Corner Trap Nonsense

[Tim Link]

Now, are products like https://www.gikacoustics.com/product/gik-acoustics-monster-bass-trap-flexrange-technology/ which purportedly reach down to 80 Hz (I suppose so long as they are actually positioned at a velocity peak for said frequency) useless in the sense of one's being better off solely using thinner broadband panels for midrange to treble absorption, or is it just a matter of positioning?

I'd put those diagonally across a corner, stacked up to the ceiling, so there's a triangular void behind them. If you can create a pressure change across a fiber panel by creating a void behind it, that's going to push a lot more motion through the fiber than putting the panel in a room null, where velocity is highest. The problem is, the velocity isn't very high in room nulls. I experimented with this using painter's plastic to create a large membrane. I wanted to see how much that membrane moved when pushed by the air in a null zone. It hardly moves at all. And this was at a very powerful null zone at the mouth of my bass horn, which causes a huge suck out in the response of my horn at 85 Hz. My attempts to absorb down the problem by using fiber resistance at the mouth of the horn were yielding very little result. The plastic sheet explained why. I could feel it vibrating even when the sound mostly disappeared because of the null, so I knew the air was moving in and out of the horn. But the plastic barely had to move at all to accommodate the motion. So, to put any brakes on it you need a solid membrane with adequate mass. It ends up more of a limp mass membrane problem. I could see a velocity absorber as being a very large limp mass membrane on a frame, open on both ends. If you put that in a null zone of a room and orient it correctly, it might break up the standing wave. Basically you've broken a large room up into two smaller rooms, at least partially, with a membrane that's acting as a soft wall that can move enough to take some energy out of the air motion in the null, and hopefully not create two smaller modes in the process. If the mass and stiffness are properly chosen it should take a lot of standing wave energy out of the room.

Edit. It doesn't really matter if it's a limp mass or a fiberous material of adequate impedance, or a solid material with perforations. So long as it offers up the appropriate impedance it'll rob energy out of the standing wave.

All absorbers must somehow reduce motion. There's no way to absorb pressure that I know of without creating a pressure differential that causes motion.

 

[Justdafactsmaam]

I'd put those diagonally across a corner, stacked up to the ceiling, so there's a triangular void behind them. If you can create a pressure change across a fiber panel by creating a void behind it, that's going to push a lot more motion through the fiber than putting the panel in a room null, where velocity is highest. The problem is, the velocity isn't very high in room nulls. I experimented with this using painter's plastic to create a large membrane. I wanted to see how much that membrane moved when pushed by the air in a null zone. It hardly moves at all. And this was at a very powerful null zone at the mouth of my bass horn, which causes a huge suck out in the response of my horn at 85 Hz. My attempts to absorb down the problem by using fiber resistance at the mouth of the horn were yielding very little result. The plastic sheet explained why. I could feel it vibrating even when the sound mostly disappeared because of the null, so I knew the air was moving in and out of the horn. But the plastic barely had to move at all to accommodate the motion. So, to put any brakes on it you need a solid membrane with adequate mass. It ends up more of a limp mass membrane problem. I could see a velocity absorber as being a very large limp mass membrane on a frame, open on both ends. If you put that in a null zone of a room and orient it correctly, it might break up the standing wave. Basically you've broken a large room up into two smaller rooms, at least partially, with a membrane that's acting as a soft wall that can move enough to take some energy out of the air motion in the null, and hopefully not create two smaller modes in the process. If the mass and stiffness are properly chosen it should take a lot of standing wave energy out of the room.

Edit. It doesn't really matter if it's a limp mass or a fiberous material of adequate impedance, or a solid material with perforations. So long as it offers up the appropriate impedance it'll rob energy out of the standing wave.

All absorbers must somehow reduce motion. There's no way to absorb pressure that I know of without creating a pressure differential that causes motion.

Pressure needs to be released. Or drained/leaked. Or turned into velocity and then absorbed. That’s what diaphragmatic bass traps do. And that’s why they work best at room boundaries

 

[Mr. Haelscheir]

While broadband bass traps like from GIK Acoustics may be reasonably "economical" for those who cannot invest the time in DIY, I suppose for tuned bass traps, unless there is a company that offers custom tunings, it may be best to go the DIY route to target exact frequencies.

 

[olieb]

I experimented with this using painter's plastic to create a large membrane. I wanted to see how much that membrane moved when pushed by the air in a null zone. It hardly moves at all. And this was at a very powerful null zone at the mouth of my bass horn, which causes a huge suck out in the response of my horn at 85 Hz. My attempts to absorb down the problem by using fiber resistance at the mouth of the horn were yielding very little result.

