In general, a hearing aid operates by using a microphone to pick up sound waves in the air and convert the sound waves to electrical signals. The signals are then amplified as needed and converted back to audible sounds for the user to hear. The hearing aid microphone, however, does not always work well in conjunction with audio devices like mobile phone handsets. The acoustic connection made between the audio device and the hearing aid can be poor and can create distortions in the sound. In addition, the surrounding noise in the area of the user is often picked up by the hearing aid and interferes with the desired audio.
Mobile phones are often incompatible with many hearing aids due to the potential for radio frequency (RF) interference in the form of scattered EM fields generated by the mobile phone while in use. When a mobile phone is in communication with its network, an electromagnetic field is present around the mobile phone antenna. When the mobile phone is in use this field pulses. It is this pulsing energy that may be picked up by the hearing aid's microphone or telecoil circuitry and heard as a buzzing sound (i.e., interference) through the hearing aid.
To address this situation the United States Federal Communications Commission (FCC) has promulgated hearing aid compatibility (HAC) rules to minimize the interference between a mobile handset and a hearing aid. As of Sep. 16, 2005 mobile phone handset makers are to include at least 4 or 25% of their mobile phone handset offerings that meet or exceed the ANSI (American National's Standard Institute) hearing aid compatibility (HAC) standard.
The FCC defines HAC for mobile phones in terms of two parameters; radio-frequency (RF) emissions (M-scale) and telecoil coupling (T-scale). Mobile phones that comply with the FCC's hearing aid compatibility rule must receive a minimum rating of M3 for RF emissions and T3 for telecoil coupling. The FCC requires that nationwide carriers offer a range of phones that comply with HAC regulations beginning in September 2005 for microphone (M) mode and in September 2006 for (T) telecoil mode.
As part of hearing aid compatibility (HAC) assessment, near-field components of electromagnetic fields are measured. These components comprise electric and magnetic fields. Tables 1 and 2 illustrate the electric and magnetic field M ratings for the FCC HAC standards.
TABLE 1M Ratings for HAC in standard unitsE-Field EmissionH-Field EmissionCategoryLimits (V/m)Limits A/m)M1149.6-266.10.45-0.8 M2 84.1-149.60.25-0.45M347.3-84.10.15-0.25M4<47.3<0.15
TABLE 2M Ratings for HAC converted to dB valuesE-Field EmissionH-Field EmissionCategoryLimits (dBV/m)Limits (dBA/m)M143.5-48.5−6.9-−1.9M238.5-43.5−11.9-−6.9 M333.5-38.5−16.9-−11.9M4<33.5<−16.9
Handsets that receive a hearing aid compatibility rating of M3 or M4 have met or surpassed the ANSI hearing aid compatibility standard as adopted by the Federal Communications Commission. An M3-rating indicates the mobile phone has satisfied the ANSI standard. An M4-rating indicates the mobile phone has exceeded the ANSI standard. The higher the M-rating the mobile phone has the lower the radio frequency (RF) emissions level and higher signal quality the mobile phone will have to the hearing aid device, assuming that the phone is still performing efficiently.
“T” rating refers to a mobile phone's coupling ability and is intended for use with hearing aids in the telecoil mode. The higher the “T” number the more likely you will be able to use your phone with your hearing aid in telecoil mode. The FCC regulations require that cell phones be manufactured for use with hearing aids on telecoil setting by September 2006. For purposes of the present invention, however, only the M rating aspect of HAC is addressed.
For a phone to be HAC compliant, it must be compliant in all US frequency bands of operation for mobile phones. These include the 850-MHz and 1900-MHz bands. Moreover, the mobile phones must, in these operating bands, exhibit, at a minimum, an M3 rating meaning RF emissions from a digital wireless mobile phone are not to exceed −11.9 dB A/m for H-field and 38.6 dB V/m for E-field.
To the inventor's best knowledge, there has not been any mobile phone that meets the M3 rating concurrently in the 850-MHz and 1900-MHz frequency bands of operation.