To be able to adjust the headphones to how we habitually hear things, we firstly have to measure and record the discolourations by our head. A dummy head with microphones in the ears is used for this. When you direct sound waves at this dummy head, you can use microphones to measure how we would hear this sound instead of the dummy head.
To ensure that the headphones do not produce a sound that always seems to come from the same direction but can instead reproduce all directions of sound equally, sound waves must be directed at the dummy head from many directions and the result must be averaged. Therefore, no direction is reproduced perfectly, but also no direction is fully suppressed.
There is an echo chamber at beyerdynamic for this purpose. It is a small, pentagonal room with suspended sound-absorbing panels on the ceiling. It looks very bare and empty. What’s fascinating about it is that it is about the size of a child’s bedroom but sounds like a cathedral. There is an octahedron loudspeaker in one corner that emits sound in eight directions. If you are far away enough from the loudspeaker, you are no longer in the direct field because of the strong reverberation, i.e. you are instead in the diffuse field of the loudspeaker, i.e. in the area in which the sound reflected from the walls is louder than that heard directly from the loudspeaker.
If you carry out the dummy head measurement in this room, many directions of sound overlap one another due to reverberation, which allows us to achieve the required averaging. This averaging (the measurement in the diffuse field) gives diffuse-field equalisation its name.
To equalise the headphones, they are placed on the dummy head and the frequency response is adjusted so that the measured frequency response corresponds to that in the diffuse field.