This is part of a post I made some time ago in another thread, but I thought I'd re-post it here. Take a look at some of the headphone measurement systems (below) that you've been checking out graphs from.
There's a lot of discussion about headphone measurements, and I think it's important, when looking at graphs/plots, to understand that there are many different headphone measurement systems and methods, and you're likely to get as much variation in measurements as there are different measurement rigs and people doing the measuring.
This is a topic I'll be discussing more and more in the coming months.
This is an oft-overlooked discussion, the comparison of headphone measurements made by different people with different setups. As catscratch said, it can make measurements made with different systems rather challenging to compare.
For example, Tyll Hertsens (Editor-In-Chief of Innerfidelity), as he describes here, uses a measurement head (the Head Acoustics HMS II.3). It is one of the industry standard tools for measurement. From that link above, you can read about Tyll's complete measurement system, including the acoustic isolation chamber he custom-built, the Audio Precision System 2 Cascade audio tester, and other components of his setup (and his methodology). Here are photos of his setup that he posted in that article (click the images below to see them in full size):
In one of the more recent headphone studies presented at AES by Sean Olive (Director Acoustic Research at Harman International and President-Elect at AES) and Todd Welti (Research Acoustician at Harman International), measurements were made using a G.R.A.S. 43AG Ear and Cheek Simulator equipped with an ITU-T type 3.3 pinna. (More specific details can be found in the full paper--Convention Paper 8744*--on the AES website.) Again, the G.R.A.S. piece and pinna is another industry standard measurement setup. Here are photos of the G.R.A.S., both without and with a ear/cheek affixed (click the images below to see them in full size):
Keith Howard in Stereophile also mentions that he uses a G.R.A.S. ear and cheek simulator; Brent Butterworth of Sound And Vision also uses a G.R.A.S. device.
I've been to several headphone manufacturer facilities, and have seen gear similar to the above-mentioned equipment in them (heads, head-and-torsos, and devices like the G.R.A.S. devices).
In their AES Convention Paper 8994* (Listener Preferences for In-Room Loudspeaker and Headphone Target Responses), Sean Olive, Todd Welti, and Elisabeth McMullin used the G.R.A.S. 45CA. for their headphone measurements.
In AES Convention Paper 9118* (The Correlation Between Distortion Audibility and Listener Preference in Headphones), Steve Temme, Sean Olive, Steve Tatarunis, Todd Welti, and Elisabeth McMullin use the G.R.A.S. 45CA, as well as the Bruel & Kjaer Head and Torso simulator Type 4128.
Here are some photos of the G.R.A.S. 45CA:
(Photo from this page on rinchoi.blogspot.com.)
The G.R.A.S. 45CA is, in a way, like having two G.R.A.S. 43AG Ear and Cheek Simulators combined, so both channels of a stereo headphone can be measured at once. One key advantage of the 45CA (versus the 43AG, or even a pair of them) is that the 45CA's "head" housing has median adult human head height and width (with cheek angles), so headphone clamping force does not need to be measured separately and then dialed into the clamp. (The single-sided G.R.A.S. 43AG's clamp mechanism can also present some challenges with open-back earcups, because it clamps down on the center of the back of the headphone earcup.)
One key disadvantage of the 45CA versus the 43AG is that the 45CA is not nearly as portable. It is much larger, and much heavier than the 43AG.
On the more homegrown fronts, purrin and Rin Choi have created their own setups.
purrin has elected not to use a dummy head or, to the best of my knowledge, a pinna. I'm not sure that his measurement system was ever discussed in great detail. When asked about his measurement system on Head-Fi, purrin called his coupler "semi-secret." (link) During a discussion at the Newport Beach audio show last year, when asked (on video) which coupler he uses (link), purrin (Marvin Chen on the video) responded: "I'm actually not using a dummy head. The reason is I actually did try a dummy head and there's some issues with that. With dummy heads you've got the ears, and the really good ones have got the tubes in them, and those create some specific resonances, that...don't necessarily represent what we hear. If I were to do a CSD measurement with one of those couplers, there would be resonances that are inherent to the human auditory system which I feel that the brain filters out. So what I actually did was I built a custom coupler. Essentially, what it does...it tries to minimize the effects of the ear, the ear canal, while at the same time trying to be realistic of what we actually hear."
Here are photos of purrin's custom coupler setup (click the images below to see them in full size):
2015-04-17 UPDATE: Marv (purrin) has updated his measurement rig to a second version. Here are photos (click photos to see larger versions):
He has described its basic structure as foam, plastic plate with hole (using a CD), and Creatology foam with hole cut for microphone. Here is Marv's description, in greater detail:
Originally Posted by purrin
- In the middle of the foam later, a piece of felt about the size of an ear. Without this piece, I found measurements too ringy. Covered completely with felt, I found measurements too damped. The felt the size of an ear also serves another function. Proper measurement of supra-aurals that rest of the ear.
- I use a strip of felt to simulate hair and sometimes another strip to simulate imperfect seal of the nape of the neck. Knowing to get a properly simulated seal is crucial.
- The hard plastic plate (use a CD) is a good bone simulator.
I filled the inside with medium density open cell foam - to simulate brains. Use something squishy to avoid interactions between L and R.
Really, you could just put the felt / foam layer on top of a CD on top of a foam brick. I just made it look nicer and named it C.U.N.T. to establish "cred". We all know that stuff that looks nice, that is shiny white, and has a euro kind of acronym like G.R.A.S. or C.U.N.T. must be good.
Rin Choi (who goes by udauda on Head-Fi) created his own dummy head that he named EURI (photos below). It is covered in a latex skin to "effectively simulate the damping effect of a human skin & insulate any acoustic leakage between the pinna simulators & the head itself." Inside of EURI can be found IEC 60318-4 occluded ear simulators. On his website, it seems to me that Rin is shooting for using industry standards (in terms of methods and gear), and, with his own measurements, compares his EURI head to commercial alternatives.
Here are photos of Rin's EURI head and the 60318-4 occluded ear simulator (click the images below to see them in full size):
Again, the measurement setups and methodologies can differ substantially from measurer to measurer. Some use more industry standard equipment and methodologies; still others claim to sidestep industry standards for individual reasons and/or claims of more accurate measurement results. So, as was stated earlier, it can be tough to make meaningful comparisons of measurements between the results of the different providers of them.
* The direct-download links for the AES papers require AES membership with AES E-Library paid access.
2015-04-17 1211 EDT: Updated with v2 of purrin's measurement rig.
2015-04-24 0910 EDT: Updated with G.R.A.S. 45CA.