Acoustic characteristics of a miniature dynamic speaker driver unit MT006B for measurement of head-related impulse responses by reciprocal method

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We measured the input impedance characteristics, input voltage versus output sound pressure characteristics, harmonic distortion characteristics, frequency characteristics, and impulse response of a currently available miniature electrodynamic driver unit (Foster Electric, MT006B) when used as a loudspeaker with an open space load. The nominal input impedance of the driver unit was 16 Ω, and its resonance frequency f0 was 2.7 kHz. At f0, the range of the input voltage level over which the output sound pressure of the driver unit increased linearly was −10 dBV (reference 1 V) or less. Below f0, the frequency response of the driver unit decreased by 15dB/oct, while above f0 it did not drop significantly. When the input signal level to the driver was −12 dBV, the signal-to-noise ratio between the sound pressure level produced by the driver and the background noise level of the soundproof room was 0 dB at a frequency of 130 Hz for a distance 0.2 m, and at 230 Hz for a distance 1.0 m. These results indicate that the MT006B can be used as an earplug speaker for a fast head-related transfer functions measurement system via reciprocity.

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In this paper, we describe a dataset of head-related transfer functions (HRTFs) measured at the Research Institute of Electrical Communications, Tohoku University. The current dataset includes HRTFs for 105 subjects at 72 azimuths  13 elevations of spherical coordinates. Anthropometric data for 39 subjects are also included. The measurement and postprocessing methods are outlined in this paper. These data will be freely accessible for nonprofit academic purposes via the Internet. Moreover, this dataset will be included in an international joint project to gather several HRTF datasets in a unified data format.
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In this work, head movements for three human subjects were measured simultaneously during head-related transfer function (HRTF) measurement for a period of 95 min each. The subjects' heads moved in all directions during measurements. Excessive head movements were observed in the pitch and yaw directions. Head movements in the roll direction were small for all subjects. Specifically, the head position at the beginning and end of HRTF measurement differed by less than 1° in roll but by as much as 10° in pitch and yaw. Consequently, HRTFs for the front position measured at the beginning and the end of the measurement session differed by 4-6 dB in spectral distortion. These results reveal that when no head-support aids are used, HRTFs might contain large errors attributable to head movement.
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An efficient method for head-related transfer function (HRTF) measurement is presented. By applying the acoustical principle of reciprocity, one can swap the speaker and the microphone positions in the traditional (direct) HRTF measurement setup, that is, insert a microspeaker into the subject's ear and position several microphones around the subject, enabling simultaneous HRTF acquisition at all microphone positions. The setup used for reciprocal HRTF measurement is described, and the obtained HRTFs are compared with the analytical solution for a sound-hard sphere and with KEMAR manikin HRTF obtained by the direct method. The reciprocally measured sphere HRTF agrees well with the analytical solution. The reciprocally measured and the directly measured KEMAR HRTFs are not exactly identical but agree well in spectrum shape and feature positions. To evaluate if the observed differences are significant, an auditory localization model based on work by J. C. Middlebrooks [J. Acoust. Soc. Am. 92, 2607-2624 (1992)] was used to predict where a virtual sound source synthesized with the reciprocally measured HRTF would be localized if the directly measured HRTF were used for the localization. It was found that the predicted localization direction generally lies close to the measurement direction, indicating that the HRTFs obtained via the two methods are in good agreement.
This book covers all aspects of head-related transfer function (HRTF), from the fundamentals through to the latest applications, such as 3D sound systems. An introductory chapter defines HRTF, describes the coordinate system used in the book, and presents the most recent research achievements in the field. HRTF and sound localization in the horizontal and median planes are then explained, followed by discussion of individual differences in HRTF, solutions to this individuality (personalization of HRTF), and methods of sound image control for an arbitrary 3D direction, encompassing both classic theory and state of the art data. The relations between HRTF and sound image distance and between HRTF and speech intelligibility are fully explored, and measurement and signal processing methods for HRTF are examined in depth. Here, supplementary material is provided to enable readers to measure and analyze HRTF by themselves. In addition, some typical HRTF databases are compared. The final two chapters are devoted to the principles and applications of acoustic virtual reality. This clearly written book will be ideal for all who wish to learn about HRTF and how to use it in their research.
