Page 43 - Fall 2005

Whatʼs new in string instruments?
such as “What makes a great guitar?” for the moment, at least, we are interested in determining what sort of structural changes make audible changes to the sound quality. In this way, we might build up a check-list of constructional aspects which makers should treat carefully and those which are of less importance in determining sound quality.
Our studies so far have thrown out a few interesting obser- vations. For example, it is often suggested that materials for guitars should have low damping, but we can see quite positive benefits from reducing Q-values of modes. Indeed, one of the major “problems” with making a good guitar is that the gener- al design principles adopted in virtually all instruments over- emphasize coupling of the strings and body.
Another observation has been to note that some of the low- order modes, which might be expected to radiate with largely monopole radiation fields, have, in fact, considerable dipole components. The relative strengths of the monopole and dipole components vary considerably from one instrument to another, dependent on the overall construction of the body (soundboard, back plate and ribs). This can have a profound effect on radiation from the instruments at both low-and mid- frequencies and may have a marked influence on balancing the various registers of the instrument.
The Five-String Banjo: James Rae, Mayo Clinic School of Medicine
Formants—Either by plucking individual strings near the bridge or by brushing all five strings at many positions between the neck and bridge, it is possible to establish a sound signature for an individual banjo. The spectra thus obtained tended to show a formant in the 200-1500 Hz range and anoth- er in the 2000-4000 Hz range. The lower and stronger one appears to be related to vibrational modes of the head, while the upper one may be due to mechanical properties of the bridge.
Total sound power—At least 99% of a banjo’s sound power occurs below 5000 Hz, and about 95% of the power comes from the sum or the first five to seven harmonics. Higher har- monics contribute markedly to the timbre but not to the radiat- ed sound power.
Fig. 1 A structural mode on a guitar by Paul Fischer imaged using holo- graphic interferometry (left) along with its associated sound radiation field (right).
Cavity tuning—Unlike most other string instruments, cavity tuning can occur after the assembly of the banjo. By installing adjustable screws to hold the top of the banjo (the pot) to its resonator, the spacing between the bottom of the pot and inner floor of the resonator can be adjusted. As the pot-resonator separation is increased, the response of the banjo is shifted to higher frequency which “brightens” the sound. We have made small spacers that our friends call “Raejustors” to facilitate this tuning.
Bridges—Most commercial bridges have a single resonance at about 2000 Hz. By using different woods and orienting the grain differently it is possible to construct bridges that show accelerance peaks anywhere from 650 to 3300 Hz.
A particularly useful bridge is one made of a vertical grain wood with a pedestal structure of different woods so that par- ticular parts of the bridge are “tuned” to the string being served.
Mandolins: David J. Cohen, Richmond, VA
The mandolin is a plucked string instrument whose ori- gins appear to go back to the medieval gittern (also known as guittarra, chitarra, and guitaire in various European countries). The modern mandolin is descended from two instruments that developed during the 18th century. The first was the mandola or mandolino, which carried six courses of two strings tuned in 3rds and 4ths and is sometimes referred to as a Milanese man- dolin. The second was the mandoline or Neapolitan mandolin, which had four courses of two strings tuned like a violin. The modern mandolin is tuned like the latter. The stiff bowls of the Neapolitans do not contribute to corpus vibrations below about 1200 Hz.
Mandolin makers have used a variety of brace patterns. Gibson mandolins generally featured longitudinal braces, whereas such makers as Gilchrist and Smart preferred crossed braces. The crossed or X-bracing pattern imparts more cross- grain stiffness than other bracing patterns. Coupling between the lowest plate mode and the air cavity resonance is stronger in f-hole type mandolins than in Neapolitans or oval hole arch- top mandolins.
The Virzi tone producer was thought by its originators,
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