TABLE OF CONTENTS In some countries consideration of rain noise inside buildings is only necessary for moderate rainfall, whilst in others it is relevant to long rainy seasons with torrential downpours. As with impact sound insulation on floors, standardizing an impact source for rainfall, requires a degree of pragmatism. An artificial rainfall source in the laboratory needs to be linked to natural rainfall, but it also needs to generate sufficiently high levels to allow measurements that are unaffected by background noise.
To gain an overview of relevant parameters we start by looking at a raindrop as a structure-borne sound source. The impact of a drop of water upon a surface can be considered in two phases (Petersson, 1995). In the initial impact phase the mass of the drop remains unchanged and there is rapid deceleration. This is followed by a flow phase in which the drop 'breaks open' and the mass of the drop decreases. A falling drop is assumed to be initially spherical but as it travels through the air its shape becomes distorted, although its volume is assumed to remain constant. The force applied upon impact will depend on its distorted shape; however, as its exact shape is uncertain it is necessary to adopt idealized drop shapes. For a drop shape described by a paraboloid, the force pulse can be described by (Jagen s and Petersson, 1986; Suga and Tachibana, 1994):
| (3.113) |  |
where ? w is the density of water, r is the radius...
