In order to have maximum control over the response curve, THD and excursion I'll be using feedback from the actual woofer. The obvious approach is to mount a piezo or MEMS accelerometer onto the woofer cone and put that in the feedback loop for the amplifier. I've been calculating the requirements for such a device and asked for feedback from Dan Wiggins at Adire and he commented there's another, potentially more interesting, relation that could make for a simpeler yet just as effective approach.

I'm however not 100% sure how to approach this so thought I'd post a few questions here to see how my fellow DIY-ers would approach this or whether anybody has any good suggestions. But let's first look at the maximum g force generated by the woofer, and then go from there.

The position of the cone with respect to time, assuming sine wave stimulus, is:

X(t) = Xpk sin(w t)

Xpk obviously is the peak excursion (not peak-to-peak, so it's 33mm here), w

is 2 pi f, and t is time.

The velocity is the first derivative of this:

V(t) = w Xpk cos(w t)

and acceleration is the second derivative of excursion:

A(t) = w^2 Xpk -sin(w t)

Maximum acceleration occurs when -sin(w t) is either 1 or -1, thus we are

left with the maximum acceleration being:

A(max) = w^2 Xpk

Let's assume the system has its largest Xpk at 20 Hz,

w = 2 pi 20 Hz

w = 125.6/s

and thus w^2 is:

w^2 = 15775/s^2

and since Xpk = 33mm, or 0.033m, then:

Apk = 0.033m * 15575/s^2

Apk = 514 m/s^2

and since g = 9.8 m/s^2, then:

Apk = 52 G

Dan commented that acceleration is also given by current times BL divided by mass (BL * i / m). So monitoring the current into the voice coil, as long as you have a constant BL and m, will give you acceleration. This got me thinking that in order to make this work all I'd need to do is to design a voltage controlled current source of sufficient power to drive the woofer and it will automatically flatten the curve, lower THD and keep excursion within acceptable limits.

My question obviously is how to approach this from the amplifier end, do I use a low-resistance sensing resistor in series with the woofer? Or something else? How exactly would I design a voltage controlled current source around the woofer? Anybody got any suggestions? Or a few pointers maybe?

Best regards,

Sander Sassen

http://www.hardwareanalysis.com