Testing headphones is a very complex topic and professional grade equipment very expensive ($10,000+). The methodology and equipment outlined here using frequency response measurements will get you to a pretty good sounding design. Best of all you can get good measurements for as little as $100-$200.
To add to the challenge, each different test rig design will give a different curve for the same headphones, even professional ones. The is largely because the geometry of the ear will affect the frequency response, and different test rigs have different ‘ears’.
As covered here, the aim is for the non-flat Harman curve. Given the point above about different test rigs giving different curves, the approach I recommend is to develop a target frequency response curve from a pair of headphones you really like the sound of, then tune your design to get as close as possible to that. Then adjust to your taste.
The $100 DIY Headphone Test Rig
While flawed because it does not have the human ear form, which affects the measurements, a flat plate type design can get you very far for not a lot of money.
The initial development of the Homebrew Headphones design was done on the following rig, with a pair of Grado SR225’s as the target response.
The microphone is a MiniDSP UMIK1, which retails for USD$120. The Behringer ECM8000 looks like a decent, cheaper option. The above rig is made from 12mm MDF, with the plates about my own head width apart.
To enable almost extreme repeatability when doing my initial work to determine the effect of different design parameters, I made a single headphone with 4 bolts and added corresponding holes on the rig. This enabled almost millimetre accurate positioning and gram accurate pressure on the cushions. The repeatability of this was better than my current MiniDSP EARS rig.
MiniDSP EARS – The $199 Option
A completely off-the-shelf option is the MiniDSP EARS tester. It is not an industry standard test fixture, so the results are not comparable to professional measurements.
I recently purchased one and initial impressions are pretty good, although I am not sure that it is much better than the plate design.
You can use the sound-card of your computer to provide the output to drive the headphones being tested. If you do, make sure all effects are turned off. On Windows, go to Control Panel -> System -> Sound -> Device Properties then disable all effects for your particular device.
A better option is the use a DAC and headphone amplifier. I currently use the Schiit Fulla.
Software For Measurements
You will need measurement software to take frequency response measurements. Room Equalisation Wizard is an excellent piece of free software for this purpose.
The position of the headphones on the test rig can have a big impact on the frequency response curve, so I would recommend taking multiple measurements. For circum-aural designs, make sure there are no gaps/leaks between the cushions and the rig, as this will affect the frequency response.
Tyll Harstens at InnerFidelity takes 5 and averages them. My approach is to take 5 or so, removing and replacing the headphones each time. This will generally give a few that are consistent and a few outliers. I delete the outliers and keep the one that looks like the median of the consistent range. I think this is a good compromise between time and accuracy.
Testing headphones is a pretty meaty topic! The following are some good resources for those who want to read further: