New InnerFidelity headphone compensation curve.

First and foremost I want to thank each and every person who commented on my initial Compensation Curve for InnerFidelity Measurements post. An InnerFidelity target curve should be considered journalism and not quite science, but that doesn’t mean I shouldn’t be juried by peers. So I very much appreciate and take very seriously you comments about this curve. Thank you.

One thing I need to say right off the bat is that the graph tool will have the ability to select one of a number of compensation curves to apply, so you won’t be stuck with the InnerFidelity curve I’m developing. Other curves will be: diffuse field; independant of direction; averaged raw; and some form of the Harman curve. Just wanted you to know you’re not stuck with what I come up with.

I’m going to take the liberty of editing, reordering, and recapitulating the some of the comments and respond here. My apologies to those I haven’t directly addressed…this topic is very complex and I’m just trying to stay focussed on the details at hand.

Tyll’s Gone Full Arbitrary!

Submitted by ————– on September 14, 2017 – 5:53pm
“Graphs are confusing, so I averaged them. The bass hump is controversial, so I got rid of it.”

Submitted by Jazz Casual on September 15, 2017 – 11:59pm
I refer to Tyll’s headphone measurements as an objective data point, but it appears that he’s just arbitrarily gone and shifted the goal posts.

Submitted by castleofargh on September 15, 2017 – 4:59am
I don’t get it. we start with a clear hypothesis about having the sound of speakers in a good room and go for it. then you don’t like the results, which is fine, it’s one idea with one room an one dummy head. then we enter the twilight zone.

Submitted by geniekid on September 15, 2017 – 7:35am
I kind of agree here. It feels like you just gave up trying to calculate the compensation curve at the low end because of an anomaly that couldn’t be explained, although I suppose it doesn’t matter as long as you’re using the graphs for comparing different headphones.

From my point of view I’ve not been arbitrary at all…let me explain.

On getting rid of the bass bump
I think we can all agree that the bass bump is somewhat controversial. Sean Olive (named Tonmeister on posts here) commented at length about this. For the most part the size and shape of this bump comes from subjective preference and hasn’t been studied enough for firm answers. Another source of this bump is that the small rooms in which most people listen to speakers get a bass boost as the wavelength approaches the room dimensions; the argument goes that because people are used to hearing it, they desire it.

The other problem is the bump my head measured does not have the same shape and amplitude as the boost measured by a calibrated measurement microphone. This may be due to room modes, or boundary gain of the head, or who knows what.

Lastly, there are numerous headphones, primarily planars, that theoretically should be flat in the bass and when I measure them they show essentially a flat response in the bass. Some run out of steam at very low frequencies, but it’s clear to me that flat on headphones is simply flat in the bass. Here are some examples:

Audeze LCD3
170918_Blog_IFCompensation01_Graph_LCD3

HiFiMAN HE1000
170918_Blog_IFCompensation01_Graph_HE1000

Mr Speakers Ether
170918_Blog_IFCompensation01_Graph_Ether

Stax SR009
170918_Blog_IFCompensation01_Graph_SR009

Oppo PM2
170918_Blog_IFCompensation01_Graph_PM2

Koss ESP950
170918_Blog_IFCompensation01_Graph_ESP950

While there are all manner of rather arbitrary bass bumps I could insert in a compensation curve, it seems to me that flat is an objective standard and one which lends itself easily to viewer interpretation.

From ~300Hz up
In this area I don’t feel I’ve been arbitrary at all. The curve in this area is basically the average of all curves measured. Except for some smoothing, I’ve not manually manipulated it all.

Still, it may not be right. Maybe it would be better to use only the stereo or left channel only, or maybe one angle is most representative of what should be heard on headphones. But which to pick? And wouldn’t that be the more arbitrary choice?

My reasoning here is selecting any one measurement or narrow set of measurements will cause the resulting curve to have sharper features created mainly by the pinna reflections. While the pinnae of my head are very carefully designed to be close representation of the average human ear, they will be different from your ears. By spatially averaging all results I’m essentially removing any sharp features created by the pinna, leaving only a general approximation of what the ear hears in front of speakers. This resultes in a fairly smooth compensation curve.

A smooth curve is good because it will be less likely to accentuate or hide fine-grained features of the headphone under test. Many of the sharper features found in the speaker measurements change amplitude or center frequency depending on position of the head. If a compensation curve has a feature in it from one particular angle, it may cause a distortion in the curve if that feature slides to a slightly different frequency with a change in head position. A relatively smooth curve will be transparent to any sharp features in the headphone response regardless of where they occur.

None the less…

Submitted by castleofargh on September 15, 2017 – 4:59am
…now that I’ve lectured you like I knew anything, it’s the best time to ask for something(I’m such a great negotiator). I’d like to see the raw front looking dummy(no angle), with stereo and with only one speaker as you seem to have both. to see how much the response changes on that head from correct “crossfeed”(IDK how that should be called)of both channels, against one channel for one ear like headphones.

