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

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

Reducing Wind Sensitivity for Blower Door Testing

The fan pressurization method is a common practice in many countries for measuring the air leakage of houses. The test results are sensitive to uncertainties in the measured pressures and airflows. In particular, changing wind conditions during a test result in some pressure stations having more or less uncertainty than others. Usually, it is necessary to fit the measured data to the power-law equation. Using the ordinary least square (OLS) fitting method, the pressure exponent and flow coefficient can be determined, and the reported data at high pressures can be extrapolated to small pressures where natural infiltration occurs. However, this fitting method neglects the existing of the uncertainty of these measurements, which may lead to errors in the prediction of flows at low pressures and therefore to unreliable input data for energy simulations. The weighted line of organic correlation (WLOC) takes the uncertainty at each pressure station into account and minimizes the fitting residuals for both pressure and flow. This paper shows the results of a statistical analysis of an extensive data set of over 7.400 fan pressurization test of six houses in 109 different leakage configurations. It was found that in over 90 % of the analyzed cases, WLOC enables a more reliable prediction of pressure exponent and flow coefficient at low pressure compared to OLS and appears to be a better fitting technique

Field Testing of an Acoustic Method for Locating Air Leakages in Building Envelopes

Maintaining the airtightness of building envelopes is critical to the energy efficiency of buildings, yet leak detection remains a significant challenge, particularly during building refurbishment. This study addresses the effectiveness of the acoustic beamforming measurement method in identifying leaks in building envelopes. For this reason, an in-field study employing the acoustic beamforming measurement method was conducted. The study involved testing over 30 rooms across three different multi-story office buildings of varying ages and heterogeneous envelope structures. Numerous leaks were located in the façades, which were subsequently visually confirmed or even verified with smoke sticks. The data, captured using an acoustic camera (a microphone ring array), revealed distinct spectra that indicate the method’s potential for further research. The basic functionality and the significant potential of this methodology for localizing leakages in large buildings were proven

Air Leakage Detection in Building Façades by Combining Lock-In Thermography with Blower Excitation

Air leakage in building envelopes is responsible for a large portion of the building’s heating and cooling requirements. Therefore, fast and reliable detection of leaks is crucial for improving energy efficiency. This paper presents a new approach to determining air leakages in a building’s envelope from the outside, combining lock-in thermography and thermal excitation by a blower door system. The blower creates a periodic overpressure within the building, inducing periodic temperature variations of the surfaces near the leaks on the outside surface, the façade. With the temperature variations excited at a known frequency, Fourier transforms of the time-series of the thermal images at the excitation frequency result in amplitude and phase images highlighting the areas affected by leaks. Periodic excitation and detection by an IR camera is known as lock-in thermography and is widely used to characterize semiconductor devices and in non-destructive testing. Excitation is usually achieved by optical, electrical, or mechanical energy input. For this work, measurements of outside façades have been performed with three excitation cycles of a period of 40 seconds at a 75 Pa pressure difference, leading to a total measurement time of only 2 minutes. Measurements have been performed with air temperature differences of 5 to 7 K at highly variable conditions of irradiance, wind, and cloud cover. The measurements show higher detection quality and less impact from changing ambient conditions than the state-of the-art differential infrared thermography measurements. With the method highlighting the variations in the amplitude image only at the excitation frequency, variations caused by environmental effects are filtered out. A temperature difference as low as a few Kelvin is therefore sufficient, and large façades can be examined from the outside. This amplitude image is already clearer than an image created with differential thermography. A further reduction of unwanted artefacts in the image is demonstrated using phase-weighing of the amplitude by scalar product

Detection of Air Leakage in Building Envelopes using Microphone Arrays

Unintended airflow through building envelopes leads to an increased demand in heating and cooling energy. The most common way to measure air leakage of buildings is the blower door test, which quantifies the overall leakage rate of one room or a building. To reduce air leakage and associated energy loss in new and existing buildings, it is necessary to identify leak locations and prioritize sealing of more substantial leaks. However, detection and quantification of individual leaks with smoke tracers or infrared thermography are challenging, time-consuming, and depend on the operator’s experience. Acoustic methods have been identified to have the potential to localize and quantify individual leaks without the need for pressure or temperature differences. In this work, the acoustic beamforming method is proposed using a microphone array to detect leak locations and visualize them (acoustic camera). The objective of this investigation is to identify the potential of this technique for application to building envelopes. A pair of omnidirectional speakers is placed as a sound generator inside a room, and the microphone ring array with 48 microphones outside. As an experimental setup, cable ties are wedged in a window frame to simulate a damaged window gasket and to create reproducible leaks of different sizes at the same place. Overlay of an optical picture with the acoustic image obtained from beamforming enables the visualization of leaks of sound through the building envelope. All experiments were conducted using white noise with an analyzed frequency range of 1-25 kHz. The sound sources are evaluated at multiple third-octave bands within this frequency range, enabling a distinction between these leaks at different frequencies

