Measurements

Headroom

HeadRoom is a website selling headphones.

They have measurements of many headphones.
They explain what these measurements indicate and enables you to compare up to four models on frequency response, distortion, impedance, isolation and square wave response.
This won't tell you exactly how it will sound but gives you a nice indication.

This graph tells you that Shure SE530 is louder at the low frequencies than Etymotic ER-4S.

It also rolled-off the highs much more so it will probably sound bass heavy.

 

Both in-ear models offer a better sound isolation than the open Sennheiser HD800

 

Both in-ears produces a lot of harmonics compared with the HD800.

 

Innerfidelity

Beside a subjective listening test, Tyll Hertsens always publish measurements for the headphone at test.

His methods for measuring are explained here.

A massive PDF with a lot of measurements

Effin'Ringin'

A critical forum of headphones addicts.

 

DIY

MiniDSP EARS $159

SImple but affordable headphone and IEM measurment rig.

References
  1. The place to discuss headphones.
  2. InnerFidelity - Tyll Hertsens Blog
  3. Effin' Ringin' - Headphone Measurements: Frequency Response and CSD Waterfall Plots