Nice surprise ending to this one.
You may remember my trip to Harman International last year to plonk my measurement head down in front of a reference level speaker system. My motive was to determine exactly what speakers looked like with my particular Head Acoustics dummy head. My basic assumption was that good headphones should measure like good speakers. If I could capture the head response with good speakers, I could then build a compensation curve for headphone measurements that would be related directly to my particular head and a speaker based reference.
Now, is there a solid, scientific reason a compensation curve should be developed like this? No, not really. As many of you know, developing a headphone target response curve or compensation curve is still in its infancy. Sean Olive at Harman International has been working hard on the problem, which I’ve reported on here and here. He did find during his research that using an approximated in-room speaker response for headphones was preferred.
However, when I tried applying the Harman Target Response curve directly to my measurements of a number common headphone it didn’t fair very well. That’s when I got really motivated to develop a compensation curve for my head in particular.
If your interested in the story in detail, check out the articles here, here, and here. To recap: I took measurements of Harmans reference system with my head at five azimuth (left to right) angles (-20, -10, 0, 10, and 20 degrees) with the head in three elevation angles (-10, 0, and +10 degrees). I did this series of 15 measurements with both speakers playing and with only the left channel playing. Here’s a recap of the measurements:
Here’s the in-room response measured with a regular measurement microphone.
You’ll notice two things about this curve: a 5dB bass boost below 150Hz; and a gentle, overall warm tilt. Simply put, this curve is a result of putting speakers that are flat in an anechoic chamber into a room. The roughly 1dB/octave tilt come from reductions in sound power in the room as higher frequencies from the speakers narrow and beam. Warren TenBrook wrote a great article about this curve and more, which includes some interview materials with Sean Olive. Highly recommended.
Immediately after taking the measurements I summed up and averaged all the stereo measurements for a first look.
Right off the bat, I’ll draw your attention to two things. First is the bass boost. It looks too big to me, and it’s got a big hump centered at about 40Hz. This doesn’t look like the room response; I’ve got problems with it.
The other thing to note is the peak at 3.5kHz and the two little sombrero bumps to either side at 900Hz and 12kHz. As you’ll see, the measurements have a lot of noise in them, but this sombrero feature continues to show up.
I then crunched some numbers to come up with some graph families of the data taken of the response with the left speaker only. These curves were smoothed in Excel with some simple moving average filters.
Note that there’s quite a bit of activity in the sombrero areas to either side of the 3.5kHz peak. It seems the area between 800Hz and 2kHz is somewhat more sensitive to elevation changes, and the area between 8kHz and 15kHz is more sensitive to azimuth change.
Thinking my simple smoothing might be creating artifacts, I got in contact with Arnaud, who crunched the numbers with more sophisticated filters. Here’s the family plots in that form.
And then looking at the family averages.
And then comparing the average of stereo vs. left channel only measurements.
It’s pretty obvious this filtering ended up poorly representing the bass response, but it did do a pretty good job of providing a cleaner look above 1kHz.
Then I spent a lot of time just staring at the plots.
In the end I came to the conclusion that trying to play any games like tuning the curve according to the lower elevation plots to bring the sound down in your head, or trying to tune out narrow peaks, is rife with problems. Simple is better, in my view. Basically, I ended up just averaging all the curves together and smoothing it a bit.
Another thing I decided is to simply use a flat bass. There’s plenty of controversy around the bass hump in the target curve, even I feel like I’d prefer the bass hump to have a different shape. Bottom line: I think it would be least confusing to simply leave the bass flat and let the viewer interpret it as they wish.
Here’s my new compensation curve (in gray) compared to the diffuse field and independant of direction compensation curve.
Here are some plots of well known headphones using the new compensation curve. I’ll show the before plot using the independant of direction compensation currently used on spreadsheet, and then the after plot using the new compensation curve.
Sennheiser HD 600 Before
Sennheiser HD 600 After
Sennheiser HD 800 Before
Sennheiser HD 800 After
Audeze LCD-4 Before
Audeze LCD-4 After
Mr. Speakers Aeon Before
Mr. Speakers Aeon After
NAD VISO HP50 Before
NAD VISO HP50 After
I’d love to hear your thoughts on this new compensation in the comments below!
And now for the surprise!
You may be wondering at this point, why the heck am I doing this? Am I going to go back and poke these new compensation curves into the 800+ measurements spreadsheets I have and republish them? No thank you, that’s way too much work, and all these .pdf spreadsheets are a pain in the butt to maintain.
No, I’m doing it because the InnerFidelity Graph Tool is in the works! Code is being written as you read this! Wa-hoooo! I can’t wait for us all to be able to compare headphones live on the web again. And I think developing a good compensation curve is the most important thing I can do before the roll-out.
Over the coming few months I’ll tell you more about the graph tool as things get firmed up. I’ll certainly be giving a hard look to what’s been measured and what needs to get measured to have as complete as possible database of measured headphones at launch time. InnerFidleity readers will, as always, have the opportunity to send in headphones missing from the databaseInnerFidelity will pay shipping both ways in the continental US.