Brody, Meredith, Food Critic has reference to this Academic Journal, PHwiki organized this Journal Binaural Hearing, Ear Canals, in addition to Headphone Equalization David Griesinger Harman Specialty Group Two closely related Threads: 1. How can we capture the complete sonic impression of music in a hall, so that halls can be compared with (possibly blind) A/B comparisons Can we record exactly what we are hearing, in addition to reproduce it later with fidelity If so, will these recordings have the same meaning as long as other people 2. What is the physics of the outer ear By what mechanisms do we perceive externalization, azimuth, elevation, in addition to timbre Are there mis-assumptions in the conventional thinking about these subjects in addition to can we do better Part 1 – Binaural Capture Has a long History at least since Schroeder in addition to Sibrasse Idea is simple record a scene with a microphone that resembles a head, in addition to play the sound back through headphones But whos head do we use How are microphones placed within it What equalization do you need to match the headphones to the listener Most people think it is possible to equalize the dummy-headphone system by placing the headphones on the dummy, in addition to adjusting as long as flat response. Un as long as tunately this does not work. The dummy in addition to the listener have completely different ear canal geometry in addition to the equalization is grossly in error.
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Some History Schroeder attempted to solve the headphone equalization problem by playing back the recording through loudspeakers, with electronic cancellation of the crosstalk between the ears. The result sounds spatially much like headphones, but the listener can use his own ear canals in addition to pinna. Un as long as tunately there are TWO pinna in the playback the dummys in addition to the listeners. And the equalization of the dummy head is still unknown. The Neumann KU80 dummy in the front of the room is similar to the dummy used by Schroeder. Note that the pinna are not particularly anthropromorphic, in addition to there are no ear canals at all. The frequency response (relative to human hearing) of such a head can be different by more than 20dB at mid frequencies. Theile Spikofski Spikofskis work at the IRT Munich promoted the idea of diffuse field equalization as the natural st in addition to ard as long as both dummy head recording in addition to headphone reproduction. The result was implemented in the Neumann KU-81 dummy microphone. I went right out in addition to bought one! To equalize headphones, put them on the equalized dummy, in addition to adjust the headphone equalization until a flat response is achieved. Good Luck Check out the KU-81 pinna in addition to couplers. Note the ear canal entrance is very different from yours. But the method did not work as long as me! Perhaps the pinna were not close enough to mine So I replaced the pinna with castings of my own. Still no go. Theile published a comprehensive paper on the subject, which suggested that one could make an individual headphone calibration by putting a small microphone in the ear canal (partially blocking it) in addition to then matching the headphones to a diffuse acoustic field. But this also did not work as long as me. The resulting headphone equalization was far from natural, in addition to unbalanced between the two ears. Theiles arguments however were compelling: It should not be necessary to measure the sound pressure at the eardrum if one was only trying to match the sound pressure at the entrance of the ear canal to an external sound field. Blocked ear canal measurements became an IEC st in addition to ard as long as headphone calibration.
Theiles method Note that the ear canal is (as usual) represented as a cylinder More on diffuse field Theiles arguments as long as diffuse field eq go this way: If headphones are equalized to match a frontal HRTF of an average listener, then ordinary stereo signals will have no room sound, in addition to be very dry in addition to unnatural. Since such signals are intended to be heard in a room at some distance from the speakers the headphones should be equalized to match the total sound pressure in the room. This implies [maybe] that diffuse field equalization is correct as long as heaphones. If headphones are equalized as long as the diffuse field, then dummy heads need to be equalized as long as the diffuse field. In this case a dummy head recording will be correctly reproduced over headphones. But not over loudspeakers! Alas this argument requires that a dummy head equalized as long as loudspeakers must be equalized to be flat in a free field as long as signals from the front. You cant have it both ways The author published a paper on this subject 20 years ago, in addition to had personal conversations with Stephan Peuss at Neumann. The result was the Neumann KU-100 dummy head. More on Theile Theiles arguments as long as diffuse field equalization are entirely Aristotelian. What if a free-field frontal equalization as long as headphones is preferred by listeners when listening to ordinary stereo In fact, diffuse field is often preferred. Free-field eq differs from diffuse field eq by having about 6dB less treble. Nearly all commercial headphones have more treble than even a diffuse-field eq. They sell better this way. Accurate is not always perceived as best. But if free-field equalized headphones were st in addition to ard, then dummy heads could also be free-field equalized. And would reproduce well over loudspeakers as well as over headphones. But all these arguments are meaningless without an accurate method of measuring headphone response on a particular individual!
