Loudspeaker Equalisation
The name of Zobel is, perhaps, most well known with regard to impedance compensation networks for loudspeakers. Clearly, his designs have applications in this field. However, none of Zobel's patents or articles appear to discuss this topic. It is unclear whether he actually designed anything specifically for loudspeakers. The closest we get to this is where he speaks of impedance matching into a transducer, but here he is discussing a circuit to equalize a submarine cable, or in another instance where clearly he has in mind the hybrid transformer which terminates a line going into a telephone instrument on a phantom circuit.
Read more about this topic: Otto Julius Zobel