in the received audio signal the transmitter 110 and the transducer 120 in the preferred embodiment have a diameter of approximately 16 mm which is nearly twice the wavelength of the ultrasonic signal at 40 khz as a result the emitted ultrasonic is spatially narrow beam e g with a 3 db beam width of approximately 30 degrees this makes it possible for the ultrasonic signal to be highly directional this decreases the likelihood of sensing extraneous signals not associated with facial movement in fact it makes sense to colocate the transducer 120 with the microphone 160 most conventional audio signal processors cut off received acoustic signals well below 40 khz prior to digitization therefore we heterodyne the received ultrasonic signal such that the resultant much lower u201c beat frequency u201d signal falls is within the audio range doing so also provides us with another advantage the heterodyned signal can be sampled at audio frequencies with the additional benefits in a reduction of computational complexity the signal 121 acquired by the transducer is pre amplified 130 and input to the analog mixer 140 the second input to the mixer is a 36 khz as in our preferred embodiment sinusoid signal the sinusoid signal is generated by producing a 36 khz 50 duty cycle square wave from the microcontroller the square wave is bandpass filtered 145 with a fourth order active filter the output of the mixer is then low pass filtered 150 with a cutoff frequency of 8 khz as in our preferred embodiment the audio channel includes a microphone 160 to acquire the audio signal in preferred embodiment the microphone is selected to have a frequency response with a 3 db cutoff frequency below.