Gottfried Behler

Variable Directivity for Platonic Sound Sources Based on Spherical Harmonics Optimization

Sound sources for measurements in room acoustics are of omni-directional type, in general. With respect to auralization applications, an omni-directionally measured room impulse response may not be the ideal choice since it does not represent the real life situation playing an instrument in the room. To achieve the directivity of a real source (like a musical instrument or human voice) with a technical sound source (like a loudspeaker) requires either to copy the entire body and the sound radiation (i.e. the surface velocity) of that particular source or to reproduce the directional pattern of the radiation using a multiple source configuration like a dodecahedron or icosahedron loudspeaker array with independent excitation of each transducer. The advantage of the latter method is obvious since one single source is able to provide a large variety of different directivities by simply changing the excitation profile. To maintain the appropriate excitation of each individual transducer, different approaches can be made. In this paper a method is described using a discretely measured target radiation pattern to calculate the frequency dependent excitation signals for each transducer. Hereby, the directivity pattern of the source transducers and a phase optimization of the energy averaged radiation of musical instruments are used. The advantage of this method is a very flexible computation that is able to match the radiation pattern at the points of measurement very well. The resulting input filters for the platonic sound source can be used either for a real time convolution or offline processing of measured signals.

Repeatability and Reproducibility of In Situ Measurements of Sound Reflection and Airborne Sound Insulation Index of Noise Barriers

In Europe, in situ measurements of sound reflection and airborne sound insulation of noise barriers are usually done according to CEN/TS 1793-5. This method has been improved substantially during the EU funded QUIESST collaborative project. Within the same framework, an inter-laboratory test has been carried out to assess the repeatability and reproducibility of the newly developed method when applied to real-life samples, including the effect of outdoor weather variability and sample ageing. This article presents the statistical analysis of the inter-laboratory test results, and the values of the repeatability and the reproducibility, both in one-third octave bands and for the single-number ratings. The estimated reproducibility values can be used as the extended measure of uncertainty at the 95% credibility level in compliance with the ISO GUM. The repeatability and reproducibility values associated with airborne sound insulation are also compared with the corresponding values for laboratory measurements in building acoustics and an acceptable agreement is found.

Sound source with adjustable directivity

Omni‐directional sound sources are used to measure room‐acoustical parameters in accordance with ISO 3382. To record a detailed room impulse response (RIR) with the aim of auralization, an extended frequency range is required that is not covered by the often‐used building acoustics sound sources. To obtain this target, a loudspeaker with dedicated sources for low, mid, and high frequencies was designed, providing a smooth omni‐directionality up to 6 kHz and a usable frequency range from 40 Hz up to 20 kHz. However, a realistic auralization of sources like musical instruments is not possible with an omni‐directional measured RIR. To include the directional characteristics of instruments in the measuring setup, the directivity of the sound source has to be frequency dependent and must be matched to the (measured) directivity of the real instrument. This can be obtained by using a dodecahedron loudspeaker with independently operating systems and an appropriate complex FIR filtering of the frequency response ...

Generation and analysis of an acoustic radiation pattern database for forty-one musical instruments

A database of acoustic radiation patterns was recorded, modeled, and analyzed for 41 modern or authentic orchestral musical instruments. The generation of this database included recordings of each instrument over the entire chromatic tone range in an anechoic chamber using a surrounding spherical microphone array. Acoustic source centering was applied in order to align the acoustic center of the sound source to the physical center of the microphone array. The acoustic radiation pattern is generated in the spherical harmonics domain at each harmonic partial of each played tone. An analysis of the acoustic radiation pattern complexity has been performed in terms of the number of excitation points using the centering algorithm. The database can be used both for studying the radiation of musical instruments itself, as well as for the implementation of radiation patterns in room acoustical simulations and auralization in order to obtain a spatial excitation of the room closer to reality.

Experimental investigations of the influence of room acoustics on the teacher’s voice

Teachers belong to the group of professional voice users who often suffer from voice disorders. One reason for a significantly increased prevalence of voice problems can be poor room acoustical conditions in the classrooms. In this study, four rather reverberant and loud classrooms in a primary school in Aachen were analyzed using measurements of the reverberation time, T20, and the speech transmission index, STI. About half of the school’s teachers were investigated with respect to their voice status by using phoniatric, logopedic, and objective voice analysis methods. The prevalence of voice problems in this group was found to exceed previous studies where subjective voice quality was rated. In a second part of this joint project the change of voice quality during the teachers’ working day was analyzed. Two of the four rooms were acoustically optimized. Members of two groups of teachers with and without voice problems were recorded before and after teaching in either one of the acoustically poor rooms o...

