Headphone Target Response Curve Research Update
Harman researchers Sean Olive, Todd Welti, and Elizabeth McMullin have continued their tireless treck towards a subjectively preferred headphone target response curve since last I reported on their efforts. The most recent paper shows their grip on the subject ever tightening and, to my eyes, giving us a clear way forward to better sounding (read: more pleasing) headphones.
In a previous paper, "Listener Preference for Different Headphone Target Response Curves" the team subjectively tested a number of headphone target response curves and showed that listeners preferred curves designed to mimic the sound of good speakers in an acoustically well designed room. The In-Room target responses for this previous paper were basically very educated guesses about what a headphone would sound like if it mimicked speakers in a room. The paper of current interest essentially tests these curves in a series of subjective evaluation experiments to home in on the exact curve that listeners find most pleasing.
"Listener Preferences for In-Room Loudspeaker and Headphone Target Responses"
Presented at the 135th Convention of the AES in October of last year, this paper strives to answer three questions:
- Are the preferred in-room loudspeaker and headphone target responses the same?
- Can the current recommended in-room loudspeaker and headphone target responses be improved upon with more research?
- To what extent is the preferred target response influenced by program, individual taste, and the methodology used to measure it?
I'll try my best to summarize the paperit's quite complicatedbut it's of such import to headphone enthusiasts that I highly recommend its purchase. You can find the paper here in the AES library. Cost is $20 to non-members, and $5 for AES members.
The first part of the paper details the calibration to absolutely flat of a reference speaker system in a listening room, and a headphone (Sennheiser HD 800) who's response was carefully compensated in order to match the flat response of that listening room. Then two tests were designed that allowed the listener subjects to adjust carefully designed bass and treble controls for listening preference. In one test, bass was set at one of two different levels and listeners were asked to adjust the treble for best listening pleasure. Then the treble was set to one of two different levels and the bass adjusted. In the second test, subjects were allowed to adjust both bass and treble for maximum listening pleasure. These tests were done with both headphones and speakers in a room. Eight trained listeners and three untrained listeners were used in the tests.
It's important to note at this point that a flat speaker in a typical home listening room does not sound flat. Due to the ever decreasing dispersion of sound from a direct radiating speaker as frequency rises and the low frequency gain of a small room (boundary effect), the actual sound power in the room is somewhat more bass heavy than the flat anechoic response of the speaker. This is normal, and it's believed that we expect and desire this warm tilt with speakers in a room.
The primary purpose of this test was to see exactly how people EQed the speakers and headphones away from flat, how much, and if there were differences between the preferred speaker and headphone response. It was found, generally speaking, that the preferred response was a warm tilt with about 8dB difference between bass and treble at the extremes. It was also found that people preferred speakers to have about 2dB more bass and treble relative to the mids than headphones.
This findingthat people actually like more bass on speakers than headphoneswas quite a surprise to me. I'll quote from the paper:
Our expectation was that listeners would prefer more bass in the headphones to compensate for the lack of whole-body vibration and tactile cues that may have been present in the loudspeaker reproduction. That was proven not to be the case.
Wow. Didn't expect that.
Now, I've left out all sorts of interesting details in summarizing here: How amazingly different recording studio reproduction systems are and how there are no comprehensive standards for control room speakers, and how that leads to serious confusion about the neutrality of recorded music to start with. How the program material used didn't seem to have too much effect on results, but listener training did. I will, however, highlight one particular and very interesting finding: The target response curve for headphones.
Headphone Target Response
With Sean Olive's permission, I give you the headphone target response shown in the paper.
The plot above shows the frequency response measured at the ear drum (DRP) of a flat in-room speaker response (dashed green line) and the new target response curve developed in the research. These curves are essentially what should be seen with ideal headphones in the raw (uncompensated) measurements on InnerFidelity headphone datasheets.
In the graph below, I've superimposed the raw responses of the NAD VISO HP50 and Focal Spirit Professionaltwo headphones that I find very neutral to my earsagainst the target response curve. It's a bit of a hack job, so please excuse the mess, but I think it's quite interesting. The NAD is the top set of gray traces, the Spirit Professional is on the bottom.
I've not spent any time (yet) looking through the headphone measurements to find others that are close, but you can bet I will. I will alsothough it will take a whilespend time creating a compensation curve for the measurement spreadsheets and stick it into a dozen or so headphones for a look-see.
Exciting Time Ahead!
In the '80s when Sean Olive (with Floyd Toole and Paul Barton) did work like this at Canada's National Research Council on developing a target response curve for speakers it sent shock waves through the speaker market. Where speaker tonal response was previously all over the map, after the studies concluded and speakers developed thereby sold well, the speaker market changed rapidly in response and speakers became much more similarand bettersounding. It's my fondest hope that this research will cause a similar rippling through the headphone market, and that we'll see many better sounding headphones in the future.
When will we see products from Harman developed in light of these findings? My communications with Sean this week have me believing it will be sooner rather than later. Keep a watchful eye on future JBL headphone offerings!
But most of all don't miss this excellent summary of their work to date in this PDF Powerpoint slide series.