Grasshopper

algorithmic modeling for Rhino

Hi!

This is very interesting, but I have some questions about the applicability / limitations of your software. What it can and what it can not do. I am an architect, used to formfinding but am not acoustically trained in any way.

I want to optimize the shape of a sound protective fence next to a rail way track to make it as small, open and geometrically interesting as possible. While still being able to prove to authorities that it will block enough noise.

Could this software help me with that? I understand the shit in shit out dilemma, and I would have to ask an acoustics consultant to verify my simulation setup. But I would rather not propose this solution to him before I know if it could work...

Excuse my acoustic ignorance but my immediate concerns about the software is how it deals with different wavelengths, especially low Hz waves that are deflected differently compared to high pitch ones. How could a raytracing algorithm deal with this?

Thanks!

Ludvig

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Hi. Thank  you for asking this very important question. I have some bad news, some good news, some bad news, some more good news, and then more bad news for you...

In general, the geometrical acoustics algorithms are ill-suited to calculating the transmission of a partially open screen. The sampling of rays required, and indeed the lack of actual wave-based movement of sound intensity makes them only suitable for large scale studies of spaces making the following assumptions, among others:

- The primary behavior of sound can be described by rays

- Diffraction lends only low significance effects

- Few, preferably no obstructions between the source and reciever

With regard to sound hitting a partially open screen, a variety of behaviors come into play. Sound moves in and out and around various points of a screen - meaning that rays can not describe the behavior of sound for such small delicate structures.

The good news is that some of the latest versions of Pachyderm also employ numerical methods. Try typing "Pachyderm_Numeric_Timedomain into the command prompt, and you'll get the controls for the Finite Volume Method. This method accounts for wave-based phenomena.

Now more bad news: The method does not have implemented an insertion loss calculation, so you would have to work in the source code to implement it, and it still does not have materials implemented (that last part may not be terribly important unless you intended to use porous sound absorptive materials).

So, in any case, I don't recommend using Pachyderm to determine the sound transmission of your design. Now for some more good news - you can do a rough calculation on a calculator, making a few assumptions, if you know the open area of your screen. Let's say that we assume your materials do not transmit at all (which they won't, but they will transmit far less than any opening in the screen). So let's assume you design a 50% open area screen. The transmission loss of the assembly, independent of octave band will be at most:

TL = 10*log10(0.5) = 3 dB

This means that the noise from your source will be 3 dB less on the quiet side of the screen than it will be on the railway side of the fence. Let's say that isn't enough... ok 20% open.

TL = 10*log10(0.2) = 7 dB

So now it is 7 dB less on the quiet side than it is on the rail side (it will probably be up to 3 dB louder at low frequencies, but this is a rough estimate).

So now the last bit of bad news - it is difficult, maybe impossible to get a strong amount of attenuation with a screen with open area. Even with a wall with no open area, the maximum attenuation will be 20 dBA. When you open it up, this will severely hamper the isolation of the screen. I hope this helps.

- Arthur

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