Primary and secondary sound transfer functions in an active noise control earplug

Active noise control in a headset or earplug typically attempts to lower the sound level at an error microphone located close to the ear. If the transmission path between the error microphone and the eardrum for the primary noise is different to the path for the secondary noise, a reduction at the error microphone may not lead to a reduction at the eardrum. In fact, if these paths are sufficiently different a reduction at the error microphone may actually lead to amplification at the eardrum. An example of a possible cause for differing transmission paths is the bone conduction effect that becomes noticeable in occluded ear canals. In this study, a probe microphone was inserted into the ear canal of human subjects. This microphone was used to measure the pressure near the eardrum for primary and secondary sound sources. The pressure was also measured at an error microphone of an active noise control earplug. These measurements will be presented and some of the implications for active noise control will be given.