igit 5. 5.3.1 The Subset Mouthpiece or Transmitter The mouthpiece converts acoustic energy (i.e., sound) into equivalent electric energy by means of a carbon granule transmitter. The transmitter requires a direct-current (dc) voltage, a minimum of 3 to 5 volts, across its electrodes. We call this talk battery. In modern systems it is supplied over the subscriber loop and derives from a battery source at the local serving switch as illustrated in Figure 5.3. Current deriving from the local switch supply flows through the carbon grains or granules that are contained just below a diaphragm that holds the carbon granules in place. This current flow occurs when the telephone is taken off-hook (i.e., out of its cradle). When sound impinges on the diaphragm of the transmitter, resulting variations of air pressure are transferred from the diaphragm to the carbon granules, and the resistance of the electrical path through the carbon changes in proportion to the pressure. A pulsating direct current in the subscriber loop results. The frequency response of the carbon transmitter peaks between 800 and 1000 Hz. This is illustrated in Figure 5.2. 5.3.2 The Subset Earpiece or Receiver A typical receiver consists of a diaphragm of magnetic material, often soft iron alloy, placed in a steady magnetic field supplied by a permanent magnet, and a varying magnetic field, caused by the voice currents flowing through the voice coils. Such voice currents are alternating (ac) in nature and originate at the far-end telephone transmitter. These currents cause the magnetic field of the receiver to alternately increase and decrease, making the diaphragm move and respond to the variations. As a result, an acoustic pressure wave is set up, reproducing, more or less exactly, the original sound wave from the distant telephone transmitter. The telephone receiver, as a converter of electrical energy to acoustic energy, has a comparatively low efficiency, on the order of 2% to 3%. Sidetone is the sound of the talker's voice heard in his own receive