Analysis of the binaural speech transmission index in classrooms
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Bachelor Thesis of Wagener, Julia
The point in time when the human ear starts to perceive sound as noise is subjective. Starting at a young age, even children are exposed to loud noises every day. However, conventional and standardized measurement microphones cannot record how complex sound or noise is perceived by humans. The first binaural models, which have been developed in psycho- and room acoustics, are only based on the body geometry and the cognitive perception of adults. It is not known yet whether these models can also be applied to children, since physiology and perception differ from those of adults. Initial research has already been carried out in this field which has not yielded any significant results yet. This bachelor thesis examines the effects of different recording methods on the binaural speech transmission index with an adult and a child artificial head, as well as with a reference microphone. An indirect and a direct measurement method according to DIN EN 60268-16 of May 2012 are used to measure the speech transmission index. The indirect method is based on a mathematical modification of the impulse response using the Schroeder equation, which is used to calculate the modulation transfer function. In the direct measurement method, on the other hand, the speech transmission index is measured using a modulated test signal. The influences of the three different receivers in both measurement methods are compared in this paper and the advantages and disadvantages are shown in consideration of the workload and above all based on the results. The research confirmed that binaural recording techniques correspond more to human speech intelligibility than recording with omnidirectional microphones. It also revealed that there is no significant difference in the values of the speech transmission index for adult and child artificial heads, neither for the indirect nor for the direct measurement method. Since the evaluation results of the indirect and direct measurement methods are comparable, it is recommended to use the indirect measurement method for room acoustic measurements of the speech transmission index, considering the required time.