How do you determine the location of highest pressure ?
Thank you in advance !

G21

For modal problems, the peak pressure will be on the surfaces associated with the mode. In other words, if the mode is vertical, then the pressure peaks would be on the ceiling and floor, so it would not be effective to place the device on the walls. If the mode is a lengthwise axial mode, then the device would need to go on the front or back walls for maximum effect. Putting it on the side walls would not be very useful. If it is a tangential mode, then you could put it on any of the four surfaces involved. And for an oblique ode (although it's rather unlikely you'd need such a device for an oblique mode... ) you could put it on any of the six surfaces.

So once you identify the mode, you'll know where to put the device.

Ok.... So, i can put anywhere on the back or front wall a bass trap. I mean close to the ceiling or close to the floor, it will work for this 55Hz main modal node located between these two walls ?

Thank you.

OK, I think I've discovered the discrepancy. Your results are off by a factor of 10 (Slot width) compared to all other calculators.

Multi-layer Absorber Calculator

Either you've entered the data wrong, or Soundflow has an error in their formula.

Yes, you're dealing with axial modes. As I showed in my pictures, I placed the traps at floor level (Primarily to avoid reflections from the traps at typical microphone heights, say vocals), but you could put them up at the ceiling-wall corner too. The axial modes will be strongest over the entire wall surface.

Soundflow is well tested, by me and others. But I guess a free tool by one individual cannot contain bugs ...

I did say "compared to all other calculators."

Chris Whealy, John Sayers, mh-audio, all come very close in agreement in putting the centre frequency between 145 and 165 Hz.

Soundflow takes more parameters into account as far as I know.

DPower, I'm not familiar with these calculators since I use Soundflow, but can you enter the density of the material into the equation?

Any calculator that doesn't allow you to enter the density of the wool (or even flow resistivity in some extreme cases) is bound to be off.

Density while not completely irrelevant, cannot account for a centre Frequency drop of 100Hz.

The basic Helmholtz formula 2160*sqrt(r/((d*1.2*D)*(r+w))) (Cox & D'Antanio Acoustic Absorbers and Diffusers: Theory, Design and Application) works out so (Metric converted to Imperial):

F=2160*SQRT(.2362/((.7480*1.2*11.8110)*(3.5433+.3262)))

Which gives us a basic centre frequency of 163.9Hz. Adding absorption to that will lower the centre frequency slightly, and lower the q, which lines up with the results from Chris Whealy's and the Multi-Layer Absorber Calculator (149 & 155Hz respectively).

Now, I'm not saying that Soundflow's formulas are incorrect. It's more likely to be user error, given that the error appears to be a factor of 10, which could be something as simple as a mm/cm mix-up somewhere.

Please enlighten me …

The fact that SF takes density into account is naturally not a small difference. It will completely change the behaviour of the system. Any calculator not including density will give other results compared to Soundflow. This is why I use Soundflow in the first place!

SoundFlow vs real meassurements:

helmholz not very popular?

Jens, I'm sure you are familiar with Occam's razor.

In this case we have an occurrence where there is a discrepancy between your results and the results of all other calculators. As I have demonstrated, this discrepancy appears to be a factor of 10. Your explanation for that discrepancy is that the density of the material is responsible. My explanation is that a decimal point has been shifted. Which is more likely to result in a factor ten discrepancy?

By all means post the math that shows that the density of the material can shift the centre frequency of a system by 100Hz, and we can throw out all the textbooks on acoustics. So, no, you can please enlighten me.

Red graph is if setting density of wool to 0:



Do you belive me now?

Soundflow also always matches real world measurements. Something I noticed was a problem before I got my hands om SF.

Well there we see nicely demonstrated the 100Hz centre frequency drop, so yes, thank you for that. It's very curious. Are Soundflow's calculations accessible? I'd like to see the calculation for density and how it relates.

Hello....

I did some changes. I calculate what to do in order to lower the 50. I fixed some slats in the front of my front corners bass traps and i started to seal the boundaries. By the end of the day the 50 was not dead but lowered a lot. But strangely, the 100 appears. And the room started to show a light reverb when talking loudly. Nice. The room seems bigger.

Tomorrow i will attach some REW screenshots.

In the meantime, can you tell me if this is normal to see the 100, 150, 200, 250, appearing. Even if we are talking about 10dB.

Thanks in advance.

Can you explain the HH array a little more? When you say the slats are 16 or 19 mm thick, do you mean the wood is 19 mm thick? You mention holes as well, is there also a perforated panel used in the construction?

HH array is made of slats or slots. You can calculate one using Acoustic Modelling - Home Page with the multi layer calculator. Select slot layer.

Inputs are : slot depth (thickness) ; slot width (space between) and slot spacing (the distance between 2 slots : 1/2 slot width + slot spacing)

I think this thread showed that you can't relate on AcousticModelling.com for such complex designs.

No, it didn't. A chart is not proof that it isn't simply a decimal error, it just points us in the direction where such an error may lay. I'm still waiting on the math.

Oh, and what this thread did show if Soundflow is correct in their equations, is that Helmholtz traps are easier to design and build, but only if you have Soundflow, as all other calculations will result in your design being off by more than 2 octaves.

Exactly what I said in this post


Regarding the Maths behind Soundflow. Ask them, and please, share the answer with us.

Regarding the differences between Acoustic Modelling and Soundflow:
You will get different results even with very simple porous absorber designs. So it's not ONLY with Helmholtz resonators.

That difference is even bigger with complex designs (Helmholtz resonator in our case).

The more it gets complex, the bigger the difference will be.

Acoustic Modelling is still a fantastic tool to get familiar with acoustic modelling. It's a great starting point. But I don't know whether professional have been using and relating on Soundflow or AcousticModelling for years for their designs...


Cheers,

J.

Correct but it gives a tendancy if not acurate.

I m going to download the soundflow trial version. I hope my room will be finished when it expires.

OK, well I've downloaded Soundflow (finally got the log in info sent to me), and I can't mach your results with the given parameters, even with entering density.

I would like to point out that in the manual, it states the following:

When the FLOW RESISTIVITY is specified, the ABSORBER DENSITY can be
entered optionally. If the latter value is given, additional effects due to the
conduction of sound by the vibrating structure are included in the result.
However, this should account for a small correction only, otherwise the plate
or perforated panel type should be used for the material.

Bolded for emphasis.

A center frequency drop of 100Hz is not what I would consider a slight correction. However, I cannot replicate this. If and when I can, I will send AFMG support an email.

I re-created this design with Soundflow and got the same results. Are you sure you enter ALL infos, specially the ones related to the perf. panels?

edit: post a screen shot here and let's see where the difference is

19mm thick Perforated Panel
Hole shape: Slit
Hole Dimension: 6.00
Porosity: 6.67 (based on 90mm spacing)
Flow resistivity in Hole: 0

Air 5mm
Absorber 300mm
Flow resistivity 8.8 kPa*s/m2
Absorber density varied between 0 and 48Kg/m3

Rigid Backing

No air gap.

Velocity based vs. pressure based absorbers

...then scroll down (but read!) until here:

Velocity based vs. pressure based absorbers

Worth reading the whole thread as well like I suggested it in my previous post here... We already discussed about it.

Also with no air gap, the results don't match. It shifts the centre frequency down as density is applied but the absorption coefficient remains below 0.7.

Screenshot please?

OK, got one that's close with a density of 30kg/m3. Close enough to show the centre freq shift well enough to ask AFMG about it.