The long, tall spike in each of these spectrograms is the single frequency sent to the speaker driver. It’s the only frequency it should be producing. As can be seen, drivers aren’t perfect. They all produce some harmonic distortion, i.e., frequencies which are multiples of the fundamental. When you look at the spectrograms, you’ll see other shorter lines popping up. Those lines are the harmonics. The more of them and the taller they are, the more harmonic distortion. Second harmonic (the first multiple to the right) is relatively benign even at 40 dB below the fundamental (1%). However, as the harmonics get higher, they are easier to hear, and therefore, the lower their level needs to be, preferably 60 dB or more below the signal, to be inaudible. Both of these tweeters are excellent. Both sound clean. Carefully comparing the two shows the $51 27TDFC consistently has less 3rd, 4th & 5th harmonic distortion than it’s very expensive big brother. Makes you wonder.
Makes me glad I did my homework before choosing a tweeter for the Parallel Audio Project. I could have fallen for the Millennium driver. It has an outstanding reputation and it’s beautiful to look at. I could have fallen for a number of other highly regarded, pricey, top-of-the-line tweeters with good reputations and good looks. But I took my time, because at the time I didn’t have any test equipment except for an SPL meter (an uncalibrated one at that), nor the desire to buy and test numerous drivers. After scrutinizing the linear response, distortion and cumulative spectral decay of dozens of drivers, I was surprised to find that there are a few moderately priced drivers of all types, not just tweeters, which perform as well or better than specialty, exotic, or stratospherically priced models. And now that I’ve tested these two for myself, it confirms what I had found, and I’m still surprised. Facts trump marketing. Go tweet that.
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