Q:

Controlling DC (Pager) Motor with Audio Signal

I'm trying to control a DC (pager) motor with a sound signal so that I can use an audio synthesizer to control the speed of the motor.  I'm hoping to modulate the speed of the motor at audio rate (1-200hz).  I need to do a few things.  

Convert AC -> DC.  A rectifier will chop the negative or fold it up, but how can I shift the waveform up, per se?  What is the circuit equivalent of f(x)+1? ( or will a rectifier be good enough)
I could send the rectified signal straight to the motor, but this might be a waste of energy.  Two strategies present themselves:

Use a comparator to send only the peaks
Use PWM (Frequency -> Pulsewidth)

Which do you think will work better?  Can you think of other strategies?  Should I implement this with, say, a small AVR chip, or with analog electronics? 
An additional constraint is that the circuit has to be small.  Ideally my motor + battery + circuit would be quarter-sized.  
Any advice would be appreciated. 
(I'll also need to do some lowpass filtering to avoid overwhelming the motors with information they can't respond to.  Thoughts on this would be appreciated as well.) 
I will be using this pager motor from Solarbotics possibly in conjunction with this one from Precision Microdrives, who specialize in vibration motors.  

A:

Use PWM to drive the pager motor, at a frequency above human hearing (so you don't get audio whine from the PWM). 
You'll need to choose  pager motor, and then characterize it to find what duty cycle produces what approximate frequency of spin.   Each motor will be slightly different, but motors manufactured at the same time should be close. 
You won't be able to get an exact frequency without some form of feedback.   If you can get ahold of 3 phase pager motors then you can set the exact frequency of vibration, but they aren't easy to find, and are harder to drive properly.   They are the same type of motor used to run hard drive platters and CD/DVD discs.   If you rip apart a CD burner you should be able to take this motor, and you may be able to hack the motor drive circuitry to do what you need - but then you have to attach your own eccentric weight to the motor, and the whole assembly is going to be much larger than a pager motor. 
But if you're just looking for a close approximation, you may be able to characterize a particular pager motor and be satisfied with that.