Do measurements matter?

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No, I wasn't thinking of starting a riot or fueling controversy with that headline. My goal is simple enough. Let's start looking at what we know matters and what we suspect does not. For example, does THD matter? The answer is not a simple one. Books could be written on the subject. But, THD is a good starting point. Total Harmonic Distortion, THD, means there's more of something than we started with. THD refers to the addition of extra tones (harmonics) not originally found in the music. Those extra harmonics are called distortion. If we put a single pure tone into an amplifier, we expect a larger—but exact—copy at the output. That's not what we get. In fact, we get more. Added harmonics. Here's a picture of what I refer to. THD "Fundamental Frequency" is our starting pure tone. All the others (H1 through H5) are added harmonics. Distortion. That which was not present before amplification. Add together H1 through H5 and you get a total. (Get it? THD. Total Harmonic Distortion). The lower the THD the fewer number of added harmonics. But before we vilify harmonics let's remember they are also naturally occurring. In fact, naturally occurring harmonics are the tones that give musical instruments their voice. Harmonics distinguish a trumpet from a clarinet, a voice from a sinewave. Here's an image of naturally occurring harmonics. Harmonics 2 Notice in this image of a guitar B-string the second harmonic is louder than the first (fundamental). That particular mix of fundamental and harmonics gives the guitar string its distinctive signature. The challenge of an amplifier is to reduce added harmonics while preserving naturally occurring ones. Today's takeaway: added harmonics are unwelcome while naturally occurring harmonics give character to sound.
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Paul McGowan

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