About quartz sound bowls
What a quartz singing bowl is
A quartz singing bowl is a vessel of almost pure silica, spun at a temperature high enough to fuse it, and tuned to hold one note for a very long time. Everything people notice about the sound follows from the material.
What they are made of
Crushed natural quartz, typically around 99.9 percent silica. The powder is loaded into a mold and spun while being heated past its fusing point, somewhere near 4,000 degrees Fahrenheit, so the grains melt into one another and the wall forms outward under rotation. What comes out is a single continuous piece of fused quartz rather than something assembled or cast in parts.
The familiar white bowls are called frosted. The surface keeps a fine matte texture from the process, which is what makes them opaque. Clear bowls are made differently and at greater cost, and are usually smaller and higher pitched for the same note.
Why the tone lasts
Fused quartz is unusually uniform and has very little internal damping, so a struck bowl loses its energy slowly. That is the whole of it. The tone does not fade the way a struck metal bowl does; it thins and goes on, and the last of it is often still there when the next bowl is struck.
It is also why the sound is felt as much as heard. A long, steady tone at a low frequency moves air for long enough that the body registers it as pressure rather than only as pitch.
What sets the note
Mostly diameter. A wider bowl has a longer path around its rim and a lower fundamental, which is why the seven bowls in a chakra set run from about 14 inches at the bottom of the scale to about 4 at the top. Wall thickness and height adjust it from there, and the maker grinds to the final pitch.
Bowls are usually named by note rather than by frequency, which hides a real difference: the same note can be tuned to more than one standard. Ours are a set, tuned against each other, which matters more in a room than the standard any one of them is measured against.
A bowl is not a pure tone. It has a fundamental and a set of partials above it that are not whole-number multiples of it, and those partials decay at their own rates. That is the shimmer you hear as a bowl fades, and it is why two bowls of the same note from different makers do not sound the same.
Harmonics, beating, and what gets called an undertone
Pluck a string and it divides itself into halves, thirds and quarters at the same time. Those overtones land on whole-number multiples of the lowest tone, which is the harmonic series, and it is why a string sounds like one note rather than a handful.
A bowl does not divide that way. It is a shell, not a line, and its modes are counted by nodal diameters and nodal circles across a curved surface. Those frequencies are not whole-number multiples of anything. They are inharmonic, and they do not fuse into a single pitch as obediently as a string's do: you hear the fundamental and you also hear tones that stay audibly separate from it. That is the shimmer, and it is the reason two bowls of the same note by different makers do not sound alike.
The pulse in a single bowl. No bowl is perfectly round. On a perfect circle a given mode would have one frequency; on a real bowl it splits into two, a fraction of a hertz apart, and which of the two is louder depends on where you strike it. The two run against each other and their sum rises and falls at the difference between them. That slow throb in a struck bowl is not vibrato and nobody is applying it. It is the bowl being slightly out of round, made audible.
Beating between bowls. Two bowls close in pitch do the same thing at a larger scale, pulsing at the difference of their frequencies. It is why intervals matter in a set: tune two bowls a few hertz apart and the room throbs, tune them a fifth apart and it does not.
Undertones. Strictly, an undertone is a tone below the fundamental, and a bowl does not produce one; nothing in the metal or the quartz is vibrating that slowly. What people hear low down is usually a difference tone, which is generated by the ear rather than by the air: sound two loud tones together and your hearing adds a third at the difference between them.
The set here makes that easy to hear. The root is C at 256 hertz and the heart is G at 384. The difference is 128, which is exactly an octave below the root. Strike those two together and a floor appears under them that neither bowl is playing and no microphone in the room would record.
How they are played
Two ways. Struck, with a padded mallet, which gives a clear beginning and a long decay. Or played, by running a mallet steadily around the rim, which feeds energy in as fast as the bowl loses it and holds the tone indefinitely. The second takes a light hand: too much pressure and the bowl chatters instead of singing.
A bowl will also respond to another bowl near its own pitch without being touched at all, which is the plainest demonstration of what the room is doing.
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