change in reverberation times.
Kind regards, Anders B
 
OP
J

janbth

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Hi!
Intereseting measurements because there are not much data to find on slatted panels with polyester backing.
Would you be willing to sehare the room volume and size of the panels as well as the details for the panel dimensions? That would make it possible to asess the field absorption coefficient for the panels based on the change in reverberation times.
Kind regards, Anders B
Hi Anders! The total area of the room is 37 m^2, but the listening space is only about one third of this. The panels cover about 3,6 m in width both on the front and the back wall. Ceiling height is 2,5 m. Each panel is 60 x 244 cm. You can find more details on the producer's home page: https://fibrotech.com/en/. There you can also find a graph of absorption coefficients (see the datasheet for the Basic panel). Not much info about the absorption measurements, but at least it is there.
 

ABuen

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Hi Anders! The total area of the room is 37 m^2, but the listening space is only about one third of this. The panels cover about 3,6 m in width both on the front and the back wall. Ceiling height is 2,5 m. Each panel is 60 x 244 cm. You can find more details on the producer's home page: https://fibrotech.com/en/. There you can also find a graph of absorption coefficients (see the datasheet for the Basic panel). Not much info about the absorption measurements, but at least it is there.
Hi! Thanks for the fast reply and the information. Thanks also for the data reference. I copied their data and your RT measurements, used these along with the room volume and panel area to get the field-measured absorption factors given in blue. The data from the supplier is the orange curve.
1679162046193.png

I have assumed 9mm PET porous material. I think one need a void of at least 50mm behind that panel to get data like those given by the supplier. I am running a project on slatted panels for a statistics based models for predicting their performance and are on version 2 on that work (The first working model was up in dec 2019). Data like this do enter the data set, given that I know the construction and that the data may be reliable. It is pretty common to see that the performance of sound absorbers in non diffuse spaces like homes, are somewhat lower than in the fully diffuse 200m3 laboratory.

Have a nice weekend!
 
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