From Workshop to Concert Hall : Acoustic Observations on a Grand Piano under Construction

Abstract

In the scope of a research project, a series of measurements has been taken on two concert grand pianos, starting with the glue-laminated soundboard planks and ending with the completed piano playing in a concert hall. The work is divided in two parts, even though dealing with different questions in detail, the two conducted experiments are interrelated by the overarching questions: How does the piano achieve its sound? What are important influential factors in the building process, as well as in concert business?

For the first experiment, two concert grand pianos have been accompanied with acoustic measurements through their manufacture to investigate the influence of successive production steps on the development of the pianos’ final sound. The utilization of a microphone array led to insights about the vibrational behavior of the soundboard: The application of ribs has a crucial impact on the propagation behavior of the initial bending waves. In direction normal to grain the propagation is not circular but rather a traveling plane wave front, as a result of superposition of reflections between the ribs. The ribs act as waveguides for higher frequencies with locally changed stiffness. The influence of the bridge is clearly observable in the initial bending wave pattern: Stiffening increases the wave velocity locally and the propagation seems to follow the bridge direction, even when curved. The energy loss could be described not only as a general signal decay but could also be given exclusively for the initial propagation on the soundboard before any reflections or drain at the boundaries. For the blank soundboard, the greatest portion of the supplied energy is preserved in grain direction. After application of the ribs the majority of the supplied energy is preserved in radial direction between adjacent ribs. After attaching the bridge, the supplied energy can distribute more uniformly due to the local stiffness increase by the bridge, acting as a waveguide in mainly longitudinal direction. The bass bridge and the connected lower treble bridge part form a loop in the upper half of the soundboard which confines significant parts of the vibrational energy (bridge loop effect). The spatial distribution of vibrational energy on the soundboard per frequency can be approximated with exponential decays. The method should be applicable to other structures for the discrimination between modal- and driving point dependent domain. The observable differences in wood material properties between the two examined soundboards are not reflected in the acoustic measurements. This leads to the assumption, that in this range of variation, the geometry plays a predominant role for the resulting behavior. An alternative approach to classic modal analysis has been presented for the description of the vibro-acoustical behavior of piano soundboards, which should be comprehensible and hopefully help piano builders to understand the effect of individual production steps for the final behavior of the instrument.

In a second experiment, measurements have been performed on a concert grand piano before and after one year in concert business in order to identify influential factors for a presumed change in tonal quality. Measurements have been performed on two occasions: First, on a completed instrument prepared for sale. Second, on the same piano after having been played for one year in a concert hall. Single notes have been recorded in an anechoic chamber. An extended ABX listening test engaging 100 players, tuners, and builders, addressed the questions whether a variation in tonal quality is audible and if so, what sound properties could lead to a perceived difference. Semantic sub-grouping allowed for indication on the vocabulary listeners of varying expertise use to verbalize their sensation. The statements gave hints on what could have changed over the year and have been used as a basis for the analysis of corresponding physical properties and psychoacoustic parameters related to the described sensations. The conducted listening test showed that a difference in tonal quality is perceivable for a piano after one year in concert usage, even for non-experts. The most stated properties used to distinguish between similar piano tones were Timbre related, followed by Pitch and Temporal attributes. Spatial and Loudness related sensations do not seem to have played an important role in discrimination, even when stimuli should have exceeded the just noticeable difference. Within the bounds, and given the described uncertainties of the study design, it can be stated with confidence that within the time frame of one year, the technician can be expected to have much more impact on the tonal development of the piano than the effects of wood aging or playing. The presented findings give the technician to the same extent the responsibility, but also the opportunity to turn a good concert instrument into an excellent one.