Not just contemporary, but also historic innovations (and I'm not saying all of these are as important as one another):
Audio-Technica:
- DADS
- Variations on 3D Wing
- Woodies (partially)
- Angled drivers (I don't know if AT has a marketing brand for this)
Ultrasone:
- S-LOGIC and S-LOGIC Plus
- MuMetal Shielding
- ULE
Beyerdynamic:
- First dynamic principle stereo headphones
- Tesla
- Variable porting (the COP)
Sennheiser:
- First open-back headphones
- Ring-radiator
- EAR (Ergonomic Acoustic Refinement)
Koss:
- First dynamic principle full-range stereo headphones (and they're also more or less responsible for stereo headphones as a consumer item as well; they did a lot to promote stereo headphone listening in the 1950s and 1960s)
- First electrostatic headphone (disputed; I think Koss can definitively claim to have produced the first CLOSED electrostatic headphones though)
- First noise-canceling headphone (I've seen counter-claims, but none are as old as Koss' claim)
- Adjustable IEMs (CC_01)
- Folding headphones (not certain on this, but I'm assuming PortaPro are the first)
- First celebrity endorsed headphones (I am not kidding; the original BeatlePhone)
- First (and currently only) WiFi headphones
STAX:
- First electrostatic headphone (disputed)
- First (only?) electrostatic IEM
Sony:
- First woodies (afaik)
- Various advances in diaphragm material over the years (bio-cellulose, LCP, nano-composite, etc)
- Magnesium-alloy frames for their headphones (very few other manufacturers do this (AT does on their TOTL), especially at a reasonable price point)
- Zobel networks in headphones
- "Personal ear speakers" (the PFR-V1; its unique, whatever it is)
- Open-air headphones
- Auranomic driver alignment (this is Sony's marketing brand for angled drivers)
AKG:
- Varimotion diaphragm
- Open-air headphones
- ESP/dynamic hybrid
- Flat-wire voicecoil
Denon(/Foster):
- Free-edge driver
- Nano-fiber driver (afaik this is unique)
Grado:
- Vented Air Chamber (basically the overall design of the Grado enclosure is unique; I'm not sure "Vented Air Chamber" is accurately how they would describe the entire thing though)
Bose:
- "Acoustic Noise Cancellation" (they're probably the only legitimate claimant to be first to ANC, over Koss, that I'd believe as well)
- TriPort design
That's all I got, off the top of my head.
EDIT:
Oh yeah, and this is a cool idea for a thread!
