WW resistor inductance

[Valvebloke]

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b) A transducer measuring speech coil movement and putting speaker in a fast enough analogue servo loop (I think this has been tried)
c) Measure sound near listener (decent electret?) and apply DSP. Initialise for room and occupants using maybe white noise and then apply feed forward dynamic transformation. I've a feeling something like this has been tried ...

The best early example of b) was the Philips Motional Feedback system. When it worked, it worked well. When it went wrong though ... gorblimey !

There are a few suppliers of room correction systems of the c) type https://en.wikipedia.org/wiki/Digital_room_correction. It's a tough job to do well though. As with all feedback control systems the best results are achieved by getting the uncorrected system to behave as well as possible before starting to apply the correction. The instruction manual for the Quad II amplifier made a point of encouraging the listener to spend a few evenings tinkering with the position of his speaker (this was the early 1950's so there would only be one speaker of course). The benefits of doing this are just as large today and, it seems, just as often ignored in place of putting the speakers somewhere convenient or visually pleasing from a room layout point of view. Durrrrrr !

VB