Limp mass max weight and wave guide efficiency

[vesalaasanen]

Hi everyone,

First of all big thanks to John. There's some amazing stuff in this forum.

I have two questions:
1: Is there some maximum suggested kg/m2 for the membrane of a bass trap? From my understanding, the efficiency (or at least the bandwidth) of the membrane trap goes down when increasing the membrane mass instead of trap depth. I'm looking for trapping 39Hz with a 22cm deep trap with some waveguides inside. This means that I would need to use about 8-9kg/m2 membrane there. I'm planning to use 7kg/m2 MLV-like material and add weight to it while tuning. Is this effective trap at all because of the high mass of the membrane?

2: Is there any rule of thumb for what is the effect of waveguides in porous absorbers? Can we say that porous absorber with waveguides acts like 20% deeper porous absorber without waveguides or so?

Cheers from finally sunny Finland!
Vesa

[Soundman2020]

Hi Vesa, and Welcome!

1: Is there some maximum suggested kg/m2 for the membrane of a bass trap?

What type of bass trap are you talking about? There are several types. Bass traps are usually built to be broad-band, in order to cover as many issues as possible. Tuned traps are only needed for specific issues, on unusual occasions.

the efficiency (or at least the bandwidth) of the membrane trap goes down when increasing the membrane mass instead of trap depth.

For a membrane trap, both the mass AND the depth affect the tuning. The bandwidth is mostly affected by the damping.

I'm looking for trapping 39Hz with a 22cm deep trap with some waveguides inside.

Now I'm confused! Why would you put a waveguide inside a bass trap, and what type of waveguide could you build that would work at 33 Hz? It would have to be enormous! How would you relate the tuned frequency of the trap itself, to the tuning og the waveguide? And where would would the waveguide be "guiding" that "wave" to?

This means that I would need to use about 8-9kg/m2 membrane there. I'm planning to use 7kg/m2 MLV-like material and add weight to it while tuning. Is this effective trap at all because of the high mass of the membrane?

It would only work at 33 Hz if you specifically designed it to work at 33 Hz, by correctly choosing the mass and the depth to tune it to that frequency.

But there's another issue with very massive tuned resonators: high mass implies high inertia. That means that the devices will need a few cycles of the wave to "get going" (overcome the starting inertia), and it will also continue to resonate after the triggering tine has stopped, once again due to its inertia... which means that it will continue to emit the SAME tone it was suppose to absorb, for a certain time afterwards. Thus, such a device can actually extend the "ringing" of a mode, if it is too massive or insufficiently damped.

2: Is there any rule of thumb for what is the effect of waveguides in porous absorbers? Can we say that porous absorber with waveguides acts like 20% deeper porous absorber without waveguides or so?

I have no idea what type of "waveguide" you are talking about here, so I can't comment. If you show your plan for the waveguide, and provide the mathematical calculations that show how it works, then I could take a look at taht for you, but without knowing what you have in mind, there's nothing I can say. I've never even heard of waveguides being build inside porous absorbers, and have no idea how you would even go about doing that. I'm also not aware of any reason why a waveguide would make a porous absorber appear to be "20% deeper". What mechanism would cause such an effect? Porous absorbers certainly do change the way air behaves, but as far as I know, waveguides don't.

It would help if you would describe what the actual basic problem is here: What type of room are we talking about, and why o you think you need to trap 33 Hz only, without trapping anything else? If you show the actual data from your room (preferably in MDAT format), along with an accurate diagram of your room, showing the shape and dimensions, along with the locations of all major parts of the room, and existing treatment, then that would help a lot,


- Stuart -

[vesalaasanen]

Thank you very much for the reply! Some further clarifications:

But there's another issue with very massive tuned resonators: high mass implies high inertia. That means that the devices will need a few cycles of the wave to "get going" (overcome the starting inertia), and it will also continue to resonate after the triggering tine has stopped, once again due to its inertia... which means that it will continue to emit the SAME tone it was suppose to absorb, for a certain time afterwards. Thus, such a device can actually extend the "ringing" of a mode, if it is too massive or insufficiently damped.

This is exactly what I wanted to know. So do you have some suggestion for the membrane maximum weight per square meter?

I have no idea what type of "waveguide" you are talking about here, so I can't comment

Sorry for the lack of info. There's two examples in the following links:

https://mixedbymarcmozart.com/2015/12/1 ... -trapping/
https://www.youtube.com/watch?v=hhN5nGq7-14

My understanding is that compared to same size super chunk trap this design causes more random incidence and therefore causes the trap to be effective on lower frequencies. I'm just wondering that is there any prediction for this effect.

[Soundman2020]

Sorry for the lack of info. There's two examples in the following links:

Technically, those are not waveguides. They are poor attempts at imitating "acoustic hangers", which is a concept originally developed by Tom Hidley, decades ago, and later refined by others, including John Sayers. Hangers do work mostly on the principle of waveguides, but they also do more than that. And the imitation shown on one of those websites would NOT work as a waveguide: I'll let you guess which one...

The theoretical principle was partially analyzed by the BBC back in 1987, and if you can find it there's a white paper they published that almost gets into it, but not very well. That was called "Theoretical and Practical Aspects of the Functional Absorber Method od Arranging Sound Absorbing Panels". It was a start, but ended up going in the wrong direction.

However, much more recently, in 2009, a much better paper was presented at a convention of the Audio Engineering Society, outlining a study conducted at the Universidad de Vigo in Spain, where they went into a real recording studio that had been treated with Tom Hidley style hangers, and carefully analyzed how they actually work, by setting up multiple transducers at many points on and around the hangers. Their conclusion is that there are six different mechanisms acting at once with hangers, most of them at low level, with the most important being the waveguide effect. They even came up with a predictive model that gives a rough idea of where the peak absorption will be, subject to cetain conditions. That paper is titled "Sound field characterisation and absorption measurement of wideband absorbers", and was presented at the 126th convention of the AES.

So that's the theory: if you want to see how that works out in practice, here's a room that we are in the process of tuning right now, and a large part of that is being accomplished with hangers: http://www.johnlsayers.com/phpBB2/viewt ... 68&start=0

Your links do not show proper hangers, nor waveguides. If you look around the forum, you'll find many dozens of studios that DO use hangers, and as you can see, the results are very effective.

My understanding is that compared to same size super chunk trap this design causes more random incidence and therefore causes the trap to be effective on lower frequencies.

Not really, no. Take a look at the AES paper to get a better view of how they do work, when build properly. Superchunks are also very good. I often use a combination of Superchunks and hangers in rooms I design.


- Stuart -

[vesalaasanen]

Excellent! Thank you very much for the information! Time to put on the studying pants again