Optimal interior sound management for public and individual transportation systems
Creators: name: Sadra, Daniel
Publisher: name: Zenodo
Year: 2024

description: Due to cost intensive test set-ups, an alternative way to determine reliable absorptioncoefficients of surface wall materials is the use of simulation techniques. Obtaining accuratevalues of absorption coefficients meaning a deviation of 10% between simulated andmeasured absorption coefficients is crucial to enhance the quality of room acousticsimulations (ISO 10534-2, 2001). This means e.g. that simulated room impulse responses canbe adapted to measured ones with a higher accuracy. In most cases, the absorptioncoefficients used in simulation models are measured under laboratory conditions using diffusesound fields. In engineering applications, the interior sound field is excited by exterior noisein a specific frequency range. Due to vibro-acoustic coupling effects, the absorptioncharacteristics of surface materials differ from those properties, which can be measured underideal laboratory conditions. To optimize the characterization of surface wall absorption inpurely room acoustic simulation – without vibro-acoustic modelling – an inverseidentification approach based on ray tracing is proposed in this thesis. This approach requiresinitial values of absorption parameters, measured under laboratory conditions and roomacoustic models of engineering applications, used to minimize the difference between thepredictive model and measurements in the sound field. This PhD thesis proposes theapplication of such an inverse approach first validated with a theoretical model, secondlyapplied to a simple interior and finally tested in a generic aircraft cabin with a volume of~11m3. This mock-up is available at the University of Applied Sciences Hamburg and iscomparable to a very light jet as one example for individual transportation. Furthermore, theenclosure is comparable to the one of a small bus which is used in public transport. To be ableto apply the inverse approach, an acoustic simulation model has to be created. Therefore,several measurements have to be performed to identify the input parameters which arenecessary to perform an acoustic simulation (e.g. absorption coefficients of the differentmaterials, the directional characteristics of the loudspeakers). The energy decay curve is alsomeasured in the enclosure, which should be simulated. In the next step, the initial values ofthe simulation model will be adapted using a least-square algorithm, until the energy decaycurves from the acoustic simulation using ray-tracing are in accordance to each other. Theresults of this thesis show the suitability of the approach and give a first idea until whichlower boundary the inverse method based on acoustic ray-tracing can be applied. A lowerboundary of 1kHz could be identified when applied to the simple interior. For this genericproblem, the room impulse responses has been reproduced ideally. For the application to theUniversity of the West of Scotland AbstractIIIvery light jet, the inverse approach also provides a very good agreement of the absorptionparameters above 1kHz (deviation below 5%). The reverberation time is underestimated bythe simulation. The most relevant result of this thesis is that for the first time, such an inverseapproach based on geometrical acoustics using ray-tracing has been successfully applied to ahigh complex, fully equipped generic aircraft cabin after the approach has been theoreticallyvalidated and tested in a simple interior with a low complexity. Moreover a lower boundary of1kHz has been identified until this approach based on ray-tracing can be applied. Withregards to the development of future interiors for public and individual transportation systemsin the industry, this approach might lead to a cost-efficient development due to knowledgeabout the interior acoustics which can be transferred from available interiors in an earlydesign stage. For public transportation systems, the approach has to be applied as well infuture work.