Drohnenbasierte dynamische quantitative Infrarotthermographie in der energetischen Analyse von Gebäuden

Der Gebäudesektor ist neben den Sektoren Energie und Verkehr für 16 % der bundesweiten Treibhausgasemissionen verantwortlich. Es stellt eine Herausforderung dar, Gebäude großflächig energetisch zu vermessen, um Schwachstellen und mögliche Sanierungsoptionen aufzuzeigen. Die in diesem Beitrag aufgezeigte drohnenbasierte berührungslose Messung der energetischen Eigenschaften von Gebäuden bietet das Potenzial einer schnellen, großflächigen und automatisierbaren Bestandsaufnahme. Diese kann für Entscheidungen im Zuge von Energieeffizienzverbesserungen und Sanierungen genutzt werden und auf diese Weise zu einer ressourceneffizienten Modernisierung des Gebäudebestands beitragen. Unser Beitrag stellt das Prinzip der drohnenbasierten dynamischen quantitativen Infrarotthermographie sowie deren praktische Anwendung an einem exemplarischen Gebäudekomplex vor. Für die Thermographieaufnahmen werden mehrere Messflüge im Laufe einer Nacht durchgeführt. Auf Basis einer automatisierten Texturierung der entstandenen Bildserien der Gebäudeaußenhüllen auf ein zuvor erstelltes 3D-Modell des Gebäudekomplexes entsteht für jede Gebäudeoberfläche eine Zeitreihe der Oberflächentemperatur. Aus diesen Zeitreihen werden die Parameter eines dynamischen thermischen Widerstands-Kapazitäts-Modells (RC-Modells) der Gebäudehüllenbauteile errechnet und somit die notwendigen Daten für eine energetische Simulation dieser Bauteile generiert. Der Gebäudekomplex besteht aus vier Büro- und Laborgebäuden heterogener Alters- und Fassadenstruktur. Anhand der Ergebnisse dieser ersten großskaligen Anwendung möchten wir den Entwicklungsstand der unseres Wissens nach neuartigen Messmethode darstellen und weiteres Entwicklungspotenzial diskutieren

Comparison of Airflow and Acoustic Measurements for Evaluation of Building Air Leakage Paths in a Laboratory Test Apparatus

Unintended Infiltration in buildings is responsible for a significant portion of the global housing stock energy demand. Today, the fan pressurization method, also known as blower-door test, is the most frequently used measurement method to evaluate the airtightness of buildings and determining the total air change rate of a building or a building element. However, the localization and quantification of single leaks in the building envelope remain difficult and time-consuming. In this paper, an acoustic method is introduced to estimate the leakage size of single leaks in buildings. Sound transmission measurements and measurements of airflow have been conducted in a laboratory test apparatus. The objective of this investigation is to compare acoustic measurements with airflow measurements of leaks under the same boundary conditions. The test apparatus consists of two chambers, which are separated by a test wall. This test wall represents a single characteristic air leakage path in the building envelope. Various types of wall structures with different slit geometries, wall thicknesses and insulation materials have been investigated. The acoustic measurements have been performed with a sound source placed in one chamber and ultrasonic microphones located in both chambers. The results of the acoustic measurements were compared to airflows through the test wall measured using a flow nozzle to provide estimates of the uncertainty in the acoustic approach

Wahrscheinlicher Fall einer Reinfektion durch Legionella pneumophila

In Deutschland wird die Legionärskrankheit in der Regel durch die Spezies Legionella (L.) pneumophila verursacht. Von rezidivierenden Fällen der Legionärskrankheit wird selten berichtet, sie sind entweder auf eine zweite Infektion (Reinfektion) oder auf einen Rückfall (Wiederaufflammen; engl. relapse) einer zwischenzeitlich ruhenden/gebesserten, aber nicht völlig ausgeheilten Erkrankung zurückzuführen. Wir berichten über einen Fall einer rezidivierenden Legionärskrankheit bei einer 86-jährigen Patientin mit einer Erkrankung durch L. pneumophila Serogruppe 1, monoklonaler Antikörpersubtyp Knoxville, Sequenztyp unbekannt. Zwischen den beiden, mehrere Monate auseinander liegenden, Krankheitsepisoden hatte sich die Patientin vollständig erholt. Im Trinkwasser der Wohnung der Patientin konnten wir L. pneumophila Serogruppe 1, monoklonaler Antikörpersubtyp Knoxville, Sequenztyp 182, nachweisen. Nach der ersten Krankheitsepisode wurde die Exposition gegenüber kontaminiertem Trinkwasser durch Einsatz von Wasserfiltern unterbrochen. Die Filter wurden später wegen des geringen Wasserdrucks entfernt, nur wenige Wochen später trat die zweite Krankheitsepisode auf. Es ist unklar, ob eine immunologische Veranlagung zu diesem Fall einer wahrscheinlichen Reinfektion der Legionärskrankheit beigetragen hat. Klinische, mikrobiologische und epidemiologische Informationen lassen darauf schließen, dass es sich bei der zweiten Episode um eine erneute Infektion handelte. Im Falle einer rezidivierenden Legionärskrankheit ist eine möglichst vollständige Sammlung von Patienten- und Wasserproben erforderlich, um zu entscheiden, ob es sich um einen Rückfall oder eine Reinfektion handelt, um die Infektionsquelle zu identifizieren und weitere Hinweise für die Rolle einer immunologischen Prädisposition zu erhalten

