carrier wave. An AM demodulator which works in this way, by rectifying the modulated carrier, is called an envelope detector. The audio frequency current produced by the detector passed through the earphone causing the earphone's diaphragm to vibrate, pushing on the air to create sound waves. The earphone was typically a piezoelectric crystal type, so sensitive that the radio receiver could operate without an electrical power supply, using only energy from the incident radio wave to drive the earphone directly, with no electronic amplification. This diagram shows a simplified explanation of how it works: (A) This graph shows the amplitude modulated radio signal from the receiver's tuned circuit, which is applied as a voltage across the detector's contacts. The rapid oscillations are the radio frequency carrier wave. The audio signal (the sound) is contained in the slow variations (modulation) of the size of the waves. If this signal were applied directly to the earphone, it could not be converted to sound, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm. (B) This graph shows the current through the crystal detector which is applied to the earphone and bypass capacitor. The crystal conducts current in only one direction, stripping off the oscillations on one side of the signal, leaving a pulsing direct current whose amplitude does not average zero but varies with the audio signal. (C) This graph shows the current which passes through the earphone. A bypass capacitor across the earphone terminals, in combination with the intrinsic forward resistance of the diode, creates a low-pass filter that smooths the waveform by removing the radio frequency carrier pulses and leaving the audio signal. When this varying current passes through the earphone piezoelectric crystal, it causes the crystal to deform (flex), deflecting the earphone diaphragm; the varying deflections of the diaphragm cause it to vibrate and produce sound waves (acoustic waves). If instead a voice-coil type headphone is used, the varying current from the low-pass filter flows through the voice coil, generating a varying magnetic field which pulls and pushes the earphone diaphragm, again causing it to vibrate and produce sound. Crystal radios had no amplifying components to increase the loudness of the radio signal; the sound power produced by the earphone came solely from the radio waves of the radio station being