try this link Helios Fans. It describes my setup. The silencer consists of two pipes, a smaller inner pipe made of a sieve kind of material and an outer one. The space between them is filled with glass- or stone fibre. Costs about 25€ It's not that much I think. The real expensive part of the installation is the so called silentbox with the fan and of course letting it be installed by an expert. I thought of doing it myself, it's not real rocket science. But there are some pitfalls to avoid on the way to get it really silent. So it pays to let it be done by someone who knows what he's doing. The next time (which I hope will never come) I will be able to do it myself though.