a. The transmitter shown in B, figure 11 has a major disadvantage in that it is susceptible to interference from
noise. This disadvantage is most noticeable when the transmitter is operated in places where the noise level is high, such
as near railroad trains, airfields, the interior of tanks, and areas where gunfire or bombardment is taking place.
b. A number of transmitters have been developed which reduce interference from noise sources. Among these
are the throat transmitter and various kinds of directional transmitters, which restrict the movements of the operator and
produce some distortion of his speech. Recently, however, a transmitter has been developed which largely eliminates
noise interference without restricting movement. It is called a noise-canceling or differential transmitter.
c. The United States Army Type T-45 lip transmitter (fig. 13) is an example of this type. In operation, sound
waves activate its diaphragm only if they are introduced close and perpendicular to the front surface of the diaphragm.
Sounds which originate at some distance enter the transmitter through two openings, on the front and back of the
diaphragm. Since this equalizes the pressure exerted on both faces, the resultant motion of the diaphragm for distant
sounds is practically zero. There is almost no change in the resistance of the carbon granules and, therefore, almost no
change in current as a result of these sounds. Since, in responding to distant sounds, the diaphragm neutralizes pressures
of relatively low frequencies more than those of high frequencies, the noises most canceled are those originating in tanks
and from gunfire, mainly in the low-frequency range. By proper design, it is possible to make the cancellation of noise
practically complete. This feature makes the differential transmitter much more suitable than others for many military
applications, and also makes it valuable for many civilian uses.