The sound we hear is, in itself, a combination of ringing tones generated by resonating objects. The character (timbre) of a sound is determined by three criteria: the frequency values of these tones, the loudness of each tone, and the relative time difference between them.
For example, the simplest form of sound we hear comes from resonance at a single frequency, such as the sound of a tuning fork. Variations in these three criteria are the reason we can differentiate between a person singing the note A and a violin playing the same note.
The frequency at which a mechanical system resonates is a fixed property. It is not related to how the system is excited. For example, striking a tuning fork hard will generate the same tone as striking it softly.
An acoustical instrument is essentially a group of resonating devices that generates many simple sound waves at different frequencies. In ouds, there are three major resonating bodies. Together, they generate hundreds of tones that combine to create the characteristic sound of the instrument.
The first set of frequencies is generated by the elastic strings. When a string is picked, its full length vibrates, but smaller sections of it vibrate simultaneously as well. Each of these vibrating portions of the string generates waves with frequencies related to that section's length. The longer the section, the lower the frequency generated.
The waves produced have frequencies that are multiples of each other, called harmonics. The lowest one, generated by the full length of the string, is called the fundamental frequency. Here, we can see how the type of string affects the ratio of fundamental to harmonic frequency content. The less flexible a string is, the fewer harmonics it can generate.
A soundboard is considered an elastic plate, and it vibrates in a manner similar to a
string, but in two dimensions. As the bridge is activated by the vibrating string, it transfers vibrations to the soundboard. The soundboard’s reaction, and the way it resonates, are affected by its material and bracing.
A simple way to demonstrate the resonance pattern on an oud is to use a method called the Chladni plate technique. It works by placing lightweight material on a flat surface. As the surface vibrates, the particles jump and settle onto the calmer, or stationary, parts of the plate. This allows the observer to see the vibration pattern. The attached photo shows a Tarablic oud soundboard reacting to the note C.
Lastly, the air body inside the oud is also an elastic object. It responds to the vibration of the soundboard and vibrates with it, moving in and out of the rosette hole, which generates its own set of frequencies.
Other factors affect this frequency “recipe” as well. For example, body material, body shape, and neck specifications all play a role. Their impact is a little more complicated to explain in this context, but they do have a significant effect on how the generated frequencies are combined and, therefore, on the overall sound.
