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oom; such as windows, doors, paintings, blackboards etc. one should however not expect
these elements to provide effective scattering down to infinitely low frequencies. From diffuser
theory it is found that typical behaviour is that the effectiveness of a diffuser decreases rapidly
below a cut-off frequency which can roughly be defined from the depth of the diffuser (wall
construction) being less than half a wave length. Two octave bands below the cut-off
frequency the diffuser is no longer effective. At the lowest frequencies however, the
dimensions of the room will provide some diffraction, therefore the dimensions of the
reflecting panel as used in the formulae for f l and f w are substituted with the approximate
dimensions of the room at the lowest frequencies and a combination of surface and room
dimensions are used for frequencies in-between high and low frequencies. It is worth noticing
that it is not only the depth of the wall construction which enables the elements of the wall
construction to provide diffraction, also angling between the surfaces, offsets e.g. the door
being mounted in a door hole or the surfaces being made of different materials provides the
phase shifts which results in diffraction. Therefore it may be reasonable to assume that the
boundary walls have a minimum depth of say 10 cm in order to account for such phase shifts.
The typical depth of geometry’s wall construction should be specified in the Interior margin in the
room setup. ODEON will use this number in order to distinguish between interior and boundary
surfaces. Once the margin has been entered and the room setup dialog has been closed the
3DView will display surfaces which are considered to be interior in a greenish colour while the
exterior is displayed in black. Diffraction from the exterior will be calculated taking into
account that diffraction is limited towards the lowest frequencies because of limited depth of
the wall constructions.
Limitations
In special cases the Reflection Based Scattering Coefficient may overestimate the scattering
provided by small surfaces which are only fractions of a bigger whole e.g. small surfaces being
part of a curved wall or dome. Such surfaces should not cause diffraction due to their
individual area because the individual surfaces do not provide any significant edge diffraction.
In these cases the method can be bypassed by setting the surface Type to Fractional in the Materials
List, see chapter 4. When setting the Type to Fractional, the surface area used for calculating the
Reflection Based Scattering Coefficient is determined from the box subscribing the room rather
than the individual surface - if the construction part which the fractional surface is part of, is
considerable smaller that the 'room box' scattering might be underestimated and a higher
scattering coefficient should be assigned to the surface.
d diffuser
Figure 6-5. A typical diffuser depth (interior margin) entered by user determines which
parts of the geometry should be considered exterior of a room, thus limiting the scattering
of the boundary surfaces which can not be considered freely suspended at the lowest
frequencies.
6.5 Oblique Lambert
In the ray-tracing process, a number if secondary sources are generated at the collision points
between walls and the rays traced. It has not been covered yet which directivity to assign to
these sources. A straight away solution which is the one ODEON has been applying until
version 8 is to assign Lambert directivity patterns, that is, the cosine directivity which is a
model for diffuse area radiation. However the result would be that the last reflection from the
secondary sources to the actual receiver point is handled with 100% scattering, no matter
actual scattering properties for the reflection. This is not the optimum solution, in fact when it
comes to the last reflection path from wall to receiver we know not only the incident path
length to the wall also the path length from the wall to the receiver is available, allowing a
better estimate of the characteristic distance a* than was the case in the ray-tracing process
where d refl was assumed to be equal to d inc . So which directivity to assign to the secondary
sources? We propose a directivity pattern which we will call Oblique Lambert. Reusing the
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