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Topic: HRTFs

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	Head-related transfer function - Wikipedia, the free encyclopedia
		The HRTF H(f) is the Fourier transform of the HRIR h(t).
		HRTFs are typically measured in an anechoic chamber to minimize the influence of early reflections and reverberation on the measured response.
		HRTFs are measured at small increments of θ such as 15° or 30° in the horizontal plane, with interpolation used to synthesize HRTFs for arbitrary positions of θ.
	en.wikipedia.org /wiki/Head-related_transfer_function (571 words)

	[No title]
		It was decided that an efficient way to obtain symmetrical HRTF measurements for both the "small" and "large red" pinnae was to install both pinnae on the KEMAR simultaneously, and measure the entire 360 degree azimuth circle.
		Assuming an HRTF is negligibly affected by the shape of the opposite pinna, these symmetrical sets should be the same as sets obtained using matched pinnae.
		Often, HRTF measurements are made at the entrance of the ear canal to eliminate the ear canal resonance.
	sound.media.mit.edu /KEMAR/KEMAR-FAQ.txt (4127 words)

	[No title]
		HRTFs for the left and right ears of a subject specify how sound from a particular direction is transformed en route to the ear drums.
		The use of non-individualized HRTFs is limited by a lack of externalization (the sounds are localized in the head or very close to the head), incorrect perception of elevation angle, and front/back reversals.
		Using the log magnitude spectra of the HRTFs as input, the analysis indicated that 90 percent of the variance in the data could be accounted for using only five principal components.
	www.nae.edu /NAE/bridgecom.nsf/BridgePrintView/MKEZ-695TMR?OpenDocument (2644 words)

	Implementation of binaural spatializer
		The HRTFs were measured in 10 degree elevation increments from -40 to +90 degrees [13].
		The HRTFs that are closest to the desired azimuth and elevation are used.
		The input block is convolved with both the previous HRTFs and the new HRTFs, and the two results are crossfaded using a linear crossfade.
	xenia.media.mit.edu /~mkc/micArray/node13.html (613 words)

	"+title+" (Site not responding. Last check: 2007-10-30)
		The primary objective of our advanced technology development effort is to develop display systems which maximize perceptual accuracy by presenting a carefully designed set of veridical acoustic cues to the listener that have been validated in psychoacoustical studies.
		The subject, seated on a swivel chair, is outfitted with blocked-ear canal microphones and HRTFs are measured using Golay code sequences.
		Initial measurements of individual subjects' HRTFs have been made at NASA Ames, but we are still developing experimental techniques for ensuring the accurate and replicable placement of the subject and the sound source during a set of measurements, e.g., by integrating a head-tracker into the measurement chain.
	human-factors.arc.nasa.gov /ihh/spatial/research/Wenzel_personalized_measurement.html (616 words)

	Frank's Thesis Chapter 4 (Site not responding. Last check: 2007-10-30)
		Wightman and Kistler have performed extensive HRTF measurements and have recently proposed a set of "principal components" from which a generalized set of HRTFs may be constructed for an arbitrary source position [21].
		For the purposes of this project, Wightman and Kistler's HRTFs for a representative subject (identified by her initials, "SDO") were adapted for use in the FIR stage of the auralization process.
		To avoid this error, HRTFs should be recorded individually for left and right ears, with the source displaced along a sphere centered on the ear, not the center of the head.
	alumnus.caltech.edu /~franko/thesis/Chapter4.html (1221 words)

	Wave Arts Purchasing
		HRTFs are organized according to measurement direction specified in polar coordinates: elevation and azimuth angle.
		Azimuth angle is measured from the medial plane; for sources on the horizontal plane 0 degrees azimuth is directly in front, 90 degrees azimuth is directly to the right, -90 or +270 degrees azimuth is directly to the left, and +180 or -180 degrees is directly behind.
		Each HRTF within the file has the same number of coefficients, as determined by the number of "b" coefficients plus the number of "a" coefficients, times 2 to account for both left and right ear responses.
	www.wavearts.com /PanoramaFile.html (1244 words)