Hammershoi in addition to Moller An excellent paper by Hammershoi in addition to Moller investigated whether the ear canal influenced the directional dependence of the human pinna system. They concluded that measuring the sound near the entrance of the ear canal captured all the directional dependence, in addition to it was not necessary to go to the eardrum. This paper has been taken as conclusive proof that the ear canal is not relevant as long as headphone equalization or dummy head recording. But Hammershoi in addition to Moller say The most immediate observation is that the variation [in sound transmission from the entrance of the ear canal to the eardrum] from subject to subject is rather high The presence of individual differences has the consequence that as long as a certain frequency the transmission differs as much as 20dB between subjects. Thus the directional dependence as measured at a blocked ear canal can be correct But the timbre is so incorrect that our ability to perceive these the true direction is frustrated. (And the sound can be awful ) Mollers ear canal Hammershoi in addition to Moller additionally say: another observation is that the data do not tend to support the simple model of an ear canal. But in spite of this, they present the following model: Once again, we see that the cylindrical model has won out over data in addition to common sense. They have assumed timbre does not matter only differences in timbre. The Hidden Assumption The work of Spikofski, Theile, in addition to Moller all rests on the assumption that human hearing rapidly adapts to even grossly unnatural timbres. That is, the overall frequency response does not matter as long as localization, only relative differences in frequency response. Alas, this is exceedingly unlikely. It seems clear that rapid, precise sound localization would be impossible without a large group of stored frequency response expectations (HRTFs) to which an incoming sound could be rapidly compared. Human hearing does adapt to timbre as we will see but adaptation takes time, in addition to needs some kind of (usually visual) reference.
A Convenient Untruth That absolute frequency response at the eardrum is unimportant as long as binaural reproduction is seductively convenient. But it violates common observation: The argument is based in part on the perceived consistency of timbre as long as a sound source that slowly moves around a listener. But perceiving timbre as independent of direction takes time. If a source moves rapidly around a listener it is correctly localized, but large variations in timbre are audible. Clearly the brain is using fixed response maps to determine elevation in addition to out-of-head impression. And compensating as long as timbre at a later step. I was just in the Audubon Sanctuary in Wellfleet at 8am, surrounded by calling birds in every direction. I felt I could precisely localize them but I could tell you nothing about their timbre. Walking under an overhead slot ventilator at Logan at about 3.5mph, I noticed a very strong comb-filter sound. When I retraced my steps at 1.5mph the timbre coloration was completely gone. In both cases the sound was correctly localized. In the absence of visual or other cues, headphones with excess treble reproduce sounds perceived as above the head. Bottom Line: Accuracy of frequency response AT THE EARDRUM is essential as long as correct localization with binaural hearing. Then why are binaural recordings often perceived as successful Binaural demonstrations are often effective especially with sounds that are to the side or rear of the head Azimuth cues derived from the time delay between the two ears, in addition to the head shadowing of the head are effective even when the timbre is grossly incorrect. When a sound source is rapidly moving the brain tends to ignore incorrect elevation cues if they conflict with the expected trajectory. If a visual cue is present at the same time it will almost always dominate the aural cues. With some good showmanship in addition to a subject who is willing to be convinced, these demonstrations can be quite convincing. But with skeptical listening frontal localization of fixed sources is rarely achieved. Head Tracking It has been noticed that st in addition to ard ear-canal-independent methods of calibrating dummy heads in addition to headphones do not work very well. It is almost universal that subjects claim frontal headphone images localize inside the top of the head. However, when a dummy head tracks a listeners head motion there is sufficient feedback that a frontal image is restored. Although the process may take a minute or so. There as long as e head tracking has been assumed to be an essential part of any dummy head recording system. But none of us need to move our heads to achieve external, frontal localization. Head motion produces azimuth cues that are so compelling that the brain quickly learns to ignore timbre cues from the pinna. But this is not an ideal solution, as issues that depend on timbre, such as intelligibility in addition to sound balance, are incorrectly judged.
