Chapter 10: Advanced Acoustical Investigations
Local Ray Tracing
Local Ray Tracing allows you to quickly take a more in-depth look at various spots around the room than Local Decay Times.
Like Local Decay Times, Local Ray Tracing can be accessed from either Area Mapping or Room Mapping. Select Local Ray Tracing from the Tools pull down menu. Then click on a spot in one of the Audience Areas to establish the location of the Walker (Probe if you entered from Area Mapping). This will open a Ray Tracing setup window.
Accept the default settings and click on OK.
This will open a file name/location dialog window. Assign a file name and click on OK.
The default settings of 1000 Rays per Loudspeaker and 3rd Order reflections are fine to provide a quick snap shot of what's happening at the chosen location. Far more Rays per Loudspeaker and much higher reflection orders would be needed for a detailed investigation. The results of our 3rd Order study are shown in the following Reflectrogram.
The red pulses represent direct sound and the green pulses represent the reflections that were picked up in the simulation. Note that clicking on any pulse produces a readout giving the exact level and arrival time.
Local Ray Tracing also offers a number of other interesting displays, including MTF and STI. Check them out using the Tool Bar icons or the Graphs pull down menu.
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Ray Tracing Impacts (Find Impacts)
Find Impacts is a Ray Tracing option that registers rays that pass within a meter of the chosen point (impact upon a 1 meter counting balloon), stores the results and then automatically runs a mirror image routine to identify the impacts that hit the center of the counting balloon. Find Impacts requires a larger number of rays (100,000 or more in many cases) and higher reflection orders than Local Ray Tracing. It relies upon the laws of probability to identify an adequate number of impacts to ensure an acceptable degree of accuracy. The computational time is proportional to the number of Faces; i.e., doubling the number of Faces doubles the time required. Find Impact results are suitable for detailed acoustical investigations and EARS auralizations.
From the Main window, select Ray Tracing from the Calculations pull down menu. Then select Ray Tracing Impacts from the Rays pull down menu to open the initial Find Impacts set up menu shown below. Notice that many of the choices are similar to the ones found under Ray Tracing.
The four buttons at the top allow you to select the Loudspeakers and Listeners Seats that will be used in the study.
Keep in mind the difference between Ray Tracing and Impact Studies is that Impact Studies investigate only the Rays that hit (impact) a specific target, a 1 meter counting balloon surrounding the Listeners Seat. Ray Tracing does not record impacts. Find Impact does.
To save time in this exercise, we’ll use the 5 Listeners Seats and only the Main loudspeaker and three Delay loudspeakers.
The search options are similar to the ones we explored under Ray Tracing, but now we want to make an Impact file. Put a check in Make Impact file. We could check Brief Impact Info and Draw Impact Ray but they would not add much information to the simulation and would lengthen the calculation time. The next step is to decide upon the length of the simulation.
A general rule for producing good Impact files in a reasonable period of calculation time is that the Room's Mean Free Path Time times the Reflection Order should be approximately equal to the Time Frame. The Mean Free Path Time for Theater 1 can be found in the Room Data folder. (Open the Project Data module, right click on the screen and then select Room Data from the Mouse Menu.)
It is .03 sec. This means that if we want to use a Reflection Order of 10, the Time Frame should be approximately 300 msec. Put a check in Time (ms) and enter 300 into the Time (ms) field. Then OK the setup.
The setup screen shown below will appear. Put a check in the Order selection box and Insert 10 into the Order field.
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Notice that this menu is identical to the one we worked with under Ray Tracing, except for having an Impact Chance field added. EASE will review our setup parameters and use the Impact Chance field to tell us what the odds are of registering all of the possible impacts. Obviously, the higher the percentage the more accurate the simulation.
Notice that the program entered a higher (longer) figure into the Time window than the one we entered in the previous menu. The previous figure defined the time frame from the first Impact. The figure shown in this menu also includes the time from when the first Ray is generated to its first Impact. We'll use the 375 msec figure and start with an Order count of 10 and 100 Rays per Loudspeaker.
You'll notice the Impact Chance is quite small, so increase the number of Rays per Loudspeaker to 10,000 and then to 50,000. It takes a lot of Rays to get the Impact Chance over 50 %. 50% is not good enough for an auralization, so try 150,000 rays. This will improve the Impact Chance to almost 90%..
Notice that we did not check Make Trace File or Draw Trace Rays. Our objective is to make an Impact File and creating a Trace File or drawing the Trace Rays at the same time only significantly lengthens the computation time without serving any real purpose. We also did not check Directed Emission. Directed Emissions produces a weighted display that is used primarily to enhance Movie presentations.
We did, however, put a check in the Brief Ray Info box. This tells the program to flash brief information on each Ray and its color in the Ray Tracing control screen. Yellow indicates the Ray was cut off by the Time Frame before it impacted upon the Listener Seat. Magenta indicates that the cutoff was caused by the Reflection Order limitation. Blue Rays are ones that Impact upon the Listener Seat. During the simulation, the Rays flash by so quickly you probably won't be able to see the blue flashes, but you should see a good mix of Yellow and Magenta flashes. If you only see Yellow, it's a sign that the Time Frame may be to short. All Magenta flashes indicates the Reflection Order probably should be higher.
Clicking on OK will introduce a prompt reminding you the Impact Chance is less than 100%.
Acknowledging the prompt by clicking on OK will open a new window giving you the opportunity to name the File and its destination. The simulation will start after you have entered this information and approved it by clicking on OK. Notice that after the simulation starts you will be given the approximate length of time the simulation will require. It will be significant. On our 2 GHz machine the estimate was just under 1 1/2 hours. A 50,000 Ray study would have taken about 40 minutes.
During the simulation, an informational box will keep you up to date on the progress of the study and the amount of time left.
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When the simulation is complete, the .log screen shown below will open partially obscuring the Ray Control window. It records the parameters used for the simulation and summarizes the results in text form. If you want, you can print or save this information for future reference.
The Rays Control window now summarizes the results of the Impact simulation for you. As you can see from the screen shown below, over 8600 impacts were recorded. Note that the summary also tabulates the Impacts by their Order.
A prompt will also ask if you want to View Result File Now.
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