kudzurunner
5149 posts
Nov 19, 2014
12:41 PM
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I wasn't familiar with the term, although I'm familiar with the general idea from Greg's other postings here. So I thought that we might use this thread to crowdsource definitions, explanations, and examples of difference tones.
Here's a video to start it off. I would appreciate Greg telling us what's going on here.
Here are a few links:
http://www.sfu.ca/sonic-studio/handbook/Difference_Tone.html
http://www.patmissin.com/ffaq/q26.html
http://johnranck.net/studio/clinic/practice_corner/difference_tones.html
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STME58
1150 posts
Nov 19, 2014
1:59 PM
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I haven't had a chance to look at the video and I can't access MBH from home so I may not get to, But I saw difference tones demonstrated at one of my sons youth symphony rehearsals. Half of the group played the root and the other half the fifth and when they were got in tune, you could hear the third, even though no one was playing it.
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dougharps
776 posts
Nov 19, 2014
2:13 PM
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Difference tones are especially evident in the bent 3-4 draw like in Hush Hush. Pat Missin wrote about it and gave examples here:
Pat Missin on Difference Tones
I switched back to compromise tuned harps after trying this with equal, just, and compromise tuned harps. The just was best for this, but sounds flat at times, so I compromised...
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Doug S.
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harpwrench
935 posts
Nov 19, 2014
3:41 PM
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The root and fifth played together doesn't give you the third, it's the root note in a lower octave.
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www.spiersharmonicas.com
High performance harmonicas.
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hvyj
2587 posts
Nov 19, 2014
3:43 PM
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In my experience, the most accessable example of difference tones or ghost notes is the ghost note that is produced when one plays octave stops. For example, playing 1 draw and 4 draw cleanly and simultaneosly as a "split" will produce a third note regadless of the temperament of the harp being played, assuming the octave intervals are in tune. This is why playing octave stops makes such a "big" sound. FWIW.
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STME58
1151 posts
Nov 19, 2014
6:41 PM
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The difference tone of a root and a fifth is the root in the lower octave. There is also a third in the next octave up that is a summation tone. This is in the example Doug's post point to. I find it interesting the the math is so simple, add the frequencies and subtract them to get the "ghost notes".
Using that in the octave the difference note is the same as the low note and the summation note is the fifth above the octave. I would agree with hvyj that this is part of why in tune octaves sound so "big".
A similar effect I like that occurs on a well tuned low harp is the sympathetic vibration of the reed an octave lower that the one being played. This effect can also occur on a guitar. It fact it is a common way to tune a guitar.
Last Edited by STME58 on Nov 19, 2014 6:42 PM
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Milsson
176 posts
Nov 19, 2014
9:37 PM
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Tartini tones!
I bet kudzu will like this video.
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Greg Heumann
2879 posts
Nov 20, 2014
8:24 AM
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I'll do my best to explain the phenomenon as I understand it:
Most of us have heard "beat notes" when two notes that are slightly out of tune with each other sound. The two sine waves slide in and out of phase with each other very slowly - a few times a second, for example. When they're perfectly in phase they are "summed" and the note we hear is louder. When they are perfectly out of phase, they cancel each other out and we hear the "difference" - the note we hear is quieter. The "beat note" you hear is this cyclical change in the sum amplitude of both waves, a few "beats" per second.
Difference tones are exactly the same - it is just that when the notes are further apart, the sine waves slide in and out of phase much more quickly - a typical example might be 45 times a second. Now the frequency has gotten high enough that instead of hearing the change as a single note getting louder and softer, we actually hear a new note at 45Hz.
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
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Last Edited by Greg Heumann on Nov 20, 2014 8:28 AM
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Milsson
177 posts
Nov 20, 2014
7:18 PM
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You´ll have to take some time to scratch the surface of fourier analysis to understand how complicated waves/sound are the sum of sevral sine waves. this video is OT to the thread but very on topic at the same time.
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dougharps
778 posts
Nov 20, 2014
8:24 PM
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Isn't the main question for us about how to access and use the difference tones in our harmonica technique? Once we have heard them while playing two or more notes, or chords, then we have to decide how to use them.
