Volker Mellert

Smoothing individual head-related transfer functions in the frequency and spatial domains

When re-synthesizing individual head related transfer functions (HRTFs) with a microphone array, smoothing HRTFs spectrally and/or spatially prior to the computation of appropriate microphone filters may improve the synthesis accuracy. In this study, the limits of the associated HRTF modifications, until which no perceptual degradations occur, are explored. First, complex spectral smoothing of HRTFs into constant relative bandwidths was considered. As a prerequisite to complex smoothing, the HRTF phase spectra were substituted by linear phases, either for the whole frequency range or above a certain cut-off frequency only. The results indicate that a broadband phase linearization of HRTFs can be perceived for certain directions/subjects and that the thresholds can be predicted by a simple model. HRTF phase spectra can be linearized above 1 kHz without being detectable. After substituting the original phase by a linear phase above 5 kHz, HRTFs may be smoothed complexly into constant relative bandwidths of 1/5 octave, without introducing noticeable artifacts. Second, spatially smoother HRTF directivity patterns were obtained by levelling out spatial notches. It turned out that spatial notches do not have to be retained if they are less than 29 dB below the maximum level in the directivity pattern.

Approximation of dummy‐head recording technique by a multimicrophone arrangement

Head‐related recording and reproduction systems rely on the two‐ear microphones of a dummy head. However, the dummy head is sometimes difficult to use, e.g., due to size and weight. Additionally, its geometry, which gives the important diffraction pattern information for sound localization, is fixed. A microphone array, which approximates the head‐related transfer functions, has fewer restrictions. Theoretical evaluation of a simple microphone array, which takes psychoacoustic‐determined frequency weights for phase and magnitude into account, is presented. Results of practical sound localization test with this array are reported for sound sources in the horizontal plane. [Work supported by the German Science Foundation (DFG).]

Regularization approaches for synthesizing HRTF directivity patterns

As an alternative to traditional artificial heads, it is possible to synthesize individual head-related transfer functions (HRTFs) using a so-called virtual artificial head (VAH), consisting of a microphone array with an appropriate topology and filter coefficients optimized using a narrowband least squares cost function. The resulting spatial directivity pattern of such a VAH is known to be sensitive to small deviations of the assumed microphone characteristics, e.g., gain, phase and/or the positions of the microphones. In many beamformer design procedures, this sensitivity is reduced by imposing a white noise gain (WNG) constraint on the filter coefficients for a single desired look direction. In this paper, this constraint is shown to be inappropriate for regularizing the HRTF synthesis with multiple desired directions and three alternative different regularization approaches are proposed and evaluated. In the first approach, the measured deviations of the microphone characteristics are taken into account in the filter design. In the second approach, the filter coefficients are regularized using the mean WNG for all directions. The third approach additionally takes into account several frequency bins into both the optimization and the regularization. The different proposed regularization approaches are compared using analytic and measured transfer functions, including random deviations. Experimental results show that the approach using multiple frequency bands mimicking the spectral resolution of the human auditory system yields the best robustness among the considered regularization approaches.

Model Calculations With a Fast Field Programme and Comparison With Selected Procedures to Calculate Road Traffic Noise Propagation Under Defined Meteorological Conditions

National guidelines for calculating noise propagation are still in use though progress was made to develop a unified scheme for sound propagation outdoors by taking more physical parameters of the propagation path into account. Besides comprehensive models simpler and faster calculations are under development in order to be used as an engineering model. Widespread classical engineering models from different European countries are compared in this study with physical wave propagation from a defined line source representing traffic noise. The numerical model is based on a fast-field programme. This scheme allows for a very fast calculation of wave propagation in a layered medium including complex boundary conditions. Source properties are carefully adjusted for comparison and the respective meteorological conditions of national guidelines are taken into account. It turns out that the different guidelines are in good agreement with exact calculations up to distances about 1000 m. The so-called downwind condition gives an excellent estimate for the conditions most favourable for sound propagation. The fast numerical calculation scheme enables easy systematic variation of e.g. source properties, meteorological conditions and ground impedances for any sensitivity analysis.

Sound and vibration at low frequencies

Equal‐loudness‐level contours are assumed to reflect the frequency sensitivity of the auditory system. Studies on equal‐loudness‐level contours show considerable differences even when the same measurement method is used. Recent studies [Gabriel et al., DAGA 96; Reckhardt et al., DAGA 97] report on the influence of measurement method, threshold, and choice of test tone level. In this study, an adaptive 2‐AFC method [Reckhardt et al., DAGA 97; Verhey et al., DAGA 97] is used for measuring equal‐loudness level contours in the frequency range of 16 to 160 Hz at 60 phon. The data are measured in a pressure chamber for 12 normal‐hearing subjects. Results are higher than the standard curve specified in ISO 226. In a study of Reckhardt et al. it is shown that the interindividual variance is decreased by taking the individual threshold of hearing into account. The results from low‐frequency measurements support this finding. In addition, this study investigates the influence of vibration on loudness perception. Eq...

