Vibroacoustics and psychoacoustics investigation of biodegradable composites for musical instruments

Due to the extinction of some wood species used to manufacture guitars, there is a trend to search new alternative materials with the same acoustic performance. There is a wide range of tonewoods: ebony, rosewood, yellow and red cedar, mongoy and sapelli, among others. These woods have high resonance qualities and the requirements are both acoustical and mechanical. In this work, alternative composite materials have been manufactured to compare dynamic and psychoacoustic parameters with the commonly used woods. This comparison will allow to conclude the possibility of using these materials as substitutive of tonewoods.


Introduction
Deforestation in some parts of the planet is becoming a threat for the use of some woods in musical instruments, specially the tropical species.The current situation makes a need of substituting these materials for these applications.Some researchers focus their works on other types of wood [1] [2].In this work, the vibroacoustic parameters of biodegradable materials, as substitutive of woods, are studied.Table 1 shows some common woods used for guitars: Table 1.Commonly used woods.
Table 2 shows the characteristics of the three materials manufactured as alternative to woods.With these materials, the goals of the investigation are: -To compare the results of the FFT.
-To compare the psychoacoustic parameters.
The results will allow to evaluate the possibilities of using these alternative materials.

Psychoacoustics parameters
Noise is defined as an audible sound which disturbs the silence or leads to annoyance.The evaluation of noise depends on the physical characteristics of the sound, on the psychoacoustical features of the human ear and on the psychological aspects of people [5].The main psychoacoustic parameters are the following: The loudness is a subjective measurement of the content of energy of a perceived sound.It is related to the decibel as it ranges from strong to weak sounds.

Sharpness
Sharpness is the frequency content of a sound.A sound with higher frequencies is sharper.

Tonality
Tonality is a quality that allows to classify a sound from low to high frequencies.

Manufacturing
To manufacture the samples, the Vacuum Infusion Process (VIP) technique has been used.It is a method to manufacture composite materials reinforced with fibers.This process utilizes the atmospheric pressure to push the resin into the mold cavity.A polystyrene resin is used as cohesive material.
The process consists of impregnating a dry preformed fiber with a thermosetting resin of low viscosity, by a pressure difference between the cavity of the preform and the deposit of the resin, sealed with a vacuum bag.The pressure difference is made by means of a vacuum pump, which forces the resin to move gradually inside the cavity.This process requires a control over the time of impregnation of the fibers, not being too fast (which would cause porosity) or too slow (causing a cure that prevents the advance of the resin in the preform).Figure 1 shows the manufacturing process of one of the samples.

Experimental Tests
Two different tests have been carried out to determine the acoustic parameters required to compare the materials: 2.2.1 First experimental set-up.
Figure 3 shows the first experimental set-up for a dynamic test.The FFT is obtained by means of this test.The working conditions of the shaker are: 5 Hz, 4 V and 0,33 A.

Second experimental set-up
Figure 4 shows the scheme of the experimental set-up to determine the psychoacoustic parameters.

Dynamic results
To compare the behaviour of the materials, the FFT are represented in Figures 5 to 11.The frequency range is from 20Hz to 4200 Hz, which is inside the human hearing field.The reference is a piano which is from 27,5 Hz to 4186 Hz.The same analysis is carried out for the alternative materials: thick wood, hemp and jute (Figures 12 to 14), so that the FFT graphs are compared to the new materials.

Psychoacoustic results
For the psychoacoustic parameters there are results from a survey in which people compare the subjective sensation.Other parameters are obtained with software dBFA.
Tables 3 and 4 show the psychoacoustic results for the common woods and for the new materials, respectively.

Conclusions
It can be concluded that the thick wool material has a similar behaviour to Engelmann fir and yellow cedar woods.The most representative peaks are at 500Hz, 700Hz, 900Hz, 1300Hz, 1900Hz y 2900Hz.
For this reason, these materials present more representative peaks in the audible range and have harmonics wider and balanced.
After the study, the thick wool composite material would be applicable as an alternative to the commonly used woods in musical instruments.

MATEC
Web of Conferences 184, 01005 (2018) https://doi.org/10.1051/matecconf/201818401005Annual Session of Scientific Papers IMT ORADEA 2018 1.1.4.Roughness Roughness is the subjective perception of the amplitude changes.It depends on the modulation of the frequency and on the sound level pressure.

Figure 2
Figure 2 shows the three test samples manufactured.

Table 3 .
Psychoacoustics results of common woods.

Table 4 .
Psychoacoustics results of the new materials.