	U.S. Patent: 6173061 - Steering of monaural sources of sound using head related transfer functions - January 9, 2001
		The monaural audio signal is filtered using head-related transfer functions (HRTFs) into the left and right outputs, these being equivalent to the acoustic HRTFs that would be generated if a source of sound were placed at the specific location relative to the listener.
		The HRTF filter function is usually obtained by using a dummy head, which is a stylized model human head, of roughly average size and shape.
		If the HRTF is to be reproduced by loudspeakers or over-the-ear headphones, but was recorded with in-the-ear microphones, then the transfer function of the ear canals must be removed before reproducing the signals through the transducers.
	www.everypatent.com /comp/pat6173061.html (5671 words)

	Adding Sound to Virtual Reality
		Once the HRTF is determined, digital sounds can be played through the function so that you hear the sound from a specific direction rather than just over speakers.
		HRTFs are usually associated with the use of headphones, but can also be played over speakers.
		For all the rays that hit the listener, the program loads the closest HRTF and combines it with the input sound file with respect to the lenght of the path.
	www.cs.nps.navy.mil /people/faculty/capps/4473/projects/01Summer/sound/SoundSoftwareDepth.htm (1661 words)

	[No title]
		HRTFs where built on measurements by which the scientists produced sounds through speakers to the listener in specific angles and different frequencies.
		A function is made out from these measurements and it is drawn for both ears which represent the differences made to the amplitude of the sound waves comming from the source by changing the frequency.
		Using a technology called "Convolving", HRTFs can be used to change sound properties before outputing sound waves to the listener from the computer, which makes the listener think that the sound is comming from a location different than the real location.
	www.arabion.net /hardware.html (450 words)

	LCAV - Audiovisual Communications Laboratory (Site not responding. Last check: 2007-10-30)
		HRTFs are filters describing the influence of the shape of the head, torso and pinnae of a person on the sound he/she hears.
		HRTFs are measured for all azimuthal and elevation angles.
		The result of the analysis of the HRTFs sampling leads to the conclusion that the necessary angle between azimuthal HRTFs is of about 5 degrees in order to reconstruct all HRTFs up to 44.1 kHz in the horizontal plane.
	lcavwww.epfl.ch /research/audio_processing/head_related_transfer_functions_interpolation.html (377 words)

	Localization of virtual sound sources synthesized from model HRTFs
		Each individual HRTF can be represented by a linear combination of these principal components in much the same way as a waveform can be represented by a linear combination of sine waves and cosine waves.
		Thus, for each left-right pair of HRTFs, the phase components of both left and right HRTFs are derived from the magnitudes and then time-shifted relative to each other by an amount estimated from the cross-correlation of the measured HRTF impulse responses.
		Five principal components (each consisting of 150 spectral values) were extracted from HRTFs measured near the eardrums of 5 listeners at 265 source positions (one directly overhead, plus those at all combinations of 24 equally-spaced azimuths covering the complete circle and 11 equally-spaced elevations from -48 to +72 degrees relative to the horizontal plane).
	www.waisman.wisc.edu /hdrl/abstract/fwieee91.htm (819 words)

	Improved HRTF-Based Psuedostereophony
		In the time domain, an HRTF is an impulse response, as a function of direction of incidence, that is convolved with the incident acoustic signal.
		In the frequency domain, the HRTF is a magnitude and phase response, as a function of frequency of sound and direction of incidence, that multiplies the frequency-domain complex-amplitude representation of the incident acoustic signal.
		In order for the use of HRTFs to yield a sensation of increased auditory spaciousness, the final delay should be at least 30 ms; this is consistent with findings from research into the effect of early reflections in concert halls.
	www.nasatech.com /Briefs/Apr00/ARC11919.html (1067 words)

	Abstracts of our Current Research Publications (Site not responding. Last check: 2007-10-30)
		The primary objectives were to quantify the differences among HRTFs, to identify listeners with similar and different HRTFs, and to test the localizability of virtual sources synthesized from the HRTFs of an individual with closely and not closely matched HRTFs.
		Listeners with similar HRTFs were identified and their ability to localize virtual sources synthesized from the HRTFs of a "similar" listener was evaluated.
		When these same listeners were tested with virtual sources synthesized from HRTFs that were identified to be "different" by the MDS analysis, both azimuth and elevation of virtual sources were judged less accurately.
	www.louisville.edu /a-s/psychology/pns/hearingscience/research/abstract/3Multi_d.htm (289 words)

