Electronic – Why are high impedance circuits more sensitive to noise

You use different speakers in different systems and even different serial and parallel configurations; that's one of the reasons there are different impedance values. 8 Ohm speakers are the standard for home, while 4 ohm are usually found in car audio systems.

If a lower impedance loudspeaker generally has better dynamic range and can achieve louder volumes, why do we still see 6, 8 or even 16 ohm speakers? Shouldn't we be "progressing" toward more efficient speakers with lower impedance?

While yes, the benefit of 4 ohm speakers is that the increased current means they have a wider dynamic range, they will be harder on the amplifier (if the amp is made for 8 ohm) and at higher volumes they will have larger THD (total harmonic distortion.) Essentially the output voltage will be unstable during high power application as the Amp will struggle to supply enough current to drive the load.

Using a 4 ohm speaker on a generic home amplifier that is made for an 8 ohm speaker will draw twice as much power and can cause the amplifier to go into protect mode or even overheat and break.

why do we still see 6, 8 or even 16 ohm speakers?

Different serial and parallel configurations are used to change the load on an amplifier. For example, you can have two 4 ohm speakers in series so that the load will be 8 ohms. Or (common in custom car systems,) you may have two 4 ohm speakers connected in parallel so the load on the amplifier is only 2 ohms, thus doubling the current.

Or in this case, four 8 ohm speakers are connected in series parallel so that the total impedance is only 8 ohms.

Are higher impedance speakers merely a product of backward-compatibility?

No they have their place, from allowing different speaker configurations, or less wear and tear on amplifiers to improved sound quality.

Using speaker with higher than minimum impedance may improve quality as the Amp will generate more stable voltage and current. Hence the THD will remain lower at higher impedance while the maximum power output by the Amp will be reduced due to higher load impedance.

Loudspeakers have impedances of 8 ohms, 6 ohms or 4 ohms (those are "nominal" or approximate values, because the impedance of a speaker changes all the time with the different frequencies of music

They are also referred to as at rest values, and if you connect a ohm meter to the speaker it should read 4 or 8 etc. ohms. Then if you gently move the speaker that reading will change. If you measure a speaker and it shows a different value than what it is supposed to then it may be defective of blown or at least slightly damaged.

This is what the article says: -

By using current signals and low impedance data acquisition devices, industrial applications benefit from better noise immunity and longer transmission cable lengths.

The article also says, in relation to devices that produce voltage signals, that: -

These devices are sensitive to the noise induced by nearby motors, conveyor belts, and radio transmissions.

Basically it's true but there are some caveats. Consider the noise induced by motors and for this, I reckon induction motors are a likely culprit. They produce magnetic fields that can induce an interfering voltage in a cable whatever the signalling type is.

When voltage signalling is used, the interfering voltage is additive to the signal just like batteries in series are additive. This adds an error.

When current signalling is used AND, providing the induced voltage is not several volts, the current flowing in the cable (due to the signal) remains exactly that current and no voltage interference is seen at the receiving end - this is because of the high-compliance of the 4-20mA current source: -

^{simulate this circuit – Schematic created using CircuitLab}

Hopefully you can see that for a high-compliance current source, interfering voltages that arise in series with the current loop have little effect.

Where does this start to go wrong: -

1. If the interference is large enough to cause the current loop transmitter to fall-out of high compliant sinking or sourcing of current
2. When the frequency is high and the current source/sink is unable to provide a high-compliance.

(1) The compliant current source may need a few volts across it to maintain performance and if the series voltage causes the minimum voltage to drop-below this point there will be glitching introduced onto the signal.

(2) At high frequencies, the compliance will change from theoretically infinite resistance to more like a small value capacitor (due to the transistors and chips in the device). This will allow high frequency interferers to circulate a current through the 100 ohm receiver (R1).

If low frequency signalling is used (with appropriate low-pass filtering at the receive end) HF interference can largely be avoided and it is advised to use screened/shielded twisted pair cable.

High energy E-field interference (as opposed to magnetic interference) tends to be seen as a voltage in parallel with the two wires and this also directly impinges on R1 so shielding and filtering is needed.