FRIDAY MORNING, 20 MAY 2005 REGENCY E, 8:30 A.M. TO 12:00 ...

9:55
5aMU5. Normal modes of a single mandolin with different bracing patterns installed. David J. Cohen Cohen Musical
Instruments, 9402 Belfort Rd., Richmond, VA 23229 and Thomas D. Rossing Northern Illinois Univ., DeKalb, IL 60115
The vibrational modes and sound spectra of a single archtop mandolin constructed with a removable back plate were studied with
different bracing patterns installed. The bracing patterns were i symmetric and ii asymmetric parallel ‘‘tone bars,’’ iii X-bracing,
and iv an asymmetric pattern previously used and studied D. Cohen and T. D. Rossing, Can. Aeronautics Space, J. 42, 48–54
2000, D. Cohen and T. D. Rossing, Acoust. Sci. Tech. 24, 1–6 2003. The results have been compared with those obtained
previously for vintage mandolins D. Cohen and T. D. Rossing, ‘‘Normal modes of different types of pre-1929 mandolins,’’ presented
at the 147th meeting of the Acoustical Society of America, New York, NY 2004 and for mandolins constructed more recently. As
was the case for the different types of vintage mandolins, changing the bracing pattern has little effect on the shapes of the normal
modes, but does affect the frequencies of the normal modes. The X- or crossed bracing pattern imparts more cross-grain stiffness than
the other bracing patterns, with the result that modes involving cross-grain bending occur at higher frequencies than is the case for the
other bracing patterns.
10:20–10:30 Break
10:30
5aMU6. Design aspects of the five string banjo. James Rae Dept. Physio. and Biomed. Eng., Mayo Clinic College of Medicine,
200 First St. SW, Rochester, MN 55905, rae.james@mayo.edu and Thomas Rossing Northern Illinois Univ., DeKalb, IL 60115
The five string banjo is a stringed instrument that uses a drum head membrane to radiate sound. The strings couple to the head
through a moveable bridge. Many have a removable resonator whose spacing from the main part of the banjo is adjustable to allow
tuning of the internal air cavity. Holographic interferometry demonstrates vibrational modes of the head that have a strong dependence
on head tension. Driving point accelerance measurements demonstrate that the ability of the bridge to be vibrated depends on the mass
and stiffness of the materials used in its construction. Peak accelerances usually occur between 1800–2000 Hz. Power spectra
measurements of the sound show that 99% of the sound is in a frequency range of 147–5200 Hz. Two substantial formants are seen
in the power spectra. The first and largest occurs in about the 400–1800 Hz range, the same frequency range where the most
substantial head modes are found. The second is from 2000–4000 Hz, a range where the series combination of bridge and head show
increasing accelerances by driving point measurements. Measurements of the transient response following a pluck quantify the
frequency content of the rising and falling phase.
10:55
5aMU7. Tuning the lower resonances of carved Baltic psalteries by adjusting the areas of the sound holes. Andres Peekna
Innovative Mech., Inc., 5908 N River Bay Rd., Waterford, WI 53185 and Thomas D. Rossing Northern Illinois Univ., De Kalb, IL
60115
Previous work has shown that with carved Baltic psalteries it is highly desirable to have a body-resonance with high radiating
efficiency between the keynote and the tone above. In many cases, it is also desirable to have a body-resonance close to the low
dominant. The frequencies of the two lowest body-resonances can be adjusted, to some extent, by adjusting the areas of the sound
holes. We describe two trials. In one case, the instrument started out with no sound holes, and their areas were increased step by step,
mostly according to listening trials, but the results were also verified by electronic TV holography. In the second case, the instrument
started with excessive sound hole area. In this case, the listening tests first determined the extent of sound holes to be covered by
sufficiently stiff material. The findings were subsequently confirmed by electronic TV holography. In both cases, a point was reached
below which smaller areas the sound quality became less bright and more dull, and above which the sound quality was perceived as
good. In each case, an optimum was reached.
11:20
5aMU8. Harp design and construction. Chris Waltham Dept. of Phys. & Astron., Univ. of British Columbia, Vancouver, BC,
Canada V6T 1Z1, waltham@physics.ubc.ca
The harp is an instrument with a set of plucked strings connected directly to the sound board. Thus the requirement that the sound
board be very light for efficient radiation is pitted against the requirement that the strings be tight enough to maintain harmonicity.
These factors have determined the evolution of the harp since Sumerian times. As materials and construction methods have improved,
strings have become tighter and sound boards lighter. Large harps have total tensions in the range of ten kilonewtons. The material of
choice for the sound board, usually spruce, is pushed to the limits of its strength, and even professionally built concert harps tend not
to last more than a few decades. The strings themselves are made from a variety of materials, wrapped or plain, to balance the physical
requirements of tension and feel, with the acoustical demands of harmonicity and tone. Historical materials such as copper and silk
have given way to steel and nylon, although gut still has a niche in the mid-range of a concert harp. These physics and engineering
issues will be considered in the context of practical harp construction.
2590 J. Acoust. Soc. Am., Vol. 117, No. 4, Pt. 2, April 2005 149th Meeting: Acoustical Society of America 2590