ACOUSTICS - Designing Acoustic Reflectors

Loading the Reflector Model

This tutorial looks at the analysis of acoustic reflectors. The design of reflectors is greatly assisted by being able to interactively manipulate objects in the model and automatically see the effect on reflected acoustic rays.

1. Open the  SimpleTheatre.eco  file from the Tutorial Files directory located in your main ECOTECT Install directory.
   
   There are three zones in this model, the default Outside zone, the Main Hall geometry and the Speaker above the stage.
   
   
2. To make sense of the acoustic rays, we want to view the model from front view. To do this select "Front" from the View menu or press the F7 function key.
   
   

Spraying Linked Acoustic Rays

These are called linked rays as they are 'linked' to the current sound source and the geometry of the enclosure.

1. Select the Linked Acoustic Rays... item from the Calculate menu.
   
   This will display the following dialog box.
   
   
   
   As we are looking in front view, the most important thing is to ensure that the Rotation setting is 90 degrees. This distributes rays around a vertical disk. Other than that, you can see that rays will be sprayed around a full 360 degrees in 1 degree increments and with 2 bounces.
   
   Once you are satisfied that the values in your dialog box are the same as shown above, select the OK button. A series of rays will be displayed within the theatre as shown below.
   
   
   
   Such a mess of sprayed rays isn't really useful in determining how specific reflectors are working. We really need to restrict rays to the objects we are interested in.

Tagging Acoustic Reflectors

1. In order to tag specific object as acoustic reflectors, we need to select them. Using drag selection, click and drag a selection rectangle as shown below.
   
   
   
   This will select only the ceiling objects. You will have to be a bit careful not to include objects that you don't want so you may need to try this several times.
   
   We next want to add in the main reflector above the stage. To add to the current selection set, simply hold down the Shift key and click-select the reflector indicated in the image below.
   
   
2. To tag the selected objects as acoustic reflectors, select the Assign As > Acoustic Reflector item in the Modify menu.
   
   
   
   You can also do this using the Tag Object(s) As group in the Rays & Particles panel.

Restricting Rays to Acoustic Reflectors

1. To restrict reflections to the recently tagged surfaces, select the Linked Acoustic Rays... item from the Calculate menu again to redisplay the Spray Rays dialog.
2. Ensure that the Only Test Acoustic Reflectors option is checked as shown below and select the OK button.
   
   
   
   The rays should now redraw themselves displaying only those that reflected off the tagged objects.
   
   
   
   Test rays are still generated over full 360 degree arc, however only rays that first hit reflectors will be tested further and drawn. This allows you to move the source around anywhere you want and automatically update the rays.
3. To demonstrate this, select the sound source above the stage by click-selecting it.
4. Make sure this is the only object selected and then press the Shift and the X key to nudge it towards the left.
   
   As you nudge the source you should see the reflections automatically update. you can use the X key by itself to move the source back towards the right.
   
   
   
   You should notice that the coverage the reflector provides of the lower audience and upper stalls area changes significantly as the source moves. This is an indication that its effect will be significantly different depending on where different speakers are on the stage.
   
   In most cases this will be undesirable as the reflector will serve mainly the lower audience when source is closest to the front of the stage and the upper rear stalls as the source moves away. When the source is closest to the front of the stage, the lower audience are the ones that least need the reflection as they are closest to the source itself.
   
   It is obvious therefore that the reflector needs some optimisation.
5. Just as you can update rays by moving the source, you can also update rays by moving the reflector.
   
   It is beyond the scope of this tutorial to describe the optimum location and angle of this reflector (mainly as this is used as an exercise in the on-line notes) however you may quickly investigate by selecting the main reflector and nudging it downwards. You can also rotate it using the interactive transformation techniques you learnt in the Modelling tutorials and then nudge it backwards a little.
   
   The animation immediately below shows the how this might look.
   
   
   
   At this stage you may want to investigate the effect of the reflector on rays in planes other than a vertical cross-section. You can do this quite simply by entering node mode and changing the direction of the source vector (dragging the arrow to point where you want it to point. This can be quite useful, however linked rays can only ever give you a sectional view of what is happening.
   
   If you want a more comprehensive view of the effects of the reflector in 3D, use the acoustic rays and particles features of ECOTECT.

Spraying Acoustic Particles

Whilst acoustic rays and particles provide a great deal of analysis potential, we are going to concentrate on the effects of the main reflector above the stage.

1. The first step is to make sure that only the main reflector is selected and to tag that as the only acoustic reflector.
   
   
2. Make sure that the Rays and Particles panel is visible in the right hand control panel section by clicking the tab.
   
   As mentioned earlier, you can assign objects as acoustic reflectors in the Rays & Particles panel as well as via the main menu. In this case, simply click the Reflector button.
   
   
3. The next step is to generate acoustic rays that travel towards the reflector. To do this, make sure the settings in the Generate Rays group are the same as those shown below and then hit the Generate Rays button.
   
   
4. Depending on you current display setting, the rays will then be displayed in the Model Canvas.
   
   To view them as animated particles, select the Animated Rays radio button in the Display Settings group and then press the Play button in the Animation group immediately below it. This should display something similar to the following:
   
   
   
   Depending on the speed of your computer you can speed up or slow down the animation by setting a different frame increment value in the Animation group. For more information on these setting as and the meaning of the particle colour coding, see the Rays & Particles topic in the The User Interface > Panels section of the main help file.
   
   What we are really interested in is the coverage of this reflector onto other objects in the scene.

Displaying Reflector Coverage

1. Select the Reflector Coverage radio button in the Display Settings group.
   
   
   
   This will display all the first-order reflection points off the specified surface, as shown below.
   
   
   
   This gives a good indication or which parts of the auditorium are receiving first-order reflections, however the random nature of the rays does not clearly show the relative distribution. We can overcome this by using evenly distributed rays.
2. To generate evenly distributed rays, change the Generate Rays settings to those shown immediately below.
   
   
   
   Note that for reflector coverage we only really need one reflection bounce. However, we are using a 1 degree angular increment which is quite fine so we don't want the overhead of 16 reflections. We may want the to see where the reflections go later on so we are going to choose 4 bounces.
   
   Also, as we are no longer targeting reflectors in the distribution method, we need to make sure the Only Test Reflector option is checked to restrict the first bounce to tagged reflectors.
   
   When you click the Generate Rays button, you will see the following distribution after a short calculation time.
   
   
   
   The even distribution of rays clearly shows any dispersal of the sound energy as it propagates. Each ray represents an equal angle so the dispersal of energy is directly equivalent to the density on incident ray over each surface. In this case the flat reflector produces a relatively even distribution.
   
   If you are interested you can now move the sound source or the reflector. As rays and particles take some time to generate you will have to manually select the Generate Rays button whenever you wish to display the effects of your changes.