Covering bass traps with a poster, am I doing this right?

[abdo123]

Long story short, my partner purchased a gigantic poster (A0, probably larger) from a famous Champaign house in France.

We had a heated argument and the conclusion was that bass traps can only go in the living room as long they're covered by posters (particularly this one).

I'm thinking of measuring which frequencies are blocked/reflected by the poster by placing the poster infront of my speaker and measure before and after.

Does this sound like a sensible strategy? What are your experiences with this?

 

[amirm]

Is the poster on paper or fabric? In other words, if you blow on it, does air go through it?

 

[abdo123]

Is the poster on paper or fabric? In other words, if you blow on it, does air go through it?

paper, air will not go through it but it's definitely bendy. my assumption that it will let frequencies up to 2KHz through unobstructed. still haven't tested though.

 

[Frgirard]

paper/card surface...acoustic properties

https://gearspace.com/board/showpost.php?p=13817057&postcount=3

 

[abdo123]

paper/card surface...acoustic properties

https://gearspace.com/board/showpost.php?p=13817057&postcount=3

thanks for sharing this! the poster is 200g for roughly 1 meter square, so it will let 80% of the frequencies in @ 450Hz. little bit lower than I expected

 

[Frgirard]

It's for a bass trap and avoid high frequency absorption , not for the reflections absorption with a broadband absorber.

 

[abdo123]

It's for a bass trap and avoid high frequency absorption , not for the reflections absorption with a broadband absorber.

I have no absorption in the room at all so I wouldn't mind broadband range (or at least till the the room stops being modal around ~2KHz).

 

[raindance]

Are the bass traps in the corners, and if so, what air gap is around them?

 

[Frgirard]

Join us here https://gearspace.com/board/studio-building-acoustics/

 

[abdo123]

Are the bass traps in the corners, and if so, what air gap is around them?

no corners and no air gap, mainly big portion of the front and back wall to deal with SBIR / LBIR.

Either GIK monster traps https://gikacoustics.eu/product/gik-acoustics-monster-bass-trap-flexrange-technology/

or the thickest 'artistic' acoustic panels they offer. since the poster seem too dense for my use.

 

[raindance]

You can always measure with and without the posters and see what the impact on decay in the SBIR area is.

Typically you'd want to get the pictures printed on fabric though for this application because absorption in the SBIR area is typically pushed up into the upper bass/low mid area by positioning speakers as close as possible to the wall.

For a true bass trap application it wouldn't make much difference except perhaps limit absorption at higher frequencies, which can be a good thing.

I use both full height bass traps with a 4" air gap all round for primary mode absorption in the front corners (my room has 2 modes that reinforce each other at 46Hz causing about a 20dB boost there) and 4 inch thick Corning 703 2'x4' panels directly behind the speakers to control SBIR. Two sides of the bass traps (facing into the room) have full length thin plywood sheets to prevent excessive absorbtion above bass frequencies. I still need to apply a notch filter at 46Hz to both my mains and my subwoofer as the bass traps make very little difference to the low bass frequency response, just the decay time. The SBIR treatment makes a decent difference though. Weirdly, if I plug the port on the left main speaker only, the primary mode is no longer an issue for ther mains and I can get away with EQ only on the sub.

Just relating my experience, hopefully it's helpful.

Another approach I've tried that worked, but took up too much space with the speakers being quite far into the room, was aligning the SBIR effect with the primary room mode. This worked with no room treatment at all, but the speakers were around 6' into the room to combat the 46Hz peak.

 

[Inner Space]

@abdo123 - this is an excellent idea for a bass trap. The targeted frequencies will pass through the poster, for absorption within, and some HF liveliness will be preserved in the room. It's the right approach, especially if you plan to use several.

 

[abdo123]

So it appears the poster is transparent till 600 Hz and then it becomes exponentially reflective.

 

[Inner Space]

So it appears the poster is transparent till 600 Hz and then it becomes exponentially reflective.
View attachment 139601

Great result. That's pretty much exactly what you want. Good sound and domestic harmony preserved - win win.

 

[abdo123]

Great result. That's pretty much exactly what you want. Good sound and domestic harmony preserved - win win.

well if you would look at the response you would see that I have problems till 2KHz, not till 600Hz.

What I want is to increase the decay time leading up to 2KHz.

 

[raindance]

Looks like SBIR effect is just below 200Hz, but smoothing makes it hard to see conclusively. Can you show a drawing of room layout or a picture?

That 1K cancellation may just be cancellation from measuring both speakers at the same time or layout related. That is what I'd look at.

 

[Inner Space]

well if you would look at the response you would see that I have problems till 2KHz, not till 600Hz.

What I want is to increase the decay time leading up to 2KHz.

OK, but you were asking about bass traps. 2kHz is outside of bass trap territory. You'll need to add regular HF absorbers or diffusors for that.

 

[abdo123]

OK, but you were asking about bass traps. 2kHz is outside of bass trap territory. You'll need to add regular HF absorbers or diffusors for that.

that's not really how it works. porous absorbers absorb all frequencies till their thickness is equal to 1/4 wavelength, once their thickness is less than 1/4 wavelength the absorption rolls off.

range limited bass traps usually have a reflective surface or a diffusion plate in front, that's why they don't absorb too much high frequencies.

However, I don't like that idea because I don't want to add coloration into my off-axis response, the way range limited bass traps absorb the entire range is not consistent.

 

[abdo123]

Looks like SBIR effect is just below 200Hz, but smoothing makes it hard to see conclusively. Can you show a drawing of room layout or a picture?

That 1K cancellation may just be cancellation from measuring both speakers at the same time or layout related. That is what I'd look at.

these measurements were not taken at the listening position, if you want to analyze the hell that is my living room then here you go. Luckily my sub-bass area is always smooth because I run multi-subs.

Speaker boundary (slightly managed with basotect for now)

Listener boundary-ish since our heads keep moving ofcourse.