Diffuse-field (equalisation). Here's a pretty good explanation from the beyerdynamic website:
Have you ever wondered why a frequency response curve is almost never included with headphones? I can let you in on the secret: they look terrible! Such an erratic frequency response graph would hardly encourage customers to make a purchase. What the customer wants in the end is something that is linear. Uncoloured. Solid.
But why do these frequency response curves look so horrible? And why do you not clearly hear these glaring leaps and drop-offs?
How we hear
From childhood on, humans are accustomed to perceiving acoustic events. We grow up with a variety of sound sources and get used to them. The baby rattle, the clatter of dishes from the kitchen, pedestrians on the street, music from loudspeakers, etc. – all of these sound sources have something in common: they are located relatively far from the ear.
Before the sound from these sources reaches our eardrum, it is coloured by the shape of our head and our ear. Depending on the angle, many frequencies are accentuated and others are attenuated. With time, we learn these frequency patterns and are able to do things such as recognise the direction in which the sound source is located. Therefore, we do not hear sound as it was produced at the source, but instead in coloured form.
Loudspeakers and headphones
When we listen to music over loudspeakers with a linear frequency response curve, we are actually hearing a spectrum that is influenced by the distinctive shape of our head. We perceive this as linear.
When listening with headphones, the headphones do not even try to generate any effects on the outer ear, since the sound source is so close to the ear. What comes out of the headphones arrives at the eardrum in relatively uncoloured form. In order for the headphones to still sound natural, the sound must be coloured so that it is as similar as possible to the colourations caused by the shape of the head and ear. In other words, the headphones must have the frequency response set so that it sounds like the sound is coming from a distant source.
In order to adjust headphones to our listening habits, we must first use technical means to measure the colourations caused by our head. For example, an artificial head with microphones in the ears is used. When this artificial head is exposed to sound, you can use the microphones to measure how the sound would be perceived by us instead of the artificial head.
So that the headphones do not have a sound that always seems to come from one direction, but instead can reproduce all sound directions equally, the artificial head must be exposed to sound from many directions and the result averaged. This does not perfectly reproduce any direction perfectly, but no direction is completely suppressed.
At beyerdynamic, there is an echo chamber for this purpose. It is a small, five-sided room with acoustic sails on the ceiling that looks quite bare and empty. The fascinating thing about it is that, although it is the size of child’s room, it sounds like a cathedral! An octahedron loudspeaker that radiates sound in eight directions is in one corner. If you are far enough away from the loudspeaker, the strong echo causes you to no longer be in the direct field, but instead in the diffuse field of the loudspeaker, i.e. the area in which the sound reflected off the walls is louder than the sound that is coming directly from the loudspeaker.
If artificial head measurements are carried out in this chamber, many sound directions overlap due to the echo, allowing us to obtain the required averaging. This averaging (the measurement in the diffuse field) gives diffuse field equalisation its name.
In order to equalise the headphones, they are placed on the artificial head and the frequency response is adjusted so that the measured frequency behaviour corresponds to that of the diffuse field.
Since the mechanical and electronic options for changing the frequency response of headphones are limited, the equalisation cannot be carried out perfectly. Different headphones are also adjusted to various tastes. It is by no means the case that all diffuse-field equalised headphones sound the same. In addition, the frequency patterns for directional hearing depend on the shape of the head and ears. For this reason, they are a little different for everyone. Hence, measuring with an artificial head is a pretty arbitrary choice.
Diffuse-field equalisation is therefore an important part of improving localisation with headphones and avoiding “in-head localisation”, but it is not guaranteed to work and is no replacement for extensive test listening.