Ingo Witew

Influence of "omnidirectional" loudspeaker directivity on measured room impulse responses.

Measured room impulse responses (RIR) strongly depend on the directivity of the sound source used for the measurement. An analysis method is presented that is capable of pinpointing the influence of the loudspeakers directivity on a set of RIRs. Taking into account the rotational symmetries of a dodecahedron loudspeaker, it detects the effects that the changing directional pattern induces in the RIR. The analysis of RIRs measured in completely different acoustical environments reveals that the influence of the loudspeakers directivity can still be observed in the very late part of the RIR-even in very reverberant rooms. These results are presented and the consistency with general room acoustical theory is revised and discussed.

A model to predict measurement uncertainties due to loudspeaker directivity and its validation

In order to improve the understanding of uncertainties in measuring the acoustics in auditoria, the influence of a sound sources directivity is investigated. In previous work a model to predict the uncertainties when measuring room impulse responses with sources of a given directivity pattern has been developed. As a result, properties of the measurement environment, i.e. the size of the room, its reverberation as well as the sound scattering behaviour of the room surfaces, were identified to be significant secondary influences. Through extensive series of scale measurements data has been collected in a reverberation room to validate the model prediction. By introducing adjustable partition panels, absorbing and sound scattering surface elements the secondary influence factors were carefully controlled over a large range of values. After a brief explanation of the uncertainty model the results of the validation measurements will be presented. The significance of the different influence factors on the mea...

The perception of apparent source width and its dependence on frequency and loudness

While it is widely accepted that apparent source width (ASW) is an important factor in characterizing the acoustics of a concert hall, there is still a lively discussion on how to refine the physical measures for ASW. A lot of experience has been gathered with interaural‐cross‐correlation and lateral‐sound‐incidence measures during the last years. As a result it was learned that different frequencies contribute differently to the perception of ASW and that the level of a sound also influences the perception of the apparent width of a source. With many technical measures having an influence on the perceptual aspect of ASW, the design of psychometric experiments becomes challenging as it is desirable to avoid the interaction of different objective parameters. In the experiments for the study presented, the perception of ASW is investigated for frequencies ranging from 100 Hz to 12.5 kHz at different levels of loudness. It is shown how the frequency and the loudness of a sound influence the perception of ASW.

Evaluation and improvement of a model to predict the measurement uncertainty due to the directivity of room acoustical sound sources

With the aim to reduce the necessary efforts to empirically determine the uncertainty in room acoustical measurements, in previous work, a model was developed that can predict the uncertainty a directivity of a sound source introduces to a measurement. As part of the validation extensive series of scale measurements have been conducted. In this contribution, the predicted uncertainty based on simulations and the empiric data is compared to each other. The results were used to improve the model. Concluding it will be discussed whether the model is suitable for a reasonable measurement uncertainty discussion.

Experimental investigations on sound energy propagation in acoustically coupled volumes using a high-spatial resolution scanning system

The aim of this work is to study the sound field distribution in an experimental scale-model of two coupled rooms. An automatic scanning mechanism moves a microphone in small grid steps to measure room impulse responses at each grid point. The measurements cover the entire two-dimensional area of the coupled rooms. Sound energy distributions can be analyzed in the form of animated visual displays, revealing sound propagation across the coupling aperture and inside each room. This paper describes the measurement results, and the analysis method, which offer deep insights into the temporal development of a sound field in coupled spaces.

Sampling the sound field in auditoria using large natural-scale array measurements

Suitable data for spatial wave field analyses in concert halls need to satisfy the sampling theorem and hence requires densely spaced measurement positions over extended regions. The described measurement apparatus is capable of automatically sampling the sound field in auditoria over a surface of 5.30 m × 8.00 m to any appointed resolutions. In addition to discussing design features, a case study based on measured impulse responses is presented. The experimental data allow wave field animations demonstrating how sound propagating at grazing incidence over theater seating is scattered from rows of chairs (seat-dip effect). The visualized data of reflections and scattering from an auditoriums boundaries give insights and opportunities for advanced analyses.

Influence of the exact source and receiver positions on the uncertainty of acoustical measurements in concert halls

Acoustical measurements are crucial to backing up theories or supporting conclusions in research and practical applications. In concert halls, however, it is well known that small changes to the receiver position yield a measurable change to the impulse response and the calculated single number parameter. This gives raise to the questions whether these spatial fluctuations limit the validity of measurements and whether there are implications for measurement applications? The presented study discusses how a measurement uncertainty approach may provide a new perspective to these problems. Based on array measurements a relationship has been established that quantifies how a change in measurement position leads to an average change to a room acoustic quantity. Strategies as outlined by the “Guide to the Expression of Uncertainty in Measurement” (GUM) are used to determine the bounds in which valid measurement results can be collected. It is discussed how these findings can be considered in applied measurement...

How reproducible are psychoacoustic listening tests on spatial impression in auditoria

In three psychoacoustic experiments, it was shown that changes in frequency (pitch) and level (loudness) of stimuli lead to significant differences in the perception of apparent source width (ASW). Due to partially identical test conditions in two of the experiments, it is possible to analyze the collected data in regard to stability of ASW perception over repeated testing sessions and to study inter-individual differences. This analysis shows that intra-individual effects of stimulus loudness and pitch are consistent across participants, whereas the absolute level of ASW perception varies between individuals and between experimental sessions. Using this data, possible influences such as listener training effect over repetition of test sessions are discussed. The presented results provide insights into possible improvements of investigating ASW in future studies.

Sampling the sound field in auditoria using a large scale microphone array

Acoustical measurements in auditoria can be laborious with microphones to be placed in large sampling areas that are divided by rows of seating and separated by balconies. As a result, practical studies are often based on measurements of a relatively small number of selected source-receiver combinations. The goal of this paper is to present a new measurement, visualization, and analysis approach for complex wave fields. The measurement apparatus, capable of automatically sampling the sound field in auditoria over a surface of 5.30 m x 8.00 m in high resolution, is described. Based on data collected with the microphone array, a case study of how sound is reflected and scattered from a concert hall’s boundaries is shown. The comparison of repeated measurements with and without the presence of chairs allows a new perspective on grazing sound propagation over theatre seating (“seat dip effect”). The presentation will conclude with the discussion of spatial fluctuations of acoustic properties as a factor of me...

Theoretic considerations on how the directivity of a sound source influences the measured impulse response

In previous investigations, it has been shown that the directivity of a measurement sound source has a significant influence on the measured room impulse response (RIR). Using a specialized method of analysis, the sources influence can be identified even in the very late part of the RIR even in very reverberant environments. These results seem to be surprising at first and contradict intuitive expectations. In this contribution, the findings are briefly discussed, and the congruence with general room acoustic theory is revised and discussed.