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Non-linear distortion is when your audio equipment (speakers, amps, headunit) plays signals that are not a part of the original recording. It is one of the defining benchmarks or specifications in audio because it generally can't be corrected or removed. It also has a direct and noticeable affect on sound quality, where equipment (especially speakers) with high distortion often sound muddy, colored, or harsh.

Let's start with a simple distortion graph. This is a plot of a high quality, TC Sounds tc2+ 12" subwoofer.



The red line is the original frequency response. Look to the bar on the left and find the column THD. That stands for "total harmonic distortion". A harmonic is a multiple of the original frequency. For example, the second harmonic of an 80hz tone is 160hz. The third harmonic is 240hz, and so on.

Now look at the green line underneath the red one. That is the second harmonic distortion. It's distortion because it's not a part of the original signal. Look to the column on the left under THD, and that will tell you how loud that second order distortion is as a percentage of the original signal. As you can see, second harmonic distortion is fairly low for this sub at less than 7%. If you want to be more detailed about it, look at the green line at 50hz. It's exactly at 7% thd. That means that while playing a 50hz tone, the sub will also generate "second harmonic" distortion in the form of a 100hz tone, that's 7% as "strong" as the original 50hz signal.

Now lets take a look at the blue line. That's third harmonic distortion. It's much lower than the second harmonic distortion. That's generally a sign of a well built driver. Take a look at 50hz again. It's roughly at 6.5% thd. That means that while playing a 50hz tone, the sub is generating third harmonic distortion at 150hz that's 6.5% as strong as the original signal.

Next, let's look at a different kind of distortion plot. This plot is a comparison of the TC Sounds tc2+ 8" and an Adire Audio Koda 8" taken at 40hz.



Ok, now you're probably thinking what the heck is this? Well, look at 40hz. That is your original test signal. This is just a simple frequency response plot. For a driver with zero distortion, and playing a 40hz tone all you would expect to see is 40hz right? But you can see there's 2 large spikes at 80hz, 120hz, and a smaller one at 160hz. These correspond directly to the second, third, and fourth harmonic. Those spikes are harmonic distortion. .. basically tones that are being generated by the speaker that weren't in the original signal.

Comparing the two drivers, you can see that the second order harmonic distortion at 80hz is roughly similar. But, notice at the third (120hz), fourth(160hz), and higher harmonics the tc2+ clearly outclasses the Koda. By how much? That's a little trickier. Look at the vertical scale on the left. The original signal is at 80db. The second harmonic at 80hz is roughly 65db. That's a difference of -15 db. Use the following table to convert db to percent distortion.

-50db = 0.3%
-40db = 1%
-30db = 3%
-20db = 10%
-10db = 30%

Or, you can use this handy calculator:

http://www.sengpielaudio.com/calculator-thd.htm

So now you're probably wondering, well that's great but how do I know what distortion level is good or bad?

Well, that's a pretty complicated subject but I'll just shoot off some very simple guidelines.

- Even order (2,4,6, etc.) distortion is more pleasing to the ear than odd (3,5,7, etc.) order harmonic distortion

- Lower order harmonic distortion is less noticeable than higher order (second and third) versus (fourth and fifth).

- The audibility of distortion depends on the frequency. At lower frequencies, we can tolerate alot more distortion because the harmonics are so much closer to the fundamental (original) frequency that our brain has a harder time distinguishing them. For example, at 20hz the second harmonic is 40hz, only 20hz away. At 2khz, the second harmonic is 4khz, which is 2000hz away. We can pick up distortion that is 2000hz away from the original signal as being more distinct than we can something that is only 20hz away.

-Distortion past 10khz is difficult to hear because the second harmonic at 10khz is 20khz... a frequency most humans can barely hear.

-Intermodulation distortion is when 2 or more tones are being played, and the distortion occurs not only at the harmonics but at other frequencies as well. For example a 100hz and 1khz tone are played simultaneously. We could expect to find intermodulation distortion at all linear combinations of those frequencies, such as 900hz (1khz - 100hz), 1100hz (1khz + 100hz), etc.

This type of distortion is far more subjectively "nasty" than simple harmonic distortion. When evaluating speakers, I always recommend testing for intermodulation distortion as well as simple harmonic distortion.
 
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