I’ve noticed, most recently when playing the guitar but also on other instruments, that when you play two notes at (almost) exactly the same pitch, you can hear, faintly, a note that is one octave higher. Eg, pluck two strings that both play C4, and you hear (a harmonic?) C5. This is very handy for telling when two instruments are in tune; we used to use it to tune brass instruments in a band I was in.
Does anyone know the explanation for this phenomenon, or know what it’s called so I can search for more info?
I have played the guitar for 40 years and have never noticed an octave playing two notes in unison that wasn’t there playing a single note.
When you play two notes exactly in unison the harmonic series of the two notes will be slightly different, since you are playing on two different strings with different tone colors, or two different instruments. But that does not create a new octave harmonic since the octave was present in each note to start with.
When you play two notes that are not exactly in unison but still close, it creates beats. A beat is a tone that sounds like the average of the frequencies of the two notes and pulses at a rate equal to the difference of the two frequencies. Listending for beats is how you tune an instrument against a reference tone (which might be another instrument). This is most likely what you experienced in tuning brass instruments. It is possible that you are in some way hearing the beats as an octave.
BTW playing any note on any instrument will generate a harmonic series–the tonic note that you are playing, then a tone one octave higher, then maybe the fifth (I forget exactly what order they are in), eventually the next octave, and so forth. (Even if you play a pure sine wave tone, the components in your ear will generate harmonics to some degree.) It is the relative strengths of these harmonics that define the tone color of an instrument. This is what I meant by saying that the octave tone was there all along.
CookingWithGas, I know what you mean about beats, they’re useful for tuning guitars (and especially pianos). The sound I hear is steady. The reason I asked is that our conducter (a few years ago, now) told us about this octave thing, so I always kind of assumed it was a recognised phenomenon. Maybe I’m just hearing with the ear of faith or it could be something to do with my specific guitar’s soundbox. I’ll check tonight and see if it’s restricted to certain notes or strings.
I had always assumed that what I was hearing was the natural octave harmonic that’s there all the time, but somehow became easier to hear when two notes were played in unison - maybe the primary frequencies interfered destructively to a greater degree than the harmonics, or something. As you can tell from the above, I don’t really know what I’m talking about, so I hope some acousticollogist dopers can help me out!
When you play an octave “harmonic” on a guitar (touch the string very lightly at the twelfth fret and pluck the string), you are suppressing the fundamental tone and allowing the octave to come to the fore, but the octave was always there . It was simply overshadowed by the fundamental.
Well, you learn something new every day. I’m still skeptical because I have never heard of it, never experienced it, and can think of no physical explanation for it. I can think of no way that two tones together could consistently reduce the fundamental leaving a stronger octave than either tone separately.
However, suppose you are positioned so that two tones reach you exactly 180[sup]o[/sup] out of phase with respect to the fundamental. This causes the fundemental to be completely cancelled. When the fundamental is 180[sup]o[/sup] out of phase, the octave will be reinforced–doubled, in fact. But this is not going to occur over and over again any time you play two notes in unison. The wavelength of A 440 is about 2.64 feet, so you couldn’t experience this with two strings on the same guitar. (I am talking about out of phase acoustically; electrical signals out of phase are a little different phenomenon that I don’t understand very well, like if you reverse the wires on one speaker on a stereo.)
(I spent one semester in high school in a special physics project on the physics of music, including hooking my axe up to an oscilloscope, giving demos to the class, etc. I also took a course at U. Mich. in Physics of Music. However, that was a very long time ago and a professional acousticist/acoustical engineer I ain’t.)
or it could be something to do with my specific guitar’s soundbox. I’ll check tonight and see if it’s restricted to certain notes or strings.