Intermodulation Distortion in automotive loudspeakers has degraded the quality of the listening experience in vehicles since the onset of audio systems in cars. Even in the current state-of-the-art systems of 30 or more loudspeakers, it can still rear its ugly head. This tutorial will define the especially egregious Amplitude Modulation distortion, show what it looks like in scope captures and simple twin-tone IMOD measurements. It will demonstrate how to use common audio tools like Room EQ Wizard (REW), Smaart, and SpectraFoo to discover what are the danger frequencies in given loudspeaker by using the Spectragraph function. It will show how even expensive speakers within their linear Bl(x) (magnet strength) and linear Kms(x) (stiffness) curves can still suffer greatly from this AM distortion, even at lower excursions than their parameters would predict. We will show how typical automotive speaker mounting and protective speaker grills actually increase the presence and audibility of this distortion. We will show a technique to measure this distortion in the vehicle. The tutorial will show visualizations of distortion-causing cone motion recorded by a laser vibrometer. Finally, we will listen to the audible effects of this distortion on different music examples. The format will allow questions during any point in the tutorial to allow for full understanding and explore how diaphragm simulation could help solve some of these issues.
