High quality dynamic microphone.
Dual capsule design (actually 2 capsules D12) provides four directional characteristics: omni, bi-directional, cardioid and two (front or rear). There is also a low-cut filter with positions 0, -7 or -12 dB @ 50 Hz
Medium-low impedance: 150Ω (omni / bidirectional) / 75Ω (each cardioid).
Its very fat with a huge margin and everything needed to capture a large bass, bass amp, a bass drum, tuba or trombone etc.. Bidirectional position proximity effect can do wonders.
It is not by chance that it was the favorite microphone for these instruments in almost all legendary recordings of Jimi Hendrix, Rolling Stones, Led Zeppelin and most famous bands that have recorded at Olympic studios in their heyday. Eddie Kramer promotes the still as the best microphone bass he has ever used.
Has equaled his (rare) big brothers, the D36 (as D30 but with the box remote control directivity and low-cut filter, such as C12) and D45 (D36 as with shockmount integrated).
Unfortunately very rare due to the fact that it was made only between 1958 and 1961 in small quantities exclusively for the European market studio / broadcast. Most were made by AKG Vienna and some AKG Munich.
I was lucky to find one in good condition and at a reasonable price, and now and that is one of the jewels of my studio.
Warning! If you come across one, make sure it has the even rarer than the microphone itself AKG taken 3 B 17/Tuchel T3007 owner! Otherwise, you end up with no way to connect to your preamp, unless you heavily modify the microphone (already stuffed inside), potentially ruin its value.