So they can hear whether the microphone is recording their voice properly.

Counting on it! ;)

They‘ll probably be popular for a while once Linus Tech Tips inevitably tests them..

I would probably not do anything and just keep using them.

> the area’s coating

Doesn't look like there's any coating. To me it looks like simple injection-molded plastic with a metal charging pad.

The water-proof part is the mesh in front of the microphone/speaker, and a seal/gasket where the shells meet so that no water enters the inside volume where it could cause electric shorts.

It‘s not fully clear from the picture - but some adhesives/glues are green. There‘s a possibility that the adhesive that was used to secure and seal the PCBA inside the earphone was leaking next to the charging pad, hence why it‘s visible outside.

Could also be corrosion, hard to tell from the picture.

It's origin is the HD580, which was released in 1993. That's 30 years ago.
The HD580 Jubilee (1995) and HD600 (1996) are small modifications of the HD580, and the HD650 (2003) has slightly less distortion.

It's old - ancient, if you were to apply the same metrics as for e.g. semiconductors.
But (passive) headphones are not semiconductors, and how good a headphone is generally doesn't correlate a lot with how old it is.

Its shortcomings have nothing to do with its age.

The HD6XX is based on the HD650 the same way that the HD650 is based on itself - it is just a different packaging and cable. Exact same acoustics.

Same as for the Harman Over-Ear target, just with the gain of the 105 Hz low-shelf filter set to 0 dB.
Or better yet: set it to whatever sounds best to you, anywhere from 0 to 15 dB is fine.

HA

Very unlikely. If it‘s not damaged, most certainly not.

How many times have you listened to them, out of curiosity?

Theoretically?

Most definitely.

But since you likely don't have the Firmware Update Tool of the chipset that is being used:

Practically? No.

The HE1 is very (!) close to the Harman Target (not by coincidence but also not because they wanted to, Sennheiser‘s internal research simply yielded very similar results to Harman‘s).

So I wouldn‘t say the Susvara tried to copy the HE1‘s sound, it just was designed with a similar target in mind (transparent, uncolored sound)

Hifiman does not have a headphone with the same timbre as the HE1.
They're all missing the bass boost that the HE1 exhibits.

> You can drive them with any amp that provides enough power.

With a load impedance that high you'll mostly be concerned about providing sufficient voltage. Current draw (and hence power) won't be high due to the high load impedance.

It's going to change the magnetic field.
This is - theoretically - predictable of course. Electrodynamics is a well understood field. But some of the parameters will be tricky to find out in your situation (magnetic remanence, for example).

> what would that do to the performance of the cans?

Any number of things could happen, but the most likely outcome is that it's not going to "better" in any quantifiable measure.

You‘re entirely correct.

I don‘t have a PhD and yes, minimum phase ends usually about an octave below 20 kHz, depending on the size of the front volume.
In-ear headphones normally are up to 20 k and higher since the front volume is so much smaller.

This just shows just how deep into the market the AirPods really have penetrated - virtually everybody knows them.

Headphones with built-in filters have been around for a decade, maybe more.

Every TWS in-earphone, every wireless headphone has that nowadays.

I don‘t think these ever had bluetooth - these are wired EarPods (older than AirPods), just with the cable being made detachable.

Those aren‘t Airpods, those are regular wired earbuds, these have existed before the AirPods. In fact the shape of the AirPods is modelled after these.

In the Bay Area you can find companies like that within walking distance regardless where you are.