Those of you who follow developments in headphone measurement will know that in recent years Harman, with Sean Olive leading the effort, has been busy investigating and defining the ideal headphone target response – in other words, the headphone frequency response, as measured at the ‘eardrum’ of an artificial ear, that results in a perceived tonal balance preferred by listeners when replaying conventional stereo music recordings. Well, I’ve news of some recent developments on that score.
Prior to Harman’s research there were two schools of thought regarding headphone target response. The first was that it should, not unreasonably, match that of a sound source located in the horizontal plane and 30 degrees to one side of straight ahead – ie where a conventionally positioned stereo loudspeaker would be. This became known as the free-field (FF) target response. Logical as the FF assumption might seem, it was successfully challenged in the 1980s by the work of Günter Theile at IRT (Institut für Rundfunktechnik – Institute for Broadcasting Technology) in Munich, Germany, which identified as preferable the diffuse-field (DF) response, as experienced in a diffuse sound field where the sound has no particular direction. Of the two, the DF response held sway until Harman began re-evaluating the issue from scratch.
When Harman generated its first target response for circumaural (over-ear) and supra-aural (on-ear) headphones in 2013 its most obvious departure from the DF response was a low frequencies, which were shelved up below about 200Hz, thereby mimicking the in-room response of loudspeakers in an acoustically well-behaved domestic space. Since then Harman has generated two further iterations of this response, in 2015 and 2017, in which the LF up-shelving and other features of the target response are similar but modified in detail. Also last year, Harman announced a separate target response for in-ear headphones (insert earphones), which incorporates even greater bass boost.
Now that I have data for the latest Harman circumaural/supra-aural target response, I thought I’d share with you not just the new response but an overlay of all four Harman target responses so that it’s clear how they have progressed. You can see this in the graph below. In the key, OE stands for over-ear and IE for in-ear, although Harman has taken to calling circumaurals ‘around-ear’ (AE), which is both a more accurate translation and a more accurate description than ‘over-ear.’ So in Harman-speak, AE equals circumaural and OE (on-ear) is supra-aural. All four responses are normalised to 0dB at 1kHz.
At high frequencies the latest OE (Harman: AE/OE) target response still quite closely resembles the diffuse-field target response although, compared to the 2015 version, the response peak at around 3kHz has reduced in level and moved up in frequency a little to 3.5kHz. Below 1kHz the 2017 target response has slightly increased in level, the 2017 and 2015 responses both being more markedly boosted below 200Hz than the original, 2013 target. The in-ear target response is very similar between 300Hz and 4kHz but notably features even greater bass boost.
Sean Olive explained the evolution of the circumaural/supra-aural target response to me as follows: “The AE-OE target curve evolved over the past four to five years. The first iteration was based on measurements of an accurate loudspeaker calibrated in our reference listening room. It was slightly modified with some method of adjustment (MOA) experiments where listeners modified the bass and treble, and also feedback from various tests with over 500 listeners from USA, Canada, Germany and China.
“The MOA tests led to about a 2dB increase in bass in 2015. In 2017-18, we reduced the treble around 3kHz by about 2dB largely based on feedback from listening tests conducted in Germany, China and USA. This new target curve was validated and tested against 31 different models, and reported in our recent AES Milan paper.
“The MOA tests have told us that the exact amount of preferred bass/treble will vary depending on age, listening experience, gender, and program material. Younger listeners and less experienced listeners generally preferred slightly more bass and treble than older, more experienced listeners. For senior listeners (55+ years old) we found they preferred on average even less bass but more treble than younger listeners. We believe this is related to hearing loss. This is an educated guess because we didn’t measure the hearing of the subjects (except Harman trained ones) and we need to test a larger sample.
“So the final target satisfies the majority of listeners, although it won’t be perfect for all programs and all people. We believe some degree of personalization is necessary to satisfy all listeners.”
When I begin my headphone testing for InnerFidelity, the latest Harman AE-OE and IE target responses will be used, as appropriate, to create corrected frequency responses, together with the FF and DF targets of old. This may seem overly complex but it allows individual readers to decide, on the basis of their own listening to reviewed models, to decide for themselves which target response is most appropriate for them. For example, I don’t generally like the full Harman AE-OE bass boost and find the diffuse-field correction more closely matches my perception of a neutral tonal balance.