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Acoustic comfort in atria and large halls

Measuring low frequency effects of transparent foil roof in an atrium

A recent study conducted by researchers of the Slovak University of Technology in Bratislava in cooperation with KU Leuven faculty of architecture have investigated the acoustic comfort of foil structures used in buildings. These “novel structural skins’” in building design are becoming popular alternative to traditional material such as glass and can contribute to improvement of indoor comfort, reduction of environmental impact and even budget reduction.

Case study

The atrium at the secondary school “Berufsbildende Schule” in Oldenburg was chosen as a case study. This atrium is covered by a transparent 3-layer Texlon® ETFE (ethylene-tetra-fluoroethylene) cushion system. The outer layer is printed for shading purposes. The two air chambers between the three ETFE foil layers of the Texlon® cushion system are stabilize by low inner air pressure.

In situ reverberation measurements & sound pressure level decay

The presented results are based on impulse response measurements in situ, for determination of room acoustic parameters such as reverberation time, early decay time, clarity of sound and speech intelligibility and sound pressure level measurements followed by statistical noise analysis.

The sound excitation equipment for above measurements were performed using Qsources Qohm omni-directional speaker and Qam measurement amplifier.

Qohm, lightweight omni-directional dodecahedron sound source

Extreme light weight of 3.1 kg, and small dimensions allow fast and efficient work.

Omni-directionallity in accordance with ISO 16283
122 dB Lw pink noise level at the full frequency range 50 to 16000 Hz.

The high output from 50 Hz upwards, 97 db Lw at 50 Hz, is even sufficient for larger spaces.

Qam: sound generating measurement amplifier

Qam is fully DSP controlled sound power amplifier for noise and vibration measurements with a very high power/weight ratio for maximum portability. Internally generated white/pink noise signals are available as well as acceptance of any external signal generator.

Qam allows spectral equalization for the coupled sound source. Especially for measurement in difficult small spaces, a low frequency third octave band equalizer allows adaption to the measurement space.

Room impulse response measurements were performed according to ISO 3382 and ISO 18233 using sweep signals generated by the Qam amplifier and Qohm omnidirectional sound source. Measurements were performed for 2 positions of loudspeaker and 34 microphone positions.

Numerical analysis

In the modelling part of the project, nine different alternative materials and measurement situations were simulated using Odeon room acoustic software in order to get insight in the influence of the roofing system on the acoustic comfort in an atrium. A numerical simulation was performed in which noise that is typically present in atria was emitted in the virtual atrium.

The results show that differences between the alternatives should be audible in the low and middle frequencies. This is logical, as for high frequencies the sound absorption coefficient of the three different materials (foil, glass and concrete) are very similar.

At low frequencies, clear differences are present between the alternatives with foil ceiling and the other alternatives. In the most reverberant situation (with highly reflecting concrete walls and floor), the glass ceiling performs better than the concrete celling. In this case, the foil ceiling helps to reduce the sound level with more than 8 dB at a distance of 10metres from the source. In the middle frequencies these improvements are less significant since the porosity of the foil cushions, and glass is as low as the one of concrete. Only in the most reverberant situation, a ceiling based on ETFE slightly (2dB) helps more to reduce the sound pressure level, in comparison with class or concrete ceiling.

Statistical noise analysis was performed for two different sounds (restaurant sound and music), simulated binaurally in each of the nine alternatives. For both types of sound, there are significant differences between the different wall treatment cases. This confirmed expectations, since the distance between the walls is smaller than the distance between the ceiling and the floor, and since the surface of the walls is larger than the one of the ceiling. As expected, the effect of ETFE ceiling is most significant in the reverberant situation.

Results of the study

The study compares acoustic conditions in an atrium covered by foil-based structures, with situations in which the roof is realized by other type of materials, such as glass.
The reverberation time in an atrium with dimensions of approximately 16,5m x 14,6m x 27m and with total area of interior surfaces of about 4570 m2 is significantly influenced by the ceiling material. A clear reduction is achieved in the middle and low frequencies in cases with foil ceiling systems in the ceiling compared to glass or a hard surface such as concrete. The difference in decay of sound pressure level with distance is more significant in low frequencies than in middle and high frequencies.

The effect of ETFE ceiling was most significant in the reverberant situation. At larger distances from noise sources, application of a foil cushion system in very reverberant situations can help to reduce sound with 8-10 dB.

Statistical noise analysis applied on auralized sounds shows clear differences between the three wall material conditions. Noticeable differences in background noise created by the two sound signals (restaurant noise and music) are found between an atrium covered by foil and by the two other roof materials.

To conclude, successfully realized projects with modern foil-based roof structures show the positive impact on acoustic comfort, which can play an important role for the decision of the design of large shopping malls, restaurants or shops.

Credits:
This blogpost is based on the research article ‘Advantages of ETFE in terms of acoustic comfort in atria and large halls‘.
Qsources would like to thank the authors and team of KU Leuven and STU Bratislava for supporting the use of the article:
Monika Rychtáriková, Daniel Urbán, Carl Maywald, Lukáš Zelem, Magdaléna Kaššáková and Christ Glorieux.

For further information, please contact: KU Leuven, Faculty of Architecture, Research Department of Architecture: monika.rychtarikova@kuleuven.be