Some guitar makers at a Cmb-lutherie school in Belgium wondered if it
would be possible to build acoustic guitars from European grown local wood
that would sound as good as guitars made from the well-known
tropical woods. This straightforward query was the starting point
for a broad and long-term international research project.

An Overview

LGRP Phase 1
From 2012 to 2014, the Leonardo Guitar Research Project conducted blind and non-blind comparative playing and listening tests on the sound perception and preferences of guitars made from tropical and non-tropical woods.
The only constant factor was the soundboard, for which the same wood and the same bracing pattern were used for both tropical and non-tropical guitars. In total, 15 classical guitars were assessed by 3 experienced guitarists and by listeners both in room and under concert hall conditions in two locations (Belgium, Finland). / See Research Report #1

LGRP Phase 2
From 2014 to 2017, we conducted new and more elaborated blind and non-blind comparative playing and listening tests with 20 classical guitars and 24 acoustic steel-string guitars. The guitars were built by 22 luthiers. Each builder made a matched pair consisting of one guitar made from local non-tropical wood and one guitar made from traditionally used tropical wood, exept for the top plate which, for all guitars, was made from European spruce. Twenty professional guitarists from three countries tested the guitars. The tests were first conducted in Belgium and then repeated in Finland and the United Kingdom. Different guitars were built in each country using the same guitar design and controlled building process, the same research design, methodology, data recording and processing procedures. / See Research Report #2
    At the end of 2017, we conducted an additional blind online listening test with the same 16 Belgian-made classical guitars as in the previous test. Two hundred twenty-six assessors participated in this test./ See Online Listening Test, Report#3

Collaborations with academic researchers.
  1) Physical sound and vibration measurements (acoustic signature) of the 16 Belgian-made classical guitars from Phase #2 were carried out and processed by François Gautier, Professor of Acoustics/Vibrations at the Acoustics Laboratory of the University of Maine.
  2) Apart from the comparisons of different tonewoods we conducted, at the end of 2017, tests to investigate the long-term stability of alternative wood species for classical guitar necks. This study inspired academic researchers at the Ghent University Laboratory for Wood Technology (UGent-Woodlab) to conduct a similar experiment using the same LGRP setup and measurement method as well as a new setup method. These two setups were compared in terms of workability and reliability.
  The UGent paper, with Bingrui Chen as main author and LGRP as co-author among others, entitled “Monitoring the long-term stability of classical guitar necks: evaluating the potential of European-grown wood species in musical instrument manufacturing” was published in “Springer Nature, Wood Science and Technology”, June 2025 / More info, see Related Research.

LGRP comparative tests / Tropical versus non-tropical.
Some results
All comparative tests (conducted in phase 1 and 2) between guitars built with common tropical woods and alternative non-tropical woods, in which all comparisons were made with a same guitar model and with European spruce for the soundboards, show that, in blind conditions, players and listeners cannot distinguish between the two types of guitars at better than chance levels.
    When all individual assessments are compared, the preferences for one type over another are relatively equal and do not deviate significantly from the average at a significance level of 5%. Combining all the results of the blind tests from the various studies shows a 50/50 preference ratio between tropical and non-tropical wood for both classical and steel-string guitars.
  In non-blind experiments, however, the bias in favor of tropical woods was strongly confirmed, especially in our first study (2013/2014), where the results showed a 50% decrease in preference for non-tropical wood guitars. This indicates that sound perception can be strongly influenced by visual information, such as the appearance of an instrument (dark versus light woods), or preconceptions about "good" and "bad" tonewoods.
    A few years later, during the "Phase 2" studies, most assessors were familiar with the LGR Project and tended to support it. This is likely why the bias in favor of tropical guitars was less pronounced, though it was still present to a lesser extent.

In the blind test, the players had no idea what type of guitar they were testing resulting in an equal preference for both type of guitars. In the non-blind tests, however, the players "knew" what they were testing, which explains the lower preference for non-tropical guitars and the bias in favour of tropical ones.

Inconsistencies and psychoacoustics.
In order to draw general conclusions about the studies and understand the observations, we must focus on some complex yet intriguing psychoacoustic phenomena that occurred during the blind tests. We must then consider to what extent these phenomena can be taken into account when interpreting the raw results.