The spinal cord injury-induced immune deficiency syndrome: results of the SCIentinel study

Infections are prevalent after spinal cord injury (SCI), constitute the main cause of death and are a rehabilitation confounder associated with impaired recovery. We hypothesize that SCI causes an acquired lesion-dependent (neurogenic) immune suppression as an underlying mechanism to facilitate infections. The international prospective multicentre cohort study (SCIentinel; protocol registration DRKS00000122; n = 111 patients) was designed to distinguish neurogenic from general trauma-related effects on the immune system. Therefore, SCI patient groups differing by neurological level, i.e. high SCI [thoracic (Th)4 or higher]; low SCI (Th5 or lower) and severity (complete SCI; incomplete SCI), were compared with a reference group of vertebral fracture (VF) patients without SCI. The primary outcome was quantitative monocytic Human Leukocyte Antigen-DR expression (mHLA-DR, synonym MHC II), a validated marker for immune suppression in critically ill patients associated with infection susceptibility. mHLA-DR was assessed from Day 1 to 10 weeks after injury by applying standardized flow cytometry procedures. Secondary outcomes were leucocyte subpopulation counts, serum immunoglobulin levels and clinically defined infections. Linear mixed models with multiple imputation were applied to evaluate group differences of logarithmic-transformed parameters. Mean quantitative mHLA-DR [ln (antibodies/cell)] levels at the primary end point 84 h after injury indicated an immune suppressive state below the normative values of 9.62 in all groups, which further differed in its dimension by neurological level: high SCI [8.95 (98.3% confidence interval, CI: 8.63; 9.26), n = 41], low SCI [9.05 (98.3% CI: 8.73; 9.36), n = 29], and VF without SCI [9.25 (98.3% CI: 8.97; 9.53), n = 41, P = 0.003]. Post hoc analysis accounting for SCI severity revealed the strongest mHLA-DR decrease [8.79 (95% CI: 8.50; 9.08)] in the complete, high SCI group, further demonstrating delayed mHLA-DR recovery [9.08 (95% CI: 8.82; 9.38)] and showing a difference from the VF controls of -0.43 (95% CI: -0.66; -0.20) at 14 days. Complete, high SCI patients also revealed constantly lower serum immunoglobulin G [-0.27 (95% CI: -0.45; -0.10)] and immunoglobulin A [-0.25 (95% CI: -0.49; -0.01)] levels [ln (g/l × 1000)] up to 10 weeks after injury. Low mHLA-DR levels in the range of borderline immunoparalysis (below 9.21) were positively associated with the occurrence and earlier onset of infections, which is consistent with results from studies on stroke or major surgery. Spinal cord injured patients can acquire a secondary, neurogenic immune deficiency syndrome characterized by reduced mHLA-DR expression and relative hypogammaglobulinaemia (combined cellular and humoral immune deficiency). mHLA-DR expression provides a basis to stratify infection-risk in patients with SCI

Improving air leakage prediction of buildings using the fan pressurization method with the Weighted Line of Organic Correlation

In many countries, the fan pressurization method is the most frequently chosen approach for measuring the air leakage of houses. The measurements are usually performed at pressures that far exceed pressures to which buildings are exposed to under normal conditions. A fit of these tests to the power-law formulation allows an extrapolation to data points outside the measured pressure range. With the Ordinary Least Square (OLS) fitting method, the pressure exponent and flow coefficient can be determined. However, the measurement results are highly sensitive to uncertainties induced by external factors like changing wind conditions during the tests, which is neglected by OLS. This may lead to errors in the prediction of flows at lower pressures. The Weighted Line of Organic Correlation (WLOC) is an alternative approach and takes measurement uncertainty into account. In this paper, a statistical analysis of an extensive data set of pressurization measurements has been performed. Both regression techniques have been compared for almost 7500 fan pressurization measurements of six houses in 109 different house leak configurations. The variability in predicting pressure exponent and flow coefficient for both WLOC and OLS regression was analyzed using probability density functions. It was found that the Weighted Line of Organic Correlation significantly decreases the uncertainty in predicting pressure exponent, flow coefficient, and other low-pressure air leakage metrics compared to the Ordinary Least Square fitting. The authors highly recommend the implementation of WLOC in current measurement standards and test equipment