Experiments on cognitive performance using binaural stimuli Janina Felsa) Michael Vorländerb) Bruno Masieroc) Josefa Oberemd) Institute of Technical Acoustics, RWTH Aachen University, Neustr. 50, 52056 Aachen, Germany Vera Lawoe) Iring Kochf) Institute of Psychology, RWTH Aachen University, Jägerstraße 17-19, 52056 Aachen, Germany Specific acoustic stimuli are required in psychoacoustic experiments on cognitive performance such as auditory selective attention. These stimuli are calibrated with regard to level or loudness and then usually presented dichotically or monaurally using headphones. However, acoustic scenes in our everyday life are binaural and complex. It can be assumed that different sound sources (noise and signals) placed at selected positions in an environment (i.e. a classroom, an open-plan office) have an influence on cognitive performance. In this case the acoustic stimuli need to be presented using binaural techniques in order to carry out experiments using headphones. The goal is to provide artificially generated acoustic scenes in a way that the difference between a real situation and an artificially generated situation has no influence in psychoacoustic experiments on auditory selective attention. For an individual binaural representation the headphones must always be adequately equalized if they are to deliver high perceptual plausibility. Because of this, individual equalizations with different microphone positions in the ear canal are measured. a)
email: email: c) email: d) email: e) email: f) email: b)
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INTRODUCTION
In a cocktail party, a typical situation is that one may listen to a single person while ignoring noise and other conversations in the background. Strong attentional selection is thus needed for the relevant information. Experiments on auditory selective attention are usually carried out monaurally or dichotically. In order to reproduce a real acoustic scene the acoustic stimuli in such experiments should be as natural as possible. This means, a binaural representation of the stimuli and the correct room acoustics (cf. Ruggles and Shinn-Cunninghamm1) should be taken into account. The goal of this study is to provide artificially generated acoustic scenes in a way that the difference between a real situation and an artificially generated situation has little or no influence in psychoacoustic experiments on auditory selective attention. In order to study the influence on the reproduction techniques of acoustic stimuli in such experiments a real situation (using loudspeakers) and an artificially generated situation using headphones will be compared. Aim of the acoustical pre-study to the project is to find a reliable system for binaural reproduction to be used for the listening tests designed by the department of psychology and described later in this paper. In a first step measurements with different kinds of headphones and microphones have been examined. Due to findings concerning the acoustical load of headphones by Cruzado2, Vorländer3, and Møller4 an open headphone (Sennheiser HD 600) has been chosen for binaural reproduction. Furthermore differences between four different types of microphones/measuring points in binaural reproduction concerning the quality of localization as well as the sound quality and realism are to be analyzed. Prior to the listening test measurements were conducted with an artificial head to analyze the repeatability and variance of the headphone transfer function. The results will be presented here, after a short description of the microphones. 2
INTENTIONAL SWITCHING IN AUDITORY SELECTIVE ATTENTION
In a past study, an experimental setup was examined in order to investigate the control of auditory selective attention (see for instance Experiment 1 in Koch et al.5). In the experiment two auditory stimuli (number words between 1-9, without 5) were presented dichotically to the subjects via headphone. The number words were spoken by a female and male speaker. The subjects had to respond selectively to the number word either spoken by the female or male speaker (selection criterion gender) and judge the relevant number word as either smaller or larger than 5. The gender of the task-relevant speaker could change from trial to trial and was indicated by a visual cue prior to the stimulus onset. Additionally, the time interval between the visual task cue and the auditory stimuli (cue-stimulus interval, CSI) was manipulated and could be either short or long (Experiment 2 in Koch et al.5). The experiment showed that shifting attention degrades performance and that these switch costs decrease with prolonging the interval between cue and stimulus. In a second experiment, the task relevant number word was either indicated by a gender cue or by a cue that indicated on which ear the relevant stimulus will appear (selection criterion location). The effect of gender and location (left vs. right ear) was compared. In a future experiment we will study a complete binaural auralized scene. The relevant cue is the position of the stimuli (see Fig 1: switching between left/right, +/- 45° in the horizontal plane and front vs. back). The stimuli will be presented in a distance of 2 m in the horizontal plane around the head of the subjects. Using this experimental setup the reproduction and