Well, anything else would have been close to miraculous.
To absorb sound below 100Hz you need quite a bit of fibrous material. How thick were your sheets/panels?

Porous Absorber Calculator

And even if you achieve considerate absorption this will only produce a (global) attenuation of sound, because you do this at the mouth of a horn (more or less one point in space) and absorption is not selective in frequency. The modes in the room will stay the same (that is valid for the horn cavity, too). Only small changes might happen.
And about the plastic sheet moving only a little bit. That is just the way sound is. That is not different with a "big" volume in the corner where air is pressed into by the bass wave. A 104dBSPL signal (already quite loud) has a pressure peak of 28Pa, that is 1/3600 of the air pressure. If you have a corner volume of 200 liters, then only about 50cm3 will be moving in and out. That is all the movement for the absorption to work upon.

 

[Tim Link]

Well, anything else would have been close to miraculous.
To absorb sound below 100Hz you need quite a bit of fibrous material. How thick were your sheets/panels?

Porous Absorber Calculator

And even if you achieve considerate absorption this will only produce a (global) attenuation of sound, because you do this at the mouth of a horn (more or less one point in space) and absorption is not selective in frequency. The modes in the room will stay the same (that is valid for the horn cavity, too). Only small changes might happen.
And about the plastic sheet moving only a little bit. That is just the way sound is. That is not different with a "big" volume in the corner where air is pressed into by the bass wave. A 104dBSPL signal (already quite loud) has a pressure peak of 28Pa, that is 1/3600 of the air pressure. If you have a corner volume of 200 liters, then only about 50cm3 will be moving in and out. That is all the movement for the absorption to work upon.

Yup. In the mouth of the horn it comes down to impedance, and any amount of broadband impedance that works is going to muffle the upper frequencies. The impedance that can work is impedance from the sides, which means extending the mouth a lot, with either thick absorption, or best just more horn. As I was playing with this stuff I could hear Darth Vader in my mind; "There is no escape." I did try extending the mouth and it just changed the problem frequency. It takes more to properly couple those horns to the corners than I have space.

Besides global attenuation, the stuffing of the mouth with all kinds of materials of various density and thickness mostly served to just shift the problem frequency slightly, which could be partially useful if some mouths were stuffed and others not. In the end I gave up. The notch is pretty narrow so it's not horrible. Long term ideas are to relegate these horns to below 80Hz duty and build a new module, which means my system will become a 4 way.

 

[MRC01]

While broadband bass traps like from GIK Acoustics may be reasonably "economical" for those who cannot invest the time in DIY, I suppose for tuned bass traps, unless there is a company that offers custom tunings, it may be best to go the DIY route to target exact frequencies.

I agree, that's what I did (link). You really need big ones if you want it to make a difference, and that's where the savings doing it yourself is in the thousands of dollars.

 

[Tim Link]

While broadband bass traps like from GIK Acoustics may be reasonably "economical" for those who cannot invest the time in DIY, I suppose for tuned bass traps, unless there is a company that offers custom tunings, it may be best to go the DIY route to target exact frequencies.

It'd be nice to find some repeatable data to target frequencies with reliability. Maybe it's out there, but I've heard it can be frustrating. Maybe a tune-able membrane trap where you can play with tension and mass loading of the diaphragm.

 

[Tim Link]

I agree, that's what I did (link). You really need big ones if you want it to make a difference, and that's where the savings doing it yourself is in the thousands of dollars.

Indeed, if you want to actually change the frequency response considerably it takes a lot of big bass trap.

 

[Justdafactsmaam]

While broadband bass traps like from GIK Acoustics may be reasonably "economical" for those who cannot invest the time in DIY, I suppose for tuned bass traps, unless there is a company that offers custom tunings, it may be best to go the DIY route to target exact frequencies.

RealAcoustix makes custom tuned diaphragmatic bass traps

 

[olieb]

It'd be nice to find some repeatable data to target frequencies with reliability.

I don't know about membrane oscillators but for Helmholtz there is this.

Helmholtz Resonator Calculator

Find the resonant frequency of an acoustic cavity with out Helmholtz resonator calculator!

www.omnicalculator.com

I found this in @sarumbears signature thread that is very interesting for optimizing room acoustics with Helmholtz resonators.

