A couple of weeks ago, I received a very interesting email addressed to Dr. Sean Olive, Acoustic Research Fellow at HARMAN INTERNATIONAL, and myself from Warren TenBrook, an astute enthusiast. Enclosed was a .pdf paper titled "An Acoustic Basis for the Harman Listener Target Curve," in which he describes an alternative way to view the Harman target response. His purpose, was to elicit some feedback on his thoughts, and, since it contained content from both Harman Intl. and InnerFidelity, to ask for permission to publish his paper on headphone forums.

I found his thoughts both elegant and provoking. It's not that there's anything particularly new in the details of his writings, but rather that his viewpoint offers a refreshingly clear way to see the issue from a different perspective.

Evidently, Dr. Olive found his perspective tantalizing as well, and over the course of the next week or so, he and TenBrook exchanged a few emails clarifying the concepts in the paper. What follows is a mash up of the original paper interspersed with further comments by Olive and TenBrook's subsequent back and forth in placed where topically relevant.

Enjoy!

P.S. Please read to the end as I have, what I consider, a very interesting announcement to make!

An Acoustic Basis for the Harman Listener Target Curve

by Warren TenBrook

Dr. Sean Olive and Todd Welti of Harman research have completed several listener testing studies to characterize preferred headphone sound under controlled conditions. They've posted their latest presentation on headphone target curves and listener preferences:

http://seanolive.blogspot.com/2015/11/factors-that-influence-listeners.html

The starting point for this work is a pair of Revel F208 loudspeakers measuring flat on axis in an anechoic chamber with wide, smooth directivity, placed at +/- 30 degree stereo positions in the Harman reference listening room. Depending on set-up, the room acoustics are capable of smooth 0.4-0.5sec RT60 reverberation across the audio spectrum in accordance with ITU-R BS.1116-1. Here is the unequalized speaker curve:

There are normal frequency response variations in the room, particularly in the bass; however, I have added a 1dB/octave trend line to highlight a gentle downward slope. For comparison, here is a graphic of a JBL Synthesis professional room correction, including subwoofers. The JBL target curve and measurements before and after EQ are plotted. Again, the entire curve family slopes 10 dB across the spectrum, 1dB/octave:

It is also instructive to compare the classic Bruel & Kjaer curve from their 1974 AES paper measuring multiple critical listening rooms and studios. There is a 6dB decrease from 160Hz to 20kHz, about 0.9dB/octave, but the pattern is consistent.

Both Dr. Olive's initial speaker measurements, the JBL Synthesis measurements, and the B&K curve agree that high-quality speakers measuring flat in an anechoic chamber tend to have a ~1dB/octave "room gain" curve when placed in a good-sounding room with no equalization. Also, JBL Synthesis (a Harman company) room correction does not equalize the speaker system to be flat in the room. They smooth the room frequency response to conform to a 1db/octave natural room gain target.

Importantly, the next step in Dr. Olive's headphone listening test method is to equalize the speakers flat in the room:

A Graz head simulator is then used to record the frequency response at the eardrum reference point (green curve):

Starting with the green headphone EQ curve, listeners were given headphones with ability to adjust bass and treble. In separate experiments, listeners arrived at the black (2013) and blue (2015) curves above, with somewhat more bass and slightly less treble, perceived as accurate or preferred.

This approach suggests two concerns, one technical, another philosophical:

First, Dr. Olive's research uses a flat in-room EQ as a reference point for subsequent listening tests and filter adjustments, which removes the observed 1dB/octave room gain of an accurate loudspeaker in a good room. I would predict that the flat in-room equalized sound would be too bright and bass-light for most listeners versus the unequalized sound of the Revels. Harman's listening test subjects agreed, and preferred increased bass and decreased treble. The flat in-room EQ does not appear intended to optimize the sound of the speaker in the room, but instead seems to provide a raw, blank canvas for the listeners to adjust the sound signature to their preference during the experiment. It is important not to confuse this 'palate cleansing' flat in-room EQ with accurate loudspeaker room response or accurate headphone EQ.

Sean Olive - I appreciate your thoughtful comments on headphone target curves. I think we fundamentally agree on how to derive a headphone target response but there is some confusion on some of the steps we took in our work.

