Hammershøi, Dorte

Møller, Henrik

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    Aalborg UniversitetInformation Interface between Sound Transmission Programs and AuralizationSystemsHammershøi, Dorte; Møller, HenrikPublished in:93rd Audio Engineering Society Convention (AES), San Francisco, Oct. 1-4, 1992Publication date:1992Link to publication from Aalborg UniversityCitation for published version (APA):Hammershøi, D., & Møller, H. (1992). Information Interface between Sound Transmission Programs andAuralization Systems. In 93rd Audio Engineering Society Convention (AES), San Francisco, Oct. 1-4, 1992General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            ? Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            ? You may not further distribute the material or use it for any profit-making activity or commercial gain            ? You may freely distribute the URL identifying the publication in the public portal ?Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: April 25, 2017Binaural Auralization: Head-Related Transfer Functions Measured on Human Subjects 3422 (M-4) Dorte Hammersh!(1i and Henrik M!c1ller Aalborg University Institute of Electronic Systems Aalborg, Denmark Presented at the 93rd Convention 1992 October 1-4 San Francisco This preprint has been reproduced from the author's advance manuscript, without editing, corrections or consideration by the Review Board. The AES takes no responsibility for the contents. Additional preprints may be obtained by sending request and remittance to the Audio Engineering Society, 60 East 42nd St.·. New York, New York 1 O 165-2520, USA. All rights reserved. Reproduction of this preprint, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society. AN AUDIO ENGINEERING SOCIETY PREPRINT BINAURAL AURALIZATION: HEAD-RELATED TRANSFER FUNCTIONS MEASURED ON HUMAN SUBJECTS Dorte Hammersh0i, Henrik M0ller Institute of Electronic Systems, Aalborg University Aalborg, Denmark Summary The result of a room analysis program can be made audible by computer generation of binaural signals. This auralization technique is based on knowledge of the Head-related Transfer Functions (HTFs). Data has been collected for 40 human subjects, measured with 97 directions of sound incidence covering the whole sphere. 1. Introduction Binaural auralization is based on the fact that the human hearing creates the three-dimensional sound image on basis of only two signals: sound pressure at each of the eardrums. On its way to the two ears, a sound wave undergoes a fi ltering caused by diffractions around the head and body of the listener. The filte ring is dependent on direction to the sound source, and may include a delay between the two ears. The hearing is able to "recognize" which filtering a sound wave has been exposed to, and thus determine the direction of sound incidence. In binaural auralization, the filtering which in the free field is carried out by the head and body, is done electronically. For each sound wave being simulated, the sound should be sent through two filters, corresponding to the transmission to each of the two ears. This filtering can be described by the Head-related Impulse Responses (HIRs) h10tt(t) and h,19t,t(t}. Head-related impulse responses are time domain representations of Head-related Transfer Functions (HTFs)- and are of course dependent on the direction of sound incidence. If the incident sound signal is given in time as y(t), then the signals z10tt(t) and z,igh,(t). which should be found in the two ears can be found by convolution: z,.tt(t) = h10tt(t) * y(t) (1) z,igh1(t) = h,igh,(t) * y(t) A detailed description of the binaural auralization process is given in [1 ]. Binaural auralization systems are attractive, because the reproduction is carried out by means of headphones. Sound examples can be recorded on traditional recorders, and they can be brought anywhere for demonstration. The technique requires a detailed knowledge of HTFs and HI Rs. Such data have been collected in our labo ratory for 40 human subjects [2] for several purposes. Apart from our interest in binaural auralization, we have interest in design and evaluation of artificial heads fo r the binaural recording technique. For the same reason , data for a number of artificial heads were collected [3]. which are also applicable for auralization. Since the characteristics of the headphone used is crucial for the correct reproduction, data for several commercial headphones were co llected in a previous study [4]. 2. Methods The methods and definitions used are thoroughly described in [2] , and they will only be summarized in this presentation. Crucial for the character and quality of HTFs and HIRs are the choice of reference point (microphone placement). It was the purpose of a previous study to div ide the free field sound transmission into two parts, a di rectional dependent part that creates all di rectional cues, and a part t hat is independent of direction. A model of the sound t ransmission from a source in the free field to the ear canal was derived [1] , and verified [5]. The directional dependent part of the model consists of the sound transmission from the free fie ld to the Thevenin sound pressure at the ent rance to the ear canal. This pressure does not normally exist physically, but if the ear canal is blocked, for example with an earplug placed with its outer end flush with the ear canal entrance, the Thevenin pressure is found outside the earplug. It was chosen to use the entrance to the blocked ear canal as reference point, because the T hevenin sound pressure contains all the directional dependent information , and it is not influenced by inter-individual diffe rences in the subject's ear canal. The free field sound pressu re at the head center position - but with the listener absent - is used fo r reference. The HTF and HIR are thus defined as: HTF = sound pressure at the entrance to the blocked ear canal sound pressure at center position of head HIR = Fourier-1{ HTF } (2) (3) The HI R is t he impulse response fo r the sound transmission f rom the reference poi nt to the measuring point, and is therefore non-causal for directions where the ear is closer to the sound source than t he middle of the head. A microphone technique, w ith a miniature microphone mounted in an earplug, was developed for the purpose of recording the sound pressure at the entrance to the blocked ear canal. 