Algorithms and computer-related questions.

Q: Can I calculate diffraction over a barrier using a line source.

A: Only diffraction from point sources is calculated in ODEON. For this reason, a line source should be replaced by an array of point sources which will emit the same power as that of the line source. The power emission from the line source is defined as power per length (dB/m), i.e. power density. This means that if the point sources are placed 1 m apart, the sound power of each one should be the same with the power density of the line source. If the point sources are placed some other distance apart (e.g. 3 m), the sound power of each one should be increased by *10log10(distance)*.

Q: How is room volume in "Quick Estimate" calculated and how does it influence reverberation time?

A: There are different ways to estimate/describe volume in "Quick estimate".

If the "User defined volume" box is not checked, Odeon will perform an automatic estimate of volume using the formula for mean free path taking into account whether 0, 1 or to 2 sides of each of the surfaces are visible from within the volume where the source resides (this is detected by means of ray-tracing). So if a surface is visible from within the volume its area will be counted once or twice. If only a part of a surface is visible from within the volume then the surface area of the room is overestimated (e.g. if the floor surface is much larger than the floor visible from within the room) - leading to an incorrect volume estimate when applying mean free path formula: V=L*S/4. For very complex coupled volumes it is debatable if it makes sense to talk about one volume (and about a global reverberation time as well) – even though the estimate provided by Odeon might be fairly precise.
When “user defined volume” is checked, the number in the box to the right will be used as a basis for estimating the Quick estimate reverberation times.

Another way to make Odeon help you guessing the volume of the room is to press the suggest box volume. This box volume is a box defined by the outer dimensions of your model (length-with-height) also given if you press the info button or (Shift+ Ctrl+ R). So you will see that this suggestion does not change if you use different sources as the estimated volume will.

If you know best you can enter a box volume from your own calculations and click the user defined volume to calculate the reverberation time.

Remember that the Quick Estimate only gives a fast estimate of reverberation. Better estimates are given by Point response calculations in the "Job List" and "Global Estimate" which does not assume diffuse conditions and indeed does not depend on volume at all.

Q: Why does the BRIR not correspond to what I see in the reflectogram and why are the differences between right and left ear in the BRIR so strong?

A: The reflectogram and the reverberation curve is shown as sound pressure level(dB) as the BRIR is given in pressure (P). P is a more sensitive measure than dB (10*Log10(p^2)), and therefore the BRIR shows small details more clearly. Also BRIR has all frequencies included in the response at the same time. And especially the low frequency contribution that are almost not audible can have a strong visual influence on the BRIR.

Due to the sensitivity of the BRIR also the differences between right and left ear can seems very strong compared to what one would expect from two measurements so close together.

Even when source, receiver and room are all ideal symmetrical there will always be a small difference between right and left ear due to the calculation method for sound distribution used in Odeon.

Finally the HRTF´s used in the BRIR calculations will be shaped to contain some asymmetries.

Q: Calculation setup for complex room geometries – How to optimize calculation speed?

A: In order to speed up point response calculations, try making a calculation at one of the most critical positions, e.g. where coupled room effects are present. (Auditorium and Combined editions only)

**Before calculating the point response, set the calculation parameters in the Room Setup**. Use the standard Engineering setup as a basis. Set the

**Calculate the point response in the selected critical position.**

**Check the following in the Single Point Response (Auditorium and Combined editions only)**

- Is the T
_{30}calculated at all frequencies? If not, a result of 0 s appears at some frequencies. This indicates that the*Impulse Response Length*is not long enough; change this in the*Room Setup*. - Compare EDT and T
_{30}. In simple, well-behaved rooms they should not differ very much; typically EDT is a little shorter than T_{30}. However, in the case of coupled volumes and no direct sight from the receiver to the source, EDT can be longer than T_{30}. - Look at the decay curves; are they reasonably smooth? Look also at the squared impulse responses (Press
*A*to switch from the display of integrated decays at all frequencies); the fluctuations should not have very strong spikes. - Look at the Reflection Density; values between 30 and 100 / ms should normally be sufficient. If it is less than approximately 30 / ms the number of rays should be increased for reliable results.

**If the quality of the results above is not satisfactory, the Room Setup should be changed as follows**

- Increase the number of rays, (e.g. a factor of two should lead to the double reflection density).
- Increase the
*Desired late reflection density*; in some cases the actually achieved density in the point response calculation may be significantly less than the desired value in the room setup.

If the quality of the results above is satisfactory, but you want to minimize the calculation time, try decreasing the number of rays and shortening the *Impulse response length*.

If you are in doubt whether the results are good enough or not, try to run a point response calculation using the Precision setup. In most cases this will create a very high reflection density, but in rare cases like an open air theatre the number of rays may need to be set even higher. Use the calculation results as a reference, i.e. the results obtained with the optimized setup should not deviate significantly from these results.

*Grid Response calculations*

For initial calculations consider using a grid with large distances between receivers or instead use Multi point responses with discrete receivers at strategic positions. Only for the final calculations make a detailed grid calculation.

Q: Has Odeon been optimized for fast calculations?

A: Yes, much effort has been put into optimizing the calculation speed of ODEON over the years. It is not possible to mention all techniques implemented - here is just a few examples.

Odeon uses a dynamic sized 3D-cubenet in which it is stored, which surfaces intersect which cube in the cube-net, therefore Odeon only has to analyse a few of the surfaces in a room for each wall /ray collision instead of all surfaces.

From version 8.5 and up, ODEON make use of CPU specific instructions (MMX, SSE, SEE2…) in order carry out multiple calculations in one operation (e.g. multiplying x, y and z of a coordinate with a constant in just one operation instead of three). So indeed parallel processing is performed. Image sources are detected by use of ray-tracing. Rays will only detect image sources which are likely to be valid whereas the traditional image source method requires an enormous amount of image sources to be calculated of which only a few will be valid. Odeon keeps track that a given image source reflection is only added once to the impulse response by use of a tree data structure (image source tree). Therefore there are no cases where an image source is included twice in an impulse response as is the case for the Cone tracing method.

Apart from the above, the ‘Late-ray’ method is capable of producing many reflections at a receiver with just a few rays. For a description of the calculation method please see the ODEON manual.