1. I think our target curve from the 2013 paper was indeed based on acoustical principles: it was first derived based on simulating the response measured at the DRP [earDrum Reference Point; as measured by a head simulator at the ear drum] of an accurate loudspeaker (Revel F208) measured in a reference listening room. We showed that the Revel F208 has a reasonably flat smooth, wide-band on-axis response and smooth off-axis sound and sound power based on the anechoic measurements.

We agree that based on previous experiments (also see Toole's recent paper http://www.aes.org/e-lib/browse.cfm?elib=17839 ) that such a loudspeaker will produce good results as long as acoustical interaction between the loudspeaker and room are dealt with below 200-300 Hz, and the bass is adjusted to produce the 10 dB or so downward slope from 20Hz-20kHz. Depending on the room and the loudspeaker-listener, this may require some extra LF gain to produce the rise below 125Hz or so. So before we "flattened" the in-room response of the loudspeaker we essentially had a headphone target response based on an ideal in–room loudspeaker response.

Warren TenBrook responds - Thanks for such a generous reply.

For point 1, I think some of my confusion comes from my interpretation of documents in your blog posts. Your reply mentions you had measured the ideal in-room response. However, I couldn't infer exactly which published curve represented this response. I was under the impression that either flat in-room EQ was being presented, or the listener preference curves.

On closer scrutiny, I see slide 5 of your presentation "Factors that Influence Listeners' Preferred Bass and Treble Balance in Headphones" includes an illustration of a 'Harman Target Curve" that looks virtually identical to my estimate.

Slide 11 also is close, with a bit less bass and slightly higher 3 kHz treble; however, it appears in the presentation after describing the F208 EQ'd flat at the listening position, so I was was not confident this was the 'ideal' in room curve.

Slide 30 is clearly labeled 'Loudspeaker equalized to flat in-room response', so that is the illustration I used to apply 1 dB/octave for my estimate.

If you could point me to the "headphone target response based on an ideal in–room loudspeaker response" my questions would be largely answered. I'd bet on Slide 5, the first graphic below. I'd also be curious if both channels of the stereo pair are driven during measurement, or perhaps summed later, so each ear in the dummy head would include cross-feed from the other channel.

Sean Olive responds - I'm glad we had this conversation to clarify some of the confusion in our work.

Below is a link to a PDF summarizing the headphone target development. Hopefully it will clarify your questions,

https://dl.dropboxusercontent.com/u/16343460/History%20of%20Harman%20Headphone%20Target%20Curve.pdf

As I explained before (and in these slides) the Harman Headphone Target Response matches the preferred in-room loudspeaker response with some tolerances ( +/-1 dB ) in the preferred bass and treble gain.

The preferred in-room loudspeaker response is a smooth curve from 20 to 20 kHz with about a 9-10 dB downward tilted slope. We agree that a flat in-room response for a loudspeaker will sound too bright and thin. We used that as a baseline for adjustment. In the end, if the loudspeaker has a flat on-axis response you shouldn't need to adjust the treble, but the bass will probably need adjustment depending on the room, and the positioning of the loudspeaker and listener.

Todd Welti is working on an explanation of how he measured the loudspeakers in the room. But essentially he used a GRAS 45 flush mounted to a head and measured each loudspeaker channel independently at the DRP with head in the sweet spot. He assumed symmetry for the head/ear and in the room and loudspeaker setup so the left ear signal for the left loudspeaker would the same as the right loudspeaker measured at the right ear. I think this is fair based on the shape of the room and the loudspeaker setup, which was symmetrical. He did a spatial average for each measured channel for +/-30 degrees and 0 degrees.

Sean Olive - 2. What seems to bother you is that we flattened the in-room response as a sort of baseline curve for the method of adjustments. It was not our intention that this flattened curve be a "reference" that listeners could hear and compare their adjustments to. The flattened curve was only a baseline from which the bass and treble adjustments were made. We never provided listeners this flattened condition as a "reference" in the listening experiments. In fact, the starting bass and treble levels were randomized in each trial so that there could be no bias effect related the starting levels. An important note is that original in-room loudspeaker response (before flattening) was one of the options that listeners could choose by adjusting the bass and treble controls to the appropriate settings. In the end, listeners preferred a headphone target response that came pretty close to the preferred in-room target response of the loudspeaker using the same programs.

