The concept of impedance is everywhere in audio – even outside the electrical circuits involved.
The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.
One of the best things about working in live-audio is that, every so often, you have a mind-blowing experience.
Sometimes, it’s a band that plays the perfect gig.
Sometimes, it’s a piece of gear that rearranges your workflow in a supremely nifty way.
Sometimes, it’s discovering something about the fundamentals of the craft – an experience where you see a “primal pattern” emerge.
Just recently, I had that experience with impedance. (Also with tuned circuits, but that was by extension.) As a formally trained audio tech, I’ve been through the requisite material about what impedance is, and why it’s important – especially for power amplifiers and loudspeakers. What I failed to see – for FREAKIN’ years – is how impedance is a primal pattern in the entire experience of live audio. That is to say, the concept of impedance (and its effect on the performance of a “loop”) has universal applicability in terms of modeling and describing the behavior of live-sound gear in “real life.”
The spark for recognizing this came from my recent post on active DI boxes. A few days afterward, something suddenly clicked in a way that it hadn’t before.
Basically, impedance is everywhere. Here are a few examples.
Acoustical Impedance Bridging
Electrical voltage is analogous to the force behind the movement of a fluid or gas. This is why there are so many electrical engineering examples that use a garden hose as a metaphor. If you increase the impedance of the hose output, the system pressure goes up, flow goes down, and you can squirt a jet of water over to that tree in the center of the lawn. By putting your finger over the hose end, you create a bridging impedance between the hose and the outside environment.
This is why we have horn-loaded loudspeakers. Although greater amplifier output has allowed us to use more direct-radiating cone drivers, pro-audio still overwhelmingly uses high-frequency transducers that are mated to horns. One major reason for this is for purposes of controlling directivity. However, I would personally argue that the main reason for using a horn is for managing acoustical impedance.
When compared to a big, heavy, LF component like a woofer, a high-frequency driver has a rather high acoustical output impedance. It just can’t move enough air to create the kind of total, in-room pressures that a big driver can manage. An HF driver without a horn is like a tiny, low-pressure pipe that empties into a giant storm-drain. Sure, there’s no opposition to the flow of air pressure, but that tiny pipe can’t fill what it’s emptying into. Fire a high-frequency transducer freely into the air of a comparatively huge room, and it’s just not as effective as it might be. The room has too low of an input impedance – you need to bridge it.
That’s what the horn is for.
Mate the HF driver to a proper horn, and what you get is a situation where the horn partially opposes the pressure flow from the driver. The acoustical impedance that the HF driver “sees” is effectively raised, which means we get much better pressure transfer to the room – just like electrical impedance bridging gives us better voltage transfer between devices. “Loudness” is SPL, or Sound Pressure Level. We do need an adequate amount of “flow,” but our main concern is pressure transfer to the room.
In this way, the horn is like our finger over the end of the hose. Our flow of sound is restricted to a smaller radiation area (directivity), but within our radiation area we get a lot more pressure (loudness). We trade the ability to hit the entire room with a little bit of HF pressure for the ability to hit a small portion of the room with much more pressure.
Another place where impedance is very important is with any resonant system. Resonant systems are damped by impedance; higher impedances prevent the system from “ringing” freely.
Resonance damping is an important factor when working with drivers mated to ported boxes. As the driver moves, the air mass inside the box partially impedes the motion of the driver. The pressurized air resists the driver’s inward travel. Add a port to the box, and you essentially add an acoustical inductor to the equation. At high frequencies, the driver continues to see a high impedance to inward motion. However, frequencies lower than the port’s resonance present very little acoustical impedance. Low impedance means a lack of damping, and this is can be a very…expensive thing. An undamped loudspeaker can have so much physical motion that it goes right past its design limits and tears itself apart.
So, when you buy a ported loudspeaker from a manufacturer, it’s important to heed the warnings about applying a high-pass filter at a certain frequency. Driving the system with material that’s lower than what the system is designed for can wreck the driver(s) in a hurry – all because the acoustical impedance is too low.
The material above is information that I had some familiarity with. I hadn’t really bothered to dig into the impedance aspect of it all, but I was familiar with the terminology and that impedance was somehow involved.
Here’s where the lightbulb really came on, though.
What happened was that I applied an analogy to a live-sound reinforcement system that I never had before: I suddenly realized that a live-sound rig can be modeled as a giant LCR circuit.
An LCR circuit is an electrical device where current flows through an inductor, a capacitor, and a load. The inductor impedes high-frequency signals, while the capacitor impedes low frequency signals. This being the case, the circuit resonates or “rings” across a certain frequency range. This ringing is damped by the impedance of the load.
This is where it hit me.
A live audio system is a (partially) closed loop. The sound from a microphone is amplified through loudspeakers, and some of that sound returns to the microphone and is amplified through the loop again. If the system gain is increased, that amplification increases. With enough amplification the most resonant frequency areas will begin to “ring” out of control. Feedback. Reduce the gain, and the feedback stops. That means that the system gain is the “load,” and that raising the gain means…
Lowering the system’s sonic impedance.
This may be a little hard to picture, so I drew a diagram:
See what I mean? The live-sound life is just one big LRC circuit, and the live-audio human’s job is to manage the impedance of the circuit. We may do it broadly (with “all-pass” gain changes), or selectively by applying EQ – which is just a smaller LRC circuit that we add to the big one.
I’ve heard it said that “everything is EQ.” I can now go a step further and say that everything is impedance.
A primal pattern is revealed.
Not only that, there’s a fractal pattern involved. It has self-similarity at all scales. The microphone is a resonant system, attached to equipment that includes resonant sub-systems, which form overall circuits that are resonant systems, which form a giant acoustically-resonant system.