Overview

LGRP Phase 1
Between 2012 and 2014, we conducted comparative playing and listening tests, both blind and non-blind, on the sound perception and preferences of guitars made of tropical wood and guitars made of non-tropical wood. 15 classical guitars were tested by 3 experienced guitarists in room and concert hall conditions at two locations (Belgium, Finland). / See report #1

LGRP Phase 2
Between 2014 and 2017, we conducted blind and non-blind comparative playing and listening tests with 20 classical and 24 acoustic steel-string guitars, made of tropical and non-tropical woods, built by 22 luthiers and tested by 20 professional guitarists over 3 countries. The tests were first conducted in Belgium and then repeated in Finland and the United Kingdom, with different guitars built in each country using strictly the same guitar design and controlled building process, the same research design, methodology, and procedures of data recording and processing. / See report #2
At the end of 2017 we organised a blind online listening test with 16 classical guitars in which 226 assessors participated. / See online listening test'

Apart from the comparisons of different tone-woods we conducted, 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 Laboratory for Wood Technology (UGent-Woodlab) to conduct a similar experiment using the same LGRP setup and measurement method plus a new setup method thanks to which the 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

Comparative tests / Tropical versus non-tropical
All comparative tests 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 blinded conditions, players and listeners cannot distinguish between the two types of guitars at better than chance levels.
The preferences for one type over another is equivalent and did not result in statistically significant differences at a 5% significance level from the average, applying to all individual assessments compared with each other per country. All the results of the blind tests in the various studies combined show a preference ratio of 50/50, for both classical and steel-string guitars.

In the non-blind experiments, on the other hand, the prevailing bias in favour of tropical woods was strongly confirmed, especially in our first study (2013/2014) where results show a marked fallback in preference of some 50% for the non-tropical wood guitars, indicating that sound perception can be strongly influenced by visually transmitted information such as the 'look' of an instrument (dark versus blond woods !), or the preconceptions surrounding ‘good’ and ‘bad’ tonewoods that their recognition allows.
A few years later, during the "Phase 2" studies, most assessors were aware of the LGR-Project and got enthusiastic about it. That is probably the reason for the less pronounced bias in favour of tropical guitars, although that preference was still present but to a lesser extent.

Comparative tests and psychoacoustics
To draw general conclusions about the studies and to understand our observations, we need to look at a number of rather complex, often occurring psychoacoustic phenomena and ask ourselves to what extent we need to take them into account in order to interpret the results.

Test results and additional observations indicate a high degree of subjectivity and inconsistency of perceptions and assessments, which vary from assessor to assessor but often also within the same assessor (with sometimes strong changing perceptions and preferences from one assessment to another about the same guitar, being under the illusion it were 2 different guitars). Despite (or because of?) the inconsistencies per assessor, however, the end result in terms of preference between the two types of guitars is always around the 50/50 ratio. The low degree of consistency in the ratings, which averages around 33%, appears to be interrelated to the finding that the sounds of the different guitars were indeed very similar, and not because the assessors had “poor” hearing.
After all, when sounds are very similar, or even actually identical, as in some parts of our experiments, the brain of the blinded assessor tends to fill in the supposed 'differences' itself, unconsciously creating and self-confirming new and different biased sonic landscapes for the same guitar. The brain doesn’t just register sounds; it decodes, interprets, and modulates the perceived audio material, with the ability to focus on certain sonic characteristics and fade others, comparable to what one could technically do with an analog or digital equalizer, but here using the ear-brain system, while one is not, and cannot even be, consciously aware of this process!
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 ).
The clear presence of an 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' brain was reset to zero between the first evaluation and the second evaluation of the same guitar, allowing for a different mental assessment process to take place. Such seemingly surprising brain mechanisms are well known in psychoacoustics and neuropsychology, and obviously cannot simply be considered as 'abnormal'. On the other hand, we may conclude that the human ear-brain system is not a reliable measuring tool, not even in blind tests.

To empirically investigate this phenomenon of shifting perceptions (which were already strongly evident in our 'Phase 1' experiments), and also to check our methodology and experimental design, we integrated (during Phase 2, Belgium) the so-called 'strange guitar experiment': an additional blind test, where a guitar of a completely different model and with a different bracing system was placed between the project guitars. All players spontaneously reported in their comments, which they were able to provide during testing, that they could now perceive clearly different and definable tonal qualities in this particular guitar compared to the others. Unlike our comparative experiments with guitars of the same design, the testers' perceptions remained consistent throughout the entire testing session, precisely because the tonal qualities were clearly provided by the nature of the instrument itself. Identifying the sound did not require the activation of creative mental 'imagery' processes.

Regarding our comparative studies we need to ask ourselves whether the degree of (in)consistency in players' ratings can also indicate the extent to which the guitars are distinguishable from each other, rather than simply statistically processing the bare numerical ratings per guitar, without taking into account the discrepancy in perception caused by undeniable psychoacoustic phenomena.
The consistency of the inconsistent ratings does indeed seem to explain the consistency of the guitars' tonal characteristics. If the sounds of the different guitars had varied more, the results would likely have been more consistent.

All results together lead to a twofold general conclusion:
1) As long the guitar design and the material and quality of the sounboard remains the same for both types, alternative non-tropical woods can be used to build equal sounding guitars as those made from the commonly used exotic woods.

2) Since our blind playing and listening tests revealed no significant difference between the two guitar types, this suggests that it is primarily the model, the top wood, the bridge design and the bracing system (as the only constant factors in our experiments) that determine the sound, and much less the back and sides, contrary to popular belief. If this is the case, it obvious explains why it is difficult to distinguish between the two guitar types, or between guitars with backs and sides made of a specific wood, regardless of whether that wood is tropical or non-tropical.
This brings us to questions and conclusions that go beyond the tropical versus non-tropical issue; the two-type concept was the starting point of the LGRP investigation, but, since it now 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. On the other hand, as long as the guitar community continues to believe that acoustic guitars sound better with exotic woods, this kind of research remains 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 suggest 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 Amerika', December 2018. (link to the paper).

Discussion and questions:
Although preferences always fluctuate around a 50/50 ratio, giving the impression that all guitars sound more or less the same, there are indeed small (but sometimes difficult to perceive) differences in sound, timbre and volume, not only between tropical and non-tropical guitars but between guitars within the same group as well, whether they are tropical or not.

Over all guitars the soundboards are as identical as possible, but they are not 'perfectly' identical, as they are made of organic material. Sometimes, for example, a particular topboard is slightly denser or less quarter-sawn than another, resulting in differences in stiffness and other parameters, which in turn effects 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?)
•Which factors play a role, and to what extent?
•Could luthiers build differently (techniques, materials) and in a more targeted way if they knew the answer?
These questions offer fascinating opportunities for further research.

Further research
One way to investigate the above questions would be to build a series of guitars with different types of wood for back and sides and with truly identical tops made from a material that can be controlled such as carbon-based composite materials or more ecological alternatives such as flax fibre, with which various builders, both for bowed instruments and guitars, have already achieved promising results.