I know ...




Yes, I chose to model a 70 mm panel for the comparison since I feared that you once again would choose to reject any simulation made by SoundFlow:
Here are some actual measurement of the same panel (3 x 3,5 meters, 70 mm wool with different slat configurations over) made in a reverberation chamber in accordance with ISO 354:


Yes, now we can actually compare apples to apples. The simulation in SoundFlow is set up like the actual measurement (and the curve is deliberately set to 1/3 oct resolution for easier comparison), and although the SF simulation is not spot on, you have to admit that it comes pretty close to the measured values, don´t you agree?





This was my last post in this thread. The climate of the discussion does not fuel the lust for sharing.

The interesting question remains:

What is the effective area of a HH absorber ?
The type with just one opening, not the type with multiple perforations (here the answer seems trivial).

Is it just the opening, or the entire device ?
In other words, is a HH capable of treating a seemingly larger area then it occupies ?

I have no clue, low frequencies behave in a very peculiar manner...

One thing that affects HH effective area is Q. The higher the Q, the higher the appearant sabins.

Jens I don't know how many times I have repeated this, but I will try again.
I absolutely believe in the usefulness of HH when applied intelligently and vigorously. Those studio pics are again beautiful and one just has to assume sound great. You might remember the high praise I heaped on the response of your tiny HH only room many years ago. It was a lot of treatment, a lot, but the response was well nigh perfect if I remember correctly.

As a passionate advocate and user of the technique, a Beta for SoundFlow, which is clearly a premium app, you are the best person around here for us to ask questions about HH.

You presented a fabulous shaped graph to make the point that HH can be broad and safe and easily done. If you don't want to share the details, for commercial reasons or indeed any reason, it's your design, please just say so.
But if you really do have a lust for sharing, jeez just do it. i.e. Your previous graph compared to simple fibre both 14" deep, please.

If you used several different resonances to achieve the wide band, don't worry, obviously 'easy' no longer applies, but it remains an excellent response curve and of remains of great interest to me and I suspect all readers. If the gross absorption of fibre is greater, and the HH effect is partly adding a little LF absorption by resonance and filtering out HF by blocking, no matter, it still is an admirable curve and HH remains valid and realistic.

Similarly a view, a shot, from anybody on the area thing would be warmly welcomed IMO. Hypothesis , wild guesses, whatever. We're jamming here.
Thank you Yannick. I have seen perf panels described as like multiple individual 'bottles'. But again what area? I suspect Opus has a finger on the truth there, higher Q, still damped enough, would mean more absorption which would make the hole sizes 'larger'.
More plse guys, take a shot.
DD

Yes, but
Helmholtz Calculator

only goes to 100% absorbtion, not over.
So, if we model a Helmholtz device using this tool, what will it behave like ?
Say a box, 60cm width, 20 cm deep, 120cm high.
With one opening in it, 3cm wide, 110cm high.

Will the box behave as:

a. an absorber of 3x110 cm
b. an absorber of 60x120cm
c. a virtual equivalent absorber of eg. 120x200 cm (if the Q is high enough)

???

In other words, a back wall full of helmholtz devices, will it outperform a 50cm deep full backwall rockwool treatment ?

In even other words, will one huge helmholtz device on my backwall, with just the one slat of just mere square centimeters be able to absorb a massive 24Hz standing wave ?
It was my understanding, for this we would need a massive surface area.

But a very low tuned Helmholtz actually has very little open area !

Hence the question Dan was asking - I believe.

You got it Yannick, although when I ask a question, particularly of experts or specialists, I assume the role of a dumb blank sheet....;-)

Opus has a link to an old thread elsewhere discussing that. PM me or him if you want it or perhaps he will post it here. It is quite a rich vein IMO, but let's continue to do our own thing here irrespective.

jcjr, Choosing between those corner trap variations, I suggest priority be given depending on which modes are to be addressed. The StudiotipsSuperSuperChunk and Square address L and W kinda equally but they present a different angle for incidence.
I would have to look up the books to have a view on angles of incidence, so I suggest again that is one best referred to Jens and SoundFlow.
Taking a shot I would go for the Square.

DD

Well I didn't want to link that thread because it was full of off-topic-ness, the real information is just buried in it. But I did get a reference from it to a book that addresses the topic directly.
Noise Reduction Analysis - K. Uno Ingard, Uno Ingard - Google Books

I have no idea if one small port can mitigate an entire axial mode for the whole wall. I would assume there are viscous losses at play.

Old thread, but after a quick read I had a thought that might contribute to the discussion. Aside from design and build difficulty, we should also consider the time it takes for bass frequencies to excite the resonant panel/membrane/chamber. While resonant bass traps will effectively absorb bass over time, they don't respond to bass transients at all. Since broadband bass absorption is easier, more affordable, and absorbs both transient and extended sound sources, it makes a lot more sense from a small studio perspective. Helmholz will certainly handle low frequency room modes, but probably won't be of much help to a small studio mix engineer struggling get the kick and bass guitar working together.

Also;

One can flip your argument: by using only broadband absorbers (even if fairly deep); there´s a big risk of over-damping the mids and highs, while not absorbing enough low frequency energy.

