Acoustics Of String instruments

Please allow a moment for images to load.

Instrument Design

The Luthier's Bench combines the latest advances in acoustical technology, while retaining the "old world" quality and craftsmanship that can only be found in a small shop. In designing an instrument, either a new instrument or a copy of a revered old world master, the relationships between the size of the soundboard, back, the volume of enclosed air inside the body, and the arching of the top and back are all carefully considered to ensure the proper balance of the tone and the avoidance of "wolf tones" or distorting notes.

Materials

The first step in building an instrument is selecting woods that not only are beautiful, but which have a great potential to become a musical instrument. For all my instruments, I hand select the finest materials available. These woods, used for centuries in musical instrument construction, have proven to be the most ideal because of their superior tonal quality and striking beauty. Acoustic Guitars and Ukulele's are built with Indian Rosewoods, Afrormosia, Honduras Mahogany, Walnut and Zebrawood, to name just a few. Guitar and Ukulele soundboards are Sitka spruce, with options for Englemann Spruce, Adirondack Spruce, German Silver Sruce, Western Red Cedar, or Redwood. Violas and Cello's use only the finest German Silver Spruce for the soundboard and figured Maple for the sides and back. All woods are air dried and seasoned for years before being selected for an instrument. Fingerboards, bridges, tailpieces, and chinrests are of choice Indian Rosewood or Gaboon Ebony. Necks typically are of select, quarter sawn maple for viola's and cello's, and Honduras Mahogany or Maple for guitars and ukuleles. Each of these tonewoods has a distinctive timbre and is selected according to the player's tonal requirements.

Tonal Response

The most important characteristic of an acoustic instrument is a full, well-balanced tone. Special requirements for a particular tone desired by a customer are "built in" and the instrument's response is customized to suit these needs.

Vibrational Modes of Musical Instrument Plates

A musical instrument plate, either soundboard or back, has many modes of vibration, and in general each one occurs at a different frequency. From more than a dozen well known modes, three or more are considered useful in the process of shaping the plates by instrument makers. For example, in guitars and ukuleles, the modes with the lowest frequency tell me where the instrument wants the main braces placed. For several different types of plates, one of the important modes I typically look at has two node lines, both approximately straight, which intersect at about ninety degrees (the mode lines below the sound hole).

In this mode, opposite sectors of the plate are going up together, while adjacent sectors (separated by one node) are always moving in opposite directions.

This sketch, though greatly exaggerated and simplified, shows an instant in the motion of the mode for the plate, which is shown above.

This mode, with its characteristic shape and frequency, though not contributing much to the total volume of the instrument, is vitally important to its sound quality.

How Chladni patterns are formed

The instruments back or soundboard, while supported on foam rubber blocks, can be made to resonate by a powerful sound wave. A frequency generator tuned to the frequency of the desired mode supplies this sound wave.

In all cases, some finely divided, lightweight material is placed on the plate. The material used here is glitter used for decorating Christmas ornaments. When the plate resonates, the motion becomes large over most of the surface and this causes the glitter to bounce and to move about. Only at or near the node is the glitter stationary. Thus the glitter is either bounced off the plate or else collects at the nodes, as shown in the photographs.

Why Chladni Patterns Are Useful

The shaping of the back and soundboard plates are very important to the properties of the final instrument. Chladni patterns provide feedback to me during the process of scraping the plate to its final shape and thickness. Symmetrical plates give symmetrical patterns; asymmetrical ones in general do not. Symmetrical patterns generally lead to better sounding instruments. Further, the frequencies of the modes of the pair of free plates can be related to the quality of the completed instrument. Many scientists have been interested in the acoustics of musical instruments for many years, and many instrument makers have been interested in science (in addition to being a luthier, I also have an advanced degree in Physics), so a lot has been written about the acoustical properties of musical instruments and their parts.