Page 46 - Fall 2005

Whatʼs new in string instruments?
Fig. 2. Six carved Baltic psalteries along with a violin and a Swedish nyckelharpa
requires that the string be strong, heavy enough, but not stiff. “Feel” is how hard the player has to pull to move the string cen- ter a given amount before release; it should be large enough that the strings do not touch, and not vary too much from string to string. Nylon or gut would probably suffice for all strings if the lower strings followed the curve set by the upper strings and became very much longer than they actually are. Gut is mechanically similar to nylon but has a warmer tone for the mid-register. Practical reasons make the harp neck a double curve (an ogive, so it isn't too tall) and so much heavier strings are needed for the low register. Nylon/gut strings would have to be very thick here, very inharmonic, and the “feel” would be so small that the strings would interfere with each other. Steel wrapped with helical copper wire is used, although there is an awkward change in “feel” at the break. The ingredients which distinguish the sound of a harp from any other plucked instru- ment, such as the guitar, are as follows:
The string is plucked in the middle, which reduces even harmonics. In some music, the harp is sometimes plucked close to the soundboard in order to imitate a guitar.
There is very strong coupling, via the soundboard, between any two strings with overlapping overtones. In the case of a large harp, many strings vibrate when only one is plucked.
The strong interaction with the soundboard causes the strings to vibrate in collapsing and expanding ellipses, which gives a long pulsed tail to the sound.
The large number of strings allows for glissandi, a feature most people immediately associate with the harp.
My current aim is to produce a thorough map of reso- nances in different harps, and to ascertain what features are desirable and what are not. In the process I hope to achieve a better understanding of how best to optimize soundboard parameters, particularly the thickness, as this is where good engineering and good acoustics seem most at odds.
Carved Baltic psalteries: Andres Peekna, Waterford, WI
The Baltic psaltery family of plucked string instruments includes the kantele (Finland), the kannel (Estonia), the kokle, (Latvia), the kankles (Lithuania), and the wing-shaped gusli (Northwestern Russia) (see Fig. 2). Over the years, we have studied the modes of vibration of several psalteries by various makers, many of which are faithful copies of ancient instru- ments. On the better instruments, the main body resonances are well distributed in frequency so that they support the vari- ous strings. Good string-to-sound box coupling also appears to play a role. A useful method for studying string-to-sound box coupling involves scanning at intervals as low as 0.1 Hz for narrow peaks within the nominal tuning range of the strings, and comparing them to their neighboring body resonances, while using electronic TV holography. Predictions of the Helmholtz resonance from sound hole dimensions and air cav- ity volume while neglecting damping in the sound holes yield upper limits when many small sound holes are involved. The locations of the sound holes, as well as their area, are found to have significant effects on sound quality and volume.
It is desirable to have a body resonance with high radi- ating 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 low- est body resonances can be adjusted, to some extent, by adjusting the areas of the sound holes. As replicas of carved Baltic psalteries are often played together with other instru- ments, such as bagpipes, violins, and accordions, this in effect imposes absolute tuning ranges. Wood is a notorious- ly irreproducible material; in many cases string instruments with the same dimensions made out of similar woods do not sound alike. Adjusting the areas of the sound holes provides a limited method for custom optimization.
44 Acoustics Today, October 2005