Demystifying Loudspeaker Design Issues Insights from the Klippel Distortion Analyzer

We often take the sound emanating from a loudspeaker for granted, assuming that if the amplifier is clean and the source material is high fidelity, the output will be perfect. But anyone who has spent serious time measuring transducers knows this is a naive assumption. There’s a hidden world of mechanical and electrical aberrations lurking within even well-regarded drivers, things that our ears might perceive as "harshness" or "muddy bass" long before we can consciously label the specific distortion mechanism at play. I’ve been spending time recently looking closely at data generated by Klippel testing rigs, specifically the Distortion Analyzer, and it forces a re-evaluation of what we consider acceptable driver performance, particularly when pushing cones near their limits.

It’s easy enough to measure Total Harmonic Distortion (THD) at a single frequency, but that only tells a tiny fraction of the story; the real devil lives in the intermodulation products and the frequency-dependent nature of those errors. When you plot distortion products against excursion or power, you start seeing where the design compromises truly manifest themselves—it's rarely a smooth curve of increasing error. Let's talk about what happens when the motor structure isn't perfectly linear, or when the suspension starts behaving non-ideally under heavy load.

What I find most illuminating when running a Klippel sweep is observing the behavior of the second and third harmonic components relative to the fundamental, especially at high output levels where cone movement is substantial. If the magnetic system—the motor—is perfectly symmetrical and the voice coil remains centered within the magnetic gap across its entire throw, we should ideally see the second harmonic (which is generally considered less offensive to the ear) dominate the lower-order distortion spectrum, perhaps with the third harmonic trailing behind. However, when I look at the measurement plots for drivers with known issues, I frequently see the third harmonic spiking disproportionately, often indicating a problem with the mechanical centering or perhaps the non-linearity introduced by the edge suspension flexing unevenly. This asymmetry in distortion generation is what separates a clean driver from one that sounds subjectively strained; the ear is surprisingly adept at picking out that specific spectral imbalance caused by mechanical asymmetry well before the raw THD numbers become astronomical. It forces the designer to think beyond simple gap symmetry and consider the radial stiffness variations across the surround as the cone moves backward and forward.

Then there is the issue of lower-frequency performance, where excursion limits are usually hit first, and this is where the Klippel system really separates the contenders from the pretenders in subwoofer design. We often focus on finding the physical Xmax limit, but the distortion analyzer shows us that the *usable* linear excursion is often much smaller than the physical limit suggests. I’ve seen cases where the third harmonic distortion rises sharply not because the voice coil has physically bottomed out, but because the radial compliance of the spider assembly changes drastically when it is heavily compressed on one stroke direction compared to the other. This mechanical asymmetry translates directly into frequency-dependent distortion modulation, meaning the distortion signature itself changes depending on the frequency content of the signal being reproduced. It’s not just about how much air the driver moves, but how linearly it manages the reaction forces from the air load and the internal mechanical restoring forces across that entire operating window. We must move past simple excursion ratings and focus on the distortion floor across the operating bandwidth to truly characterize performance.