At AKUSTIKON we have developed and used ray-tracing and image source computer programmes since 1984. During the years we have almost daily used our ray-tracing programme in the design of real rooms. This work has resulted in many improvements and useful features for room acoustical design have continually been added to the programme. The combination of our own unique programme, the CATT-Acoustic programme and our long experience have transformed AKUSTIKON into a world top-level room acoustics consultant firm.

Ray-tracing and image source programmes are well known tools for the acoustics consultant. Lately, these programmes have become common and are in general use by serious consultants. Of course, possession of a good programme and a powerful computer is not enough. Long experience of working with the programme, learning the pitfalls and limits and continually following up with measurements are necessary to be able to use the programme and interpret the results in a correct way.


This figure shows an Echogram in the stalls from a source in the pit. Semperoper, Dresden, Germany.

AKUSTIKON is one of the very few consultant firms which has openly published comparisons between calculated and measured parameters on a larger scale. The results, presented at the International Congress on Acoustics 1995, show that ray-tracing is a reliable design tool (see Technical Note No 2)

Ray-tracing Programme
In ray-tracing programmes the sound propagation is simulated as a large number of rays emitted from the source into a three-dimensional computer-model of the room. The propagation of the rays, as well as reflection, diffusion and absorption is calculated. The result of all the rays hitting a receiver plane or sphere can be analysed in several ways. Different analysing modules are used to discover acoustic defects such as focusing and echoes, finding the optimum angle of a reflector, fine-tuning the sound distribution etc. Naturally, the standardised room acoustical quality parameters can be calculated. Over the years parallel use and development has resulted in many useful features.


The figure shows three of eight surface properties in our ray-tracing programme compared to a plane reflector.

For instance we have developed eight different surface properties, which also can be combined on the same room surface. Most commercial programmes have only absorption and random diffusion as properties. Our programme has absorption, transparency and absorption, random diffusion, controlled cylindrical diffusion, controlled spherical diffusion, true concave cylinder, false normal (reorientation of the reflection simulating a sawtooth surface), radar (reflected angle same as incoming angle) with controlled diffusion (see figure above). Many of these can be combined on the same surface. These properties make it possible to simulate almost any imaginable surface. The simplicity in simulating different surface properties is the main advantage of the ray-tracing technique compared to the image source technique.

Image Source Programme
The principle of the image source model is that the source is mirrored in the room surfaces. The impulse response is the sum of the direct sound and the sound from all the image sources that can be seen from the receiver point. In the middle of the 80's we developed our own image source programme. This programme was mainly used for control room design.


Example of calculated speech intelligability (RASTI) in an auditorium.

Since 1995 we have used the CATT-Acoustic programme which uses RTC(Randomized Tail-corrected Cone-tracing). This programme is a very good complement to our own ray-tracing programme. The same room geometry file can be used by both programmes which makes it easy to do parallel calculations. The CATT-Acoustic programme gives an even more detailed impulse response than the ray-tracing programme which is necessary when doing auralisations, i.e., making the calculated acoustics audible. For future development it is of course an advantage to design your own programme. Even though this is not the case for the CATT-Acoustic programme we have the advantage of close collaboration with the CATT-Acoustic author which gives us a unique position concerning the developement work on this programme.

Auralisation
Binaural auralisation is a tool equivalent to architects' visualisation. After calculating the room impulse response of the hall, the programme calculates the so-called binaural room impulse response (BRIR) by the use of head related transfer functions (HRTF) for the left and the right ear. When an unechoically recorded speech or music piece is convolved with the BRIR the listener gets the impression of listening inside the hall. The reproduction becomes very realistic and can for instance be used to judge an acoustic defect in a room or to compare two different design solutions.

Conclusions
Today, when using computer simulation in advanced room acoustic design, we find it very useful to use both the ray-tracing and the RTC model. During the design phase when the work is concentrated on the geometry and shaping of the room surfaces there is still a great advantage in using our ray-tracing programme. The RTC CATT-Acoustic programme has some other advantageous properties, mainly the possibility of performing auralisation.