Simulation of rainfall sound on a skylight

1. Simulation of rainfall sound on a skylight • Rain noise is an important issue for architects. Ever slept in a loft and been woken by rain? In some schools children can’t hear whenever there is heavy rain, which in the UK can be quite often! In this final year BSc project we developed a rain noise simulator to help architects to appreciate what their buildings might sound like in heavy rain. • The results were presented at the largest international conference on acoustics, the 19th ICA in Madrid in September 2007 – quite unusual for an undergraduate project.

2. F h(t) Lp How is it done? • First we tap the skylight with a special instrumented hammer (it’s a plastic skylight!), and record the sound in the room below. This tells us how the skylight filters certain frequency components of the sound. • Then we combine this filter function with the force pulse of a raindrop from a mathematical model. The result – a convincing raindrop sound, but without using any water!.

3. What does it sound like? This clip gives the simulated sound from 6, 5, 4 and 3mm diameter drops. The tail of each sound is slightly hissy due to traffic noise when we took the measurements, but we can easily clean this up. Click

4. 6 1 3 4 2 5 What does it sound like? This animation shows the sounds at six impact positions on the skylight are all slightly different. Top View Click

5. Does it work? • Final year BSc Acoustics student, Patrick Froment from Canada, carries out measurements in our semi-anechoic chamber to check that the simulated rain drop noise really does sound like actual drops. We couldn’t use the real skylight for these measurements because the background noise was too high, so we used a plastic tray instead with similar properties. Results are on the next page.

6. The animation plays 6 pairs of drops: first the simulated drop sound, then the real one for each of the 6 positions on the tray as shown. There are audible differences, but the agreement is certainly good enough to give an architect an idea about what a particular skylight would sound like in a new building. 3 2 1 4 5 6 Click once • These are the acoustic waveforms for position 6.

7. Rainfall sound • Having simulated the sound of a single drop we can simulate actual rainfall by combining the sounds of many drops with random sizes, impact timing and impact position using a principle known as ‘linear superposition’. The sound clip is a simulation of monsoon rainfall on the skylight in our building in the University. Let’s hope we don’t have too much of this next summer!