# A simple question on converting sound wave into electrical wave

#### AlwaysNumber1

Joined Dec 4, 2016
52
When sound wave is converted into electrical wave, is the electrical wave a sinusoidal with constant frequency and amplitude ?

#### BR-549

Joined Sep 22, 2013
4,938
Only if you are a good whistler. Connect a mic to a scope and watch. The electric "wave" follows the frequency and amplitude of the sound wave.

#### AlwaysNumber1

Joined Dec 4, 2016
52
Only if you are a good whistler. Connect a mic to a scope and watch. The electric "wave" follows the frequency and amplitude of the sound wave.
But why then everyone else draws this signal as constant while describing modulation ?

#### BR-549

Joined Sep 22, 2013
4,938
Because it's easier to see, and understand. Also unlike our voice, our test generators use nice sine waves. Our voice waves change too fast for our eyes to see. But if we use a symmetrical sound pattern.......it's easier to use and measure.
Does that make sense?

#### AlwaysNumber1

Joined Dec 4, 2016
52
Because it's easier to see, and understand. Also unlike our voice, our test generators use nice sine waves. Our voice waves change too fast for our eyes to see. But if we use a symmetrical sound pattern.......it's easier to use and measure.
Does that make sense?
Yes, it does
But if the picture that we see in the oscilloscope represents our real voice, how will the diode cut off bottom part of the signal without destroying it >

#### BR-549

Joined Sep 22, 2013
4,938
A diode does not destroy a signal. It rectifies it. It cuts it in half and only allows one side to pass. It will select either the positive part or the negative part. The positive part is a mirror of the negative part.......and vice-a-versa. They are mirrors. They are equal.....just inverted. SO.......we only need one. This would be called half wave rectification.

But if we use more than one diode........we can flip the half that was discarded........and add it to the one we saved.

That's called full wave rectification.

#### AlwaysNumber1

Joined Dec 4, 2016
52
A diode does not destroy a signal. It rectifies it. It cuts it in half and only allows one side to pass. It will select either the positive part or the negative part. The positive part is a mirror of the negative part.......and vice-a-versa. They are mirrors. They are equal.....just inverted. SO.......we only need one. This would be called half wave rectification.

But if we use more than one diode........we can flip the half that was discarded........and add it to the one we saved.

That's called full wave rectification.
Aaaaa
So even though the signal is constantly changing, the upper part is still the morrow of the down part ?

#### BR-549

Joined Sep 22, 2013
4,938
Yes......but see how much easier it is to understand with sine waves? The voice wave is too fast and irregular to try and see it. But with a sine.....you can see it very well. Whistle steady into the mic and see.

#### BR-549

Joined Sep 22, 2013
4,938
When we listen to audio.......our brain does not listen to every little component of the sound. Our brain detects the average amplitude and average frequency. It listens and detects the envelope (average) of the sound. Our brain catalogs envelopes. These envelopes are patterns. This is why we can understand a word that comes from different people. The individual sound is different.......but the pattern is the same.........or close to it.

#### AlwaysNumber1

Joined Dec 4, 2016
52
When we listen to audio.......our brain does not listen to every little component of the sound. Our brain detects the average amplitude and average frequency. It listens and detects the envelope (average) of the sound. Our brain catalogs envelopes. These envelopes are patterns. This is why we can understand a word that comes from different people. The individual sound is different.......but the pattern is the same.........or close to it.
Thanks a lot then !

#### Papabravo

Joined Feb 24, 2006
13,528
When sound wave is converted into electrical wave, is the electrical wave a sinusoidal with constant frequency and amplitude ?
No, it will vary in frequency and amplitude.

#### Papabravo

Joined Feb 24, 2006
13,528
But why then everyone else draws this signal as constant while describing modulation ?
Everyone does not do this. A carrier maintains it's frequency, but the envelope reproduces the audio modulating signal.

#### Papabravo

Joined Feb 24, 2006
13,528
Yes, it does
But if the picture that we see in the oscilloscope represents our real voice, how will the diode cut off bottom part of the signal without destroying it >
The modulating signal does the same thing to the positive half cycle as the negative have cycle. They are mirror images of each other.

#### PhilTilson

Joined Nov 29, 2009
81
Both BR-549 and Papabravo have said "The positive and negative half cycles are mirror images of each other". Whilst I understand why they have said this, and whilst it makes it easier to explain the basic case to the OP, it's not actually true!

This only applies to a constant repetitive waveform. In the case of, for example, human speech, whilst the positive and negative half cycles will be very similar, by the very nature of the input, they will not be identical or mirror images. I only mention this because, if I don't, someone else will, and probably won't be as polite!

#12

#### crutschow

Joined Mar 14, 2008
24,725
Both BR-549 and Papabravo have said "The positive and negative half cycles are mirror images of each other". Whilst I understand why they have said this, and whilst it makes it easier to explain the basic case to the OP, it's not actually true!
It is true for an AM modulated signal, which is what they are referring to.