Originally Posted by Xymordos
Is TWFK really the best mid-high driver? I mean, looking at the spec sheet Knowles provide, its frequency response isn't great...
Knowles doesn't do measurements specific for audio (unless you're a manufacturer and you request them privately), so you are probably not seeing the relevant information. They might night not be using the IEC711 coupler, and rather a straight up 2cc coupler, and their microphone might not be rated beyond 10k. There are a lot of factors that may cause a graph to be irrelevant to high-end audio.
Secondly, there are definite issues with the TWFK; it has a dip between 3.5-4 kHz, which is very audible and probably its biggest flaw. It's very difficult to compensate for this dip. I've heard several TWFK variants, including several where Knowles has an integrated low and high pass applied to the two drivers, and although partially minimized, the problem is still there. Thus, the TWFK driver isn't really the best for an 'accurate' response, but if you want high end extension in a tiny size, there's no other driver that can provide this type of response.