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Headphone acoustic impedance, FEC criterion and PDR - any easy way to measure?

post #1 of 3
Thread Starter 

FEC criterion and PDR were described in "Møller, H. et. al.: “Transfer characteristics of headphones measured on human ears”  J. 

Audio Eng. Soc., 1995, #43(4), 203-217". However, I do not have access to that and don't know the info first-hand.

What I've found was an article on designing an headphone specifically for binaural reproduction.


Actually, it was what I've found first and from that article I've got information on FEC criterion and PDR.


From what I seem to understand, FEC stands for "Free-air Equivalent Coupling" and PDR is "Pressure Difference Ratio". They are related to the fact that ear transfer function is dependend on acoustic impedance, which is changed by headphones. PDR is difference of frequency responce at eardrum in free air vs in headphones. FEC criterion is passed when PDR stays within some limit, however, I do not know what is that limit, as I haven't seen the original paper, but it seems it's within +- 10 dB, while headphones often have +-30 dB. PDR at +-0 dB means acoustic impedance equal to free air impedance across whole audio band. Impedance seems to be affected not only by pressurized chamber effect, but also by standing wave resonances. Impedance plot allows to estimate PDR plot and vice versa, to some extent.


The FEC criterion seems to be a very important factor for determining headphones ability to allow resolving the length of a sonic radius-vector from recording, i.e. depth of scene in all directions. Without passing the criterion, you could have ecxellent separation and positioning, but acoustically perceived distance from you to sound sources would be incorrect.


My main question is how to measure headphone's acoustic impedance? Preferrably in an more or less easy way, possible at home using only the headphone, electret mic and some baffle. Knowing the impedance plot one would be able to tune HP's acoustic impedance to allow for more realistic (and safer, without huge HF peaks) reproduction, particularly in scene departament.

post #2 of 3
Thread Starter 

Well, there are several links about acoustic impedance measurements:






And some more applied to headphones: 

http://genuix.es/desarrollo/wp-content/uploads/headphones.pdf - sadly they do not disclose headphone models, would be interesting to know the correlation of subjective scene with impedance delta measurement.



I haven't yet dug deep into them, but the measurement device is more or less obvious, the processing method for data is a bit less obvious but understandable - detect pressure(=SPL) peaks and detect how far apart they are in time on the measurement mics - to determine speed of sound.


Given we want to assess an relative and not an absolute measurement, I suspect calibration is not really needed. Perhaps, we wouldn't even need a pinna like in the first headphone-related link, though I don't yet know wether pinna-less delta spectrum result would be equal to pinna-equipped one. Nevertheless, it should provide insight wether there is at least any acoustic impedance difference and would show how it changes with modifications done to headphones. Given the ideal result would be zero delta across whole audio band, we would know that less delta is always better.


I'm not planning to conduct any experiment very soon, most likely I'll try to begin them after about one month from now, wouldn't have time before that.

post #3 of 3

Hi Nevod and thanks a lot for those links, I'm working on my master thesis and this will help greatly. 

Did you find new stuff (tried measurements, papers, links...) since then on acoustic impedance applied to headphones, and headphones designed specifically for binaural spatialisation use ? 


Thanks again. 

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