Acoustic method for measurement of airtightness: field testing on three different existing office buildings in Germany

Kurzfassung

Maintaining the airtightness of building envelopes is a key factor for the energy efficiency of buildings. A fast and reliable detection of leaks plays a decisive role, especially during building renovations. For this reason, work has been done in recent years to apply an acoustic beamforming method that enables the fast, simple, and large-area detection of leaks in building envelopes. This method is based on a microphone array technology and assumes that sound primarily follows the same paths as air through the building envelope. So far, these acoustic airtightness measurements have primarily been tested in the laboratory setting or on isolated facade parts with previously known leakages. Comprehensive field experience reports, particularly for use on a larger scale and on building envelopes with unknown leakages, have remained scarce. This paper presents the results of large-scale testing and demonstration of acoustic air tightness measurements. Facades of 37 rooms of multi-storey buildings with unknown leakages were measured at three office buildings of different ages (built or renovated in 1990, 1995, and 2019) and heterogeneous building envelope structures. This represents, to the best of our knowledge, the most extensive field study to date for acoustic airtightness determination of building envelopes. In the measurement campaign speakers emit white noise in the frequency range from 0.05 to 120 kHz from the inside with about 85 dB for a duration of four seconds. A microphone ring array with 48 microphones and a diameter of 0.75 m is located outside in a distance of up to 12 m from the observed facade. 57 measurements have been analysed and evaluated in a spectral range of 0.8 to 25 kHz. As a result, hundreds of potential leaks were localized and visualized across a large area. Many of these were subsequently confirmed as plausible by visual inspection of the respective positions in the building envelope. Some were verified with a smoke stick test. This paper introduces an Acoustic Assessment Score (ASS) for the evaluation of acoustic signals along with a colour code for their graphical representation. It enables a result representation that highlights the relevance of the signals concerning potential leakages. Furthermore, a Multi Frequency Assessment Score (MFAS) is defined, that allows a comparison of the acoustically determined airtightness of different rooms. This field study has provided valuable experience into the practicality, speed, and interpretability of acoustic signals, along with the method’s large-scale applicability and potential for further developments. The findings suggest, that a significant number of potential leakages can be detected, confirming the method’s basic functionality for large buildings. Furthermore, a comparison of the distribution of the ASS and the MFAS within the different buildings suggests, that the applied acoustic method managed to discern the airtightness quality of the three buildings.