My understanding is that with these traps the size of the cavity compared to the amount of dampening has a pretty big effect on both the Q, so in the case of ported speakers, which are generally rather small, the timing isn't as much of a concern, but I've definitely heard some very, very laggy (cheap) ported speakers before. When you put a lot of dampening in the chamber, I suppose that could reduce the response time by decreasing the air volume that's being pushed around, but at least in the case of a membrane/panel absorber, wouldn't there also be an increase in air pressure? That of course would improve the absorption, but wouldn't that also require more sound pressure to excite the resonator? I honestly have no idea how that would affect the amount of time required for the trap to respond, but it seems like it would.

As for overdamping the highs and mids, your point is well taken. Most of the really good commercial corner traps I've seen lately, like some of what GIK and RealTraps are making, have reflective and/or diffusive surfaces to mitigate that potential problem, but I suppose whether or not it's really a problem is extremely situational. Lots of folks with really tiny rooms like mine are often thought to be better off deadening just about everything if keeping any liveness means not controlling the bass, but there are certainly other opinions. Someone else mentioned that getting the decay times at the different frequencies as close as possible is ideal, and a nearly dead space might be the only way to do it.

However, helmholz or membrane traps may more easily pass the wife test than big chunks of fiberglass in the corners :P

Thanks for the informative response, I love to learn.

Spot on... As a company we also sell pressure based absorption , but in a small room they are generally used in conjunction with broadband. And like you correctly pointed out there are ways to design broadband to retain/scatter/diffuse a lot of the upper frequencies.
https://www.gearslutz.com/board/7597560-post146.html
FlexRange Technology
I will point out though that there are a lot of ways to skin a cat and Jen does some nice skinning himself.

Well, when you look at how ineffective these shallow porous based "basstraps" from GIK, Realtraps, Acoustimac, etc. are, the total result will never be great. They are basically absorbents with some (and very low) effectiveness in the bass area. If one is using porous material for bass trapping one need understand that it takes serious size. Much more then what these commercial companies are telling you. DIY is the only viable route if porous material is the choice.

If you are looking to absorb at 40hz then something like your Soffit (broadband) will work (lab tested) or pressure base (Scopus) as I pointed out. I can't speak for all companies but we feel pretty confident from our lab and room testing that our other bass traps do a great job below 100hz.

Testing below a certain frequency means nothing in these labs. That's marketing. I saw your deleted post: "heh I think our customer feed back from actual customers keeps me feeling pretty confident about what we produce."
Customer feedback! Lol. They don't have anything to compare to other mostly untreated rooms, extremely thin absorption or egg cartons. My mum is happy with her Sony ministereo but that doesn't mean there isn't something better out there.

Oh by the way, I'm one your former customers.

I never said there was nothing out there better, which is the reason we also have a pressure based line. I totally respect how you moved forward with your room. It seems to be working for you which is great.

Jens, I suggested to a fellow looking for diffusors to use your Optiffusors on the back wall (distance about 2 m).

Above and below the diffusors to have sealed compartments, 22-25 cm deep filled with glass wool (Flexibatts or Isover Piano), as the Optiffusors are 22 cm deep. He would then have the alternative to put slats in front of the wool and get a broadband HH in the bass / lower mid region and reflective above, -instead of purely resistive absorbers.

Suggested slats for esthetics were pine in standard dim 70x28 mm. These can be stained for a better look than painted MDF and no paint clogging up the open areas. They have a corner radius of about 3 mm, for good or worse considering “mouth correction”. Would it be possible to get a diagram from Soundflow for 70x28 and 22 cm glass wool behind? -To show the difference compared to the KTH diagram above with slats 70x16 mm in front of 70 mm glass wool. I guess 3 or 4 mm gap between the slats would be suitable (?). (Back wall is 375 x 218 cm, with a 70 cm door 1/3 in from corner, both corners are Superchunks with the same glass wool at 45 degrees, also compartmentalized. Room length if I remember correctly 721 cm.)

Better to know what frequency range he would like to address, and I´ll suggest a suitable gap between the slats (or should I assume that he would like to address the second length mode (1st is a bit too low to treat with HH array if not very deep) of about 48 Hz?

The best thing would be to use the area behind the diffusers as a membrane absorber targeting the 1.0.0 mode (about 24 Hz based on the room dimension provided) and the area around them, to target the 2.0.0 mode it using HH arrays (slotted panel for instance).

Good idea. I’ll tell the guy to measure the empty room for lowest problematic length mode.

The room is still under construction and has not been measured in REW yet. It is a basement with concrete walls, partly below ground level. Front wall and side walls are concrete, length to back wall is (should be) 721 cm. Back wall is a newly erected double stud wall, 2 layers of gypsum towards listening room, 20 cm distance between zig-zagged studs. These 20 cm are filled but not packed with glass wool + 2 more layers of gypsum. In front of the stud walls are the sealed compartments for diffusor / glass wool / slat absorber. Behind the studded wall is a storage area and it should be close to 10 m between the shell of concrete walls.

LOL, that page brought me here.

I was re-reading this great thread...and remember I couldn't got the same result using Soundflow. Even using the same characteristics (density, GFR...and thickness). What did I miss?
I don't have Soundflow here (on this computer) but was only to get the same result with 20" (if I remember correctly).