The Ohm B is a dynamic speaker.
Robert C (November 26, 2013): The Ohm B was Ohm's first production speaker. I still have a pair I bought new in 1973. While the Ohm A pre-dates it, Ohm considers the Model A a prototype.
The Ohm B is a 12" two-way system with a 1" polycarbonate dome. The crossover is at 2200Hz. The slope is 18 db/octave on the tweeter and 6 db/octave on the woofer. There is probably additional woofer mechanical roll-off.
My particular pair of Ohm B's are very early ones and they use CTS woofers and Philips tweeters. I don't know if later Model B's used different components.
The woofer cone is treated with silicon damping material to control cone break-up. It works unbelievably well and allows the woofer to go up to 2KHz with a remarkably smooth, natural, and "un-boxy" midrange.
The damping on the woofer cone also adds mass which, affects its Thiel parameters. The free-air resonance of the woofer is approximately 16Hz. Installed in the cabinet, the system resonance is approximately 38Hz. By comparison, the AR-3 - which is virtually the same exact size - has a system resonance of 42Hz.
Because of this added mass, the efficiency is low. Ohm says 86 dB/watt and I believe them.
The tweeter is mounted in such a way that its entire front face is utterly flat with front panel. Part of this is due to the way Ohm filled in the "trench" that runs around the outside of the tweeter if it's merely countersunk. Just using countersinking without filling in that trench creates resonances around 7KHz which give this tweeter a sound sometimes characterized as "aggressive."
Dome tweeters are linear down to 2KHz or lower and the use of such a low cross-over frequency contributes to the smoothness of the midrange (along with the treated woofer.)
There is one weakness to this design: The efficiency is rather low - but the power-handling capacity is also low. This is due to running the dome tweeter down to 2.2KHz.
There were fairly high failure rates on Ohm B tweeters. Ohm rated these speakers as being able to handle up to 100 watts, which was a mistake (in my opinion) They cannot handle that much power (my opinion). If you limit the input to 30-35 watts, they are beautiful.
These speakers are now very very rare.
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