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Disclaimer: I don't now much about music theory.

Anyway, my wife just bought one of those, which I think is called a "tongue drum":

enter image description here

What strikes me is that, as bad as I am at playing music, whatever I randomly do on it, however I play it, it seems to make harmonious music. Or at least, much more harmonious than what I could randomly do on a piano or a guitar. My son, who is 5, plays with it too, and it just works. It seems this thing prevents anyone from playing dissonant music.

I played some other versions of this instument, which I'm pretty sure had a different number of notes, and probably organized differently, and the result was similar.

So, is there a particular method of selecting/organizing notes on an instrument which will make any sequence played with it sound harmonious? Is there a theory behind this?


Edit: So I pushed the investigations further, as the notes still seemed a bit alien, compared to what we usually find on classical instruments.

I used some phone app that measures the frequency of what it gets from the phone mic (typically made for tuning instruments) to get a map of what we had, and here are the results (the raw frequency figures are certainly a bit inaccurate, but I'm pretty sure the resulting notes are correct):

enter image description here

So it seems based on the regular major diatonic scale, but not arranged as the typical CDEFGAB: it has missing notes, a few additional bass and treble notes chosen further away from the main group, and the notes are all over the place.

I don't know what to conclude from this. But it really feels like this organization makes melodies easier on the ear than what you could randomly do with 9 consecutive white keys on a piano. But maybe the clear sound of the instrument itself is also playing a role, here. I don't know.

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    I'd guess the note selection is either diatonic (e.g. only the white keys on a piano) or pentatonic (e.g. only the black keys on a piano). – Your Uncle Bob Sep 9 '19 at 12:28
  • @YourUncleBob I'm pretty sure I can manage to make some rather ugly melodies only with the white keys on the piano. It doesn't seem to be the case with this instrument. – dim Sep 9 '19 at 12:57
  • @ dim please provide a link to a place which sells that, or better yet, to a "user manual" . Having never seen this, and your picture being not-very-clear, we can't comment on what notes it produces. – Carl Witthoft Sep 9 '19 at 14:55
  • @Carl Here is a wikipedia page: en.m.wikipedia.org/wiki/Steel_tongue_drum. And here is a random shop (there are lots of them, but those are mainly handmade/artisanal products, so there is typically no user manual/datasheet): metalsounds-shop.com/en/7-steel-tongue-drum – dim Sep 9 '19 at 19:33
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    Well, Wikipedia says "The steel tongue drum is often tuned to pentatonic scales but can be tuned to the diatonic scale, the chromatic scale, or any set of notes the maker chooses." If even a 5-yo can't make it sound bad, my money's on pentatonic. – Your Uncle Bob Sep 9 '19 at 19:43
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Like the commenters, my money is on your tongue drum being tuned to the pentatonic scale. To answer your very final question, yes - there is theory behind why the pentatonic scale sounds so "universally good" and why it is generally 'easy' to utilize and play against melodically and harmonically...

A major pentatonic scale has an absence of tritone due to the missing 4th and 7th intervals (it is missing the fa and ti within the do-re-mi-fa-so-la-ti-do.) This also allows for a distinct absence of tension, or "pull," from one note to the next within the scale itself...

This is not to say that a pentatonic scale cannot be dissonant, however. If you smash all of the notes of the pentatonic scale at once on a keyboard, for example, it does not sound "good." There is dissonance... But, there is so little dissonance that it doesn't sound "terrible" either...

According to the wiki:

Pentatonic scales were developed independently by many ancient civilizations—an indication that pentatonic scales are based upon a naturally occurring phenomenon.

So, the other part of this question rests on the way one plays the specific instrument itself. Unless you or your son bash the thing against a wall, it's generally always going to "sound good." As for your first question, it would be way more easily answered if you provided the exact brand that your wife purchased, or the exact random shop she bought it from... But, it's probably pentatonic.

EDIT

So, it looks like your tongue drum is tuned to a C major scale, with multiple octaves for the tonic (C) and the 5th (G). There are plenty of explanations of the special relationship between the tonic and the 5th so, I won't go into that here. But, there are other explanations as to why you 'can't fail' when you play this thing. Firstly, 7 out of the 9 notes will 'always sound good' together - again, take away the F4 and the B3 and you have the pentatonic scale...

But, for the most part, I'd say the can't-fail-phenomenon rests in the way this instrument is played, as I said before. One creates the sound by striking the tongues with two hands - it creates a kind of melody and rhythm simultaneously. The only harmony that something like produces is in the resonance of the notes as they sustain until the tongues are muted or struck again. If you and your son play the thing together and bang only the B, C, and D all at once, it's the closest you'll likely get to having it sound 'bad.'

Finally, the can't-fail-phenomenon is based on the fact that you are playing the tongue-drum by itself, with no other voices... I suspect if you played along with other instruments playing in Bb minor, for example, it would likely not sound so great.

If any instrument can keep you relatively within one octave, while adding only octaves for the tonic and the 5th, then it will always sound good. But, it's not going to be very versatile as far as playing in different key signatures...

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    For your information, following your answer, I made further experimentation to find the actual notes. There is some pentatonic there, indeed, but still with very unusual things. You can check my edit. – dim Nov 3 '19 at 20:59
  • Actually, I was confused, it's not the pentatonic scale, I meant the regular heptatonic scale. Or is it called diatonic? Somethingtonic, anyway... – dim Nov 4 '19 at 10:52
  • Hmmm... I just read your edit. I'll edit my response a bit. as well – Tim Burnett - Bassist Nov 4 '19 at 11:33

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