I am writing to offer a solution to the interference problem created when digital cellular telephones are used by those who wear hearing aids. Because this problem involves elements in both the aid and the phone, the solution must involve changes in both products.
In reading the telephone industry’s comments to the FCC proceedings 01-309 regarding hearing-aid-compatibility, it appears that the makers of digital cellular telephones blame the interference exclusively on hearing aids lack of RF immunity. (This is an understandable misconception since detection of the interference generated by the telephone is impossible: the pick-up, demodulation, and amplification in the hearing aid’s amplifier is present whether the hearing aid is using the microphone or the telecoil.) The fact is that these are two sources of interference: RF interference generated in the hearing aid; and magnetic interference to the telecoil generated in the telephone.
One hearing aid manufacturer, Siemens, has developed a completely RF immune hearing aid and discovered that there is still magnetic interference to the telecoil. This source of magnetic interference is generated by the pulsing or varying battery current. In the battery itself as well as the leads from the battery to the telephone’s electronics is responsible for that pulsing current. This magnetic interference can be eliminated or lessened to the extent that it will not be heard when using a digital cellular telephone by employing the magnetic self-canceling technique described below.
If a magnetometer is held near a lamp cord carrying an AC current there will be a small reading. If, however, the leads are separate by more than a fraction of an inch, the reading on the magnetometer will increase greatly. This is true because the currents flowing in the two conductors are flowing in opposite directions and the magnetic fields generated in each are in opposite directions and tend to cancel each other depending upon the spacing of the leads. Carrying the concept still further, if the current is passed through a coaxial cable, the magnetic field generated by the inner conductor will be completely cancelled by the magnetic field generated by the outer conductor.
This concept can also be utilized to cancel the magnetic fields generated in the cells making up the telephone’s battery by covering them with conducting materials such as copper connected as explained below for a battery composed of two cells in series.
The method of connecting the cells to the covering material can be explained as follows:
Call one cell “Number 1” and the other “Number 2.” Of course, it can be extended to as many cells as required. Connect the positive contact of Number 1 to the negative on Number 2. Connect the negative contact of Number 1 to the covering material. The output of cells is now connected to the positive contact of Number 2 and the covering material. Any current flowing in the cells will now be flowing in the opposite direction to that in the covering material and the field generated by each will be cancelled.
This concept was tested by placing two AA cells in one-half-inch copper tubing connected as above and the magnetic field generated by the battery was measured with a magnetometer to be very small.
In order to complete the coaxial idea there must be some means of assuring self cancellation in the connections between the battery and the cell telephone that will allow easy replacement of the battery. Some mechanical engineering must be done to assure coaxial connector or leads so close to each other that self-cancellation is assured at all connections.
George DeVilbiss
Falls Church, VA
George DeVilbiss holds BEE and MSE degrees from the George Washington School of Engineering. As a U.S. Naval officer he was designated Engineering Duty Only (Electronics) and served as had of the Electronics Division of the Research and Development Directorate of the Navy’s Bureau of Ships. Since retirement from civil service he has been involved in modifying mass-produced electronics for use by people who are hard of hearing. An example is a wireless doorbell which flashes a light when the doorbell is pushed to get the attention of the person with hearing loss.