equalization techniques of the binaural stimuli will be examined. We will study a loudspeaker reproduction versus individually equalized headphone reproduction (using individual HRTFs (Masiero et al.6 and Pollow et al.7) and individually measured headphone transfer functions) and generic equalization techniques (using artificial heads). This will lead to a new complementary approach to study the tolerances of equalization of signals and stimuli. 3
HEADPHONE EQUALIZATION TECHNIQUES
3.1 Microphones Four different microphones and positions are taken into account in this study. Hammershøi among others showed in Hammershøi8 already that measurements of head-related transfer functions, HRTF, can be carried out at the entrance of the ear canal without any loss of spatial information. Either the microphone is placed inside the ear canal or otherwise the microphone is located outside the ear and connected to the ear canal by a probe. Following the used arrangements are explained. The microphones used for the measurements are Sennheiser products called KE4 211-2 and KE3. The first type was used for the probe microphones. A tube connected to a cone that holds the microphone was built at the Institute of Technical Acoustics, Aachen (ITA). Two lengths of tubes were constructed. A shorter tube was built to measure at the entrance of the ear canal, while a longer one was built to measure at about halfway the length of the ear canal. The second microphone type was placed directly in the entrance of the ear canal without the help of any probe. It was fixated at this position either with an Open Dome, which gives the possibility to measure at the entrance of the ear without closing the ear canal (i.e. changing the impedance at the ear drum). Usually it is used for listening aids. On the other hand it was fixed by an individually molded cast to block the ear canal, which is also called “otoplastic”. 3.2 Variability of headphone repositioning The variability of headphone repositioning, as well as microphone repositioning have been analyzed. The microphone and the headphone have been removed from the head and have been replaced in as close as possible to the same and best fitting. The measurement has been conducted on an artificial head, which makes it difficult to find the most comfortable position. This procedure has been carried out in total ten times for every microphone. In Fig. 2 the mean value and its uncertainty due to gauss' error propagation is shown. The uncertainties can be seen for all microphone techniques. For open and closed ear canal the deviations are very small for lower frequencies, about +/- 3 dB in the range of 5-10 kHz and above these frequencies even up to +/-7 dB, which leads to an overall acceptable range of tolerance. However the probe microphones show an enormous discrepancy. The positioning of the long probe microphone is rather stable, since it is limited by the shaping of the ear canal, where the short probe microphone which is placed at the entrance of the ear canal has more opportunities to rotate and twist. The measurements prove this assumption and show a much higher deviation for the short probe microphone, from +/-5 dB in frequencies up to 10 kHz and larger standard deviations above, than for the probe microphone placed in the middle of the ear canal. The deviations of the long probe microphone are at least +/-2 dB greater than the deviations of the miniature microphones. In a second analysis only the headphone has been replaced ten times using the procedure described above. The variations tend to be smaller than 2 dB for all microphones up to 10 kHz.
In detail the miniature microphone and the open dome seem to have a slightly greater variability. For frequencies above 10 kHz the standard deviation can even reach +/- 10 dB. This deviation is caused, however, by the different positioning of the headphone. These results show like other publications that a headphone equalization is necessary for all microphone techniques (cf. e.g. Masiero9, Møller10). 3.3 Pressure Division Ratio and Equalization The Pressure Division Ratio, short PDR, was introduced by Møller and Hammershøi10 and further analyzed by Völk11. This article will not present a detailed description of this parameter, but will summarize its methods shortly and use it to evaluate measurements with the different kinds of microphones. An artificial head with an ear coupler was used to perform these measurements. The transfer function from a loudspeaker positioned in front of the head to the build-in microphone inside the head and to the probe microphone/ miniature microphone was measured. Furthermore the transfer function from the headphones to the microphones is measured. Simplified the pressure division ration can be written as
PDR
TF TF TF TF
TF TF
∙
TF TF
,
(1)