	Sound Demonstration Files
		There is a HRTF for every angular direction; one example is shown here [48 kb].
		It is not difficult to manipulate the HRTFs to remove the relative time delay, or remove the relative amplitude difference, and that is what I did next to create two new files.
		The second file uses HRTFs where the relative time difference between the right and left ears has been removed, and the third file removes the relative amplitude differences.
	www.silcom.com /~aludwig/Sound_demos.html (2222 words)

	Coding Horror: 3D positional audio and HRTFs
		The HRTF is a linear function that is based on the sound source's position and takes into account many of the cues humans used to localize sounds.
		The HRTF is used to develop pairs of finite impulse response (FIR) filters for specific sound positions; each sound position requires two filters, one for the left ear, and one for the right.
		HRTF functions magically convert stereo sound into 3D sound, but they are computationally expensive.
	www.codinghorror.com /blog/archives/000494.html (1932 words)

	Receptive Fields and Binaural Interactions for Virtual-Space Stimuli in the Cat Inferior Colliculus -- Delgutte et al. ...
		elevation coincided with a trough in the HRTF at the CF.
		with a peak in the HRTF at the CF.
		Measuring HRTFs in animals with moving pinnae such as cats is not straightforward because the positions of the pinnae are
	jn.physiology.org /cgi/content/full/81/6/2833 (9102 words)

	HeadWize - Technical Paper: Virtual audio for headphones by Alastair Sibbald
		The HRTF can be synthesised electronically, using digital signal processing, and used to process various sound sources which can be delivered to a listener's ears via headphones (or loudspeakers).
		The resultant HRTFs are proprietary to Sensaura systems and, in their neutral state, have been optimised for universal headphone usage.
		If this is carried out correctly, then he or she perceives the original sound to be at a position in space in accordance with the spatial location of the HRTF pair which was used for the signal-processing.
	headwize.com /tech/sibbald_tech.htm (2082 words)

	Frank's Thesis Chapter 8 (Site not responding. Last check: 2007-10-30)
		At the present time, the HRTF recording process is cumbersome, requiring the use of an anechoic chamber and other specialized equipment which preclude incorporation into end-user auralization systems.
		One approach to increased HRTF compatibility is to provide a selection of several general HRTF sets, with a means for the user to select the HRTF which generates the most effective results for him or her.
		Although presently prohibitively expensive, it is conceivable that a system could be developed to scan each individual's ear to determine the major physical structures; an HRTF could then be constructed from the physical dimensions of the ear, as in the work by Han [30].
	alumnus.caltech.edu /~franko/thesis/Chapter8.html (1480 words)

	Current Research : Head-Related Transfer Function (HRTF) Research at the FIU DSP Laboratory
		The study of HRTFs is a rapidly growing area with potential uses in virtual environments, auditory displays, entertainment industry, human-computer interface for visually impaired, aircraft warning systems and many others.
		We measure 72 pairs of HRTFs for every individual (12 azimuths and 6 elevations) which can be analyzed to find out how the HRTF changes from person to person.
		We are currently pursuing the identification of the pattern of frequency attenuation and enhancement that seems to depend on the protrusion angle of the pinna.
	dsplab.eng.fiu.edu /DSP/Research/Research_007.html (709 words)

	Abstracts of our Current Research Publications (Site not responding. Last check: 2007-10-30)
		Each HRTF is formally a ratio of the spectrum of the sound wave at the eardrum to the spectrum of the same sound wave in free-field.
		Since the pattern of peaks and notches in the HRTF magnitude is different for each source direction that pattern could serve as a code for direction.
		Both the magnitude of the HRTF for any given direction and the pattern of change in the HRTF with direction are radically different from one person to another.
	www.louisville.edu /a-s/psychology/pns/hearingscience/research/abstract/3bruss96.htm (471 words)

	HeadWize - Technical Paper: 3D Audio and Acoustic Environment Modeling by William G. Gardner
		Each HRTF, typically consisting of several hundred numbers, describes the time delay, amplitude, and tonal transformation for the particular sound source location to the left or right ear of the subject.
		However, measuring HRTFs is a complicated procedure, so 3D audio systems typically use a single set of HRTFs previously measured from a particular human or manikin subject.
		The crosstalk canceller used by Wave Arts 3D audio is based on actual HRTF measurements and thus accurately models the crosstalk that occurs with human listeners.
	www.headwize.com /tech/gardner_tech.htm (5096 words)

