Sound in Circuits

Discussion in 'Homework Help' started by DiodeMan, Feb 11, 2013.

1. DiodeMan Thread Starter New Member

Feb 3, 2013
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This isn't necessarily a "homework" question, but I'd figure I'd ask here because I'd probably get more help.

I'm just wondering how a sound wave in an electronic circuit would function, for example a note coming from a guitar or piano. Since the note is a sine wave with a specific frequency, would it act similar to an AC source? Would the peak-to-peak voltage be the sound intensity? Or would the sound intensity come from the current?

Also, if I were to put this wave through a common emitter amplifier, would the inversion of the wave cause me any changes in the tone, or would it not be affected?

I know these are kind of general questions, but I'm not looking for anything overly specific, just yes or no, maybe with a very brief explanation.

2. slodnulius New Member

Feb 11, 2013
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Well, I have built some pedal effects for my bass, and for testing them, I hooked up an A.C. power supply with an oscilloscope. I'll say that a note acts like an A.C. source with an specific frequency (the higher the pitch, the higher the frequency). I could be wrong, so don't take my word for a fact.

Also, I hooked my oscilloscope to output of the pedal (but this time using my bass guitar). The harder I pluck the strings, the larger Vpp I measured with the oscilloscope.

Using a common emitter amplifier, I actually didn't heard any difference in the sound. But one of my teacher (talented bassist player and also he has a very good ear for this kind of stuff) told me that my bass sounded slightly different using the pedal. Probably this is because of the "cheap" components that I used, or the cables... or the input/output. I'm not sure about this one.

I hope this helps!

3. KnRele New Member

Jan 7, 2013
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Once the microphone is being affected by the pressure variations in the air that we know as sound, it will deliver an alternating voltage corresponding to these pressure variations.

So yes, the microphone with some instrument playing into it can be modeled as an AC voltage or current source delivering power to the subsequent amplifier stage. The Thevenin model of a voltage source with a series internal resistance or impedance is probably the easiest to work with, but there is nothing keeping us from viewing it as per Norton's theorem, as a current source with a parallell internal resistance or impedance.

Different types of microphones may present different source impedances.

The inversion that takes place in a common-emitter amplifier is not likely to be audible in itself. What slodnulius' teacher heard was probably effects of distortion in the amplifiers, not the inversion as such.

Now, a bass guitar produces signals much closer to a pure sine wave than a piano would do -- the way the hammer in the piano hits the string sets it going at several different frequencies at the same time.

4. blah2222 Well-Known Member

May 3, 2010
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Yeah, so in order to realize the sound of an instrument into a measurable medium there has to be some form of transducer to convert the mechanical vibration into an electrical signal.

The louder the sound, the larger the amplitude of the signal.

In terms of putting the signal through an inverting configuration and then listening to the sound, if it is amplified the only difference in sound from the original signal will be its volume. The frequency does not change so there will be no change major change in how it sounds.

Now say that you amplified the original signal with both an inverting pathway and a non-inverting path with the same gain and then summed the two signals before the output. Assuming they had no relative phase differences, they would exactly cancel each other out and there would be no output sound.

Things to keep in mind when playing around with sound and transducers is input/output impedance matching. Normally the transducer passes a very weak signal identified by having a high output impedance. When you attach the output of a transducer (high output impedance) to the base of a CE BJT (low to moderate input impedance) you get a sort of voltage divider:

$V_{base} = \frac{R_{bjt}}{R_{bjt} + R_{trans}}V_{in}$

If Rtrans >> Rbjt then there is not much signal that will travel to the base as the majority of it will be across the output impedance of the transducer.

Just some things to keep in mind.

5. WBahn Moderator

Mar 31, 2012
18,087
4,917
Depending on the circuit, it could be represented as either a voltage signal or a current signal. Since, in most parts of most circuits, there is a strong correlation between voltage and current the information is actually present in both. But most circuits are designed specifically to respond to one or the other and the effort goes into keep that representation as distortion free as possible.

It's quite possible there were inversions elsewhere, too, such as in the transducer circuit. It has no effect on the perception of hearing because the ear responds to pressure variations about the mean. As noted earlier, any differences that are perceived are the result of distortion in the circuits.

One thing to always keep in mind is that whenever we use a small signal model to design, say, an amplifier we are using the behavior of the circuit at a particular DC input level and assume that this behavior is the same for a range of input signals about that point. This is pretty much never true since we are dealing with a highly non-linear device in the case of transistor circuits. But our goal isn't perfection, it is to be "good enough" for the application at hand. But we can reduce the distortion by designing the circuit so that the variations about the bias point are small enough to keep the non-linear affects below some tolerable level.

I doubt soldnulious made any effort at doing so in his simple common-emitter amp while the people that designed the amp it was being compared to did. That is probably the difference and not the quality of the components used (which can be a factor, but probably not in this case).

6. crutschow Expert

Mar 14, 2008
13,498
3,374
Inversion as such doesn't affect the sound, but if you are directing the signal to more than one speaker, each speaker has to have the same phase signal or the sound from one will tend to cancel the sound from the other (particularly at the bass frequencies).

7. MrChips Moderator

Oct 2, 2009
12,635
3,454
Have you never examined the grooves on a vinyl record with a magnifying glass?
Or are you so young that you have never seen one?
Then imagine how the grooves on the record gets transformed into music by a needle that gets jiggled by just following the groove on the vinyl record.

8. #12 Expert

Nov 30, 2010
16,690
7,330
This is one of my pet theories. Inversion is a phase shift of 180 degrees and humans can not detect the phase of a single tone. They can detect changes in phase, as in a Leslie speaker or human speech, but a single tone is not interpreted by humans as to phase.

9. DiodeMan Thread Starter New Member

Feb 3, 2013
13
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Hahah I've definitely seen a vinyl record before, I never thought to examine one with a magnifying glass though

Also, thanks for all the help everyone, all my questions regarding this have been answered