Generation of a reference radiation pattern of string instruments using automatic excitation and acoustic centering

Radiation patterns of musical instruments are important for the understanding of music perception in concert halls, and may be used to improve the plausibility of virtual acoustic systems. Many attempts have been performed to measure the spatial response of musical instruments using surrounding spherical microphone arrays with a limited number of microphones. This work presents a high-resolution spatial sampling of the radiation pattern of an electrically excited violin, and addresses technical problems that arise due to mechanical reasons of the excitation apparatus using acoustic centering.

Design of the new public address (PA) system for the cathedral of Münster, Germany

One of the most renowned cathedrals in Germany, the Dom St. Paul in Munster was completely closed for renovation for almost one year. During this time the entire electro acoustical sound reinforcement system has been renewed. As for most buildings of this type the acoustical situation is far away from optimal. This mainly is due to a huge reverberation time which makes the understanding of spoken words almost impossible. Moreover, the old concept of sound reinforcement by using distributed loudspeaker systems all over the church is not satisfying anymore with respect to nowadays demands for quality and speech intelligibility. Due to the situation that the number of people in the cathedral during service times is varying from only some hundreds to over 2500 a more flexible PA system is required, that takes into account that only occupied areas inside of the cathedral should be supplied with amplified sound. To achieve the target an entirely new concept for the sound reinforcement based on digital signal di...

Representation of the musical instruments directivity using dodecahedron loudspeakers.

Room‐acoustical measurements in general are performed with omnidirectional sound sources. With respect to auralization, such an impulse response may not be ideal since it does not represent the situation playing an instrument in the room. To achieve the directivity of a real source (such as an instrument or human voice) with a technical sound source (a loudspeaker) requires either to copy the body and the surface velocity distribution of that particular source or to reproduce the directional pattern of the radiation using a multiple source configuration like a dodecahedron loudspeaker array with independent excitation of each transducer. The advantage of the latter method is obvious since one single source is able to provide different directivities by changing the excitation profile. To maintain the appropriate excitation of each individual transducer, different approaches can be made. The method described here uses spherical harmonics decomposition of the target radiation pattern and a subsequent calcula...

Variable Source‐Directivity Using Dodecahedron‐Loudspeakers

For room‐acoustical measurements dodecahedron loudspeakers are commonplace to achieve a uniform directivity. Therefore all transducers are fed with the same signal. If the signals for the twelve transducers are individually adjustable, the variation of amplitude and phase offers the possibility to achieve a predefined directivity. The goal is to calculate the twelve frequency dependent amplitude‐ and phase‐coefficients for any given directivity with the least possible error. A simple approach like superposition unfortunately does not reveal a correct result, since all transducer interact with each other. The decomposition of spherical functions into spherical harmonics, however, leads to an analytic solution for the prediction of the sound radiation. The acoustical components ‐ like sound pressure and sound velocity ‐ are split up into weighted, orthogonal base functions which can be combined in a way that the mutual coupling between different membrane vibrations is respected. Under these conditions compl...

Real‐time audio rendering system for virtual reality

Systems of virtual acoustics are more and more coming into the focus of research and application. Creating a virtual sound scene with spatially distributed sources requires a technique for adding spatial cues to audio signals and an appropriate reproduction. A real‐time audio rendering system is introduced that combines room acoustics, dynamic crosstalk cancellation, and multitrack binaural synthesis for virtual acoustical imaging. The room acoustics component takes into account the full up‐to‐date algorithmic approach of specular reflections and scattering, but with real‐time processing. The real‐time performance of the software was reached by introduction of a flexible framework for the interactive auralisation of virtual environments. The concept of scene graphs for the efficient and flexible linkage of autonomous‐operating subscenes by means of so‐called portals has been incorporated into the existing framework, combined with an underlying BSP‐tree structure for processing geometry issues very fast. U...