In the 2013, paper they preferred slightly less bass than the loudspeaker. In the more recent paper, the average preferred bass level was the same.

Second, I support listener preference research, but I feel that subjective listener data using two broad bass and treble filter adjustments should remain focused on documenting listener preferences alone. I don't support extrapolating these subjective preferences to broadly-applicable target curves for design, testing, or purchasing headphones as this contributes to what Dr. Floyd Toole dubbed 'The Circle of Confusion'. In my opinion, a measurement specification like a headphone target curve should be derived from objective physical and acoustic principles; listening tests can then inform us if the objective curve is reasonable and consistent with preferences, i.e. "musical".

Sean Olive - 3. Your second argument, is that preference experiments should NOT be used to define target responses for headphones, in part, because of circle-of-confusions issues. All of the loudspeaker research (Toole @ Olive) and headphone research (Olive & Welti) have involved preference ratings to help support acoustical hypotheses of what the ideal headphone and loudspeaker should look like in terms of acoustical measurements.

Whenever you do listening tests you have to use music programs and cannot avoid the circle-of-confusion issues. So you try to choose neutral, sensitive programs and use many programs and use statistical averaging. Ideally, we should have used more programs statistically sampled from what people purchase but we don't have the time and resources to do so. We acknowledge this in the paper and say that the ideal target response is a moving one that will depend on the recordings you listen to.

If the professional recording industry used calibrated, standardized monitors that were well-defined acoustically to record and master music, then it would be simple to define the target response of headphones and loudspeakers: you simply copy their standard. But because no such standard exists we have to define a statistical average of what they use and hope the recordings were made using that average.

The good news is that statistical average of most professional monitors is probably close to something like a Revel F208 (flat on-axis, smooth sound power) or a JBL LSR632 or M2, which means our headphone target response is pretty close. What we did find from this research is that the preferred bass and treble levels depends on the age, listening experience and gender of the listener. Younger people with less experience preferred more bass and treble. Older people preferred more treble (to compensate for hearing loss?). This is helpful in explaining why some listeners may not prefer a headphone or loudspeaker that is preferred by others.

Warren TenBrook responds - Point 3 is most interesting. Again, when I could not readily pick out the measured 'ideal in-room loudspeaker response' curve, I began to almost view the listener test data as obscuring what I wanted to learn about the in-room measurement, and that led me to imply that you were largely basing the target on listener data, and de-emphasizing the in-room measurements which were my primary interest. I wanted to convey my opinion that listening tests are not an end of themselves, but a confirmatory tool to make sure the technical approach holds water.

Then I read your insightful reply "...If the professional recording industry used calibrated, standardized monitors that were well-defined acoustically to record and master music then it would be simple to define the target response of headphones and loudspeakers: you simply copy their standard. But because no such standard exists we have to define a statistical average of what they use and hope the recordings were made using that average..." This caused me to recall the work at NRC on listener testing of loudspeaker characteristics. Dr. Toole and other's work to find correlations between measurements with listener preference changed the industry. (I still have my trusty Paradigm 5se bookshelves from years ago. I think their design was based on Floyd Toole's workand they still sound great). My impression is we are getting close to a de facto speaker standard building on this research. But it never could have happened without listener tests to guide what measurements were relevant to good design. Clearly this is important to a headphone target as well.

Even without a generally-accepted standard for mastering studiosand assuming a majority of recordings weren't mastered on accurate speakers in a good roomI'm glad Harman is putting a stake in the ground and proposing that its speaker products and reference room reflect state-of-the art and are suitable to derive the headphone target curve.

Sean Olive - 4. In conclusion, I believe our listening tests confirm that the preferred headphone target response closely matches the ideal in-room target response of a loudspeaker measured at the DRP.

I would argue we defined the curve based on acoustic principles (measuring an anechoically flat speaker calibrated in a reference listening room) and confirming that people like it through a method of adjustment listening test. In the listening experiments, the green curve (flattened in room curve) was only one of many options including the ideal 10 dB-octave sloped in-room curve. It was not a "reference" that listeners could access as an anchor but rather one of many options within the limits of adjustment. One of the adjustment options was the so-called ideal in-room target curve.

