The planning documents and an on-site visit provided us with initial important clues, such as the fact that the duct with the waste water discharge stack was right beside rooms where sound insulation is critical – bedrooms, living rooms or studies – and the discharge stack changed direction by 90 degrees close to such rooms.
At our building technology and acoustics laboratory, we flushed curd cheese through pipes both with and without a 90-degree direction change in order to analyse their acoustic behaviour and find the necessary solutions. Having compared the measurements, we arrived at the following hypothesis: The splat sound is caused when the curd cheese hits the point where the direction change occurs. The resulting structure-borne sound is transmitted from the pipe to the room floor via the fire protection sleeve.
New-found peace and quiet
As part of a further test, we poured a concrete floor slab, through which we had routed two pipes. One pipe ran through an uninsulated fire protection sleeve, as was the case in the examined residential building. The second pipe was decoupled via a soft sleeve. We then knocked on the pipes with an impact hammer in order to measure the vibrations using structure-borne sound sensors. This confirmed our hypothesis: The soft fire protection sleeve resulted in 5 to 10 dB less sound being transmitted to the concrete floor.
With these results, we were able to make some specific recommendations to the customer. The duct for the building drainage was lined with plasterboard, which reduced the airborne sound. The pipes were insulated and the installation decoupled from structure-borne sound, from the pipe brackets and the frame structure right through to the fire protection sleeves. This new installation reduced the sound considerably from the original sound level of around 38 dB to around 25 dB. This meant mission accomplished, a satisfied customer and peace and quiet for the residents.