# Recent Posts

# Tuning Fork - Part 1

The very first episode in which we introduced Elmer was the Elastic Modes of a Metal Bar episode. In that episode we introduced Elmer by solving a linear elasticity eigenproblem, and mentioned how vibration is an integral part of acoustics, being vibrating bodies one of the principal causes of airborne sound radiation. In this new series of episode we will explore vibration further, integrating in it what we learned so far, and we will explore vibro-acoustic coupling with Elmer.

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# Project Refactor

This project has now quite a few articles and simulations going. Unfortunately, the way the Julia code repositories were set up is not very functional and is not scaling well to many repositories. For this reason, this site will undergo a period of refactoring.
The Status Quo At the moment there are few additional Julia code repositories that contain helper code, mainly for postprocessing and to create the plots in this website.

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# Frequency Responses

In the Home Studio - Part 3 and Home Studio - Part 4 episodes we referred to the results of our frequency sweep as frequency responses. In those episodes we calculated the steady state pressure disturbance in a room at different frequencies and sampled the field at a couple of locations so to be able to plot the steady state magnitude at those locations as a function of frequency. It is intuitive to refer to this result as a frequency response, in the sense that the field magnitude as a function of frequency gives us an understanding of how strong the steady state disturbance is at any given frequency in our study.

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# Home Studio - Part 4

In the Home Studio - Part 3 episode we simulated the steady state pressure field for a realistically shaped room governed by the linear wave equation. The boundary condition for the room was that of ideally rigid walls. We seen that the effect of such a boundary condition is an extremely uneven frequency response. This due to how sharp the resonances of the room are. In this episode we will introduce slightly more realistic boundary conditions and see what the effect on the solution is.

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# Home Studio - Part 3

In the Home Studio - Part 1 episode we gave a first look at the response of a realistically shaped room with rigid walls. We understood how to develop and run a model with an uniform velocity sphere source placed somewhere in the room. We run the model up to $400$ $\text{Hz}$ with the help of the convergence considerations outlined in the Dealing with Convergence Issues episode. In the Home Studio - Part 2 episode instead we used Elmer’s WaveSolver to compute the eigenmodes of the room.

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# Intro to meshio

In the previous episodes we manipulated some of the result fields from Elmer with meshio, a very nice Python package that allows us to do many useful operations with meshes and fields. However, in the episodes we did not focus into the details of how to use the package. Whilst this webiste is perhaps not the best place to do so, as the best place to understand how to use a package is, of course, its official documentation, it is useful to collect here few examples and remarks about using it with Julia, the programming language we are using for this project.

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# Home Studio - Part 2

In the Home Studio - Part 1 episode we computed the steady state field of a realistic, but still rigid walled, room. We put a source in the room, modelled as a flux boundary condition, and run the study at few different frequencies. The solutions that we found were reminiscent of modal patterns, which is expected as the low frequency response of a room is dominated by its resonances. However, that kind of study does not inform us on the actual resonance frequencies of the room, which are properties of great interest, as well as how the associated modal shapes (eigenmodes, or eigenfunctions) look like.

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# Interpreting Helmholtz Solver Solutions

In the previous episodes we obtained various numerical FEM solutions for Helmholtz problems. Although we discussed their meaning in the previous episodes it is useful to discuss that again more deeply in a single page, which can act as a quicker reference. This episode will focus on the Helmholtz solver solution fields, while other episodes will focus on the output of other solvers.
The Physical Meaning of a Solution Field The meaning of a solution field largely depends on the solver itself.

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# Rigid Walled Room Revisited - Part 3

In the Rigid Walled Room Revisited - Part 2 episode we reviewed the solution provided by the Elmer Wave Equation solver. In that study we solved for the eigenfrequencies and eigenmodes of a rectangular room by using a first order mesh. The results we found were very accurate already, with eigenfrequencies within $1$ $\text{Hz}$ from the exact value and eigenmodes accurate within $1.4\text{%}$. However, we seen in the Mesh Order and Accuracy episode that we can significantly increase the accuracy of results by acting on the mesh fineness and, more effectively, the order.

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# Rigid Walled Room Revisited - Part 2

In the Rigid Walled Room Revisited - Part 1 episode we seen how to setup an eigen system problem with Elmer by making use of the Wave Equation solver. In this episode we will review the simulation results and check the agreement with the analytical solutions in the Acoustic Modes of a Rectangular Room episode.
Project Files All the files used for this project are available at the repositories below:

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# Rigid Walled Room Revisited - Part 1

We covered the rigid walled rectangular room previously in the Rigid Walled Room episode. In that episode we solved for the steady state field in a rectangular rigid walled room as driven by a source placed somewhere in the room. This allowed us to see how the steady state field is sustained by a modal superposition, the purer the lower the driving frequency (assuming that this driving frequency matched an eigenfrequency of the room).

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# Mesh Order and Accuracy

In the Dealing with Convergence Issues we made use of first order meshes in order to ease convergence of our simulation at high frequency. However, the accuracy of FEM solutions is higher the higher the order of the mesh, so doing so will come at the expenses of accuracy. Still, we argued that the accuracy is mostly controlled by the mesh size, so as long as we have more than ten elements per wavelength the solution should be reasonably accurate.

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# Dealing with Convergence Issues

In the Home Studio - Part 1 episode we faced convergence issues when dealing with the highest driving frequency for our room ($400$ $\text{Hz}$). This meant that we could not quite trust the solution, and so we discarded it. In this episode we will look at what to do in this cases, and how to reach convergence. Rather that dealing with the issues in abstract and general terms (which would require writing an entire book about it) we will use the Home Studio - Part 1 episode to introduce the problem and figure out how to deal with it practically.

