Biomedical Engineering Reference
In-Depth Information
Figure 6-4.
How FM systems provide improved signal-to-noise ratio
(SNr). In each box is a gray shaded area to represent the background
noise. the word “Hi.” represents the speech signal of interest. From left to
right, we can see the effect of increasing distance on SNr from the origi-
nal source without the FM system. that is, the SNr decreases from +6 to
+2 dB. this is represented visually with the word gradually fading into the
noise. With the FM system, SNr remains constant at +10 dB regardless of
increasing distance. this improvement (or maintenance) of SNr can also
be seen with other hearing assistive technologies, such as induction loop
systems and infrared (Ir) systems.
in conjunction with personal hearing aids
or implantable devices with an integrated
(built-in) FM receiver or add-on FM
receiver unit (audio shoe; Figure 6-6) to
be used with a compatible microphone/
transmitter (Figure 6-7). Regardless of
the personal FM product package used,
the fundamental operation is the same.
The microphone/transmitter will be
placed on the person who is speaking
(e.g., teacher, parent, spouse, or friend)
or it can be placed near a sound source
of interest (e.g., next to a television/radio
or on a podium). Unless there is only one
frequency channel, the user will need
to make sure that the microphone and
receivers are “tuned” to each other by
being on the same channel.
It may not be apparent at first, but it
is important to understand the different
coupling options with FM receiver units
(unless an integrated FM or FM audio
shoe is used). Figure 6-8 shows a number
of different coupling options, with fur-
ther description as follows: For individu-
als without hearing aids or implantable
devices, a pair of headphones or earbuds
will provide acoustic coupling, and users
should be able to adjust the FM receiver vol-
ume control to optimize listening comfort.
