Most of us are familiar with harmonic distortion (HD). Most every amplifier, receiver, or other electronic audio component has published its harmonic distortion figures. Most electronics have vanishingly small amounts of it, but speakers produce much, much more. Yet as important as harmonic distortion is, it’s not the worst of the pack. Harmonic distortion is similar to the natural harmonics musical instruments produce, and therefore, small quantities are relatively benign. Intermodulation distortion (IMD), on the other hand, is not so benign. It’s audible at lower levels than HD.
Unlike harmonic distortion (multiples of the fundamental frequency), IMD is the sum and difference of two or more simultaneous frequencies. These sum/difference frequencies are not harmonically related, which is why they stand out more audibly. Music is made up of many notes played at the same time. Even a single note sung or played contains many harmonics from which IMD can rear its ugly head. IMD is a critical distortion.
So then, how can intermodulation be reduced? One way is to reduce the bandwidth (frequency range) of the driver and its amplifier. With a narrower range and fewer frequencies to interact, there is less intermodulation. This is why single driver speakers are particularly prone to intermodulation. Two-way speakers (one tweeter & one woofer) are an improvement. To further reduce IMD, a three-way configuration makes greater strides towards avoiding the problem. This requires a crossover, a series of electrical circuits that splits the spectrum into narrower bandwidths to feed the tweeter only high frequencies, the midrange only middle frequencies, and the woofer only low frequencies. Problems solved, right? Not yet. You still have the IMD from the amplifier. (Not to mention the mountains of other issues created by a passive crossover reprocessing the amplified signal.) There’s one more easy step we can take to bowl over intermodulation—split the bandwidth before amplification, when it’s easier to do, avoids the high level crossover issues, and send each limited band to individual amps. Now the problem is tamed not once, but twice.
Why is this distortion another one of those fahgeddaboutit problems? It’s not for lack of consequence to the sound quality. No, together these factors separate the good from the outstanding. Rather, it’s for convenience. These issues get complicated. And that’s not the only reason they’re not talked about. The number one reason is that manufacturers think you’re too stupid to understand, and what’s more, if you did understand you’d cross their speakers off your short list in a heartbeat.
Well, you’re not so stupid, and now you do understand. The Parallel Audio Project uses an active crossover and three amps, one each for the woofer, the midrange, and the tweeter. Intermodulation is greatly reduced allowing you to hear clean music.
Next topic in part 3 : resonance.