negative acoustical feedback, anybody?

[xpert]

Hi,

Once I tried to apply feedback to a bass speaker via microphone sensor. It worked to some degree.

One shortcoming was the marginal bandwith in which it was useful. Going to high in frequency negative feedback evolved to positive FB (resonance ;-) due to phase shifts due to coil inductance at most.

Second shortcoming was the confinement in closed boxes due to low frequency phase shift > 180° with reflex. Seconded to some extend by the challenge to summ up cone/pipe parts.

By now I use much bigger and more costly drivers than then. A feedback system would not contribute to much to the total cost. I discovered that the higher harmonics won't be to much of an issue with a decent driver due to masking by human hearing. But subharmonic components would matter - perhaps. And further on I've got an idea how to sneek around that phase issues with reflex. In the end I would like to take a new chance.

Anybody to share experiences with? Since that all ain't rocket science I won't worry to much about patents and such. On the other hand I won't discuss things like "feedback always comes to late" the 1001th time again.

Comments appreciated

[bear]

Generally speaking you are talking about what is commonly called "servo control".

There are a number of means of detecting the motion of the cone. A microphone is one. It needs to be very close to the cone in order to work well. Others include accelerometers, light pick-ups and inductive pick ups. The dual voicecoil is part of that.

All of them have to deal with the same phase shift issue that you noticed.

So, generally speaking most feedback schemes confine themselves to a narrow range, usually for Bass and Sub-Bass to avoid the phaseshift problem.

You might want to search here and online and look at the various servo-control projects, methods and patents...

_-_-bear