Chapter 2. From Analog Sound to Digital Audio
2.3. A Note on Impedance
Once converted into voltage by the microphone capsule, our sound enters the electrical realm; for a clear explanation of how electronics work, see here. We have talked about voltages so far because that is what is physically changed in the processes we described; however, when those voltages are transmitted along conductors like cables, it is more convenient to discuss about currents; both are linked by resistance or impedance.
There, components transmitting the current, like cables, can alter it. They have electrical characteristics like resistance, which has some resemblance to friction in the mechanical world. Because currents transmitted by audio equipment vary with time (alternating currents are either generated or modulated by transducers), the correct concept to use is impedance instead of resistance; see this article for a complete discussion of the concept.
Why? According to the laws of physics, two phenomena appear when tensions (or currents) change with time: a magnetic field appears (induction effect) and electrical charge is stored in the conductor (capacitance effect), both increasing the factors impeding (hence the term “impedance”) the current from flowing to its destination: inductive reactance (from the induction effect) and capacitive reactance (from the capacitance effect) form the reactance :
with XL = 2 * Π * ν * L and XC = 1 / (2 * Π * ν * C) .
L is the inductance (measured in Henrys) of the material used and C is its capacitance (measured in Farads). Note that inductive reactance is proportional to the audio frequency while capacitive reactance is inversely proportional to it: this means that at low frequencies, inductive reactance is small but capacitive reactance is large, the opposite being true at high frequencies.
Impedance is simply the sum of those two effects, resistance and reactance. The symbol for impedance is Z and it is measured in Ohms (Ω), like resistance:
where R is the resistance, X is the reactance. That is where the “Z” on your audio interface comes from.
Because different devices translate pressure into electricity (like microphones) or transport electricity (like cables) in different ways by design, they have different impedances and, thus, transport current with varying friction. There generally are two types of impedances related to audio equipment: high impedance (usually labelled HiZ) and low impedance (usually labelled LowZ). Low impedance devices (0-10kΩ, 1 kΩ = 1000 Ω) include most microphones, interface inputs for microphones and most interface outputs. For example, my interface has three types of input impedances: microphone at 2400 Ω, mic/line at 10kΩ and guitar at 1 MΩ; the output is at 100 Ω. High impedance devices (> 10 kΩ) include instrument outputs (generally between 7 and 20 kΩ) and interface instrument inputs (in the 1 MΩ range).
Because in some situations, knowing the source and destination device impedances (destination impedance can also be called the load) can help resolve audio issues like loss of sound quality. Most of the time, impedance issues are transparent because of the way audio equipment is built: to ensure a problem-less signal transmission, the target impedance is at least 10 times that of the source. This ensures that voltages are properly transferred across multiple devices so that the signal quality is preserved.
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