Since you asked so nicely….

170918_Blog_IFCompensation01_Graph_CenterLeft

170918_Blog_IFCompensation01_Graph_CenterStereo

The problem I have with these curves—and the speaker measurements in general—is the big dip between 1kHz and 2kHz. After staring at the graphs for a long time it seems to me this trough is elevation sensitive and the feature most prominent when the head is straight ahead. It seems possible to me that the lack of this feature in headphones may be partially responsible for headphone imaging being high in the head. The hardest place to get localization is directly in front and this feature may show a cue that is sensitive in the forward direction. Unfortunately my data isn’t really clear enough to make that call.

None the less, when we sit in front of a stereo system we want to hear the acoustic image fill the entire space between the speakers. If headphones were tuned for the center position, it may not portray information off-center properly.

500Hz to 2kHz
Submitted by mrspeakers on September 15, 2017 – 12:22pm
One thing I note is that the curve appears to create a notch/discontinuity around 1K. I suspect it very unlikely almost all these headphones have a midrange discontinuity like that, it’s a pretty critical area, and so it looks to me like there’s an issue there that might create concerns about midrange linearity.

Yes, as mentioned above I see a problem with any target with significant inflection in this area. In fact, I tend to think the Harman Target is probably right on 600Hz to 3.5kHz. For reference, both the Diffuse Field and Independant of Direction curves have deflections in this area as well.

170918_Blog_IFCompensation_Graph_IFDFIDCurves

I think we’ll have to see how this shakes out over time as people get to use the graph tool. I encourage folks to listen carefully in this area and comment as time goes on.

Height of peak at 3.5kHz
Submitted by Tonmeister on September 14, 2017 – 11:00pm
3. The other source of controversy could be the trade-off between bass boost and 3kHz bump. Many open-back headphones cannot produce the preferred 4-6 dB bass bump that are easily achieved with closed back headphones. So manufacturers trade-off the the bass bump for a reduction of energy at 3 kHz. If you read the paper by Gaetan Lorho ( see http://www.aes.org/e-lib/browse.cfm?elib=14966) he took a headphone equalized to the diffuse field response (ie. flat bass) and had listeners adjust the level of the 12-15 dB peak at 3 kHz (measured in an ear simulator) according to taste. He found listeners on average preferred the 12-15 dB peak at 3 kHz adjusted down to about 3 dB if I recall. So listeners were essentially trading off bass for less upper midrange Several studies have shown that Lorho’s target is less preferred to a target response like the Harman target that has more bass (4-6 dB below 125 Hz) with an otherwise DF-like response above 200 Hz.

Hm….interesting. Thanks, Sean. Personally I do tend to like this peak at about +12dB rather than +15dB…maybe you’ve identified why. At the moment I’m going to stick with the data gathered, but this is certainly something to think about moving forward. Eventually we might be able to have more than one target curve developed that covers the case of boosted bass or not.

3.5kHz to 8kHz
Submitted by brkitup on September 14, 2017 – 8:17pm
Thank you so much for your work on this. The graphs have traditionally not represented very well what the end user hears, and the new graphs are much more representative of how these headphones sound, especially in the treble. The one thing that still bothers me slightly, though, is the dip around 4-5kHz that seems common to all headphones when measured and compensated. I’m not sure how accurate that part of the compensation is.

Submitted by Argyris on September 14, 2017 – 8:55pm
Same here. It’s smaller in width and depth in the IF compensation than on the DF or ID compensations, but it’s still there. I wanted to see what this sounded like filled in, so I tried it on my HD 600 with a parametric EQ. I dialed in a simple peak centered around 5 kHz, with 0.3 octave width and 5 dB boost–it doesn’t perfectly fill the hole, but it’s a quick and dirty test of concept. It didn’t sound horrible, like filling in the much larger and deeper trough left by the other compensations does, but it did add an uncomfortable edge and glare to everything.

Submitted by kais on September 15, 2017 – 3:00am
Currently the 5K range looks suspicious.

I quite agree, this area is tricky. It seems some headphone makers try to dial down this area to rid the cans of the piercing sound that can occur here, while others seem to keep the level up in order to improve speech intelligibility. I tend to see a lot of variation here. Fortunately, the way the numbers shook out kind of splits the difference and I do feel pretty good about it. Again, time will tell.

8kHz – 13kHz
The proposed curve does have a little detail in this area. This is likely some ear canal resonances. I was tempted to just erase them as they may create some artifacts, but as I looked at a lot of headphone plots I do see some seemingly common features in this area. Fortunately in the process of averaging all the speaker measurement curves these features are relatively small in the compensation plot so they shouldn’t cause too much trouble and will likely reduce some of the noise in this region.

