THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

10.7 Prosthetics for Hearing 233computer to provide the frequency equalization curve on the basis of the resultsfrom testing the patient’s hearing. Algorithms in the DSP can continuously dividesounds into frequency channels and emphasize the higher frequencies containingvital consonant sounds in speech over the distracting rumble of low-frequencynoise. Algorithms also manage noise: owing to the fact that speech sound intensitiescan change radically in a millisecond whereas noise is more acousticallystable over a comparatively longer time. On a time basis, DSP can reduce the levelsof continuous sounds such as traffic noise and household appliances. And it simultaneouslyreadjusts when changes occur, restoring amplification when shorterduration sounds are detected. In relatively quiet surroundings, the digital algorithmcan detect the consistency of softer environmental sounds from ventilationsystems and appliances, and it also automatically reduces amplification in theappropriate frequency range, immediately restoring proper levels when the soundpattern changes. Digital hearing aids carry the disadvantage of considerably highercosts and may not provide enough amplification as the other types of hearing aids.Implantable hearing aids, referred to by the acronym, IHA, can be used bypeople having sensorineural hearing loss but a healthy middle ear. The IHA, whichis still in the stages of final development, converts sound to vibrations inside themiddle ear. This gives the IHA several advantages of traditional hearing aids. Ifthe bones of the middle ear can be directly used, sound quality may be improvedalong with much less feedback. Also, there are no external components to the IHA,which is, after all, completely implantable.Three versions of IHA are being tested in human trials. Either the receiver,which collects the sound energy, or the entire hearing device is surgically insertedinto the middle ear. The surgical procedure is as follows: skin and tissues aroundthe ear are laid back, and a magnet (which may no may not include electronics) isnestled into the bone behind the ear. A component is then attached to one of themiddle ear bones. One prototype is called the Envoy Totally Implantable HearingRestoration System. This device utilizes the eardrum as the microphone that sendssound energy to the piezoelectric crystal in the middle ear. Two other prototypesuse electromagnetic technology. The Vibrant Soundbridge (made by SymphonixDevices, Inc.) has already received an Investigational Device Exemption (IDE)from the Federal Drug Administration (FDA). IDE represents the preliminary stagein receiving FDA approval. Another system, Otologies, is also in the IDE stage ofthe FDA process. Both of these systems are partially implantable; only the receiver,consisting of the magnet and transducer coil, is implanted. The microphone andamplifier are worn externally. These devices may be more suitable for a widerrange of hearing losses than the one using piezoelectric technology, which can beused by those having not more than a moderate (50 dB) hearing loss.Cochlear ImplantsIn Section 10.2 we have examined how the cochlea produces signals to the cochleanucleus. When air cells become so damaged that they cannot be stimulated cellsof the spiral ganglion, hearing is lost. Without regular activity, the portion of