2 Measurements were carried out on subjects standing in an anechoic chamber. 8 loudspeakers placed in an arc with a radius of 2 meter and an angular distance of 22Y2° were used. Subjects were rotated in steps of 221h0 , to yield measurements from 97 different directions of sound incidence, covering the whole sphere. Impulse responses were measured using Maximum Length Sequences. Two general purpose measuring systems, known as MLSSA, were coupled in a master-slave configuration with a custom designed synchronization unit, allowing sample synchronous measurements for .both ears. 3. Results A few examples of the extensive data material is given in the following. Figure 1 is an example of HTF and HIR shown for one subject with soun coming from the left side. As expected, the signal at the right ear is attenuated compared to the left ear, especially at high frequencies. It is also seen that the impulse response is non-causal for the left ear, because this ear is closer to the sound source than the middle of the head. The interaural t ime difference is approximately 600 µs. The inter-individual variation in the HTFs and HIRs is seen in Figure 2 and Figure 3. In Figure 2 the HTFs for both ears of the 40 subjects are shown for sound incidence in the horizontal plane, 22Y2° left of frontal incidence. In Figure 3 the corresponding HIRs are shown, given a time offset so that they all start at the same time, and the time axis is thus relative. Although the HTFs and HI Rs vary from subject to subject, there is a common general structure in both frequency and time domain. For most auralization systems, it is desirable to minimize the length of the HI Rs. It is diflicult to set an objective criteria to determine the length of the responses. A subjective judgement of our data gives a maximum length of 1 ,5 ms, measured from the beginning of the responses. 4. Summary Presently, we have determined HTFs and HIRs for 40 human subjects and several artificial heads. Our work will proceed with psychoacoustic experiments, for evaluation of the influence of the inter-individual variations. Measurements will be carried out to develop and verify interpolation procedures, and implementation of a binaural auralization system. Effort has been taken to design a standard information interface, between sound transmission programs and auralization systems [6]. so that the auralization system can be used with a variety of sound transmission programs. 3 References [1 ] M0ller, Henrik (1992): Fundamentals of Binaural Technology. Applied Acoustics, Vol. 36 (in press) . [2] Hammersh0i, Dorte, Henrik M0ller, Michael Fri is S0rensen, Kim Alan Larsen (1992): Head-related Transfer Functions: Measurements on 40 Human Subjects. Presented at the 92nd Convention of the Audio Engineering Society, Vienna, 1992. Preprint 3289. [3] Hammersh0i, Dorte and Henrik M0ller (1992}: Artificial heads for free field recording; how well do they simulate real heads? To be presented at the "14th International Congress on Acoustics", Beijing, September 1992. [4] M0ller, Henrik, Dorte Hammersh0i, J0rn Vagn Hundeb0II, Clemen Boje Jensen (1992): Transfer Characteristics of Headphones: Measurements on 40 Human Subjects. Presented at the 92nd Convention of the Audio Engineering Society, Vienna, 1992. Preprint 3290. [5] Hammersh0i, Dorte and Henrik M0 ller (1991 ): Free field sound transmission to the external ear: A model and some measurements. In: Fortschritte der Akustik -DAGA'91 Bochum, April 1991, pp. 473-476. [6] Henrik M0ller: Interfacing room simulation programs and auralization systems. Paper on request at the "International Symposium on Computer Modelling and Prediction of Objective and Subjective Properties of Sound Fields in Rooms". Special issue of Applied Acoustics (in press). 4 Amplitude (dB] Amplitude (Pa/Pa] O dB •-w•--•- ••-"' -~ •----..... • •w• •, •" • '• t 10 dB \:'.\/\ .. . .. , ,,., ',\ ' i .· -, ! ------~ --- --. -- _, ·----· ·~---~--- ------ -l 100 1k 1 Ok 20k 0 Frequency [Hz] Time (ms] Figure 1. Examples of HTF and HIR shown for one subject for sound coming from the left side. Both ears are shown, dashed line is right ear. Amplitude [dB] O dB O dB 100 1k 10k 20k Frequency (Hz] Figure 2. HTFs for 40 human subjects, upper curves left ear and lower curves right ear. Sound incidence in the horizontal plane, 221/2° left of frontal incidence. Amplitude [Pa/Pa] 0 ' 1~----~-a/Pa 0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 Time (ms] Figure 3. HIRs overlayed for 40 human subjects, upper curves left ear and lower curves right ear. Sound incidence in the horizontal plane, 22Y2° left of frontal incidence. Note a time offset has been applied so that they all start at the same time, and the time axis is thus relative. 

Information Interface between Sound Transmission Programs and Auralization Systems

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Information Interface between Sound Transmission Programs and Auralization Systems

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