The effect of feedbass, measured.

A project log for Feedbass! (microphone in loudspeaker)

Using a microphone in speaker cabinet as feed-forward, to extend low frequency range of audio system

DeepSOICDeepSOIC 03/05/2018 at 20:230 Comments

So, I hoped to get something like this...

And of course... it's not really what happens. This is what I measure:

("full on" means feedbass pot is at the point just below where self-oscillation begins)

So. Does it work? well, NO. It does something different to what I wanted. But what it does do is it makes the sound of this little speaker much more massive. Not quite massive as in absolute, but relative to what it was, the improvement is pleasant and very noticeable.

Now, why doesn't it do what I expected it to? well, there is a big thing, that I hoped would be small. When compensating for a spring, I need to put out Force. Force is proportional to current through voice coil. But the amplifier outputs voltage, and voltage is related to current as follows (ignoring coil's inductance):

I = (U - U_backEMF) / R

My original hope was that the drivers are crappy, and their back-emf is negligible, so output voltage and coil current are in direct relationship. However, as it is hinted by quite a few observations, U_backEMF, which is voltage generated by the coil due to its movement in magnetic field, is not negligible at all. One of the things that suggests it is the lack of resonance peak in frequency response. The resonance is dampened because the coil is essentially shorted by amplifier, and it brakes like a shorted DC motor. That resonance appears if I connect the speakers through 40 Ohm resistors.