An extensive set of head-related transfer function (HRTF) measurements of a Knowles Electronics Mannequin for Acoustic Research (KEMAR) has recently been completed. The measurements consist of the left and right ear impulse responses from a Realistic Optimus Pro 7 loudspeaker mounted 1.4 m from the KEMAR. Maximum length (ML) pseudorandom binary sequences were used to obtain the impulse responses at a sampling rate of 44.1 kHz. In total, 710 different positions were sampled at elevations from -40 deg to +90 deg. These data are being made available to the research community on the Internet via anonymous FTP and the World Wide Web.
Detection thresholds to warming and cooling were measured in 13 regions of the body in 60 adults aged between 18 and 88 years. From these thresholds were constructed maps of thermal sensitivity homologous to body maps of spatial acuity (in the older literature two-point discrimination), long known to the somatosensory scientist. Maps of cold and warm sensitivity for young, middle-aged and elderly adults, show how sensitivity changes with age in the various body regions. Three characteristics emerge, irrespective of age: (1) sensitivity varies approximately 100-fold over the body surface. The face, especially near the mouth, is exquisitely sensitive, the extremities, by comparison, poor, other regions, intermediate. (2) All body regions are more sensitive to cold than to warm. (3) The better a region is at detecting cold, the better it is at detecting warm. With age, thermal sensitivity declines. The greatest changes take place in the extremities, especially the foot, where thresholds often become too large to measure. Central regions give up their sensitivity with age more slowly, and even (as in the lips) inconsequentially. Similar age-related changes have also previously been shown to characterize spatial acuity.
To determine how the ear-canal sound pressures generated by earphones differ between normal and pathologic middle ears. Measurements of ear-canal sound pressures generated by the Etymtic Research ER-3A insert earphone in normal ears (N = 12) were compared with the pressures generated in abnormal ears with mastoidectomy bowls (N = 15), tympanostomy tubes (N = 5), and tympanic-membrane perforations (N = 5). Similar measurements were made with the Telephonics TDH-49 supra-aural earphone in normal ears (N = 10) and abnormal ears with mastoidectomy bowls (N = 10), tympanostomy tubes (N = 4), and tympanic-membrane perforations (N = 5). With the insert earphone, the sound pressures generated in the mastoid-bowl ears were all smaller than the pressures generated in normal ears; from 250 to 1000 Hz the difference in pressure level was nearly frequency independent and ranged from -3 to -15 dB; from 1000 to 4000 Hz the reduction in level increased with frequency and ranged from -5 dB to -35 dB. In the ears with tympanostomy tubes and perforations the sound pressures were always smaller than in normal ears at frequencies below 1000 Hz; the largest differences occurred below 500 Hz and ranged from -5 to -25 dB. With the supra-aural earphone, the sound pressures in ears with the three pathologic conditions were more variable than those with the insert earphone. Generally, sound pressures in the ears with mastoid bowls were lower than those in normal ears for frequencies below about 500 Hz; above about 500 Hz the pressures showed sharp minima and maxima that were not seen in the normal ears. The ears with tympanostomy tubes and tympanic-membrane perforations also showed reduced ear-canal pressures at the lower frequencies, but at higher frequencies these ear-canal pressures were generally similar to the pressures measured in the normal ears. When the middle ear is not normal, ear-canal sound pressures can differ by up to 35 dB from the normal-ear value. Because the pressure level generally is decreased in the pathologic conditions that were studied, the measured hearing loss would exaggerate substantially the actual loss in ear sensitivity. The variations depend on the earphone, the middle ear pathology, and frequency. Uncontrolled variations in ear-canal pressure, whether caused by a poor earphone-to-ear connection or by abnormal middle ear impedance, could be corrected with audiometers that measure sound pressures during hearing tests.
Conference Paper
This paper describes a public-domain database of high-spatial-resolution head-related transfer functions measured at the UC Davis CIPIC Interface Laboratory and the methods used to collect the data.. Release 1.0 (see includes head-related impulse responses for 45 subjects at 25 different azimuths and 50 different elevations (1250 directions) at approximately 5° angular increments. In addition, the database contains anthropometric measurements for each subject. Statistics of anthropometric parameters and correlations between anthropometry and some temporal and spectral features of the HRTFs are reported
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