Impedance is a measure of resistance at frequency.
But there's more going on inside of a voice coil.
In this guide, we'll discuss
- DC Reisistance
- Nominal Impedance
- Peak resistance at a driver's Fs
- Voice Coil Inductance
- Box Rise, or how an enclosure affects impedance
- The effects of passive filtering on a subwoofer's parameters
Voice Coil DC Resistance
DC Resistance is an easy parameter to measure. Simply set your digital multimeter to DC Ohms and probe the leads. For a 4ohm driver, the digital multimeter should read anywhere from 3.0 ohms up to 4.5 ohms, and anywhere in between.
The DC Resistance is different than Nominal Impedance because that's a measure of impedance at zero ohms. There are methods for measuring impedance vs frequency, but that's a discussion for another day. But understand that there are free modeling tools we can leverage to discover a woofer's impedance curve.
Nominal Impedance
Simply put, Nominal Impedance is how much resistance the subwoofer provides to the amplifier most of the time. A subwoofer's nominal impedance is a very basic and useful measurement for matching a subwoofer system to an amplifier. Nominal impedance will tell you what impedance to expect over the loudspeaker's useful range. If the nominal impedance is too low, the amplifier will go into protect or overheat since the amplifier's circuitry is designed to operate with a certain amount of impedance on the loudspeaker leads. You can use this number in place of resistance to compute series and parallel circuits to wire up multiple subwoofer drivers to an amplifier, too.
We use "nominal" because that's how the subwoofer is expected to behave for a majority of frequencies in the driver's usable range. The impedance will go up and down across the frequency range for both mechanical and electrical interference reasons.
Peak Resistance at Resonance
A subwoofer is an electro-mechanical transducer, converting electrical signals into physical sound waves that travel to our ears. The mechanical part of the device has a resonant frequency. Resonant frequency is the "natural" frequency based on a woofer's cone and suspension. If you've ever tapped on a woofer cone, you can hear the low frequency "thud". That thud's tone is centered around the woofer's resonant frequency.
This thud is the equivalent of a cymbal's ring or the sustained sound from a plucked guitar string. It wants to keep ringing.
Just like the cymbal or guitar string, the woofer cone and suspension has a tendency to keep ringing. This tendency to continue ringing pushes voltage back through the woofer's motor assembly, introducing additional impedance to the voice coil around that frequency. If we were to use a tool like a woofer tester or set up a testing circuit to measure impedance at frequency, we would notice an incredibly high impedance at that resonant frequency, making a peak in the woofer's impedance curve. This impedance at resonance may be 10x the woofer's nominal impedance!
Voice Coil Inductance
An inductor is also known as a "choke coil". It's a coil of wire that passively increases impedance as frequency increases. The inductor is used by loudspeaker engineers to curtail high frequencies sent to a loudspeaker as part of a crossover network. The inductor works by quadrupling the impedance for each doubling of frequency. The term for this is "-6 decibels per octave slope", as the output goes down -6db for each doubling in frequency.
Each doubling of power would result in +3db. As such, each halving of power results in -3db. Doubling the resistance halves the power. Quadrupling the resistance (doubling again) results in -6db.
A subwoofer's motor assembly is a coil of wire surrounded by a magnet. As such, the motor assembly will exhibit properties of an inductor, causing the impedance to rise as the frequency increases. This will cause the subwoofer's impedance to rise as the frequency increases.
"Box Rise", or How an Enclosure Affects a Subwoofer's Impedance
An enclosure affect's a subwoofer's impedance in much the same way an enclosure affects the subwoofer's output. A sealed subwoofer enclosure will increase the subwoofer's overall resonant frequency and will also increase the "ring time" of the driver. Both of these properties will increase the impedance seen at the new resonant frequency.
If one were to include a port on the subwoofer enclosure, we would add an additional resonance to the loudspeaker system. This additional resonance is centered around the port tuning and adds an additional ringing. Just like the ring of a woofer adds resistance within the voice coil, this resonance at port tuning also adds resistance to the voice coil centered at that frequency.
This port tuning adds an additional peak to the impedance curve, centered around the port tuning frequency. There are now two peaks in the subwoofer's impedance curve
When one adds another chamber and resonance to the enclosure (in the case of a sixth order band pass), one adds yet another resonant frequency peak, totaling up to three.
These additional peaks in the impedance curve are frustrating to SPL competitors who design a subwoofer system's output to dovetail into a vehicle's sonic signature and provide maximum boom at the "burp frequency", only to find the impedance at the "burp frequency" curtails the amount of power the amplifier can send through the voice coils. This additional impedance is known as "box rise".
Box rise can be introduced through heat in the motor assembly during a long demo or a db drag race format.
Effects of Passive Filtering on Parameters
Passive Filtering (like in the choke coil example discussed above) will change how the system reacts. Any additional components will introduce a bit of a change in the system. The addition of a choke coil to curtail higher frequencies from the subwoofer driver will introduce additional DCR. This additional DCR in the circuit will change the Qes parameter of the subwoofer driver, and then subsequently change the Qts parameter. This change will affect how the subwoofer driver will perform in an enclosure. The higher the DCR of the passive components in line with your subwoofer driver, the larger the change in Qes/Qts. A large shift in Qes will change the efficiency bandwith product. That will change what kind of enclosure your woofer will perform best in, shifting towards a sealed box.
This complication illustrates why it's recommended to use a dedicated subwoofer amplifier, instead of trying to drive all loudspeakers from a single amplifier with passive speaker-level filtering.
Some Final Thoughts on Impedance
A subwoofer's nominal impedance is only a rough estimate of how the subwoofer will present a resistance to the amplifier. The subwoofer voice coil, enclosure and external components will all effect how the subwoofer performs. Most of the time we don't need to worry beyond the nominal impedance matching up to the amplifier, but in some cases we should take it into consideration.