If you play chords on a harp tuned to just intonation the difference tones are quite apparent and the chords sound sweet and full. Compromise intonation, a little less strong, and equal intonation less. Tube amps with good low frequency reproduction really pick up and emphasize the difference tones.
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Doug S.
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colman
323 posts
Nov 23, 2014
9:30 AM
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guitarists use overtones a lot ,they project the overtone by slightly dampening the base note to let the overtone sing.i`m sure with control any instrument can project the overtones.also the overtone series naturally creates the blues scale ,starting with the flat 7th.,next octive flat 5th,and next flat 3rd... a jazz theory teacher taught us this...
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The Iceman
2252 posts
Nov 23, 2014
11:01 AM
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If difference tones are created by the interaction of overtones from two notes, has anyone scoped out a vibrating reed to see if these overtones exist in a strong sense?
They do on stringed instruments, as a vibrating string will subdivide along its length ans create smaller independent vibrational sections coinciding with the overtone series, but a reed is a fixed at one end small strip of vibrating metal.
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The Iceman
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Greg Heumann
2882 posts
Nov 23, 2014
7:53 PM
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@Iceman - Difference tones are lower in amplitude than the primary tones that make them. The ones we STARTED talking about above come from the fundamental frequencies of two reeds. Once you start talking about the production of difference tones from those reeds' own overtones (aka harmonics) - they're going to be really hard to hear. They MIGHT color the sound, but I don't think you could really hear them. The bass note that is manufactured from a 2-3 draw, however - once amplified with a good amp - is very audible.
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
Bluestate on iTunes
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STME58
1153 posts
Nov 24, 2014
6:57 PM
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Iceman, I think what you are talking about are vibration modes. For a string, the first mode is the whole string moving with the part making the largest excursion in the center. Another mode, which you can excite by touching the string lightly exactly mid span as it is plucked, has both sides of the string moving in opposite directions and a node, or stationary point in the center. This sounds one octave above the first mode. There are torsional modes also. I read on the UNSW website that an experienced bowed string player matches the torsional vibration of the string to the transverse vibration.
Reed modes are a bit more complex but calculable. I posted some calculated mode shapes and frequencies in this thread. These are not as easy to observe as the transverse modes of a guitar string. They also occur at higher frequencies and may not be easy to excite. I have wondered if this is what causes some reed squeal. If I ever capture one on camera I will post it.
Difference tones occur no matter how the sound is produced and are independent of overtones. You would be able to hear them even with pure sine tones produced by a synthesizer. Greg brings up an interesting point about the possibility of difference tones of the overtones. I agree with him that you would be unlikely to perceive these.
Last Edited by STME58 on Nov 24, 2014 7:09 PM
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The Iceman
2254 posts
Nov 25, 2014
5:22 AM
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As this discussion split into two directions, I was attempting to pull it back...
Am very familiar with a vibrating string and all it's wonders and mysteries, as I was a Piano Tuner/Tech for 20 years before changing direction as well as acoustic science cadet with early synthesizers in college.
Have always been curious about a vibrating reed and its overtones, as conceptually I can't visualize there being much going on here.
Yes, it's understood that difference tones are not dependent on overtones.
I like discussions like this...tickles the brain.
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The Iceman
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STME58
1154 posts
Nov 25, 2014
8:33 PM
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I have been thinking along these lines and wonder what others have discovered;
How much of a harmonicas sound is due to the motion of the reed directly causing a sound wave (like a string or drum membrane) and how much is due to the reed acting like a valve (like a siren) and sending pulses into the air.
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the_happy_honker
195 posts
Nov 27, 2014
9:56 AM
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My money is on the siren model. A tuning fork makes very little sound in free air. But when the energy of the vibrating mass is transferred to a resonator, it can be quite audible.
The same is true of a vibrating reed. A plucked reed in free air is barely audible. But rather than being coupled to a resonator, the reed draws energy (sustains its vibration) from air rushing through a slot. The air is chopped into pieces in the process and this is quite audible.
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