Finally, you can calibrate your headphones to your ears.

[Marko_EarEqual]

This is definitely interesting and I appreciate all the discussion and explanation so far. Noob question here: I understand the need for open headphones to create a valid measurement with this mic. Does this also mean the resulting correction is only valid for those headphones, or could it also be adapted to closed back cans?

Specifically: I have a set of Focal Elex that might work for the measurement but I'd also want to correct my Audeze closed-backs.

Good question. Two separate things here.

First, the correction is specific to the headphone you measure. An Elex measurement corrects the Elex, it won't carry over to the Audeze, because each headphone has its own response at your ear. So you measure each pair you want to correct and get a correction for each.

Second, yes, you can measure the closed Audeze too. The "open headphones needed" idea is a simplification. The blocked measurement assumes free-air equivalent coupling (FEC), and no real headphone meets that perfectly. Møller et al. measured 14 headphones and found that all but one deviated from ideal FEC on the order of 2 to 4 dB above 2 kHz. It tends to be a bit larger for closed backs, but open vs closed doesn't reliably predict it. Hammershøi herself made this point, an open headphone isn't automatically FEC and a closed one isn't automatically bad.

The alternative, measuring in an open canal, trades that FEC error for the problem of holding the mic at exactly the same spot every time, which is difficult and prone to errors. So for a closed back you're picking between two imperfect methods, and the repeatable blocked measurement can be the better one. Neither is perfect, neither is wrong.

Short version: measure both, correct both, and the Audeze being closed rules it out.

 

[morillon]

Some do it manually and subjectively, using slow, targeted sweeps—through trial and error, and so on.

 

[listener650]

Great discussion here, many thanks to @Sean Olive, @solderdude, and of course @Marko_EarEqual for the enjoyable Saturday morning read.

Question for @Marko_EarEqual; do you think the issue of repeatability is one inherent to open canal microphones, or do you think there's a possibility that the magnitude of variation we've seen in the literature is partially down to how the open canal microphones people are using are designed?

I ask because there are plenty of blocked canal mic designs that can also have incredibly unreliable positional consistency, so I'm wondering if you've maybe seen (or done) any comparative evaluation of different open canal microphone designs similar to what Brinkmann et al. have done for blocked canal designs.

 

[Marko_EarEqual]

Great discussion here, many thanks to @Sean Olive, @solderdude, and of course @Marko_EarEqual for the enjoyable Saturday morning read.

Question for @Marko_EarEqual; do you think the issue of repeatability is one inherent to open canal microphones, or do you think there's a possibility that the magnitude of variation we've seen in the literature is partially down to how the open canal microphones people are using are designed?

I ask because there are plenty of blocked canal mic designs that can also have incredibly unreliable positional consistency, so I'm wondering if you've maybe seen (or done) any comparative evaluation of different open canal microphone designs similar to what Brinkmann et al. have done for blocked canal designs.

Thanks, and good question.

For an open canal measurement you want as little as possible perturbing the canal, so there is not that much room to "design" the sensor itself. The smallest electret capsules are around 3 mm diameter and 1.5 mm tall with two wires. A MEMS is similar in footprint. Probe tube approaches have smaller footprints but tend to be noisier.

But in my experience the dominant source of variability is fixation: keeping the mic in the same position between the reference measurement and the headphone measurement. The hard part is that the headphone cushion presses on the wires and the skin and moves the mic. And because the open canal pressure field has steep spatial structure at high frequencies, a small shift means your two measurements were effectively taken at different planes. So a lot of the variability is really the field being position sensitive combined with the cushion guaranteeing a position change.

To your direct question, I have not done a controlled comparison of different open canal probe designs. What I can speak to is my own near eardrum work, where we placed a mini mic close to the eardrum with an otologist's help. It was very hard to position the mic accurately or consistently across subjects, and the cushions always moved it. So my read is that fixation under a headphone is the harder problem than the sensor itself.

There is also a separate class of semi open designs where the mic is supported by some structure bracing against the canal walls. That opens a different topic which I would rather leave for another time.