Pressure based motional feedback.

[Circlotron]

Originally posted by bobo1on1
...a bass reflex is a resonator, which means the sound builds up slow when signal is applied and decays slow when the signal is turned off. This means the feedback circuit will applie huge amounts of power at the beginning to get the resonance started and it will apply huge amounts of power at the end to stop the resonance.

I imagine that if you had a driver with low Qes and suddenly applied a LF signal at the resonant frequency of the box+port, while the port output is gradually building up it doesn't place much load on the driver and so the cone excursion is large, and gradually reduces as it's load increases by the port coming "up to speed". What the port momentarily lacks, the larger cone movement makes up for. Is this true in practice???

A mfb of some sort will work for a sealed box where cone movement equates to spl but with a bass reflex at the tuning point for example the cone movement gets very small and most of the sound comes out the port. MFB is going to make a mess of this because if it keeps the cone movement directly proportional to signal level you will end up with a =huge= peak at box+port resonance.

[bobo1on1]

Originally posted by Circlotron


I imagine that if you had a driver with low Qes and suddenly applied a LF signal at the resonant frequency of the box+port, while the port output is gradually building up it doesn't place much load on the driver and so the cone excursion is large, and gradually reduces as it's load increases by the port coming "up to speed". What the port momentarily lacks, the larger cone movement makes up for. Is this true in practice???

Not really, the output on my woofer is 10 db lower than the output from the port at resonance, that's why it needed a bassreflex enclosure in the first place

About two years ago I build an mfb sub using a piezo loudspeakerthingy and a schematic from elektor, it worked pretty good but I had some problems with clipping and I didn't have a scope yet so I didn't know what the problem was.

It was pretty cool though, if you used the volume pot from the amplifier you could turn the woofer from tight to boomy without changing the volume.

[phase_accurate]

The principle brought up by Calvin has been discussed several times here. NP for instance used it for active absorbers and he gave some comments on it as well.
The owners of the original patent - B & M - used an electret mic capsule that was glued to the driver itself in order to achieve the lowest delay possible. They placed it in the grove between cone and dome.
And most importantly: It was monted sideways !!!!!!!!!!! Otherwise it would work partially as an accelerometer instead of a pure pressure detector.

There is still a commercial sub available that uses the principle: The Manger subsonice.

Here you will find an extract from the German mag Funkschau describing the (then new) B&M:

http://www.johannes-krings.com/funkschau2

Regards

Charles

[tade]

this is cool. I think ill buy a mic and borrow an oscilliscope and see what my sub is doing.
Thanks

[bobo1on1]

I just did a small test with a microphone, I could set the feedback to about 70% microphone signal and 30% line signal, that is the maximum feedback I could use without oscillation.

The frequency response with and without feedback is included, remember this is in room with the mic close to the sub.

[bobo1on1]

And the schematic.

I need a better lowpass to prevent oscillation.

[tade]

looks good!

[bobo1on1]

One other interesting feature I noticed is that it linearizes the phase as well as the amplitude, this should make the woofer faster.

[bobo1on1]

I just tried a second order lowpass but it doesn't do any good.

I did put a shelving lowpass at the input, you can see the feedback effect even better now.

Now all I need is a highpass

[phase_accurate]

What frequency was it oscillating at ?

In order to increase stability you'd have to put your mic as close to the driver as possible. High filter orders in the loop don't help to increase stability either. Even your first order LPF might probably be working better if changed to a lag filter. I.e. connect a small resistor in series with C1 like 1 k for instance.

Regards

Charles