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# Home Studio - Part 1

In the Rigid Walled Room episode we seen how to model a rectangular room with rigid walls. We driven the room at the modal frequencies and compared the solution field with the theoretical modal shapes, finding that the results matched single modal shapes real well until, at a frequency high enough, the contribution of multiple modes (in addition to that related to the driving modal frequency) became important. In this episode we will look at making the model more realistic.

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# Intro to ParaView

In the previous episodes we often made use of the ParaView postprocessor to visualise our solution field from the Elmer solver. ParaView can do all sorts of cool visualisations and animations, as well as providing the way of doing quantitative analysis. It is by far the best option to visualise and postprocess results from Elmer. It also allows to export data in various formats, such as CSV, that allow us to do any additional kind of postprocessing or verification, by either using Julia, Python or any other language, or even spreadsheet software if you fancy that (for whatever reason…).

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# Rigid Walled Room

In the Acoustic Modes of a Rectangular Room episode we explored the analytical model of a rigid walled room with some Julia code. We focused on finding the resonance frequencies (or eigenfrequencies) of the room and calculating the related modal patterns (eigenfunctions). Now that, thanks to The Pulsating Sphere episode, we know how to setup Helmholtz problems with Elmer we can approach the problem with the FEM method. In this episode we will solve for the modal superposition in a rectangular rigid walled room and use the results from the Acoustic Modes of a Rectangular Room episode to check the accuracy.

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# Elmer Model and Solver Parameters

In the previous episodes we solved a few equations with Elmer. We did some choices when we setup the solver parameters. What those parameters do, and how should we set them? This is perhaps the trickiest part in FEM (beside making the mesh right). In this episode we will step back and look at those solver options more closely. This post is really not meant to be an exhaustive explanation. For that, refer to the Elmer documentation.

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# The Pulsating Sphere

In this episode we will build a model of a pulsating sphere source. The pulsating sphere source is an ideal source which forms the base for the development of point sources. In essence, a point source is a pulsating sphere in the limit of $a$, the radius of the sphere, approaching $0$. For this reason, although abstract, the pulsating sphere is a very powerful theoretical tool that enables the study of point sources which in turn, through integration and wave propagation principles, enable to study of any arbitrary acoustic field source.

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# Refining the Metal Bar Model

In the last episode we seen that our model solution for the modes of vibration of a metal bar wasn’t looking particularly good for the highest mode. In fact, when we clipped into the bar, we seen few bubbles of discontinuity in the displacement field that are not expected for linearly elastic homogeneous and isotropic bodies. To understand this we have to step back a little and think about FEM some more.

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# Elastic Modes of a Metal Bar

In the last episode we examined the analytical solution of the acoustic modes of a rectangular room and we are now ready to take steps into moving in the world of FEM modelling. Elmer is a powerful package, but it is not extremely user friendly. So, it is best to have a gentle introduction to it first before dwelling into the intricacies of FEM modelling of acoustic fields. One of the simplest problems to solve with Elmer is that of the elastic vibration modes of solids.

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# Acoustic Modes of a Rectangular Room

In this episode we will look at how to make a simple Julia model of one of the simplest systems in acoustics, a rectangular room with rigid walls, assuming adiabatic wave propagation. Even if this system is among the simplest in acoustics it is actually already very complicated. As such, we will focus only on the modes, one part of the problem, without attempting impulse response simulation or other fancy things like that, for now.

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# The FEM Pipeline

In the previous article an overview of the open-source ecosystem for scientific and technical computing was presented. In this article the focus will be on FEM. We will first look at how we build a FEM problem, and then select among the packages we listed the software that is best suited for implementing FEM studies.
The Basic Pipeline The diagram above shows the process of creating a FEM study in its simplest form.

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# FEM in a Nutshell

In the previous episodes we seen that physical systems are described by PDEs. However, PDEs are seldom solved analytically, unless for certain simple cases, the reason being that finding the solution of a PDE on a complex, realistic, domain with realistic boundary and initial conditions is extremely complicated, or downright impossible, even when the PDE is known to have analytical solutions from theoretical considerations. For this reason, numerical methods have arisen in order to handle these problems.

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# What is Acoustic Modelling

Welcome to the first actual episode of the series about acoustic modelling with open-source software. We will first try to understand what modelling acoustics means. In reality it doesn’t mean just one thing, as many phenomena of acoustic wave production and propagation can be modelled and simulated in various different ways, with higher or lower degree of accuracy. However, the core of the modelling problem resides in partial differential equations. This post will be a very, very, brief, intuitive and nonrigorous introduction to the topic, mainly to give context to those that are not accustomed to the concept.

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# Setting Up an Ubuntu Modelling Environment

In the first episode of this series the open-source ecosystem for the modelling of acoustics was covered. As we will see in the next episodes, we will make extensive use of FEM, at least initially, to model acoustic phenomena. This episode focus on building a simple simulation workstation based around Elmer, a powerful multiphysics FEM suite with excellent acoustics capabilities.
Why an Ubuntu Based Workstation? Ubuntu Linux is a very convenient operating system for this purpose as all the needed packages are straightforward to install, especially Elmer and Salome, which can be quite complicated to get up and running in other distributions.

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# Modelling Acoustics With Open-Source Software

One of the backbones of the scientific paradigms is the repeatability of results. Ideally, all software used for scientific research should be open-source, so to allow complete repeatability of data analysis and models. And in fact, a lot of scientific software is indeed open-source, and a lot of this software can be used to model acoustic phenomena.
What follows below is a panoramic overview of the scientific open-source software ecosystem. This overview is far from comprehensive.

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