LG Stylo 2 Plus LG-K550 - Manual (Page 108)

For Your Safety 107
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’s 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.
To ensure that the Hearing Aid Compatibility rating for your phone is
maintained, secondary transmitters such as Bluetooth® and WLAN
components must be disabled during a call.
For information about hearing aids and digital wireless phones
Wireless Phones and Hearing Aid Accessibility
http://www.accesswireless.org/
Gallaudet University, RERC
http://tap.gallaudet.edu/Voice/
FCC Hearing Aid Compatibility and Volume Control
http://www.fcc.gov/cgb/dro/hearing.html
The Hearing Aid Compatibility FCC Order
http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-168A1.pdf
Hearing Loss Association of America [HLAA]
http://hearingloss.org/content/telephones-and-mobile-devices