Investigations of Cavity Noise Generation on a Cylinder

Cavity noise generation is of particular interest for e.g. the aeronautical and automotive industry where a lot of different shaped cavities occur on curved surfaces. In this work the flow over a small rectangular cavity (L/D ≈ 1) on a cylinder at velocity ramps up to 45 m/s was investigated in an acoustic wind tunnel. Acoustical and PIV measurements indicate a sudden transition of the circulating flow to a turbulent boundary layer at the typical Reynolds number leading to cavity noise. The onset of the acoustic radiation is contemporaneous with this transition and shows a strong hysteresis. Pressure measurements directly on the cylinder surface reveal a hydrodynamics and acoustics coupling point in front of the cavity.

Free‐field reciprocity calibration of microphones in ultrasonic frequency range

Exposure to airborne low‐frequency ultrasound occurs in many industrial applications such as cleaning, welding plastics, measuring distances in buildings, and by using consumer devices such as camera rangefinders, automatic door openers, parametric ultrasound loudspeakers, etc. Exposure to ultrasound in air may be dangerous to the hearing and may have negative bio‐effects on humans. Care should be taken in its use. In order to establish appropriate limits and to measure the output of ultrasound devices, there is a need to develop a sound‐pressure standard in the frequency range from 20 kHz to about 160 kHz. A calibration project for quarter‐inch microphones by the reciprocity method has been started in the PTB, and a new automated measurement setup for free‐field reciprocity calibration of quarter‐inch microphones has been established. A procedure for a free‐field reciprocity calibration of quarter‐inch microphones in the frequency range 20 to 160 kHz is described. First results of free‐field calibration of quarter‐inch microphones are presented, giving the sensitivities and the repeatability of results. The effects of reflections and cross talk on the accuracy of the measurements will be explained.

Factors influencing equal‐loudness‐level contours

Data on equal‐loudness‐level contours taken from the literature show considerable differences between the results of different laboratories, even when using the recommended alternative forced‐choice (AFC) method. Some of these differences can be attributed to free parameters in the procedures that the experimentator could change. This work investigates the influence of the experimental setup on the resulting curves obtained with an adaptive two‐alternative forced‐choice procedure for the 30‐ and 50‐phon curves in the range from 200 Hz to 1 kHz. For the adaptive procedure, the amount of change in the resulting equal‐loudness‐level contours is measured for different free parameters (e.g., initial step size, starting level). It is shown that for a simple adaptive procedure the influence of the starting level is smaller than the range effect when using the method of constant stimuli. The effect of the starting level is reduced when using large initial step size in the adaptive procedure. However, the effect c...

Effects of helicopter and aircraft interior noise and vibration on passengers’ comfort sensation and subjective well‐being

Apparently passengers’ sensation of comfort and subjective well‐being are key concepts in the research on user acceptance of helicopter and aircraft systems. The design and development of advanced air transportation technologies require a fundamental, theory‐based understanding of these concepts. From the passengers point of view, helicopter and aircraft interior noise and vibration are primary physical factors underlying the perception of comfort and well‐being. In various judgmental contexts the influence of exposure to combined interior noise and vibration in helicopters and aircrafts will be analyzed by means of combined psychoacoustical, socioacoustical, and psychological research methods: Field experiments in a flight simulator and during real flight situations as well as laboratory experiments using a sound and vibration reproduction system will be carried out in order to examine the interactive effects of noise and vibration. The aim is to develop a passenger comfort or well‐being index for helico...

Multimicrophone arrangement as a substitute for dummy‐head recording technique

Natural sound is usually recorded in head‐related stereophony by ear microphones of a dummy head in order to ensure a virtual reality, e.g., for subjective sound‐evaluation procedures. However, in many applications the use of a dummy head is disturbing. Additionally, it is difficult to change the geometry or shape of a dummy head in order to adjust a recording individually. The spatial information of the sound field, which is perceived in appropriate stereophonic reproduction, is encoded in the two signals of the ear microphones by recording the modifications of the wave field from the near‐field diffraction of the obstacle ‘‘dummy head.’’ A diffraction pattern can be approximated by a sufficient number of monopole sources at appropriate locations, and adjusted in amplitudes and phases. This method of source substitution is used for an arrangement of pressure microphones. Each microphone signal is processed twice and added in the stereophonic channels. If locations and transfer functions are optimized, th...