	Improved Localization of Virtual Sound by Spectral Modification of HRTFs to Simulate Protruding Pinnae
		A well-known shortcoming of HRTF sound spatialization is the presence of a "Cone of Confusion".
		These HRTFs were helpful in resolving the cone of confusion when used by subjects with average or small pinna protrusions.
		For example, a subject with protruding ears showed a larger difference between the HRTFs from azimuths 30° and 150°, compared to the HRTFs from a person with smaller or average protrusion angle.
	dsplab.eng.fiu.edu /DSP/Publications/publication.asp?num=88 (300 words)

	AES Barcelona 2005: Paper Session A (Site not responding. Last check: 2007-10-30)
		This hypothesis was evaluated in a series of listening tests, where the two cues were manipulated in HRTFs used for binaural synthesis of sound in the horizontal plane.
		HRTF switching involves switching of spectral characteristics and timing characteristics (ITD), and the audibility of these were studied separately.
		To derive the algorithm we study the dual problem where sound is emitted from the listener's ear and the generated soundfield is recorded along a circular array of microphones around the listener.
	www.aes.org /events/118/papers/session.cfm?code=A (826 words)

	Are CDs lossless? - Hydrogenaudio Forums (Site not responding. Last check: 2007-10-30)
		To hear how differences between people can affects things, The CIPIC HRTF Database for binaural recording is interesting to play with, to hear how differences between people plays a role concerning perception.
		The goal is as you say, to make both perceive the same thing, but that may not work if the HRTF generated with person A is used with person B. As person B's hearing is trained with his own body/HRTF.
		Even for canalphones (say Etymotics) the ear canal acts as a (somewhat closed) resonant chamber and as we know, the length of ear canal is not standard from human to human.
	www.hydrogenaudio.org /forums/index.php?showtopic=11829&st=25& (5317 words)

	Factors influencing acoustical localization
		Stoerung der individuellen Lokalisationsparameter zu untersuchen, werden weiterhin psychoakustische Diskriminationsexperimente beschrieben in denen spektrale und temporale Eigenschaften der HRTFs variiert wurden.
		The physical properties of HRTFs and the effect of spectral smoothing on the physical localization cues are investigated in another section.
		Lastly, the effect of a monaural spectral cue in the spectrum of the sound source (that could be introduced by 'spectral scrambling') on the elevation perception is investigated.
	www.physik.uni-oldenburg.de /docserver/diss/metadocs/200134416502.html (261 words)

	2pPP13. Efficient representation of head-related transfer functions. (Site not responding. Last check: 2007-10-30)
		Head-related transfer functions (HRTFs) contain information that is vital both for the analysis of acoustic cues used in sound localization and for the application of virtual-sound synthesis techniques.
		As the HRTFs measured for a representative set of source positions constitute a large body of data, the application of suitable data reduction techniques is of crucial importance.
		An obvious method, which takes the properties of the human ear into account, is the averaging of the frequency spectrum within equidistant intervals on a logarithmic frequency scale.
	www.auditory.org /asamtgs/asa97pen/2pPP/2pPP13.html (226 words)

	[No title]
		HRTFs play an essential role in the localization of sound, which is the determination of where a sound source is located based on auditory cues.
		HRTFs essentially contain the spectrum of the frequency response that our ears, head, and shoulders impress upon an incident sound depending on which direction it comes from.
		I do not find the front/back one entirely convincing, but with the others it is very easy to pick out one voice when there is a "distance" separating the sources (this is known as the cocktail party effect).
	ccrma.stanford.edu /~bdangelo/150/project.html (789 words)

	Principles of binaural spatial synthesis
		Usually, the HRTFs are equalized to compensate for the headphone to ear frequency response [26, 17].
		The HRTFs used for sythesis are often a generic set and not the specific HRTFs of the listener.
		Because we are used to seeing sound sources that are in front of the head, it is difficult to convince the perceptual system that a sound is coming from the front without a corresponding visual cue.
	xenia.media.mit.edu /~mkc/micArray/node12.html (500 words)

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