AlwaysForward
Headphoneus Supremus
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Yup, to further explain, here’s a copy/paste from an important post on reddit by an acoustic engineer who has shared a lot of knowledge and experience with the hobby community:
differences between the Harman curve and Diffuse Field
I could write a long article about this (and I already have as part of my thesis), but I'm gonna try and keep it short, so it's more easy to understand.
The first question is: "what should a speaker sound like?" (in terms of frequency response). The short answer is (after decades of research) is: A speaker should produce a flat frequency response in an anechoic room. When the same speaker is placed in a "normal" (slightly reverberant) room, the frequency response will be a little tilted - about 4 dB more bass, and about 2 dB less treble. The debate about this is basically over, the question has been answered, and indeed, virtually all "good" speakers show this behaviour (flat on-axis, controlled sound power output).
And since recording studios use good speakers (studio monitors) to record, monitor and mix the music that consumers listen to later, it makes intuitively sense to listen to the music on similarly performing speakers - because that is what the music is supposed to sound like, this is what the artist and recording engineers decided "sounded good".
So the target for speakers is: Flat on-axis, controlled sound power output (smooth directivity).
Now, the same question can be asked for headphones: "what should a headphone sound like?" (in terms of: What is the ideal frequency response of a headphone"), and the short answer is: "it's not that simple".
The answer is simple for speakers (not that simple really, but it has been answered), but for headphones it is much more difficult.
The first difficulty is "how do you measure it?". It's easy with speakers - put a calibrated microphone at a standardized distance. With headphones this isn't possible (much of the sound depends on the shape of the head). The general consensus is to measure headphones on artificial heads, with artificial ears and artificial ear canals. The problem with this is, that head shape, ear shape and ear canal have significant influence on the acoustics, most prominently a 10-20 dB boost at 3 kHz. The important thing is: We "hear" this boost even when listening to speakers - because our ears are always there. When the artificial head measurement shows a high boost at 3 kHz, this sounds "flat, linear" to us, because this is what our ears hear. But how should this boost look like exactly? What is the target frequency response?
There have been many approaches to define the "target" for headphones.
Historically, it started with the ITU's recommendation of a "free field curve". This was measured by placing a good speaker in an anechoic room, and placing an artificial head in front of it. Then we measure the response of the speaker, but not with a measurement microphone, we measure with the artificial ears of the artificial head, so we can "see" what a human "hears" when he stands where the artificial head stands.
The resulting target frequency response curve has a 15 dB boost at 3 kHz, and is very wobbly above 5 kHz, due to specific resonance and phase effects that occur at specific distances and angles. It's hard to manufacture headphones that reproduce all these wobbles exactly right.
So another approach was taken: The diffuse field curve. Instead of putting a single speaker directly in front of the head, we place the head in a very reverberant room, so that sound arrives at the head from all angles and from all directions equally. The reasoning behind this idea was that sound arrives from all angles as well when wearing headphones - simply because the headphones cover the whole ear.
Diffuse fields are hard to set up, because you need to carefully position a lot of speakers and reflectors in a room with very hard walls to avoid any direct reflections, leaving you with only reverberation. Usually we use speakers that radiate in all directions, to further excite the diffuse field. The frequency response in the room is still linear and flat - but the sound is coming from all directions and not just from the front (as in the free field).
Now, when we measure the frequency response of the diffuse field with an artificial head, the resulting curve is much smoother above 5 kHz.
Free field and diffuse field in comparison.
When we build headphones that are tuned towards the diffuse-field curve, they sound neutral but a bit bright. Examples are the AKG 240DF, Beyerdynamic DT880 and most famously the Etymotic Research ER4-series. But also the Sennheiser HD800 is tuned for diffuse field response (but a very modified one).
But the question is not yet fully answered. Enter a scientist named Sean Olive currently employed at Harman. His hypothesis was that neither the Free Field nor the Diffuse Field curve were "correct" (read: Neither were ideal), since both the concept of FF and DF are very abstract and don't happen when listneing to music. He proposed another way of creating a target curve:
Placing a pair of good speakers in a "regular" listening room similar to the control rooms of recording and mixing studios, and measuring the frequency response with an artificial head. Harman's reference room is neither fully reverberant nor fully anechoic, it features a reverberation time of about 0.4 seconds, very similar to what professional recording and mixing studios use (the rule of thumb is 0.3 seconds).
Now if we measure a headphone on that same artificial head and the headphone were to have the same frequency response that we measured in the room, then this frequency response would be ideal, or so Sean Olive proposed. And further research proved that he was right, the majority of both trained and untrained listeners prefer this target curve over any other target curve.
The difference to DF and FF curves is that the room will slightly boost low frequencies due to reverberation, but high frequencies do not reverberate as much as they are more easily absorbed.
This comes much closer to what the artist and recording engineer heard in the studio, and what they based their judgement on in order to shape the sound of the music.
In other words: The Harman Target is basically the same sound that the artist and engineers heard when creating the music that we hear.
Damn, I did write morethan I intended to, and there's still a lot more to be said about the Harman Target (for example why there are currently four different Harman Targets...).
Any questions? ask away.
