We are told that singing bowls have been used for centuries in different cultures and religions for meditation, healing, and relaxation purposes. Commonly these bowls, also known as Tibetan singing bowls or Himalayan bowls, are believed to have originated in the Himalayan region over 2,000 years ago. The bowls are usually made of metal, and their unique sounds are generated by striking or rubbing the bowl’s rim. But how exactly does a singing bowl make sound? In this article, we will explore the science behind singing bowls and the factors that influence their sound.
As mentioned earlier, the material of construction is one of the most important factors in determining a singing bowl’s sound. The bowl must be made from a material that will remain resonant, providing a structure that can be activated by either the friction of rim singing or an excitation through gong-style striking of the bowl’s rim. Materials such as brass, aluminum, bronze, steel, titanium, nickel, and quartz are all great examples of resonant materials. Copper, on the other hand, is too soft for this application and will not produce the desired sound.
The design of the bowl also plays a significant role in the sound it produces. The thickness of the bowl and the shape of its base, walls, and rim are all factors that affect the end sound of a hand-hammered bowl. The shape of the bowl, for example, determines its tone. A bowl with a rim that curves inward will have a higher tone than a bowl whose walls gently slope outward. Additionally, each bowl is designed to have a unique set of variables when it comes to the way the instrument vibrates. When you take a top-down view of the vibrating bowl, you will notice the vibration on a pair of axes. The bowl appears to flex inward at 12 and 6 o’clock while flexing outward at 3 and 9 o’clock. It is these regions where we notice the changing shapes that are the fundamental or principle modes of vibration. The combination of material, thickness, and shape, with the additional variables imparted by the hammer marks, all conspire to make the one-of-a-kind sound from a hand-hammered bowl.
The sound of a singing bowl is generated by the vibration of the bowl’s material. When the rim of the bowl is struck or rubbed, it begins to vibrate, creating a sound wave that travels through the air. The frequency of this sound wave is determined by the bowl’s shape, size, and material. As the bowl vibrates, it creates standing waves that have nodes and antinodes. The nodes are the points of zero displacement, while the antinodes are the points of maximum displacement. These standing waves are also known as normal modes.
How the oscillations are created, as different driving forces will excite different normal modes. The sound of a struck singing bowl and a bowl that is brought to sound through rim singing is visible on a spectrograph. Each can bring about unique variations in the sound of the bowl. 
If you change any one of these factors, the sound may be quite different. Making the bowl out of a different material will influence the vibrations that the bowl makes in response to a tap, and may excite more than one normal mode (especially if “defects” like hammer marks are present, as vibrations can reflect off of them). Making a singing bowl of a different shape will change the presence a/or frequency of the normal modes. Making a singing bowl vibrate by a different method (for example, hitting it hard against a hard object, or hooking it up to a mechanical oscillator tuned to resonate with a higher normal mode) will change the normal modes that are excited. Hearing the singing bowl vibrate with an ear (or a microphone) that has a different response curve will also change the perceived sound. 

The geometry of the bowl changes the number and frequency of the available normal modes. A singing bowl can have four or more modal points visible in its vibration. The best way to observe this is from directly above. Try filming the rim above and watch the playback in slow motion. You will be able to see the points at which the rim of the bowl is flexing. Different driving forces will excite different normal modes. The sound of a struck singing bowl and a bowl that is brought to sound through rim singing is visible on a spectrograph. Each can bring about unique variations in the sound of the bowl.
The hand hammered construction breathes a unique sonic fingerprint into each bowl. This labor intensive process creates instruments that carry common characteristics like the multi tonal layering built into the sound of each instrument. The fiery creation of the bowl through hammer blows determines that it will be highly unlikely to find two bowls that have the same spread of notes from fundamental, through the grouping of harmonics, meaning that each bowl can have a unique sonic footprint allowing for highly specialized combinations of tones. Whether the you find the tones arranged as a harmonious chorus or a mysterious blend of tones that touch on more pensive or introspective emotional content, if you haven’t found the bowl that sings the song you are looking for, it is probably still out there.
In summary, the sound of a singing bowl is determined by the material of construction, the design of the bowl, and the method used to excite the bowl’s vibrations making the hand hammered bowl a unique instrument who’s construction leaves tuning up to chance. By changing any one of these factors, the sound produced may be quite different. Making the bowl out of a different material or changing the alloy will influence the vibrations that the bowl makes in response to a tap, and may excite more than one normal mode (especially if “defects” like hammer marks are present). The hand hammered singing bowl is a one of a kind instrument who’s unique voice is worthy of a closer listen.