Apparatus for use in a hearing aid wherein a first microphone is connected to the hearing aid case and exposed to free air and a second microphone is connected to the hearing aid case and sealed from free air. The prescribed threshold value is dependent on the reference block. Upon a signal, from a starting point, the players commence play in a determined order and designated direction of play. The computing device determines the scores for the first set of candidate strings based on probabilities of the candidate strings being entered. The abrasive material may be sandpaper and the throughbore may be threaded for engagement with a mandrel.