There have been numerous discussions on the boards about the sound of the BK5A or B on percussive sounds, kick drums, and other sources. It seems to deliver the punch, the kind that hits you in the chest, that is defined and not a muddy thump.
The BK5s use an acoustic labyrinth that provides a blocked off acoustic impedance to the rear of the ribbon, which is then more sensitive to incoming sounds from the exposed front. Like a pillow, it cuts off the rear lobe, at least in the mids and highs, but not very much in the bass region. The more focused hypercardioid response is sometimes easier to use but the tradeoff is that the mic begins to sound like a conventional dynamic. The reason for this is twofold: First, there is an increase in off-axis coloration, and second, spatial cues that the ear and brain interpret as "space" are absent vs the figure 8 configuration. Since the spatial component of ribbon mics is one of the more recognizable attributes there is less of that ribbon sound.
One thing is preserved however: The top end smoothness produced by the linear, restricted mode movement of the ribbon is still present, and that alone contributes to the lack of tizziness and lateral modes which plague most round elements of any appreciable size. That would include large diaphragm condenser and dynamic types. Chladni patterns on round plates clearly show that complex, distortion producing lateral modes (that are mostly avoided with a ribbon) and that attribute alone may be reason enough to choose a ribbon over the tympanic form of microphone.
Read about Hugh Tripp's investigation here.
About that tizz. It is clear to us that the newer generation of engineers and producers have recognized tizz for what it is, and no longer confuse the fizzy shhhh with "air" or "tops", especially in vocals. We have all heard many songs where the vocals are just too harsh for this reason, and it is of course an engineering goal to keep listener's ears on the song, rather than have them tune away. Tizz-air has become an affectation. I can think of one recently recorded album (which is incredible, fabulous, perfectly mixed, beautiful and monumental) where the singer's very famous voice is presented in a way that sounds like there might be some bees buzzing around. Only on a track or two. A detail.
Ribbon microphones have gone mainstream in the last few years, and the reasons for that are many. Improvements in ribbon mics, which have been largely left unimproved for decades, are on the way too, so the whole scene is changing. We refer to the beloved classics - like this BK5, from the set of The Tonight Show with Johnny Carson - as milestones but also as starting points.
More about the BK5A from NBC here.