More on the necessity of accurate timbre As we will see, human hearing adapts to timbre relatively quickly. But in my experience inaccurate timbre while monitoring a recording with headphones results in recordings that are far to reverberant. Intelligibility is often reduced by upward masking, which is a result of the mechanical properties of the basilar membrane. Boosting the treble increases the intelligibility of speech in addition to music. This effect is not compensated by adaptation. This is one reason that headphones with excessive treble are often preferred. But they do not make successful recordings And they are misleading when used as long as hall research. There is a headphone eq method as long as head recording that works! We need to go back to basics. record the sound pressure at the eardrum of a listener in addition to then reproduce the exact same sound pressure on playback This is not particularly difficult. And the result can be amazingly realistic. After failing with Theiles method 20 years ago, the author constructed the purple probe microphone on the right to measure the sound at my own eardrum. It is uncom as long as table, but it works! The black model to the left is a probe from 3 years ago. It works well, but is slightly uncom as long as table, in addition to the S/N is not great. The bottom one is the latest. It is com as long as table in addition to works well. Probe Microphones 1mm from the eardrum Compact probe microphones can sit very close to the eardrum with no discom as long as t, in addition to no disturbance of normal hearing. They are also quite discrete
Probe construction The probe mike is made from a Radio Shack Lavaliere microphone with a 6cm length of 18 gage PVC clear tubing glued with epoxy to the end. The PVC is hard enough that there is no sound leakage through the tube a problem when the whole tube is silicon. A ~1cm length of ultra-soft silicon medical tubing is then press-fit into the slightly exp in addition to ed end of the tubing, in addition to cut to length so it sits just in front of the eardrum. The silicon is soft enough that it can be touched to the eardrum without consequences! details Microphone is a ¼ omni capsule. (See bare capsule in the middle of the picture.) They come assembled into a lavaliere mike from Radio Shack, model 33-3013. The case unscrews as shown. Remove the damping cloth. The hard tubing is 18 gage PVC, carefully bent above a small c in addition to le flame. I use a 2mm nail to burn a hole in a piece of bicycle inner tube, in addition to cut a washer that fits over the mike end of the PVC. Gently heat the nail in addition to as long as ce it into the other end of the PVC to exp in addition to it as long as the silicon tube, Helix Medical REF 60-011-04, .030 ID, .065 OD. The mike is re-assembled, in addition to finished with epoxy to hold the PVC in place. Probe Equalization This graph shows the frequency response in addition to time response of the digital inverse of the two probes as measured against a B&K 4133 microphone. Matlab is used to construct the precise digital inverse of the probe response, both in frequency in addition to in time. The resulting probe response is flat from ~25Hz to 17kHz. A mathematical inverse can sometimes have sharp peaks that produce audible artifacts. I minimized these artifacts by truncating the measured response as a function of frequency. (These probes were early models. Current probes have a much smoother response.)
Mike calibration I tape the finished probes to the tip of a measurement microphone in addition to record a sine-sweep. Matlab is used to FFT the resulting sweeps. Dividing the reference FFTs by the probe FFTs gives you the inverse frequency response of the probes. Although you dont need to know the probe responses if you follow the procedure shown in the next slides as long as calibrating headphones, I prefer to correct my recordings as long as the probe response, in addition to then correct them as long as the response of my outer in addition to middle ears to a frontal flat loudspeaker. This process is described in later slides. The result is a response that is flat to the front of a listener up to about 6kHz. I leave the horizontal localization notches in place, as they vary too much to correctly equalize. The result are recordings that play successfully over loudspeakers or un-equalized headphones. Recording Completed probe system plugs directly into a professional minidisk recorder. 4 hrs of compressed audio, or 1 hour of PCM can be recorded on a single 1GB disk. Record level can be digitally calibrated as long as accurate SPL. Equalization of the playback headphones Carefully place headphones on the listener while the equalized probe microphones are in place. Measure the sound pressure at the listeners eardrums as a function of frequency, in addition to construct an inverse filter as long as these particular phones. If this is done carefully, the sound pressure during the recording will be exactly reproduced at the eardrum With several tries, a successful equalization can be found. For accurate vertical localization a precise mathematical inversion is probably necessary. But as long as use with other people I prefer to construct an inverse filter using a small number of minimum phase parametric filters.
