basic signal question

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yellowmania

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doing experiment in the lab at school using harmonic synthesiser, oscilloscope and Vrms voltmeter to create different types of waves. one part involved entering a loudspeaker into the circuit and turning it on and off to see the effect it had on the waveform on the oscilloscope screen. i noticed a drop in Vrms on the voltmeter when the speaker was turned on. my worksheet said this is expected but i am struggling to understand why? im thinking its something to do with the impedance of the speaker but id rather fully understand why the Vrms drop happens, could anyone point me in the right direction? thanks!
 


The loudspeaker load results in a voltage divider with the source resistance of the synthesiser. If the the no-load output of the source is Vo, the source impedance is Zs, and the load (loudspeaker) impedance is Zl, then the voltage acroos the loudspeaker speaker will be Vo(Zl/(Zs + Zl).
 

To put a simple example of what kral said. you may consider that your generator has a series resistance of 50ohms and your speaker 8ohms (this values are very common). Then you will get about 14% of your no-load output.
 

The better way to understand is to consider a voltage drop between two resistors in series
 

Without the speaker connected, the resistance is very high due to the huge resistance of the voltmeter. So current flowing through the series resistance of the synthesizer is very small. So voltage drop across the series resistance is very small. However with the speaker connected, the overall series resistance (and impedance) is much smaller, current flowing through is larger and so voltage drop across the internal series resistance is higher. So voltage across speaker is smaller and you notice the voltage drop.

Hope this helps.
Tahmid.
 

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