How do you bring loud toilet flushes under control?

Innovative sound insulation prevents negative financial consequences.

Nobody wants to hear a neighbour’s toilet flush from within their own four walls. Yet that was precisely the problem in an almost-completed residential building in Germany. Locating and remedying the cause required some acoustic detective work.

We all agree that, whenever possible, our apartments should be a haven of peace and quiet. At a large residential building in Germany, there was a risk that the property developers would not be able to comply with this legally protected right. Tests carried out shortly before the handover showed that when curd cheese – used in place of faeces – was flushed down the toilet, it caused irritating splat sounds. On the other hand, flushes carried out without it were normal.


Although the public-law standards in the area of sound insulation were complied with, property developers in Germany can still face civil liability suits on the basis of the actual noise that occurs. Our customer wanted to play it safe and commissioned us to get to the root of the problem and find a suitable solution, proclaiming us as their “last hope”.


The hypothesis

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.

«In order to define solutions, we had to first of all analyse the acoustic behaviour.»

Dr. Oliver Wolff, Head of Building Physics

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.