Ricardo R. Boullosa
Centro de Instrumentos UNAM, Circuito Exterior CU,
México DF, CP 04510, MEXICO.
Popular version of paper 2aMU10
Presented Tuesday morning, December 2, 1997
134th ASA Meeting, San Diego, CA
Embargoed until December 2 1997
Enjoyment of musical performance is affected by many factors which include, among others, the technical and artistic abilities of the musician, the quality of the musical instrument, the room, etc. Some of these aspects, specially with regards to the musical instrument and the room, involve physical properties (mechanical, acoustical, etc.) which can in principle be measured in a very objective and precise way. However, in the end, the enjoyment of music remains by its very nature a purely subjective matter. Something which surely cannot be meaningfully reduced to a series of quantitative measurements. In spite of this, it is still desirable to establish certain correlations between measurable quantities and subjective preferences, if only to provide guidelines for the design and construction of musical instruments and concert halls.
A research program of this kind involves consideration of the following issues:
In the case of musical instruments, the research on subjetively important factors has been developed to a somewhat lesser extent, but has already produced very interesting results, specially with respect to the violin, the piano and other instruments. Previous research on the subjective quality of classical guitars has already revealed some degree of correlation with physical characteristics such as atack and decay times of the tones, sound pressure level (related to loudness), certain parameters of the mechanical and acoustical resonances of the guitar, etc. However, physical parameters will no be discussed any further in what folows.
The work described below is a comparative study of two different methods for the evaluation of the subjective quality of a set of classical guitars. One method is based on "blind" listening tests by a general audience and another is based on direct inspection and playing by guitar teachers and students. Both tests were carried out on the same set of four guitars, ranging in quality from "low" to "medium-high" (their prices in USD range from $50 to $500 approximately). In what folows, the guitars are identified by the three-letter codes: CHF, APL, RRB and FOB (named after their owners and listed by price with CHF being the cheapest).
A reasonable low proportion of X-votes was obtained which gives some confidence to the relevance of the results. On the other hand, the proportion of X-votes is not altoghether negligible and may reflect the difficulty of the test which was expressed by some of the subjects. Also interesting is the fact that the proportion of B-votes is clearly shown to be larger than the proportion of A-votes. This seems to suggest that a short-term memory effect is taking place, in which listeners tend to prefer the second guitar (the more recently played) over the first, in spite of the fact that listening to each pair lasts less than about 40s. The following figures show different ways of quantifying the subjective quality calculated as a percentage of the number of votes obtained by each guitar, and where 100% means a guitar that obtained the maximum number of votes possible in each case (this particular way of counting implies that the percent values do not add to 100% in this case).
Differences between the guitars are less clearly seen when they appear first in a pair, which is consistent with the previous observation regarding the bias towards the second guitar in a pair. The label of votes "as A or B" corresponds to percentages of the total number of votes, while "as A and B" corresponds to a special count in which ambiguous votes have been eliminated (a subject emits a pair of ambiguous votes when (s)he prefers different guitars in reversed presentations of the same pair of guitars). There is a significant difference (about 50%) between the total voting and the unambiguous voting which is surely due to the bias towards the second guitar mentioned above. In any case, the unambiguous voting "as A and B" can be taken as the more reliable result from this test.
The rating of the guitars shows a great consistency among the different criteria. The second figure shows that the combined rating according to the first five aspects, labeled "All aspects", coincides very well with the rating of the overall subjetive quality. The rating of the guitars is also consistent with the results of the listening tests.
These observations will have to be confirmed by more extensive tests with the participation of more subjects and with instruments of higher quality. On the other hand, physical tests carried out on the same guitars will have to be correlated with subjective preferences in order to establish the precise mechanical and acoustical characteristics that define a fine classical guitar. Overall, the results of our tests fit nicely with each other, lead to reasonable conclusions and, in particular, suggest that the old wisdom: "ask the expert" seems to be firmly grounded.