Test results, additional observations and comments from players reveal considerable inconsistency and subjectivity in perceptions and preferences. It is quite understandable that this differed per session as in the first blind session, the players were not given any information about the type of wood used for each guitar, whereas in the second blind session they were. Session three was non-blind. However, even within the first session without any information, players' perceptions changed from one assessment to another for the actual the same instrument. They were led to believe that they were testing two different guitars in a group of five, when in reality there were only four.. This double test for the same guitar is referred to as a 'test-retest'
see Fig. 1 

Depending on the calculation method, and based solely on the two test-retest scores from the first blind sessions (where the players had no information about the woods used and therefore could not be biased by external stimuli), the degree of consistency ranges from 27% (identical scores) to 41% (gradual similarity) within the same session for the same player on average.

However, we need to look at these numbers in a broader context.
From the comments that the players made during the evaluations, we could clearly deduce that when they gave similar, consistent-looking scores for the physically identical test-retest guitars, they did so for various reasons related to perceived sound characteristics. This shows that similar scores do not necessarily correspond to similar perceptions; usually not. This finding puts the calculation of consistency based on preference ratings into perspective.
    Furthermore, we must take into account that, on average, within a single sessions, the players' scores differed only within a small margin of 3 to 4 points on a scale of 1 to 10, creating a high probability of coincidental score similarities. Consequently, the physically different guitars and the test-retest guitar often received the same scores within that 3- to 4-point range, although the two test-retest scores were identical in only 27% of cases. Either the players' preferences for the different guitars were relatively close together due to accurate observations, contradicting their comments, or the differences in sound were very vague, possibly introducing an element of randomness. The latter seems to be confirmed by the inconsistent test-retest results, as well as by other results such as the 50/50 preference ratio outcome of tropical versus non-tropical guitars.
    Although the above factors are difficult to quantify accurately due to their varying degrees of probability, we can conclude that there was little to no consistency in perception. This was most likely because it was hard to tell the guitars apart, and not because the musicians had "bad hearing."

The so-called "bad hearing" phenomenon requires further clarification.
After all, when guitar sounds are very similar, or even identical, as in the test-retest experiment, the blinded assessor's mind has a strong tendency to fill in the supposed differences itself. As a result, through a cycle of self-confirming expectations and biases, it unconsciously creates new and sometimes very different soundscapes for the same guitar. Confirmation bias is a powerful and treacherous force that can help explain many things within the context of our experiments. It enables people to mislead themselves by selectively filtering facts and manipulating analysis to match expectations and beliefs.
      The brain doesn't just register sounds. To some extent, depending on several factors, it decodes, interprets, and modulates perceived audio material. It has the ability to focus on certain sonic features and fade out others. This is comparable to what one could technically do with an analog or digital equalizer. However, in this case, it is the ear-brain system that does this, and one is not, and cannot even be, consciously aware of this process! ¹
      Paradoxically, professional musicians, due to their more developed skills, experience, knowledge and thus their advanced ability for 'mental imagery'¹ regarding sounds and music making, seem to be able to perform these processes 'better' and 'more creatively' than non-musicians or even amateur musicians (according to academic studies investigating auditory neurocognition in musicians ¹ ² ³ ).
   In the 110 blind test sessions that took place, not a single one of the 20 players noticed that two guitars in the same session were identical, even when they were tested one after the other (this configuration sometimes occurred because, after becoming familiar with the five guitars, the players could choose the order in which they were tested).
     The auditory illusion was complete. The clear presence of that illusion confirms the existence of cerebral mechanisms that influence the reception and perception of the same sound material during the decoding and assessment process, and in our tests, mostly with a different result for each assessment for the same instrument.
    Believing it to be a different guitar, the assessors' brains were reset to zero between the first evaluation and the second evaluation of that same guitar. This allowed them to start a different mental evaluation process creating feedback loops that reinforced the initial misperceptions. This was observed in nearly all players.
    These brain mechanisms, which may seem surprising at first, are well known in psychoacoustics and neuropsychology. They cannot simply be considered abnormal; quite the contrary. However, we must conclude that the human ear-brain system is 'not always' a reliable measuring tool.