My acoustically treated Home Theatre (updated)

Most recent update is here. I have been posting about benefits of acoustic treatment of a room, especially in larger rooms. I believe that if done right it is a better, and if you have basic carpentry skills, a much cheaper option then using multiple subwoofer setups. I have the degree to do it...

www.audiosciencereview.com

 

[Andysu]

I was wondering what ASR thinks of this video.

TLDR:
- Bass modes are formed by reflections from walls. Not the corners.
- What % of the wall is treated by the corners? Answer: a very small surface area.
- Corner traps are a marketing con designed to sell traps to people to put in corners, where there is usually nothing anyway

Sadly, no measurements.

click n' chalk and blackboard bait thumbnail again
doesn't he ever do an actual practical video showing a with microphones the before and after , sort did one myself

 

[Wesseling Audio Lab]

I designed, built and installed the treatment in both of the rooms in the attachments.
The home owner(room with diffusers) had talked to Foley on the phone before I got involved. He said foley was very cocky and after one conversation the home owner was not interested.

The room Foley did with for the guy with the steel outbuilding was a rip off. He over sold his product and man hours..

 

[Wesseling Audio Lab]

For room modes
Complete surface coverage with 3-4 inch Rockboard 40/60 with as much room size reduction that is acceptable to each individual.
This is how my listening room is being done. New framed walls so nothing attached inside the room..

For rockboard thickness, spacing and air gap decisions I use(for the most part) the studies in "The master handbook of Acoustic" F. Alton Everest
Also, Ethan Winer and his Forum

FWIW, for an experiment I was interested in reducing or removing floor bounce between speaker and listening position. A measured a major reduction(near ilimination) with 3" rockboard 60, 2x7 wide panels. 8 of them piled on the ground. Mic at 2M

 

[Batterhead]

New to this forum, but have been going back down the rabbit hole re: Corner trapping, wall trapping, etc. Wanted to share a few videos I think may explain some things (and some of which may sound controversial ie corner traps might be debunked). Would love to hear feedback on them if you have time!

In case you all haven't seen, they explain better than I can. I'm a fan of Master handbook of acoustics and Ethan Winer as well, and as a non-acoustician, please take this as informational only "what about the bass" has some interesting bits that make me want to trash my diy corner traps lol.

 

[jlo]

The topic is very interesting and I mostly agree with this "corner trap nonsense"
Maybe I'm a bit late to explain why.

Imagine a room in infinite space and cut out all corners (ie 1m in x,y,z)
So sound arriving at corners will completely disappear in the empty space (in other terms, will completely be absorbed)
But all walls, floor and ceiling are still here and so are main modes (especially axial modes).
So cuting out corners will not suppress or even decrease those modes at listening position
Theorically, at first glance, total absorption in corners will not change room modes effects at listening position

But when you measure rooms before and after absorption in corners, you notice some (generally low) effect at listening position. Why ?
I think that, because this absorption is not total, the reflected (not-absorbed) sound is diffused in many directions and has some influence at listening position.

 

[Chromatischism]

Obviously, sound energy escapes a room with open corners.

 

[MRC01]

... Imagine a room in infinite space and cut out all corners (ie 1m in x,y,z)
So sound arriving at corners will completely disappear in the empty space (in other terms, will completely be absorbed)
But all walls, floor and ceiling are still here and so are main modes (especially axial modes).
So cuting out corners will not suppress or even decrease those modes at listening position
...

I find this thought experiment unconvincing. If you stand in the corner of a room you can hear a lot more bass. It's not subtle but obvious. And it's real, you can measure it on a mic. Cut out all the corners and that effect likely disappears. All that extra bass energy you were hearing is escaping instead of being reflected back into the room. This leads one to question: how much do the corners of the room support the bass modes defined by the wall to wall distances?

That said, what's intuitive to one person is not to another. We don't need to trust thought experiments. It's not difficult to measure.

 

[Chromatischism]

That said, what's intuitive to one person is not to another. We don't need to trust thought experiments. It's not difficult to measure.

And it's calculable and visualizable:

amroc - THE Room Mode Calculator

HTML5 room mode calculator. Read and hear the tune of axial, tangential and oblique modes. Scientific sources included.

amcoustics.com

 

[Flaesh]

A real experiment can be carried out, easier and better in a cuboidal room. Identify the modes and compare the influence of axial (especially longitudinal) and tangential.
Even calculators like Roomsim and Amroc symbolically represent modes with lines of different heights))

 

[krabapple]

Ok, me also. Like a metamaterial basically.

It's comically unclear to me from that paper what the absorber actually is/looks like. A cylinder made of (?) with a silicon membrane on top and a rigid bottom? Partly filled with AC?