Measurements, errors and reproducibility
That the results of the earplugs fit need not be explained further, for this the initial solution was explicitly produced and also used for some time in a laboratory. We see here the curves for the left channel (yellow) and the right channel (violet) as superimposed graphs.
But that’s tricky with the over-ears. Let’s start with the rubber ear on the padded plate (my simulated head) I am using nearly unsmoothed curves here for illustration. In the first run, I put the headphones on, took them off, and put them on again four times, so I measured them individually a total of four times and then superimposed the curves.
The first three measurements use the edges I mentioned as a guide when positioning, although I then deliberately placed the earpiece at a slight angle on the fourth and last attempt (yellow curve). All three measurements with the positioning aid really coincide almost perfectly, at least within the range of acceptable tolerances. Only the lovelessly placed listener deviates a bit, but not too much either. Target achieved, I would say.
By the way, this curve also shows the complex filtering effect of the head, the outer ear (pinna) and the trunk. The slight exaggeration at about 3 KHz is, after all, what I always preach in my sound tests of headphones for localization and spatiality as the most sensitive area of the auditory system and what is also known as HRTF (Head-Related Transfer Function). The amplitude evaluation in this frequency range is, after all, the essential basis of our “spatial” hearing in addition to the also occurring travel time differences between the ears and not artificial 5.1 surround gimmicks.
If you now simply remove the inserted ear and leave the large opening as it is, you will unfortunately measure completely unusable rubbish. One gets a mismatched, huge inner volume and even records a mode-like effect with partial cancellation and exaggeration in the low frequency range. It is completely worthless
So you can’t get there with that, and if you want to leave the ear out and just measure lying on it, you have to use a thinner cushion with a specially adapted opening so that the distances are right again. It’s nice that my system is modular and you can attach or easily replace many parts. The result of this new measurement then looks like this with the same headphones (purple with ear, yellow with flat pad) that it makes more sense. Nevertheless, in a direct comparison, I would prefer the version with the replicated ear canal, since all sounds from the upper mids upwards are reproduced much more vividly and realistically. The overall picture of this variant is still a bit too dull. But I will definitely optimize this part, I have to test and measure the distances exactly.
Summary and first conclusion
What do you actually want to measure? Of course, you always have to answer this question for yourself first. Do you want to measure the exact frequency response produced by the transducer directly at a measurement microphone by placing the ear flat on a plate with the microphone inserted? Then you’re about where I myself was already with the dummy head and the OKM, because the microphones sat on the outside and you could never include the ear canal. Or do you really want to measure what really arrives at a (replicated) ear by including anatomical conditions? I think I’ll go for the rubber ear version after all, because it’s also subjectively much closer to my hearing sensation.
I’m not completely satisfied yet, but almost and also really happy that I managed such good reproducibility for the over-ears. I still have to decide whether and how to change various things in advance for the headphone tests that follow later. Maybe your feedback will help me a bit more. But that with the drill stand, the leather and the foam plates has almost real entertainment value. Mighty powerful, Egon, Benny, or Kjeld would surely say. If anyone else knows them. 😀
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