Impulse response is the same as frequency response, just visualized in a different domain (time vs. frequency). The only time the impulse response is a nice addition is with multi-driver headphones with crossovers that delay different parts of the signal differently.
Electrical impedance and phase does not change how the headphone sounds, nor does isolation.
Square waves, as has been shown in other threads, are difficult to "interpret" because inaudible phase shifts can change their shape significantly. Also, if you have the impulse response you can also calculate how the square waves would look like (without nonlinear distortion).
And for non-linear distortion there is the THD plot, which is probably the second most important measurement.
Well, I disagree here as well. I doubt plastic has better acoustic properties than other materials, and even if it had they wouldn't have to put the spray painted plastic on the outside.
The HD280, for example, is made of plastic as well and looks sturdier but has serious resonance problems due to the plastic. I guess the enclosed volume and isolation are significantly more important.
A couple things:
Yes, impulse response is similar to frequency response, in a way. It gives us a way to tell what the highest reproducible frequency
could be. However, it cannot be faked/changed based on EQ or any other processing. And not only do we care about the impulse speed, but also the size of the oscillation that happens afterwards (distortion).
And the plastic case in the HD800, the plastic actually
does have better acoustic qualities -when you design for it
. Not all plastic is equal- nylon, teflon, PVC, PLLA, PA66, etc all have different acoustic qualities (and yes, I actually used to work engineering new types of plastics, though you wouldn't want to use the stuff I developed in your headphones as it's both carcinogenic water soluble). What's in the HD800's is a rather expensive type called
Leona, made by Asahi Kasei, and much of that is actually filled with glass fibre (again, fine tuning the acoustics). It's actually
very expensive to engineer headphones in this way.
Now, to be fair, they really could have used a better silver paint... I have no idea why they didn't think that one through.