Indeed, "not always." To gain more insight into the phenomenon of shifting perceptions, which was already evident in our 'Phase 1' experiments, and to empirically investigate it, we incorporated the 'strange guitar experiment' into our 'Phase 2' experiments in Belgium. This involved a blind testing session comparing a guitar with a different design to the project guitars, which were all of the same design.
   The research question was:  'Can players distinguish a guitar with a different design from the other guitars, which were previously difficult to distinguish from each other?' During testing, all players spontaneously reported in their comments that they could clearly perceive different, definable tonal qualities in this particular guitar compared to the others. In contrast to our comparative experiments with guitars of the same design, the perception of that 'strange' guitar remained consistent throughout the entire test session.
    Identifying the sound did not require the activation of creative mental 'imagery' processes, precisely because, in this experiment, the tonal qualities, compared to the other guitars, were clearly provided by the nature of the instrument itself.

Conclusions
1) Regarding the comparative studies, we can conclude that the degree of (in)consistency in players' ratings and the underlying perceptions can also provide an indication of the extent to which the guitars are distinguishable from each other, rather than only looking at the raw numerical and statistically processed rating data.
The broader context in which inconsistent perceptions frequently occur do indeed seem to confirm the indistinguishability of the different guitars and thus the consistency and similarity of the sound characteristics of both tropical and non-tropical guitars. If the sounds of the two types of guitars had been more varied, the ratings would likely have been more consistent.

2) The above findings together with the 50/50 ratio of tropical to non-tropical preferences demonstrate that that as long as the guitar design and the material and quality of the soundboard remains the same for both types, alternative non-tropical woods can be used to build guitars that sound as good as those made from the commonly used exotic woods. This confirms the initial research question.

3) Since our blind playing and listening tests revealed no significant difference between the two guitar types, this suggests that the model, top wood, bridge design, and bracing system (the only consistent factors in the experiments) primarily determine sound differences, and much less the back and sides, contrary to popular belief. This, in turn, explains why it is difficult to distinguish between the two types of guitars or between guitars with backs and sides made of a particular type of wood, regardless of whether the wood is tropical or non-tropical.
      This leads us to questions and conclusions that extend beyond the issue of whether guitars are tropical or non-tropical. The two-type concept was the starting point of the LGRP investigation. However, as it has been confirmed that, in blind tests, one type cannot be significantly distinguished from another, this classification, based on widely held but incorrect beliefs, appears to be purely artificial and essentially no longer valid. Nevertheless, as long as the guitar community believes that acoustic guitars sound better with exotic woods, this kind of research will remain relevant.

Note: A similar study led by Lancaster University (UK), comparing "traditionally prized, expensive, and rare woods" with "cheaper and more readily available woods" came to a comparable conclusion: "The poor ability of guitarists to discriminate under blinded conditions between guitars with backs and sides made of different woods suggests that back wood has only a marginal impact on the sound of an acoustic guitar."  The study was published in 'The Journal of the Acoustical  Society of America', December 2018. (link to the paper).

Discussion and questions.
While preference scores consistently hover around a 50/50 ratio, creating the illusion that all guitars sound more or less the same, there are, in fact, subtle variations in sound, timbre, and volume, not only between tropical and non-tropical guitars but also within the same type, whether or not they are tropical.

The soundboards of all guitars are as similar as possible, but because they are composed of organic material, they are not "perfectly" identical. For instance, a soundboard may be slightly denser or less quarter-sawn than another, resulting in variations in stiffness and other characteristics that affect the sound.

The questions we currently have no answers to, are as follows:
● Are the (small) differences in sound between the guitars used for the tests, apart from the fact that they belong to the tropical or non-tropical group, the result of using different types of wood for back and sides, or are they the result of inevitable differences in the different spruce tops? (or in both?)
● What factors play a role, and to what extent? (this question also applies to other elements within the instrument)
● Could luthiers build differently (techniques, materials) and in a more targeted way if they knew the answer?
These and many related questions offer fascinating opportunities for further research.

Further research
One way to answer the above questions would be to build a series of guitars with different types of wood for the back and sides, but with an identical top made from a controllable material, such as carbon-based composite or an eco-friendly alternative like flax fiber. Various builders of stringed instruments and guitars have already achieved promising results with these materials.
      This is just one example of an experimental design that uses constant and variable elements for comparative empirical research on the acoustic properties of wood throughout an instrument. Many variations are possible...