How Are Handpans Tuned?

 

The simple answer is…. with a hammer!  However, it is much more complicated than most would expect.

Handpans, and other tuned steel instruments, such as steel pans, are quite unique musical instruments.  They rely on specific mechanical properties only found in a few metals.  Steel is the most commonly used metal that has the preferred properties that make it ‘tunable’.  In order to be tunable, the metal must be able to maintain high compressive in-plane stresses.  These stresses are what allow the metal to vibrate.

Tuned steel notes are utilizing a buckling effect similar to the lids of mason jars.  However, the buckling seen on mason jar lids is not controlled, whereas it is controlled in handpans in order to produce the unique sounds we hear emanating from them.

Once in this controlled buckling state, multiple frequencies can be adjusted at will by the tuner.  Tuning these frequencies is made possible by creating several consistent modes of vibration in the note.  These modes of vibration are created by careful shaping of the note itself.  To make a sound that is enjoyable to our ears, the frequencies of these modes are tuned to basic harmonic intervals.  Commonly, at least 2 harmonics of the note will be tuned to specific frequencies; the octave (2 times the fundamental frequency) and the compound fifth (3 times the fundamental frequency).

It is an important concept to understand that these frequencies are not naturally occurring modes of vibration in a tuned steel note.  Natural sound generation is not linear in steel notes, as opposed to other vibrating systems such as tensioned strings (violins, pianos, guitars, etc).

This is where the difficulty in tuning begins.  Adjusting the fundamental pitch, the first harmonic, and second harmonic can be a challenging endeavor, as adjusting each one will also affect the other two frequencies to varying degrees simultaneously.  To add to the complexity, tuning one entire note can affect its neighboring notes as well.  Getting these frequencies to where they need to be in as few hammer hits as popular is the end goal.

But wait, how does a tuner adjust those frequencies?  The simple, yet vastly deceiving answer is…… with a hammer!  Each tuner has their own preferences as to hammer size, weight, head contour, etc.  However, the core concept remains; the stresses in each note are adjusted by carefully placed hammer hits of varying force.  The locations of these hits can literally be anywhere on the surface of the instrument.  It is the tuners job to know how best to adjust the frequencies in the most effective and efficient way possible.

Tuning those frequencies is only the first challenge.  Next comes the timbre.  Timbre is how the note actually sounds.  It is entirely possible for a note to be in tune, but have poor timbre.  Is the note stable under a soft, medium, and hard strike?  Do any of the frequencies modulate excessively when the note is struck?  Do any of the tuned frequencies have higher amplitudes (louder) than the other frequencies?  Some of these factors are more desired (such as ‘strong’ notes that can take hard hits without modulating), while other factors such as the amplitudes of some frequencies can be desired by an individual tuner that makes their sound unique to their style or musical taste.

Lastly, we have the actual stability of the tuning; how well can the notes stay in tune?  A note can be in tune, and also have good timbre, but be completely un-stable so when it is played, it will go out of tune quickly, even with just a single strike.  It is the tuners job to ensure the stability of the note.  Stability is often checked by the tuner by striking the note forcefully with a rubber mallet.  If the note remains in tune after heavily striking the note multiple times, then the note can be considered stable.  In general, the harder hits a note can take without going out of tune, the more stable it is.

Even once a note is tuned with good timbre and good stability, it will slowly drift out of tune over time.  This drift is due to a naturally occurring process in steel known as “strain aging”.  In the building and tuning process, the steel has undergone a large amount of hammering.  This hammering introduces stresses that will slowly try to resolve themselves over time.  The strain from these small stresses will slowly reduce the compressive forces at play in each note, thus causing a change in the tuned frequencies.  The effect of strain aging can be reduced by heating the instrument during the build process between multiple tuning sessions.  After heating and cooling, these stresses relax, requiring the notes to be re-tuned.  After repeated tuning and heating, this drift in tuning will become less and less after each repetition.

Even though this strain aging process can be largely expedited by performing multiple stress relieving sessions as mentioned above, there will still be a slow drift in tuning over time.  This drift can be attributed to the fact that the notes are always under a compressive stress, and this stress will slowly cause a strain in the steel and a drift in the tuned frequencies.  This is similar to why a piano goes out of tune; many components, including the strings, are under a constant force which will slowly cause small but permanent changes in the structure.  Conceptually, the handpan is no different.