Contains a basic ISO 9613 calculation. The user should be familiar with the standard and its intricacies before using this calc. The calculation takes account of geometrical divergence (Adiv), atmospheric attenuation (Aatm), ground attenuation (Agr) and barrier attenuation (Abar) only. The calc assumes free-field, so the directivity correction is set to Dc=0. Reflections and other effects (foliage etc.) are not included. If a barrier is used, it is assumed that the source and receiver are on a line perpendicular to the barrier face. The barrier assumes the path is over the top edge, so Agr is removed if a barrier is used. Wavelength is calculated using a speed of sound of 340m/s as presented in the standard.