Mechanical Properties of Nylon Harp Strings

Abstract

Monofilament nylon strings with a range of diameters, commercially marketed as harp strings, have been tested to establish their long-term mechanical properties. Once a string had settled into a desired stress state, the Young’s modulus was measured by a variety of methods that probe different time-scales. The modulus was found to be a strong function of testing frequency and also a strong function of stress. Strings were also subjected to cyclical variations of temperature, allowing various thermal properties to be measured: the coefficient of linear thermal expansion and the thermal sensitivities of tuning, Young’s modulus and density. The results revealed that the particular strings tested are divided into two groups with very different properties: stress-strain behaviour differing by a factor of two and some parametric sensitivities even having the opposite sign. Within each group, correlation studies allowed simple functional fits to be found to the key properties, which have the potential to be used in automated tuning systems for harp strings.

Frequency Measurement of Musical Instrument Strings Using Piezoelectric Transducers

“…The red section of the response indicates a period when a desk lamp, with a 60 W incandescent bulb, was placed near the string to provide some local heating. Raising the temperature of a settled nylon string makes it go sharp [3]. In this case, however, the effect of heating merely offset the underlying creep for a while.…”

“…This had the advantage of allowing plenty of time for the string to settle. While variations in the string tension and frequency could be expected to result from changes in the ambient temperature [3], the string was (at least for the purpose of these tests) no longer creeping. To take advantage of this stability, the test sequence was organised to complete as many measurements as possible before making any tuning adjustments.…”

“…Calculating the length of string to be wound onto or off the winding shaft to make a given tuning adjustment requires knowledge of the string's Young's modulus: specifically, the string's "tangent modulus" (the stress/strain ratio for small perturbations). For nylon strings this quantity varies significantly with the string stress, but the previous study of nylon harp strings led to an expression which can be used to predict the Young's modulus with sufficient accuracy to provide a stable tuning control algorithm [3].…”

“…The bridge consisted of a semi-cylindrical steel spacer, 4 mm in length, holding the string about 2.5 mm above the transducer disc. The mass of the spacer was about 0.2 g. The termination points at each end of the vibrating length of string (the vertical bridge pin and the horizontal spacer) were set at 90°to each other so that the effective vibrating length of the string was not affected by the polarisation direction of the transverse vibration [3,4]. The string used for these tests was a centreless-ground nylon string from the Bowbrand "Pedal Nylon" range, with an unstretched diameter of 0.84 mm and an unstretched linear density of 0.593 g/m.…”

“…A study of the mechanical properties of centreless-ground ("rectified") nylon harp strings has shown that holding such strings at a constant tension may be enough to keep them acceptably in tune [3]. Many players, however, prefer natural gut strings, which are much more sensitive to changes in humidity and can change in unpredictable ways: on the same instrument, a given gut string may go sharp while an adjacent string goes flat.…”