In am receivers, we detect the modulating signal by using envelope detector. So, the diode rectifies the signal, and the detected signal has doubled frequency than the original. Does this mean we will hear distorted sounds, if we connect audio amplifier. Is it possible to decrease the frequency by two?
In AM you have a waveform typically formed like this:
Rectification chops off the negative portion of that signal, thus:
Filtering then removes the high frequency component:
A capacitor then removes the DC offset:
Nowhere in that does the frequency change.
Even if you were to use full-wave rectification, only the frequency of the carrier would change - the modulated signal frequency will be just the same as it was.
In the image you provided in your comments:
The modulated signal is modulated twice - once on the positive axis, and once on the negative axis. This is the same as the top image above. However, the modulating signal has an amplitude that crosses the zero axis, so you actually end up with two signals crossing over each other, like this:
When you then rectify and filter that waveform you get just the positive portions of both waves:
With pure audio modulation (modulating two audio signals together) this can be desirable as it produces very noticeable affects and artefacts (you would never typically demodulate this signal, it would be the finished audio product in its own right). In RF modulation though it's not wanted, so the incoming signal should have an amplitude of no more than 50% of the carrier frequency, and be off-set to half-way up (and down) the carrier wave so the two sides of the wave don't cross over.