In the end, the listening test data shows that listeners rejected the flat in-room curve, which has been shown in previous studies, and preferred something that resembles something you propose. You could go through the exercise of measuring many other speakers in many different rooms to acoustically derive a new target curve. But if you look at all the studies together (see Toole's paper), there seems to be consensus on what the ideal in-room loudspeaker response should be: in rooms, cars, cinemas and hopefully soon, headphones too.

An Acoustic Basis for the Harman Listener Target Curve (continued)

I wondered if there was a physical/acoustic approach that would favorably compare with the Harman listener test preferences. What would happen if I restored the 1dB/octave room gain slope to Dr. Olive's measured flat in-room 'green curve'? Here is the result:

The adjusted curve is similar to the Harman listener curves in key respects:

The adjusted curve has a gentle rising midrange from 200 hz to 3000 hz.

Treble is tamed compared to the flat in-room speaker measurement and returns to baseline level at 10 kHz.

The 1 dB/octave room gain bass rise from 200 Hz to 20 Hz is similar to the Harman listener curve, without a bass 'shelf' under 100 Hz. The 'shelf' might be related to the bass filter shape used for the Harman studies (another reason not to derive target curves from the two filters used in the listening tests).

Tyll Hertsens has commented on bass emphasis in several headphones bleeding toward the 200+ Hz range can lead to upper bass and lower midrange thickness that is not neutral. The adjusted curve backs off the upper bass level slightly more than the Harman curves, particularly by eliminating the bass shelf.

Here is a comparison of the curve with some sample headphones. First, the Oppo PM-3 follows the curve very closely, with treble somewhat rolled-off as observed by Bob Katz:

Next, the Ether C is slightly under the curve in the bass and has a bit more energy at 3-4 kHz, but the midrange and treble follow the curve very closely:

Sennheiser HD650, an open headphone, follows the curve well, with a bit less bass extension and slight midbass warmth:

It appears that measuring accurate loudspeakers in a room with good acoustics and natural 1dB/octave room gain would provide a measured target curve close to that preferred by Harman listener tests. I predict an accurate objective target curve for headphones could be derived from:

Rooms with controlled reverberation time meeting ITU specifications. Testing in different rooms meeting the specification would be beneficial.

Speaker placement at +/-30 degree standard stereo.

Relative speaker distance to room boundaries and to listener position selected to approach smooth room response from the stereo pair.

EQ, if used, should be limited to smoothing the natural room gain of the speaker, but not force the speaker to be flat at the listening position. A flat EQ at the listening position is too bright. Instead, room gain of approximately 1dB/octave downward slope is expected.

Testing with different artificial head/torso apparatus meeting IEC specifications would be beneficial to bracket the range of potential measurement variations.

A profile of several well-designed conventional dynamic loudspeakers is preferred, representing mastering studio quality (anechoic flat with wide, controlled directivity). Each good speaker will have somewhat different directivity that will offer a range of potential interactions in a good room. Examples might include: JBL M2 Studio Monitor KEF Reference Series Revel Salon 2 Dynaudio Evidence TAD Reference One



By performing various measurements with the same general approach to room acoustics, selection of reference-quality speakers and simulated head measurement techniques, a family of curves could be presented, including a best fit average curve. One convenient approach could be to engage several well-engineered mastering studios and do multiple torso?head measurements in each.

I predict the resulting curves would be similar to the Harman listener preferences and would be an excellent technical definition of a neutral headphone curve.

Editor's Note: First, thank you Warren for permitting me to publish this work-in-progress. I look forward to your further thoughts on this subject, but ultimately felt there was so much good information here that publishing your thoughts, and Sean's responses as is was a worthy endeavor.

Further, Warren has pushed me over the edge. I've been, for a long, long time, very curious about what a good set of speakers in a good room would look like when measured by my head. As part of my effort to take a good hard look at artificial head measurements, I asked Sean Olive if it would be possible for me to visit Harman's research facility with my gear and take some measurements with my head. He has graciously replied that I could do so. I'm stoked! The measurement session will be in early June. Woot!