# – What are oscilloscopes measuring, exactly? I know the y-axis is voltage and the x-axis is time, but what does that mean? How can they display, say, sound waves? What does sound have to do with voltage?

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Also, if you plug a probe into it then to an outlet, what is the oscilloscope displaying? The difference in electric potential between what?

In: 0

In normal use the x axis indeed is time based.

The input to the Y axis has to be a voltage. You can measure many different types of signal (sound, temperature, current… blah blah) BUT you need to convert it to a voltage first.

In the case of sound – you are probaly happy with using a microphone.. this converts sound into an analog voltage which could be fed to an amplifier to hear it – OR to to a scope in which case the Y axis will be scaled for amplitude, and the X will give an indication of frequency.

Other examples –

a thermocouple provides a voltage signal based on temperature

a pressure sensor gives signal based on pressure..

most sensors provide a voltage output based on some physical input….

As for your probe – it probably has a shield or ‘ground’ clip associated with it. The voltage is normally refernced to this, or to the 0V of the scope power.

An oscilloscope measures oscillations in voltage over time, as you noted.

When an o-scope is display a “sound wave”, it would be in a situation where it was connected to a device like a radio or microphone that was generating an audio signal. While it can’t measure the actual audio signal you hear, since that’s vibrations in the air and not electrical, it can measure the changes in voltage that are sent to a speaker that would in turn produce that sound.

A speaker is basically a cone of some type of material that pushes air around. At the bottom of the cone, it’s usually glued or secured to a magnet which is in turn inside a coil. When an electrical signal is sent through the coil, as the voltage goes up, it moves the magnet one way, and as the voltage goes down, it moves it the other way (this is extremely simplified). When that back-and-forth action is fast enough and strong enough, the speaker cone pushes the air back and forth, making vibrations your ear detects as sound.

As to the question about putting the probe into an outlet, first:

DO NOT DO THIS.

If you know what you’re doing, it can be done safely, but if you’re a novice, it’s a good way to hurt yourself, your equipment or give yourself a lethal shock.

That said, electricity delivered to homes in the US is “alternating current”. It goes back and forth sixty times per second in the US, and fifty times per second in the UK.

If you were to measure this voltage with an oscilloscope, you’d see a “sine wave”, which is a smooth up-and-down transition of voltage from zero volts to the peak voltage (for technical reasons the actual maximum value is about 170 volts), then back to zero, then down to the minimum voltage (negative 170 volts).

The actual voltage you can get out of an AC circuit is a sort of average called root-mean-square (RMS) voltage, which is 120 volts in the US. That’s what you’ll measure at the outlet.

As to what it’s actually detecting, it’s the difference in electrical potential- called voltage- between the two conductors. The reason it’s going back and forth is that it’s more efficient to send electricity over long distances with alternating current.