Martin Guski’s research while affiliated with RWTH Aachen University and other places

Performance spaces are characterized by a complex sound field, due to the presence of absorptive and diffusive surfaces. In situ evaluations of the acoustic effects that these surfaces have on the objective acoustic parameters and on sound perception have not yet been fully understood. To this aim, acoustic measurements have been performed in a variable-acoustic concert hall, the Espace de Projection, at the Institut de Recherche et Coordination Acoustique/Musique. These measurements have allowed the effects of one single wall to be determined. A diffusive and a reflective condition of one of the long lateral walls of the shoebox-like hall have been considered, while the other surfaces have been fixed in absorptive mode. Measurements have been carried out at different distances from the test wall, using an artificial head and an array of omnidirectional microphones. Objective acoustic parameters, such as early decay time, reverberation time (T 30), clarity (C 80), definition (D 50), and interaural cross correlation, have been compared between both conditions. In addition to the objective indexes, a perceptual evaluation has been performed using listening tests that had the purpose of determining the maximum distance from a diffusive surface at which acoustic scattering effects are still audible.

In room acoustics measurement, calculating reverberation time from room impulse responses is often done, aided by software. This paper compares the performance of nine software implementations for calculating octave band reverberation time, including two written by the authors. Synthetic impulse responses are used to test decays without and with a steady noise floor, and an impulse response from a real measurement is also used for comparison. Results indicate no significant reverberation time calculation problems for noise-free exponential decays, and for exponential decays leading to a steady noise floor. Frequency selectivity is identified as an area for potential improvement in filter-bank design, and a highly selective octave band filter-bank is shown to be effective without introducing errors. Testing with a real measured impulse response, which had been used in a 2004 study comparing reverberation time analysis implementations, showed greater agreement between software than was found previously. This might reflect an improvement in software performance in the years between the two studies. However, it also might reflect the smaller scale of the present study. Nevertheless, the results can contribute to confidence in current software implementations.

Sweeps used as excitation signals for impulse response measurements show various advantages compared to other excitation signals regarding time variances, non-linearities and signal to noise ratio. The main disadvantage of sweeps is the sensitivity to impulsive noise during the measurement, because the noise affects specific frequencies with rather high noise levels. The fact that the occurrence of impulsive noise is hard to detect complicates the problem. Maximum length sequence or similar techniques, where the signal excites all frequencies (pseudo-)randomly, are more robust against impulsive noise, because any kind of background noise is spread over the full time range of the impulse response with relatively low levels. In this paper a detection method for impulsive noise in sweep measurements is presented. The excitation signal is removed from the recorded signal to obtain an estimation of the background noise during the measurement. In this processed signal the impulsive noise is clearly visible against the stationary background noise. After the measurement, this detection can be automatically executed and allows a repetition of measurement, if necessary.

Patients and medical staff are exposed to high noise levels in ICUs, which may have a negative impact on their health. Due to the diversity of noise sources present, including the operating noise of medical devices, staff conversations and the unwrapping of disposables, noise profiles are varied. Psychoacoustics deals with the analysis of sound, focusing on its effects on physiological perception and stress. The aim of our study was to examine and to classify noise and its psychoacoustic properties in different locations in our ICU at different times. The impact of noise on subjective parameters and stress-related physiological data was also assessed with and without interventional methods. A randomised, controlled, single-blinded clinical trial SETTING: University Hospital, from November 2010 to May 2011. One hundred and forty-four patients in the ICU. In the first part, multidisciplinary psychoacoustic measurement was performed on the patients in our ICU. In the subsequent clinical trial, patients were equipped with effective earplugs, less effective earplugs and no earplugs. Thereafter, active noise cancellation headphones with or without sound masking were employed on a third patient population. Cortisol and α-amylase in saliva, skin conductance measures, vital signs, psychoacoustic analyses and two standardised questionnaires [State-Trait Anxiety Inventory (STAI) and Hospital Anxiety and Depression Scale (HADS)] were assessed. In the first part, the mean ± standard deviation (SD) subjective loudness was 9.2 ± 4.0 sone. Although absolute sound pressure level and loudness were lower during the night, the number of loud events increased significantly. Skin conductance in the earplug groups was significantly reduced in comparison to that in the control population but not the active noise reduction groups. Nevertheless, noise reduction was found to be comfortable for most patients. Noise in the ICU is of high clinical relevance. Diverse noise reduction methods, such as earplugs and active noise cancellation, are available. The avoidance of unnecessary noise, however, should be the primary focus. German Clinical Trials Register (DRKS00000534).

Noise that is part of the measured impulse responses is inevitable but can have a large impact on the evaluation of room acoustic parameters. Five well-known and widely used noise compensation methods are discussed and their performances are compared in this study. Two evaluation approaches are used to test the different methods: A simple parametric model to simulate the envelope of an impulse response including measurement noise, and in a second approach, special designed long-term measurements. These were conducted to be able to evaluate the errors as a function of the noise level. The results that are obtained using the model and the measurement approach are consistent with each other. When these methods are used to suppress noise effects, their performances differ significantly. This is also true for the three methods that are compliant with ISO 3382. Four methods cause systematic errors depending on the peak-signal to noise ratio. The reverberation time is more sensitive to noise than energy parameters such as clarity or definition. A comparison of the different excitation signals that are used for the measurement approach shows that there is no difference with regard to sine sweeps and maximum length sequences, if no impulsive noise or nonlinearities occur.

Besides clarity and definition, the reverberation time is the most common room acoustic parameter. The latter is also an essential quantity for acoustic measuring techniques (i.e., sound insulation, scattering, or diffuse absorption). The unavoidable occurring noise in every measurement is the most significant factor that causes incorrect evaluated parameters. Therefore, it is important to respond to the effects caused by noise. In this study, different noise compensation methods are compared theoretically and based on measurements. At first all methods are investigated theoretically utilizing a simple parametric model impulse response. As a second step, long-term measurements have been conducted in an auditorium to analyze the performance of the different techniques under realistic conditions. Therefore, the excitation signal has been varied in volume to obtain measurements with different noise levels, and the evaluated room acoustic parameters are examined as a function of peak-signal to noise ratio. Theoretical and measured results coincide with each other for each analyzed method. The performances of the examined evaluation methods differ clearly. In particular, the three methods defined by ISO 3382 show different behaviors. The advantages and the limitations of each noise compensation method are presented.

The assessment of the range of uncertainty for room acoustic parameters is an ongoing research topic. The ISO 3382 demands the calculation of the uncertainty according to the GUM. However, the separation and de- termination of the main influence factors and their contribution is still not fully solved. Mainly the position of the sound source and the microphones, background noise and the loudspeaker directivity pattern are currently ad- dressed due to strong deviations observed in the past. In this contribution we explain an existing modeling technique of non-linear systems that is used to simulate the effect of loudspeaker distortion in im- pulse response measurements. Generic impulse responses are used to simulate a room acoustic measurement. We will analyze which room acou- stic parameters are potentially affected. The evaluation of the room acou- stic parameters leads to a scenario with controllable degree of loudspea- ker distortion without the influence of the other uncertainty factors priorly addressed. This simulation approach is validated by measurement results with different amplifications in auditoria.