Energy Transp. Group, Inc. v. Wm. Demant Holding A/S WDH, Inc., No. 11-1487 (Fed. Cir. 2012)Annotate this Case
A jury found that Demant and Widex infringed the 749 and 850 patents. The court entered judgment finding noninfringement of the 749 patent, finding that prosecution history estoppel barred the finding of infringement under the doctrine of equivalents. The patents relate to technology for reducing acoustic feedback in a programmable digital hearing aid. All hearing aids have: a microphone that picks up sound and converts it to an electrical signal, a speaker (receiver) that converts the signal back into sound waves, and sound processing circuitry that adjusts received sound to compensate for any hearing impairment. Some of the amplified sound from the speaker may also travel back to the microphone via an “acoustic feedback path” and is amplified along with all other sound arriving at the input microphone. The resulting cycle of amplification causes the whistling sound known as “feedback.” The common specification. with a 1986 priority date, describes a method of reducing feedback by creating an electrical feedback path and inserting a programmable filter in that path to mimic the effects of acoustic feedback on the phase and amplitude of a sound signal in the transmission channel. This electrical feedback signal then cancels the acoustic feedback signal. The Federal Circuit affirmed.