Cochlear implant processors do not traditionally make any sound, rather they send electrical impulses to the implant inside the cochlea that stimulates the nerves and is perceived as sound. At the end of the movie, when Regan holds her processor to the microphone and it's amplified through the loud speakers, it shows that the processor is making a feedback sound that the creatures can hear rather than just electrical interference that they can otherwise detect. This could only be possible though the modifications that Lee made to the processor. In addition to the small amplifiers he says he added from the stereo (which only convert sound from the microphone into a digital signal), he would also have needed to add a receiver that he may have sourced from the pile of hearing aids it is seen he has, which can then convert the digital signal into an audible sound.

Film

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