Rossing Et al.
Thomas D. Rossing of Northern Illinois University, and a number of his colleagues and associates: Craig A. Anderson, Richard S. Christian, Robert E. Davis, Garry Kvistad, Ronald I. Mills and Arnold Tubis published the results of numerous studies on timpani acoustics between 1976 and 2000. Rossing et al. conducted extensive research on the vibrational modes of timpani, how the bowl influences the sound and how the sound radiates from the instrument. 18
Ideally, a timpano bowl should not directly affect the vibrating head itself in as much as it should affect the air inside of the bowl that the head displaces when it vibrates. Rossing’s et al. research in this area provided much data on the subject of comparing the vibrational frequencies of a timpano, a timpano head without a bowl and and ideal membrane. The charts in this section have been extracted from the article Acoustics of Timpani which Rossing published in the Percussionist [Percussive Notes Research Edition] 19/3 (Fall 1982): 18-31. The timpani used in this particular study were two identical 26 inch (65cm) drums, one with a bowl and one without. Rossing does not give an exact model of the timpani, but it is probable that these were Ludwig Professional Symphonic timpani with suspended bowls and external spider: the model which he used in his Acoustics of Timpani: Preliminary Studies as well as his article The Physics of Kettledrums, which was published in Scientific American, November 1982. In the Endnotes, Rossing gives credit to the Ludwig Drum Company for furnishing the drums.
One of Rossing’s conclusions was that the frequencies of the preferred modes are only somewhat influenced by the bowl; even without the bowl, an air loaded timpano membrane conveys a fairly definite sense of pitch. The preferred ratios 1.00, 1.48, 1.92, 2.36 and 2.78 are not quite as harmonic as those preferred mode frequencies with a bowl, but still tolerable.
Timpani without bowls is not a new phenomenon. In 1860 Adolphe Sax patented his timpani without a shell 19 and currently Marcus de Mowbray of London England markets a Tour Timp which is essentially a timpani without a bowl. There are probably any number of reasons why this type of timpani is not preferred by most timpanists. Perhaps it is due to the overall sound quality and lack of projection since the bowl contributes significantly to both.