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device is relatively immune to interference noise. The hearing aid
and wireless phone rating values are then added together. A sum
of 5 is considered acceptable for normal use. A sum of 6 is
considered for best use. In the above example, if a hearing aid
meets the M2 level rating and the wireless phone meets the M3
level rating, the sum of the two values equal M5. This should
provide the hearing aid user with "normal usage"± while using their
hearing aid with the particular wireless phone. "Normal usage" in
this context is defined as a signal quality that is acceptable for
normal operation. The M mark is intended to be synonymous with
the U mark. The T mark is intended to be synonymous with the UT
mark. The M and T marks are recommended by the Alliance for
Telecommunications Industries Solutions (ATIS). The U and UT
marks are referenced in Section 20.19 of the FCC Rules. The HAC
rating and measurement procedure are described in the American
National Standards Institute (ANSI) C63.19 standard.
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