where TF stands for transfer function, ls for loudspeaker, hp for headphones, mic denotes the probe or miniature microphone and refmic the build-in head microphone. For more detailed and complex information see Völk11. The aim is that the PDR equals one over the whole frequency range (accordingly: 0 dB). The measurements completed with the open ear canal show the best results. Up to 14 kHz the PDRs show no greater deviation than +/-2 dB. Up to 8 kHz the measurements done with the open dome show good results as well, with deviations smaller than +/-2 dB. Results found by Møller and Hammershøi in Møller10 report similar effects. They did not measure with probe microphones though. As to see in the named figures the pressure division ratios tend to have greater deviations from 0 dB in the frequency range above 1 kHz/2 kHz. While the findings for the short probe microphone seem to be acceptable up to 7 kHz, the results for the long probe microphone vary enormously. As already published in Masiero9 a robust headphone equalization was developed to balance great variations due to repositioning. Through further improvements which are not completed yet the following pressure division ratios calculated with the equalization are compared to the measured PDR (c.f. Fig. 3). For the measurement with the open dome and the closed ear canal the findings are very good and only small differences between the PDR calculated only from measurements and the PDR calculated including the equalization are visible. Völk12, who only used a closed ear canal, had similar outcomes. 4
CONCLUSIONS AND OUTLOOK
In this study the uncertainties and variations of four different procedures and microphones for the individual equalization of headphones are examined. On account of the greater uncertainties, variations, errors and problems due to the measurements with the probe microphones, which have been found in the measurements, in later listening experiments they
will not be taken in consideration. Next to the presented results a light coloration due to the damping of higher frequencies by the tube has been noticed and is also mentioned by Cruzado2. Even though the measurements with the open dome show a slightly worse performance, this measuring procedure will be continued to use for a listening test, because of its advantages in mainly unchanged impedance. The next listening test will study the ability of localization of sound sources using the individual headphone equalization and individual measured HRTFs versus loudspeaker presentation as well as the naturalness and acoustic color. 5
ACKNOWLEDGEMENTS This work is supported by the Deutsche Forschungsgemeinschaft DFG Grant FE 1168/1-1.
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REFERENCES
D. Ruggles and B. Shinn-Cunningham, “Spatial selective auditory attention in the presence of reverberant energy: Individual differences in normal-hearing listeners,” JARO-Journal of the Association for Research in Otolaryngology, vol. 12, no. 3, pp. 395–405, 2011. 2. C. G. Martin Cruzado, “Influence of the acoustic impedance of the headphone on psychoacoustic effects,” Master’s thesis, RWTH Aachen, 2006. 3. M. Vorländer, “Acoustic load on the ear caused by headphones,” Journal of the Acoustical Society of America, vol. 107, pp. 2082–2088, 2000. 4. H. Møller, C. B. Jensen, D. Hammershøi, and M. F. Sørensen, “Design criteria for headphones,” in 94th Audio Engineering Society Convention, 1995. 5. I. Koch, V. Lawo, J. Fels, and M. Vorländer, “Switching in the cocktail party: Exploring intentional control of auditory selective attention,” Journal of Experimental Psychology, 2010. 6. B. Masiero, P. Dietrich, M. Pollow, J. Fels, and M. Vorländer, “Design of a fast individual HRTF measurement system,” in Fortschritte der Akustik - DAGA 2012, (Darmstadt), 2012. 7. M. Pollow, B. Masiero, P. Dietrich, J. Fels, and M. Vorländer, “Fast measurement system for spatially continuous individual HRTFs,” in 4th Int. Symposium on Ambisonics and Spherical Acoustics, (University of York, UK), Sunday 25th March – Tuesday 27th March 2012 2012. 8. D. Hammershøi and H. Møller, “Sound transmission to and within the human ear canal,” Journal of the Acoustical Society of America, vol. 100, pp. 408–427, 1996. 9. B. Masiero and J. Fels, “Perceptually robust headphone equalization for binaural reproduction,” Audio Engineering Society, vol. 8388, p. 7, 2011. 10. H. Møller, D. Hammershøi, C. B. Jensen, and M. F. Sørensen, “Transfer characteristics of headphones measured on human ears,” Journal of the Audio Engineering Society, vol. 43, pp. 203–217, 1995. 11. F. Völk, “System theory of binaural synthesis,” Audio Engineering Society, vol. 8568, p. 17, 2011. 12. F. Völk, “Messtechnische Verifizierung eines datenbasierten binauralen Synthesesystems,” in DAGA 2010, 2010.
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Fig. 1 – Possible positions of the acoustic sources in an experiment of auditory selective attetention.
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Fig. 3 – Pressure Division Ratio (PDR) measured and equalized for the four different types of microphones.
