All steel tongue drums have a different degree of sound purity. Having studied these instruments for seven years, I have not yet come across a steel tongue drum with a perfectly clear sound. The exceptions are the RAV vast, Orion, and Kaa-tone drums, but they aren't actually steel tongue drums but representatives of the senior tonguepan (tongue drum) class.
Why is the sound different in purity? Because it's all about overtones. Any sound in nature consists of a basic tone and many of its overtones.
Physics and mathematics deal with sound purity. The frequencies of the main tone and its overtones should perfectly correlate with each other, consist in the correct proportions of 1:2:3:4: 5, etc. That is when their sound waves will beautifully and harmoniously merge into a single mellow tone, complementing and amplifying each other. Otherwise, the sound waves will conflict, extinguish each other, and create a dissonant mood.
The sound saturation (or mellowness) is formed by the number of overtones, and the tone quality (its structure) is formed by the balance in the sound volume ratio of all overtones.
Now, let's go back to steel tongue drums.
In steel tongue drums, the following things contribute to the sound purity:
1) Tongues: they are tuned by notes;
2) Steel tongue drum body (the remaining area between the tongues): it also produces its side tones.
[🛸] First, let's deal with tongues.
As a rule, only one note on each tongue is tuned on most steel tongue drums — it's the so-called "Main Tone". In this case, overtones are not tuned, but this does not mean that they do not exist, and they do not sound.
In this case, overtones are defined by the tongue's shape, its position on the drum body, its thickness and the metal alloy.
Let me explain: when it comes to steel tongue drums, their overtones mean a high metallic sound. It can be clearly heard if you hit the instrument with something hard (e.g. a wooden stick, a ballpoint pen, or a knuckle). If you hit a tongue with a soft ball, you will also hear that sound, but it will be less pronounced compared to the main tongue tone. Nevertheless, it is present, and it always sounds like a background sound.
The next question is what it sounds like.
It sometimes happens that it sounds "to the point". It can occur for two reasons:
1) The overtone frequency has got into the desired note by coincidence (according to the tongue's shape, size, metal thickness, position on the drum body). This is a rare thing.
2) Although the frequency hasn't got into the right clean note, it has given its own peculiar side tone. Even if this sound is dissonant, it can sometimes be perceived as [relatively] sounding "to the point", as in such a way it forms a certain instrument's mood. As a rule, this is a gloomy and mystical mood.
Another thing is when overtones are tuned up in pure proportions. Then, instead of a "dirty metal side tone", you can hear a crystal sound similar to high-pitched bells. Such a sound type will be a beautiful addition and background to any instrument mood: mystical, oriental, or light and cheerful. Moreover, you then get new ways of playing and phonation.
As for the steel tongue drum body, I'll be concise in its description. Each part of the drum body, which is the area between the tongues, also has its own sound, and usually sounds "out-of-place", creating a dirty background tone. Very few people are currently working on resolving this issue. The main way that is common now is to mute the body sound with various "dampers". Therefore, the sound of such side tones gets quieter.
[✅] That's all for now.
Let me know if you are interested in this topic in the comments (put "+", or just tell me what you'd like to learn about in detail), and I will come up with a video review, where I will share my experience in much more detail and give examples.
[🔔] As a result, I'm attaching a video featuring the steel tongue drum sound, which was made on the basis of my observations and the knowledge I collected. In my opinion, this is still the best drum that we have managed to make. However, I'm not saying it's perfect. We are working on the sound and will try to make even better instruments.
Model: KOSMOSKY DVUTONE, 14" inch, B-minor 11a.