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April 21, 2020

Charts, Graphs, and Tools

What are those weird charts? How do I read them? What do they mean? Is this really necessary? People have been asking, so here are the answers.

I take audio testing very seriously, and fortunately, PC-based tools can make the testing economical. I've had to build some specialized equipment, however, to analyze the sound coming from the earbuds and to properly test the audio performance of digital audio players under load.

This chart is a spectrogram, and it shows the audio spectrum over time on the horizontal axis against frequency on the vertical axis. The intensity of the sound is plotted as a color, and the legend is on the right, with red being the loudest and blue being the quietest. Here's a larger view of a different player under test:

The larger chart shows extensive harmonic distortion. The slanted bars above the main bar are harmonics of the fundamental frequency. There are at least ten of them, so that's a lot of distortion. It's also pretty strong. By matching up the predominant color in the harmonic bars to the key at the right, you can see that the first harmonic is 10 to 15dB below the test tone, which means that it's plainly audible. Once the harmonics are into the blue range, they're very hard to hear. They still color the music somewhat, and they also rob power from the fundamental frequency.

The blue vertical bars in the lower right corner are faint noise bursts, not harmoncially related to the test tone, but a symptom of some other problem with the player. they sound like a faint, soft-edged popping noise.

This next chart is the frequency response, plotted in 1/3 octave increments:

This plot can either be stereo or mono. Here it's just one channel. The vast majority of players have absolutely identical left/right performance. You can see the roll-off in the low frequencies, which in this case starts at 100Hz. That's in the midbass region, and there's a 10dB drop down to 20Hz. There isn't much music below 40Hz (the low E on an electric bass or upright bass), but organ pedals, 5-string basses, and some bass drums have fundamentals down there.

You often see this kind of plot in audiophile magazines centered, with plus and minus dB regions, greatly magnified. I don't think those little half-dB increments are very important to digital audio player performance.

This plot is made by recording the peaks of bars as the test tone sweeps through the audible range. It cannot show noise or harmonic distortion.

In another spectral view, the peaks are recorded continually, not in 1/3 octave increments. I sometimes do a screen capture while running the sweep tone, as in this case:

The main wave moves across the screen, creating the peak trace at the top. But it's also pushing waves of higher harmonics in front of it. In this particular case, the left and right channels have diverged. (They were convergent at lower and higher frequencies.) The multicolor spectrogram generally does a better job of recording these anomalies, except that it can't display the left/right divergence.

Here's a graph you won't likely see anywhere else. I created this test to explore the low-frequency performance of a number of players, prompted by the essentially perfect bass performance of the iPod shuffle. It's a 40Hz square wave, instead of the usual sine wave, which is much harder for the player to reproduce accurately. Music is full of harmonic content, and a square wave contains an infinite number of harmonics. Adding harmonics that aren't in the source is a form of distortion, and removing harmonics that should be there is another.

In theory, looking at low frequency square wave performance is not much different than the "droop" at the low end of the spectrum that you see in the octave and spectral views. In practice, it tells me about the stability of the player's power supply, its bass performance, and the characteristics of the output section. The player above has no earphone load, but the square wave is still slightly flawed.

This is the same player, with the provided earbuds plugged in. The load of the earbuds prevents the player from sustaining the square wave over time. If the wave sinks all the way down to zero, the bass performance will likely be noticeably lifeless, either lacking in lows or unable to "punch" the bass frequencies convincingly.

Posted by machrone at April 21, 2020 08:46 PM