Finding Flat
Submitted by mrspeakers on September 15, 2017 – 12:22pm
In the absence of a standard “flat” compensation curve what about publishing a difference curve relative to your target? Musing out loud, people would be able to easily see where the response is on target or off and thus even if they’re not able to intuitively process what the 4K dip really sounds would a differential plot would make it more intuitive?

Maybe I haven’t been clear, but this is what I’m doing. The proposed curve will be subtracted from the averaged raw measurements to portray the difference between the two. The assumption is that if a headphone is tuned like speakers it will sound flat. Problem is, headphones aren’t speakers, and there may be psychoacoustic reasons for headphones to have a somewhat different response at the eardrum than speakers.

Submitted by zobel on September 15, 2017 – 8:39pm
Nope. You cant measure what we see by putting a camera in an eyeball, any more than you can measure what we hear by putting a microphone in an ear.

Subjectivity rears its beautiful head here. The brain subjectively interprets the SPL data at the eardrum, and trough the entire inner ear as it is converted into a representation of SPL in nerve impulses. It wouldn’t help to measure those impulses on the way to the brain either since the processor (brain) actually synthesizes our picture of the sound, with it’s own correction curve, which is as individual as our set of ears are.

We will just have to make tests like Harmon did to find averages among listeners in comparing cans to a reference system.

Submitted by zobel on September 15, 2017 – 8:27pm
Mr Speakers asks the same old question that we all have been for many years, “Why not portray flat as a flat line, using raw data to provide the difference curve between measurements, and the perception of flat. It of course is not possible to do this scientifically or subjectively, but that attempt needs to be made to provide a more meaningful and representative curve. It would be an averaged curve, like Harmon’s, based on listener’s trials with a reference speaker system in a room, followed by headphone listening to determine the closest match to the room sound.

Yep. That sure would be nice. Unfortunately that task would be way out of my job description. I can only defend my methods here by saying I am unaware of any standardized curve for my head other than diffuse field, free field, and independant of direction, and those curves are simply wrong for headphones. Measuring my head in front of speakers, right or wrong, seems to me the best I can do. As Sean says:

Submitted by Tonmeister on September 14, 2017 – 11:00pm
In conclusion, I think the main reason for this bass bump controversy is elated to these 3 issues (no target will satisfy all tastes and all programs) and there is lack of published scientific evidence to support any one target. Hopefully, this will change soon.

Amen.

Odds and Ends
Submitted by kais on September 15, 2017 – 3:00am
I would like to see more headphone’s curves compensated with this target, specially those models that are considered close to neutral like the STAX SR009.
If you average a lot of those the result should be something close to a straight line.

I’m not sure I’ll be able to average a big wad of headphones, but next week I will publish a couple dozen or so compensated plots so people can have a larger survey of headphones.

Submitted by markus on September 16, 2017 – 12:59am
I wonder how the frequency response would change if the dummy had arms and an upper body.

This is a good point. If you look at the proposed curve at the top of the page you can see the transition from flat to the upward rising portion at about 450Hz is rather abrupt. If my head had arms and a torso this might not be the case.

170918_Blog_IFCompensation01_Graph_HeadAndTorsoContributions

The dashed line labeled #2 in the plots above show the contribution of the torso to the ear drum response. You can see it does elevate response from below 200Hz. In the long run it may be that the IF compensation curve needs some messaging from 100-500Hz…but I don’t want to be any more arbitrary than I have to be—I’ve already ruffled enough feathers.

Submitted by Argyris on September 14, 2017 – 6:38pm
The HD 600 and HP50 graphs match up pretty well with what I heard (and still hear with the HD 600) from both headphones, much better than the previous compensation curve.

Submitted by kais on September 16, 2017 – 3:28am
If the goal is that a perfect headphone shows a straight line in the measurement this new correction curve approach comes closer than the one we had before. Still some way to go.

Thanks. Seems to me it tends to warm things up a bit too much when applied, which means the proposed curve is a bit too cool. It might be interesting to play with tilting the curve some, but I’ll wait ’til there’s far more evidence to make that move.

Submitted by Pokemonn on September 14, 2017 – 6:24pm
Is web graph tool underdeveloping? Hallelujah!

Submitted by lerrens on September 14, 2017 – 11:52pm
Innerfidelity has the best headphone measurement database right now and the Graph Tool will make it super awesome! I’m so excited I have to create an account just to tell you this: Thanks for all your work! Really looking forward to this 🙂

Submitted by castleofargh on September 15, 2017 – 4:59am
…and congrats for the graph tool being made. that’s really good news.

Hallelujah indeed. I’m stoked!

Please feel free to add more comments below, I will be watching carefully.

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