Headphone type I did a series of experiments at Aalto University in Helsinki in addition to at Rensselaer University in New York to find headphones with the least individual differences in response as measured by loudness matching. Sennheiser on-ear headphones were the clear winners, particularly the noise-cancelling model 250-2. The noise canceling circuit equalizes the low frequency pressure in the concha. With extra equalization they can respond flat to below 30Hz. The circuit also makes the response relatively independent of how you put them on. Circumaural phones are not recommended as long as binaural reproduction, as they reproduce the 90 degree azimuth, zero elevation HRTF, in addition to this is impossible to equalize away. In addition the response varies every time you put them on. Phones such as the AKG 1000 also reproduce the 90 degree azimuth notch Results Recording a scene with probes at the eardrums, in addition to then equalizing the playback using the same probes, results in startling realism with no need of head motion tracking. This is the ideal method as long as an electronic memory as long as sounds of any kind. I have been doing recordings of this type as long as several months, in addition to have interesting results from many halls. I would be happy to share these with you. Problems The biggest problem is that no-one (in their right mind) will put anything in their ear! Bigger than their elbow But if a madman equalizes a system as long as himself, can others obtain the benefit Considerable benefit is obtained. Most individuals say the headphones sound more realistic in timbre. But frontal imaging may not work well. In my experience there are large differences between individuals in the way high frequencies couple from headphones to the eardrums. The consequences of these individual differences [as described by Moller] in addition to what can be done to mitigate them are the subject of the next section of this talk. In general, a non-invasive equalization procedure is frequently sufficient to make a more accurate playback.
Part 2 Binaural Hearing Practical questions: Is it possible to measure HRTF functions with a blocked ear canal Maybe. Partially blocked ear canal measurements appear to capture the directional dependence of HRTFs. But timbre (the overall equalization) needs to be correct when the signal is played back. Because the actual ear canal trans as long as m is unknown, timbre ( in addition to elevation) is usually not accurate with headphones. Is it possible to achieve out-of-head localization in addition to frontal imaging with headphones without a head-tracker Yes – we do it with our own ears every day. When timbre is accurate it is also possible with headphones. With some adjustment to headphone response non-individual HRTFs will work as long as most people (not all ) Is it possible to achieve out of head perception without using a measured HRTF Yes but beyond the scope of this talk What HRTFs (or dummy head) should be used in concert hall or car modeling Several probably work well. There is probably more variance in ear canal geometries than in pinna. Some kind of individual matching as long as timbre is needed as long as playback. What is the meaning of flat frequency response The sound pressure at our eardrums is not at all flat, in addition to is different as long as each individual, in addition to as long as each sound direction. Do we all hear the same sound as spectrally balanced Maybe – Our impression of response is adaptive but there are limits. Technical Questions: Is it true that a blocked ear canal captures all spatial differences Does a blocked ear canal measure headphone response accurately How can we equalize a dummy head such that recordings can be played over loudspeakers Is it possible to match headphones to a listener through subjective loudness If we can do this, is it be possible to play both binaural recordings (equalized as above) in addition to st in addition to ard stereo material with equal realism How adaptable is timbre perception Another great question but also beyond the scope of this talk. Research Methods We make probe microphone measurements at the eardrum of any person willing to try it. New probe tubes are very soft in addition to audiologists make this kind of measurement 10 times a day. It is simple, easy, in addition to painless. We constructed a new dummy head with an accurate physical model of the ear canal in addition to eardrum impedance. We have live recordings with probes on the eardrum, or with the accurate dummy head. You have got to hear it to believe it. Subjective response calibration with noise b in addition to s. A simple octave b in addition to equalization process works surprisingly well to match headphone timbre to individuals, allowing non individual HRTFs to work.
Conclusions Dummy head recordings from heads with anthropromorphic pinna can give good results if the head is properly equalized in addition to headphones can be matched to an individual listener. Finding the correct equalization as long as the dummy can be difficult but can sometimes be done by spectral analysis post-recording. All available dummy head models will give inaccurate results when used to equalize headphones. Headphones can be accurately equalized as long as a particular listener using eardrum pressure measurements with probe microphones. Or using a dummy head with accurate ear canals. Such an equalization appears to sound better as long as most listeners than other available alternatives. Loudness matching appears to be a viable alternative as long as matching headphones to an individual listener without invasive probes. With some luck an individual headphone equalization can give frontal localization in addition to realistic reproduction of timbre from non-individual recordings.
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