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On a standard tuned guitar, if you play the 12th fret on the top E string and the 16th fret of the B string at the same time, you get a rather impressive warbling sound. It seems similar to if you "gargle" two harmonics with a tremolo bar.

The two notes are E and Eb. I looked into the frequencies, and it's 659hz and 622hz respectively. I found a particular site that let me generate audio from given frequencies, punched them in, and sure enough, the same warble.

So I'm curious why it does this, and how I might learn to cause it in other places on the guitar naturally.

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Possibly the reason this stands out at more with those particular notes is because of the resonance of the open guitar strings. The 12th fret E is a harmonic of both the bottom E and A strings. If the guitar is not "perfectly" tuned, the harmonics of those strings may also beat with each other and add to the "warbling" effect. – alephzero Mar 13 at 4:58
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Try any two notes that are one semitone apart. – Matthew Read Mar 13 at 5:42
    
this answer to a different question goes into detail about how the sound waves combine. This is not a duplicate, but the information in that answer would be an interesting read on a very similar topic. – Snowman Mar 14 at 5:19
up vote 5 down vote accepted

The "warble" is a beat frequency. Any two notes that are only a half step apart will produce a similar beat frequency. This particular one might be more noticeable on your guitar because of intonation and/or differences in timbre that highlight that particular beat frequency.

Actually any two notes played at the same time produce some sort of beat frequency, but it's hard to notice unless the notes are fairly close together. When tuning instruments or voice by ear, one listens for beat frequencies and tries to eliminate them, since two tones that are exactly the same pitch will have essentially no beat frequency.

Notes that are very close together (almost in tune) have such a slow beat frequency that it's hard to hear. Notes that are far enough apart (more than a half step or whole step depending on other factors) can have a beat frequency so fast that it becomes another audible pitch. Those beat frequencies (usually called combinations tones when they become separate pitches) are a part of what makes chords sound the way they do.

If you have an electric guitar, you can usually create many beat frequencies very easily. Dial in a little distortion, if you can, and then play some unison bends. As you bend the unison bend up and down a little, you'll get faster or slower beat frequencies. With enough finger strength and/or fairly light strings, you can do unison bends on the B and high E strings on acoustic guitar as well.

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