Subject: Re: Headphone to ear canal Transfer Function From: jan schnupp <jan.schnupp(at)PHYSIOL.OX.AC.UK> Date: Tue, 2 Nov 2004 12:10:23 +0000Dear Sergio, think about it this way: if you are unlucky and the headphones you use are not perfectly symmetric in their transfer function, then they will introduce their own, (entirely unphysiological !) interaural spectral pattern differences, and that would most definitely affect the fidelity of your VAS. Most headphones have some "manufacturing tolerances", i.e. the transfer function of the left and right ear transducer are not exactly identical. Whether these asymetries are large enough to create a problem is not something you'll know until you try, but because of these potential problems, most serious researchers will calibrate their individual set of headphones, and will work the head phone calibrations into their virtual acoustic displays. It is conceivable that specialist headphones, like the tube phones from etymotic, have transfer functions that are so flat and reproducible that you may not need to worry about this individual calibration, but with "bog standard" headphones designed for domestic audio this is definitely not the case. Also, etymotic headphones are "guaranteed flat" only for a limited frequency range, and you would need to check whether your signals remain within that range. HRTFs have quite complex spectral patterns at the high frequency end, but the short wavelengths of high frequencies mean that they can also easily be influenced by standing waves / reflections & interference effects. Calibrating a headphone system for the high frequency end is therefore particularly tricky. In other words, the more high frequencies you have in your signals, the more you have to worry about getting the calibrations right, and if you want to guarantee VAS fidelity all the way up to 20 kHz then your best (perhaps only) option will be to rig up small microphone capsules that you can insert in the ear canal of the subjects under the headphone so that you can calibrate the headphones in situ, as it is done for example in the Wightman / Kistler lab. Look at their papers for details. Regards, Jan Sergio Rodríguez Soria wrote: >Dear list members, > >I work with HRTF models and in the next months need to do some >listening tests in order to validate the models. Normally I don't take >into account the headphone to ear canal Transfer Function (which includes >the headphone transfer function). Is that very important for reproducing >virtual sound through headphones? If it is so, I will need the >headphones free-field transfer function (for compesating just the >headphones), and/or, if there exists in public domain, the headphone to >ear canal transfer function (for compensating the headphones and the >transmission path between the headphone's output and the ear canal >entrance - given that the HRTF model reproduce the transmission between >the "free field" and the ear canal entrance). Do I am right? Does anyone >know about a public database for these kind of signals? > >Besides of that, what headphones are recommended for these tests? > > >Thanks in advance > > >Sergio Rodríguez Soria >Graduate Student >Signal Processing Laboratory >University of São Paulo > > -- -------------------------------------------------------- Dr. Jan Schnupp University Laboratory of Physiology St Peter's College Oxford University New Inn Hall Street Parks Road, Oxford OX1 3PT, UK Oxford OX1 2PL Tel (01865) 272513 Tel (01865) 278889 Fax (01865) 272469