Pascal Dietrich

On the in situ impedance measurement with pu-probes—Simulation of the measurement setup

High-quality numerical simulations in room acoustics require a detailed knowledge of the acoustic reflection characteristics of the materials in the room, in order to realistically model the interferences between multiple sound reflections at the room boundaries. While different standardized measurement methods exist for the determination of the absorption coefficient and reflection factor these methods can generally not be applied in situ. Thus time-consuming laboratory measurements and the supply of material samples are required. Driven by the obvious demand for a reliable in situ measurement technique, a pu-probe based method has emerged during the last years, which derives the reflection factor based on the simultaneous measurement of sound pressure and velocity. However, previous investigations of the setup and publications by other authors have shown that the measurement results are affected by various uncertainty factors. The present study aims at the identification, separation, and quantitative assessment of the uncertainty factors related to reflection and diffraction effects at the loudspeaker, sensor, and the absorber geometry. Therefore, a purely simulative approach will be used that replicates the actual measurement situation in every detail, including the geometries of sensor, loudspeaker, and absorber. The simulation setup is validated by measurements and is used to systematically separate the different uncertainty factors.

Including directivity patterns in room acoustical measurements

Room acoustical measurements according to ISO 3382 require source and receiver to be of omnidirectional sensitivity. Therefore, radiation patterns of natural sources and receivers (although audible) are not accounted for when using the obtained room impulse responses (RIRs) for room acoustic analysis or even auralization. In order to include this spatial information in the RIR, it is necessary to measure the RIR for each pair of radiation patterns of source and receiver. This could be done by electronic beamforming during the measurement using array systems or by mechanical modification of the transducer (as, e.g., a dummy-head with its corpus). In this contribution, an alternative approach is shown, using the superposition of a set of sequential measurements done with a spherical sound source. At the cost of longer measurement times, the obtained data can be used universally to synthesize RIRs of arbitrary directivity up to a certain maximal spatial resolution, as long as the room is considered as a line...

Synthesis of room impulse responses for arbitrary source directivities using spherical harmonic decomposition

The acoustics of rooms can be accurately described by the room impulse response, stating the temporal succession of the room reflections encountered for a specific transfer path in the room. The measurement is usually performed with a single channel spherical loudspeaker—ideally with an omnidirectional radiation pattern—providing an objective measure for a given combination of a source and receiver position in a room. The measurement result can be used to auralize sources in the measured room by convolution with a dry signal or to derive standardized room acoustical parameters. However, every acoustic source has a distinct frequency dependent radiation pattern, which is to some extent responsible for the perceived characteristics of the sound source. The conventional measurement of room impulse responses does not account for this effect. Hence, a method is proposed to obtain room impulse responses for specific radiation patterns by superposing measurement results of an electro-acoustic sound source with a known radiation pattern for different orientations. The limitations of the method are examined in terms of frequency range and spherical harmonic order. The assumption of an LTI system is studied by special measurements. Finally results of measured room impulse responses for a target source are compared to the synthesis results obtained by the proposed method.

Black box measurements − Using a family of electrical circuits as a tool for self-guided learning in acoustical engineering

A partnership between Brazils first undergraduate program in Acoustical Engineering and the Institute of Technical Acoustics of RWTH Aachen University yielded a didactic project that uses the engineering software MATLAB with the ITA-Toolbox to teach acoustic measurements. Simple electrical circuits are used to mimic typical behavior of acoustical systems. This low-cost solution has proven to be didactically very effective since it helps students to identify themselves with the measurement tasks. Two hardware solutions were developed-a simple oscillator circuit integrated into connectors of audio cables and a desktop box containing seven different transfer characteristics ranging from ideal linear and time-invariant to nonlinear and time-varying behavior. Undergraduate students of Acoustical Engineering used both devices in classroom experiments for self-guided learning by comparing their results to published results. Students were able to learn the fundamental concepts of acoustical measurements and to h...

Comparison of Strategies to Model Spatial Fluctuations of Room Acoustic Single Number Quantities

In a number of independent empiric studies it was shown that room acoustic single number parameters vary severely with small changes of the source and microphone position. Presently there is no evidence that these spatial fluctuations can be modelled using simulated impulse responses. As a result there is limited knowledge about the origin and the contributing influence factors of this variance over space. In this contribution the results of simulations using wave based as well as ray tracing simulations are compared to each other. It will be discussed if these simulations are able to predict the fluctuations that were found in measurement series taken in a number of different auditoria.

Measurements of electrical transfer characteristics with soundcards as classroom activity

Measurements of transfer functions and impulse responses of different types of systems are an important part in acoustics and should be an integral part in acoustics education. Nevertheless, usually expensive equipment is required to carry out such measurements making it difficult to realize hands-on measurement classes. Contrarily the transfer characteristics of electrical systems can be measured much easier. Electrical systems and acoustics meet in electro-acoustic equipment such as soundcards, which can be found as onboard and external devices and in a wide variety. Input and output of soundcards can be connected by different types of transmission systems that might use different types of electrical circuits adding a variety of transfer characteristics. The inherent transfer characteristics of the given soundcard can be analyzed and compensated before assessing the transfer characteristics of different electrical circuits (black boxes). Thus, the experimental measurement of soundcards together with dif...

Measurement uncertainty of scattering coefficients in scale models.

The measurement of the scattering coefficient is defined in ISO 17497‐1. The method evaluates random‐incidence absorption coefficients obtained from measured reverberation times for four different configurations. Measurements in a scale model reverberation chamber are preferred to real scale chambers for several reasons. Requirements in terms of time invariance are easier to fulfil and the handling of samples is simpler. Although the use of a scale model renders the measurements easier, results are still affected by an uncertainty that is currently not considered when using these results for ongoing simulations or predictions. Moreover, air absorption at very high frequencies causes the measured reverberation times to be very similar, thus influencing the validity of the results in this frequency range. This paper aims to determine the measurement uncertainty of the scattering coefficients in a scale model reverberation chamber. A simplified uncertainty model will be presented taking into account the unce...

Influence of various uncertainty factors on the result of beamforming measurements.

In a previous publication [Mller‐Trapet and Dietrich (2009)], a virtual measurement environment for sound‐source localization on vibrating structures was presented. Based on surface velocity data obtained from laser‐scanning‐vibrometry measurements, the boundary‐element‐method is used to simulate the sound radiation from a vibrating plate toward a microphone array under ideal conditions. The advantage of this approach is that the measurement conditions can be perfectly controlled and real sources can be considered, without restrictions on the type of source. The virtual measurement environment will now be used to investigate the effect of some of the uncertainties that can be encountered during beamforming measurements. For the most common planar array geometries, the beamforming source maps will be calculated for varying signal‐to‐noise ratios and different array imperfections (uncertainties in the location of the microphones and deviation from the omni‐directional directivity pattern of the microphones)...

Modeling Measurement Uncertainty in Room Acoustics

This paper investigates a way of determining and modeling uncertainty contributions in measurements of room acoustic parameters, which are commonly used to describe the acoustic situation of a room in an objective manner. If the range of uncertainty and the confidence interval are not given, the results remain incomparable to other measurement teams, since modern PC-based measurements still show appreciable sources of measurement errors. The Guide to the Expression of Uncertainty in Measurement (GUM) defines a unified guideline for determining uncertainties in all fields of measurement. Its application is increasingly required by modern measurement standards. However, the GUM procedures have not been applied to room acoustics yet. Hence, a scalable linear approach for calculating the combined uncertainty of room acoustic parameters with regard to the input quantities is proposed. In-situ measurement results of specially designed experiments show the significance of the main influence factors and are used to build the uncertainty budget.